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Engine Sweet Spot

Dayton Murdock

Well Known Member
Hi All
Is there a sweet spot for engine operation, I run most for the time at 2500 RPMs, 20" MP , at 9.5 GPH @ 8500 DA. Is there a better combination for cruise power?
 
Depends on what you mean by "sweet"

If you mean just for the engine, regardless of the overall effect on speed and economy then I suggest you look for some charts in the engine documentation. Specifically, I can offer the example of my Superior IO-360. Buried in the various information that comes with it is a chart that points out that the best SFC is at 8 GPH. There is more, of course, but your engine is probably at least a bit different anyhow.

The sweet spot should be where you can run LOP and get the lowest (=best) SFC which is measured in pounds per HP per hour. Proper use of LOP lowers cylinder pressures, elongates the power pulse a bit, keeps temps down, probably minimizes vibrations - at least the ones caused by the engine.
 
Hi Dayton,

There's a series of articles by John Deakin called the Pelicans Perch that is absolutely fundamental for understanding where and more importantly why you should run your engine at a particular setting.

https://www.avweb.com/flight-safety/pelicans-perch-index/

I would say the best place for you to start is article #63, then #64, #65, #66, but then again they're all worth reading and some more than once.
In the 20+ years I have been flying aircraft, this series of articles is unquestionable the most valuable assembly of literature I have discovered. Operating under this modus operandi will ensure your engine runs cleaner and your wallet remains fuller.

Regards,

Tom
RV-7
IO-360M1B
 
Hi Dayton,

There's a series of articles by John Deakin called the Pelicans Perch that is absolutely fundamental for understanding where and more importantly why you should run your engine at a particular setting.

https://www.avweb.com/flight-safety/pelicans-perch-index/

I would say the best place for you to start is article #63, then #64, #65, #66, but then again they're all worth reading and some more than once.
In the 20+ years I have been flying aircraft, this series of articles is unquestionable the most valuable assembly of literature I have discovered. Operating under this modus operandi will ensure your engine runs cleaner and your wallet remains fuller.

Regards,

Tom
RV-7
IO-360M1B

I absolutely agree about Deakin's articles, based on a lot of solid testing and experience, as well as sound science. But it is primarily directed at injected engines for which the injectors have been adjusted for narrow "GAMI spread." If Dayton has a carbureted engine, he may be able to get slightly LOP, but probably can not follow Deakin's methods.

I'm also guessing from Dayton saying that he typically runs 2500 RPM that he has a fixed-pitch prop. For those of us with constant speed props, better efficiency is to be had at lower RPM. The cool thing is that once you are LOP, the power output is primarily a function of fuel flow and RPM no longer plays a role in power delivered. (you get all the energy out of the gas converted to power, and that's all you get. So 2500 RPM and 2350 RPM produce about the same power if the fuel flow is the same and you are 50 degrees LOP) But with a fixed pitch prop, your cruise speed is directly linked to your RPM. Reducing RPM means slowing the airplane down.
 
Hi Steve,
I would suspect your guess that Dayton is running a fixed pitch prop is part of the reason he has reached out to us.
Dayton used to run a fixed pitch prop and now runs a CS. Thus, he obviously has a tendency to operate where he has formerly been comfortable, which is not necessarily the "sweet spot" for CS operation, and at 9.5GPH at 20" and 2500rpm I'd say it's not LOP, nor optimal for this engine in cruise.

Here's Dayton's kit log:

http://www.mykitlog.com/n9174l

If you look at the manual, page 3-9 has a chart that clearly shows the optimum brake specific fuel consumption occurs slightly lean of the peak EGT.

https://www.lycoming.com/sites/default/files/O-320 Operator Manual 60297-30.pdf

This is backed up by the chart on page 3-18 that shows a step change in efficiency when dropping below 75% power.

Even if you lean to the peak EGT as the manual recommends when less than 75% power, this assumes that you are looking at the EGT from one exhaust, so likely some of the cylinders are operating LOP anyway.

Here's the specific leaning language from the manual:


1. LEANING TO EXHAUST GAS TEMPERATURE GAGE.
a. Normally aspirated engines with uncompensated carburetors.
(1) Maximum Power Cruise (approximately 75% power) – Never lean beyond 150°F on rich side peak EGT unless aircraft operator’s manual shows otherwise. Monitor cylinder head temperatures.
3-4
LYCOMING OPERATOR’S MANUAL SECTION 3
O-320 SERIES OPERATING INSTRUCTIONS
(2) Best Economy Cruise (approximately 75% power and below) – Operate at peak EGT, or if desired, drop 50°F on rich side of peak EGT.
2. LEANING TO FLOWMETER.
Lean to applicable fuel-flow tables or lean to indicator marked for correct fuel-flow for each power setting.
3. LEANING WITH MANUAL MIXTURE CONTROL (Economy Cruise, 75% power or less, without flowmeter or EGT gage).
a. Slowly move mixture control from “Full Rich” position toward lean position.
b. Continue leaning until engine roughness is noted.
c. Enrich until engine runs smoothly and power is regained.
As shown in Figure 3-1, if engine speed and throttle setting are kept constant at normal cruise conditions, the effect of leaning on engine power and engine temperatures will be as shown. Power drops rapidly when the engine is leaned beyond peak exhaust gas temperature; also, best power is attained on the rich side of peak exhaust gas temperature.


So in summary, the manufacturer supports running at peak or slightly lean there of and all John Deakin does is better explain the theory behind why. I'm now flying a C182 with a O-470U that has spent its former life drinking fuel ROP. We now run it LOP with a little loss of speed but massive reduction in fuel consumption and it runs as smooth as silk.

If I was to sum up how I run my IO-360 engine in cruise at any level, I pull the MP back to 22" (I use this number as it is what I see at 8000' when WOT and I run RON98 mogas so I give myself a good detonation margin) and set my RPM to the smoothest/least vibration setting (between 2350 and 2400) then do the big mixture pull and richen until smooth. If I want more power, I richen further, giving me a higher CHT and EGT as a result.

Tom.
RV-7
IO-360M1B
Also an aerospace engineer, but still not nearly as smart as the likes of DanH and rv6ejguy.
 
Hi Steve,

So in summary, the manufacturer supports running at peak or slightly lean there of and all John Deakin does is better explain the theory behind why. I'm now flying a C182 with a O-470U that has spent its former life drinking fuel ROP. We now run it LOP with a little loss of speed but massive reduction in fuel consumption and it runs as smooth as silk.

.

I think this is a pretty severe mis-characterization of what Deakin says. Unlike the factory leaning procedure, Deakin will tell you that is the worst place to run the engine. Yes, below 75% power, you won't hurt it. But it still not best for engine life or fuel economy. You really need to be 50F LOP. But again, with a carbureted engine, all bets are off as to how lean you can actually get.

Glad to hear you can run your O-470U slightly LOP. I have a C182B with a O-470L and I can not. I lean it as far as it will go, running pretty rough, then richen until it smooths out. My GEM says that is at peak EGT. As much as I would love to run leaner, I go 25F ROP from there. this is all WOT at 8,500 or above.
 
I don't believe there is a sweet spot so to speak. Our engines are designed to run at a variety of Pwr settings, it's personal choice. I never run LOP, am old school, been operating engines ROP for 40 years, I prefer best power. My 8 which has the usual IO360 CS prop I drive 23/2400 ROP, to me fuel is the only known factor of running an engine, each to their own:) At that Pwr setting it runs like a Swiss watch:)
 
Better MPG

I guess I should of given you more information, My set up is:

O360 lycoming Carburated with C/S

currently I see 184kts true at 8500 alt , 2500 rpm 20" MP and 9.5gph

Is there a better setting for MPG, I have slowed down, change alt but my MPG seams to stay around 21ish MPG?

Will a slower RPM help?
Will a higher ALT help?

What is your cruise settings?
 
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O360 lycoming Carburated with C/S

currently I see 184kts true at 8500 alt , 2500 rpm 20" MP and 9.5gph

You seem to have a very fast plane. You're approaching VNE in cruise (191kts I think??).


EDIT: looks like you have an RV-4. I presume an O-360 is a large engine in that plane???
 
Is there a better setting for MPG
What is your cruise settings?

Yes, LOP will get you the best MPG but you won't be going as fast. And the higher you go, the better MPG, perhaps by a very appreciable amount.

Just to give you some numbers for my engines
with my RV7A (IO360, CS), I use to cruise at around 154k true and burn 6.6G on a X-country trip

With my RV14A (IO390, CS) I am getting 165K true and 8.0 if I go high and 8.5G if I stay around 12K

These are LOP numbers.
 
I absolutely agree about Deakin's articles, based on a lot of solid testing and experience, as well as sound science. But it is primarily directed at injected engines for which the injectors have been adjusted for narrow "GAMI spread." If Dayton has a carbureted engine, he may be able to get slightly LOP, but probably can not follow Deakin's methods.
...

Steve, Dayton has dual P-mags that have been tuned with his EIC on his O-360. Same setup as I have, only he has a CS prop and I have a FP prop.

The P-mags will allow the O-360 to run LoP very smoothly. I typically run 50? LoP, sometimes higher. (I'm looking at a picture of me cruising at 45% power, 143 knots true, burning 5.6 GPH at 7300' DA.

360's seem to have a better intake plenum than the O-290 and 320 as I could not run LoP with my 290 and some of our customers report that they cannot run LoP with their O-320's.
 
Hi Dayton,

There's a series of articles by John Deakin called the Pelicans Perch that is absolutely fundamental for understanding where and more importantly why you should run your engine at a particular setting.

https://www.avweb.com/flight-safety/pelicans-perch-index/

I would say the best place for you to start is article #63, then #64, #65, #66, but then again they're all worth reading and some more than once.
In the 20+ years I have been flying aircraft, this series of articles is unquestionable the most valuable assembly of literature I have discovered. Operating under this modus operandi will ensure your engine runs cleaner and your wallet remains fuller.

Regards,

Tom
RV-7
IO-360M1B

Thanks for that post Tom! I read all of the engine articles and will be referring to them a few more times for sure. I have to say Deakin?s method to descend using mixture and RPM to manage power is new to me and interesting to say the least. Very good reading.
 
Happy Place

Rather than look at the science of an operational sweet spot, I take a gut feeling.

Personally, every airplane I’ve flown feels best at some combination of throttle, mixture, altitude, etc.

My current RV-8 has a carbureted O360 with a custom air box and dual magnetos and a fixed pitch propeller - a [mostly] basic setup.

If feels good at about 2550 RPM and 160kts (+/- a little).

As you can tell, it’s not the fastest possible setup. However, given the airframe, engine, and propeller combination, this is where it feels good to me. It can go faster but feels like it’s working harder than it needs to. It can fly slower with lower fuel burn but that feels like it’s loafing.

I suspect, another pilot flying my RV-8 may well feel it is operating best at some other combination of settings. I say, do what feels right :cool:
 
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Yep

Rather than look at the science of an operational sweet spot, I take a gut feeling.

Personally, every airplane I?ve flown feels best at some combination of throttle, mixture, altitude, etc.

My current RV-8 has a carbureted O360 with a custom air box and dual magnetos and a fixed pitch propeller - a [mostly] basic setup.

If feels good at about 2550 RPM and 160kts (+/- a little).

As you can tell, it?s not the fastest possible setup. However, given the airframe, engine, and propeller combination, this is where it feels good to me. It can go faster but feels like it?s working harder than it needs to. It can fly slower with lower fuel burn but that feels like it?s loafing.

I suspect, another pilot flying my RV-8 may well feel it is operating best at some other combination of settings. I say, do what feels right :cool:

I agree with this. I tested a whole bunch of stuff to get a feel for my plane after all is said and done I just like certain altitudes and I basically run at about 9.0GPH@2400RPM and let the speed fall out. Sometimes I choose 8.5 and sometimes 9.5, but basically if just feels comfortable.
 
No worries Eddie.

Running 2450rpm, 11gph and 171kts used to feel right to me, because running ROP was all I had ever been taught, but since become enlightened regarding the issue of ROP vs LOP, what feels right is now 2350rpm, 22in MAP LOP and 163kts for 7.5gph. Sacrificing 4.7% of speed for a 31.8% reduction in fuel burn just makes sense and now extended periods or ROP operation feels dang wrong.
Part of feeling right is often rooted in understanding, which is what the OP had reached out to obtain. Those Deakin articles were a prime part of the pathway to getting me to that new point of comfort.

T.
 
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Dayton. One of the things I do not see mentioned is the throttle position. I, like almost everyone with a CS prop, always run WOT in cruise. This is the basis of everything else. From there power is adjusted using RPM and mixture. The WOT is often a surprise to those used to running a constant pitch prop.

Personally, I use WOT, 2400 rpm and about 125 ROP EGT or best power. I am generally not concerned about LOP. The fuel savings is not that important to me. I built a fast airplane to fly fast. I find that my Hartzell BA is nearly as fast at 2400 as it is at 2700, only about 2-3 knots difference, and yes 2400 “feels” right. I fly at 10500, -11500 and I think I get 24 miles to a gallon last time I checked IIRC

And yes understanding the Deakin articles are essential.
 
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Let's assume good reciprocating and rotational balance, mechanical issues not subject to inflight control. The two smoothness factors which may be influenced by the pilot are cycle-to-cycle variation and resonant vibration.

Not every cylinder charge lights and burns at the same rate. There are multiple factors at work here. The mixed fuel and air presented to the spark plug at the desired moment of ignition may not be homogeneous, being either too rich or too lean in the immediate vicinity of the spark, so initial inflammation may be slow. The overall mixture may be very lean, both more difficult to light and slower to burn. The spark duration lasts for a finite time, through some range of crankshaft rotation, and may achieve ignition at some point later than the planned initial spark discharge. And of course sometimes a cylinder fails to light at all, a misfire. Regardless of root cause, peak pressure and the point of peak pressure vary from optimum, and cylinders make an uneven contribution to torque. We call it roughness, and fix it with the mixture knob.

Resonant vibration happens when a periodic driving force is applied to an object, and the period frequency is at or near the object's natural frequency. When the driving frequency is within a range of (caution, rough example) 0.9 to 1.1 times the natural frequency, the object vibrates powerfully, with the peak amplitude happening at Fd = Fn. In theory, amplitude can reach infinity, and doesn't only because the world is full of damping factors.

Probably the most significant pairing in the context of "engine sweet spot" is one in which the prop blades are the resonating victim. It's the basis of the "Don't Operate Here" RPM and MP combinations.

Less appreciated is the fact that our airplanes are full of structures, all of which have a natural frequency. Any of them with a natural frequency which happens to match one of the multiple driving frequencies emanating from the flailing and whirling parts up front may vibrate, some with considerable amplitude. Here's the takeaway; it is possible to have a very smooth running engine, and still have an irritating vibration notable in the cockpit. Although the powerplant is vibrating at low amplitude, the magic of resonant amplification can result in an airframe component with high amplitude vibes.
The pilot's perception, based on what he feels, can be misleading. He may make a power change (notably RPM in this case) until the vibe subsides, when the real cure may be to add some damping to an airframe component, or change its natural frequency.

Steve, Dayton has dual P-mags that have been tuned with his EIC on his O-360. Same setup as I have, only he has a CS prop and I have a FP prop. The P-mags will allow the O-360 to run LoP very smoothly.

Two points please.

Tom and Steve mentioned Deakin articles. Advanced timing for LOP is not within that school of operating thought.

In the fixed timing approach, peak pressure is purposely moved further from TDC by slowing combustion speed with lean mixture, then an inch of MP is added to compensate. Easy to do in your turbocharged whatever.

With variable timing, we add advance, moving peak pressure back to where it was at best power mixture, ballpark being 15 ATDC.

With the former, peak pressure is lower near TDC and a bit higher near exhaust valve opening. With the latter, peak pressure is high near TDC and lower at valve opening. Nothing wrong with either approach, but they are quite different. You can see it on the gauges. Fixed timing slow burn is low CHT and higher EGT. Advanced timing to compensate for slow burn is high CHT and lower EGT.

Second, timing advance is not required to run smoothly when LOP. The ability to light a lean mixture is a function of healthy spark across a wide plug gap, not timing. It can be done with mags and narrow gaps, but in our world, it means an electronic ignition. However, we should not conflate the typical EI's two entirely separate features, (a) high spark energy and (b) spark timing.

Break

Dayton mentioned 8500 alt, 2500 rpm 20" MP and 9.5gph. That should be around peak, rather than significantly LOP. Nothing wrong with that either.
 
How can we thank DH enough?

For teaching us what many are capable of but not investing the effort to do so,,,,THANKS DAN
 
Wow Dan, that is a lot of thought going there. Reminds me of the time I was sitting in a gondola with a ski instructor checking out two fledgling instructors. Head guy asked “ what are the five techniques for executing a turn”? The two guys proceeded into a 5 minute dissertation on turn techniques. As they finished, the gondola doors opened and as I exited I looked back and said “ ya, and don’t forget to follow the tips of the skis”

Set WOT,, pick an allowable RPM you like, set EGT to best power or LOP, sit back, enjoy View.
 
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I like flying my 8 with O-360 just a tad below WOT (to close enrichment circuit) at 2280 rpm, 7.2 gph and 182 mph TAS. Typically at 19-20 inHg MP (12,500 - 10,500 ft MSL). CHTs circa 320 F, EGTs circa 1380F. Smooth, quiet and plenty fast for my needs.
 
I think a key take-away for Dayton should be that now that he has a constant-speed prop, he should lower the RPM significantly, something in the 2400 range. This is a more efficient operating point for the prop. Then, to make up for the power loss from the lower RPM, increase the manifold pressure. Yes to WOT!

Then, mixture is the other variable. leaning more makes you a little bit slower, but saves a lot of fuel. Richening to best power mixture gives you the best speed. You get to pick, depending on your preference and specific circumstances.

My normal cruise is WOT, 2400, 7.8 to 8.4 GPH, depending on altitude. This gets me a pretty consistent 25 MPG.
 
today?s test

Hi All
Thank you for all the responses on my thread. Today I tried a new cruise setting 2350 RPM with wide open throttle at 8500 feet.
22.2 Inches of Manifold pressure 180 knots true airspeed 8.8 gallons per hour.Then I throttle back to 20 inches of manifold pressure at the same altitude with 172 knots true airspeed and 7.7 gallons per hour fuel burn. I believe I?m going in the correct direction for better fuel economy just slowing down a few knots and changing the engine RPMs has made a significant difference in my fuel burn .I found a picture in my phone from earlier this year of my EFIS I was at 7500?20? of manifold pressure 2500 RPM and burning in 9.9 gallons per hour and 172 kts . Please share your engine settings
 
Dan has the combustion nailed. Regarding a sweet spot, it is (the engine) kinda designed to have one at the recommended continuous power speed, or cruise speed. That, back when designed, was 1800 ft/min piston speed. It is where the pumping and friction losses are rising rapidly with RPM, and might be otherwise called the peak torque speed (if actually measured). That is precisely 2468 rpm for the 360. Most comments here in are in that ball park.

A data point for ya: I just came back from a cross country and cruised at 11.5k DA (10k BA), 2490 RPM, 7.5 GPH and 161KTAS. Standard Slicks, stock M1B, wishing for some EI and advance. That is better than I was getting on the way down at 9400 DA(8k BA). I just ran 2490 to keep the yellow alert from flashing at 2500, gotta reset that.:eek:

In theory, and practice, best BSFC will be LOP until it hooks up due to the misfire. Variable/proper timing will definitely help the SFC, until it misses. Whether an engine is damaged at peak EGT is more a matter of its physical design. Might be fine, might not. Not much advantage to run there, not best SFC or best power. It is the only place we can run and know exactly what power we are making though so it is good for flight testing.
 
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There is a speed for best L/D that will give you best MPG at whatever altitude you are at but it's too slow for most of us most of the time.

The next best speed is Carson's which is approximately 1.32x best L/D. The explanation is too long for here but can easily be looked up. Some of us would want to go even faster but at the cost of fuel economy.

Then there is where the drag curve intersects the best SFC for the engine. The best SFC for the engine will be altitude dependent to some extent. It will also be best LOP.

You can find the best SFC for some engines in the manual. For the rest, if you have good glass panels you can usually find it even if the actual number reported is in error. The best will be easy to find.

Once you know the best SFC for the engine, you can vary the speed up or down to suit your preferences for speed vs. dollars. Somewhere between Carson's and best (lowest) SFC is the answer. If you have a CS prop you have choices of RPM/MAP for the rest of us the choice is altitude.
 
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Hi there Dayton, I flew over your hangar twice today in and out of Minden-Reno :) Glad to see your results, and thanks for the thread, it's been a good one for me to follow along with.

I enjoyed reading Dan's comments a few posts ago. Bringing electronic ignition specifics into the discussion are right up my alley. I have one P-Mag and one Slick. If I can justify the cost I'll probably replace the Slick with another P-Mag within a year. I consider my Lycoming experience as transitional because it was in a training environment (with the exception of flying 402's for a thousand hours but that was formulaic at best with company power settings, etc). I went to turbines about 26 years ago. That constitutes old school and out of date on Lycoming expertise. I want to continue to learn how to "absolutely best" care and feed a sngle-engine Lycoming powered aircraft that I fly over mountains in, as we know today.

My gut instinct is to run ROP because that's what I remember back in the old days. I see all the data on the pressures and wear ROP. I've played with "the big pull" a few times below 60 percent power up above 10k - and I think I'll need to sort out some gami injectors to dial in more even EGT's (My split on EGT temps LOP seems a lot worse than ROP and I don't want to push it until I really know what I'm doing). Any thoughts on that? For now, I'm ROP by about 150 degrees up high and about 200 down at sea level on climbs (using altitude and % power concepts to stay out of the "red box" in relation to EGT and percent power). It seems my EGT's are peaking out about 1450 or so on the warmest reading up high where I can experiment outside of the red box (momentary peak reading to verify). A sea level climb at full rich has me at about 1220F EGT for the first few minutes until DA increases (about 2 flights a month at sea level). 75 percent of the time I'm at 5000 feet and above though since I live in Reno.

Anyhow, thanks to all the guys in the know on this. I'm really loving getting back into the fray again and look forward to really dialing in my knowledge base on how to keep those numbers pegged in the right setting for my engine. My goal is longevity and reliability of course.
 
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Hi Eddie,

If your fuel distribution isn't perfect, there are ways to deal with this without going the full GAMI injector route.

Inside each lycoming injector there's a tiny little restrictor. Lycoming have done their best to sizing each of these to ensure they result in even fuel/air ratio to each cylinder. In the case of my engine, they were smack on the money (within 0.1GPH). You can assess how well balanced they are in your aircraft by performing the GAMI test, instructions below:
https://gami.com/gamijectors/leantest.php
If you have an EFIS recording one second data, you only have to do the leaning process once, then you can go back to your computer and see exactly what fuel flow which cylinder hits peak EGT.
If you have some running too rich and others too lean, you can always do a swap restrictors and complete another subsequent check. Alternatively you can contact a guy called Don at Airflow Performance and he will supply you with a restrictor to give you the fuel flow based on your GAMI lean test results. You don't need fancy new injectors, you just need this restrictor size changed, which is an order of magnitude cheaper.
Tom.
 
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Hi All
Thank you for all the responses on my thread. Today I tried a new cruise setting 2350 RPM with wide open throttle at 8500 feet.
22.2 Inches of Manifold pressure 180 knots true airspeed 8.8 gallons per hour.Then I throttle back to 20 inches of manifold pressure at the same altitude with 172 knots true airspeed and 7.7 gallons per hour fuel burn. I believe I?m going in the correct direction for better fuel economy just slowing down a few knots and changing the engine RPMs has made a significant difference in my fuel burn .I found a picture in my phone from earlier this year of my EFIS I was at 7500?20? of manifold pressure 2500 RPM and burning in 9.9 gallons per hour and 172 kts . Please share your engine settings

2350 and WOT numbers sound pretty sweet to me Dayton - lean it as far as you can to save fuel, riches it up to go faster. We can?t keep up anyway.....

No need to make it complicated - these engines are wonderfully simple once you get above 8,000....it?s pretty hard to hurt ?em.
 
Good point

I run 2300 rpm and WOT at 8000 or so and above, and couldn't be happier with the fuel burn and performance of my 9A and its carbureted O-320 Lycoming. :)

I have noticed that if I lean really aggressively at that altitude and prop/throttle setting, one of my cylinders (#2) gets substantially warmer than the rest (+50 degrees or so). It is right in line with the others in all other scenarios. I've been wondering if I'm seeing the result of uneven fuel distribution, or maybe something else.

I do know that I never would have noticed this in my old Warrior, with basically the same engine, since my engine instruments consisted of oil pressure and oil temp -- and that's it!

2350 and WOT numbers sound pretty sweet to me Dayton - lean it as far as you can to save fuel, riches it up to go faster. We can?t keep up anyway.....

No need to make it complicated - these engines are wonderfully simple once you get above 8,000....it?s pretty hard to hurt ?em.
 
Whether an engine is damaged at peak EGT is more a matter of its physical design. Might be fine, might not. Not much advantage to run there, not best SFC or best power.

Why do you say so Bill? My own sweet spot is peak EGT or very slightly LOP. Here I'm defining "sweet" as "lowest fuel flow possible without notable airspeed loss". It is not the most economical, but rather, the most economical way to run a fast airplane fast.

Re damage, Lycoming has long recommended peak EGT for cruise. Gas temperature, valve temperature, and EGT mirror each other. An EGT 50 lower says the valve is 50 cooler. In the context of valve temps, 50F is less than 3%, which does not suggest 50 LOP (as compared to peak) offers a notable advantage in long term durability.

There would be difference between parallel and angle valve engines, based on the angle's typically lower CHT; valve seat temperature is probably lower, good for durability.

For sure, at high power peak EGT is beyond the detonation limit (assuming pro-detonation conditions per standards), which would cause damage. It also means best BSFC is not attainable under those conditions.

Re the location of best BSFC, I'm looking at plots showing the BSFC curve as very flat near peak EGT. Absolute best cruise power BSFC as only a wee bit LOP, like 20F max. It's also about where the power curve becomes a steep slope downward. The airspeed loss becomes significant.

My photo upload capability is crippled right now. I'll get the charts up when I can.
 
SNIP
Absolute best cruise power BSFC as only a wee bit LOP, like 20F max. It's also about where the power curve becomes a steep slope downward. The airspeed loss becomes significant.
SNIP

+1 on Dan?s post.

Take a day to balance the injectors and run ~20 degrees LOP, 2350-2500 RPM (simple swap out of $26 injector nozzles from AirFlow Performance). The 20 degrees LOP achieves:
- Significant drop in CHTs
- Significant reduction in fuel flow
- Minimal speed loss

I spend 90% of my time at WOT and this LOP setting. 2350 RPM if low, 2500 RPM if high (above 14K?) and heavy to maintain power level. Typical cruise is 170-173kts TAS.

As Dan points out, not the most economical engine settings but the most economical if you still want to clip along at 170kts or so. This is why we have fast planes, right?

Carl
 
2350 and WOT numbers sound pretty sweet to me Dayton - lean it as far as you can to save fuel, riches it up to go faster. We can?t keep up anyway.....

No need to make it complicated - these engines are wonderfully simple once you get above 8,000....it?s pretty hard to hurt ?em.

Good point. Actually you can pretty much lean with impunity under 22? MP- roughness, fuel consumption, and power become the issues of concern. I?m now installing APFI with the hope of better fuel distribution, but with the carb I usually cruised WOT at altitudes producing MP<22?, straddling peak EGT, with one cyl. ROP and the other 3 LOP. That produced smooth running, good fuel economy and speed for my 7A.

Btw- max power is seen at about 100? ROP, along with max wear and tear, max CHTs, min engine life. I would guess the numbers cited in the first post of this thread coul be in that region- not the best strategy for full time operation.

Flying with CS, For a given power output level, RPM is inversely proportional to BMEP, which in turn is directly proportional both to efficiency and wear and tear. I think THAT is the place to look for a ?sweet spot?. Lugging the engine is at one end of that spectrum and needlessly high RPMs at the other. - Otis
 
+1 on Dan?s post.

Take a day to balance the injectors and run ~20 degrees LOP, 2350-2500 RPM (simple swap out of $26 injector nozzles from AirFlow Performance). The 20 degrees LOP achieves:
- Significant drop in CHTs
- Significant reduction in fuel flow
- Minimal speed loss

I spend 90% of my time at WOT and this LOP setting. 2350 RPM if low, 2500 RPM if high (above 14K?) and heavy to maintain power level. Typical cruise is 170-173kts TAS.


Carl
Based on the recent trips, I am learning more and that the top EGT and LOP numbers are not a standard value for the giving engine. We have been cruising higher (17k+) and I learned that there was only a max 10-15 degree of LOP possible before the engine was running rough where as I can go 90+ LOP down low (8K and up). Changing the PMAG curve to advance to 13 degrees in stead of the usual 10, was giving a bit better LOP numbers from the top EGT but not much change in the speed. Likewise, going ROP did not have much effect on the speed either.
 
You mean MPG? With the fancy electronic engine monitoring today you can get real time economy based on many factors, including ground speed.

There is no way to answer your question because of so many variables. On the other hand there are some practical limits to what power you can set with your Lycoming especially fixed pitch. If you look at the Lycoming manual for your engine for power settings you will see where the sweet spot is.

In general you want to fly at 8500 feet and above at full throttle. You will get MP of about 21 inches (better if you have good ram air box). Your RPM with fixed pitch prop is what you get. If it is too high (more than 2600 RPM) then you have to throttle back. If you have CS I typically set what is smoothest or allowed by the prop limitation, 2300-2500 RPM is typical. Higher RPM more nose and fuel, for a little gain in speed.

If you are cruising down low and you can set what you like (assume CS prop) I like squares, 23 square for example. However if you want to get out of town 25 square is good, but that is burning way more fuel.

Can you fly LOP? Well below 75% power. Can you fly OVER SQUARE (more MAP than RPM). See your engine and prop manual for limitations.

Assuming an RV, average Lycoming, no or light winds, there are go to altitude and power settings we all use for different trip lengths. That is by trial and error.

How and what to set RPM, MAP and mixture has been written about in great detail. Not that a O-360 can't burn a whole bunch of extra fuel if not managed well. A lot of this economy really pays off with +500 cu-in engines. The bottom line is experience and experimentation.
 
We have been cruising higher (17k+) and I learned that there was only a max 10-15 degree of LOP possible before the engine was running rough where as I can go 90+ LOP down low (8K and up).

You're running constant flow injection on your 390? I recently made some measurements. Down low, throttle partially closed, the injector bleed air deltaP is high, so atomization is good. Up high, at WOT, the deltaP is lower, and the nozzle delivery is more blobby.

Marvin McGraw and I compare notes a fair bit. His 390 has SDS EFI, so he is not dependent on bleed air for a nice fuel spray. With 40+ PSI rail pressure and a pintle injector, EFI always has a fine fuel spray. Marvin can run smoothly far, far more LOP up high, and Dave Anders has reported the same. However, remember it's a maximum MPG sweet spot; although smooth, it's also very slow. Power is way down. so the ability to run very lean doesn't get used much.
 
You can fix this!

You're running constant flow injection on your 390? I recently made some measurements. Down low, throttle partially closed, the injector bleed air deltaP is high, so atomization is good. Up high, at WOT, the deltaP is lower, and the nozzle delivery is more blobby.

Snip


We have seen this phenomenon on the Rocket for quite some time; it shows up as fuel staining around the injectors. The better ram air inlet systems REALLY need help. Our plan to address this is to switch to a turbo rail sort of arrangement. The best setups have a pitot line about in the middle of the cowl inlet (one on each side). I set mine up with silicon tubing, and put the inlets on the back side of the air box. No more fuel dripping out of the screens on the injectors, and much smoother running LOP.

You are correct - the high pressure injectors on the SDS system will be immune to this sort of requirement. The std aircraft setup needs to mix air with the fuel inside the injector, and it needs positive relative pressure to work well.
 
You're running constant flow injection on your 390? I recently made some measurements. Down low, throttle partially closed, the injector bleed air deltaP is high, so atomization is good. Up high, at WOT, the deltaP is lower, and the nozzle delivery is more blobby.

Marvin McGraw and I compare notes a fair bit. His 390 has SDS EFI, so he is not dependent on bleed air for a nice fuel spray. With 40+ PSI rail pressure and a pintle injector, EFI always has a fine fuel spray. Marvin can run smoothly far, far more LOP up high, and Dave Anders has reported the same. However, remember it's a maximum MPG sweet spot; although smooth, it's also very slow. Power is way down. so the ability to run very lean doesn't get used much.

Thank you Dan for the info, although I am going to read up more to really understand it fully.
To answer your question, yes as I understand it Airflow Performance is a constant flow injector.
To make this a bit more meaningful and for the community to compare notes, my numbers based on two separate flight each about 3.5 hours, at 17500, my MAP at WOP is about 15" and running at 2500 RPM I burn about 7.3G and speed is about 162-165k. My speed picks up about half way thru the flight and I assume it is due to getting lighter as I burn more fuel but wasn't sure if that is the factor or other things beyond my understanding of things.
One question I have, is there a realistic chance of hurting the engine running so close to the peak EGT at that level of power?
 
We have seen this phenomenon on the Rocket for quite some time; it shows up as fuel staining around the injectors. The better ram air inlet systems REALLY need help. Our plan to address this is to switch to a turbo rail sort of arrangement.

The most recent measurements were taken with turbo injectors and rails.

Lots of folks have tried it, but nobody actually measured anything. Turns out (when measured with rails) there is only one very short period in 720 crank degrees during which port pressure exceeds bleed air pressure, the old explanation for fuel staining. Without rails there were probably more random periods of low bleed pressure, due to ordinary turbulence in the upper plenum. I'm too lazy to take the rails off just to map randomness.

Feel suggests the rails let me run leaner while maintaining smoothness, but measurements are better.

...my MAP at WOP is about 15" and running at 2500 RPM I burn about 7.3G and speed is about 162-165k.
One question I have, is there a realistic chance of hurting the engine running so close to the peak EGT at that level of power?

At 15"? None.
 
In theory, and practice, best BSFC will be LOP until it hooks up due to the misfire. Variable/proper timing will definitely help the SFC, until it misses. Whether an engine is damaged at peak EGT is more a matter of its physical design. Might be fine, might not. Not much advantage to run there, not best SFC or best power. It is the only place we can run and know exactly what power we are making though so it is good for flight testing.


Why do you say so Bill? My own sweet spot is peak EGT or very slightly LOP. [ is this with your EI?] Here I'm defining "sweet" as "lowest fuel flow possible without notable airspeed loss". It is not the most economical, but rather, the most economical way to run a fast airplane

I do agree, Dan. My "Sweet Spot" definition is where the SFC is lowest yielding power and efficiency. The standard curves provided by Lycoming show the BSFC flattening at peak EGT, but that flattening is due to the specific settings and specifications of spark energy and timing, not due to combustion physics. The lean charge is more difficult to ignite, but the sfc can continue to drop below EGT when reliably ignited.

Re damage, Lycoming has long recommended peak EGT for cruise. Gas temperature, valve temperature, and EGT mirror each other. An EGT 50 lower says the valve is 50 cooler. In the context of valve temps, 50F is less than 3%, which does not suggest 50 LOP (as compared to peak) offers a notable advantage in long term durability.

The bold type was for Ross and his comment of not running his O470 at peak EGT or LOP. Any aircraft engine should be able to do that, but there may be some designs that are not happy. A little odd, as Continental , internally, really promoted the LOP operation and more than 50F. Maybe it was for specific models. Not referring to our Lycoming/Clones

There would be difference between parallel and angle valve engines, based on the angle's typically lower CHT; valve seat temperature is probably lower, good for durability.

For sure, at high power peak EGT is beyond the detonation limit (assuming pro-detonation conditions per standards), which would cause damage. It also means best BSFC is not attainable under those conditions.

Re the location of best BSFC, I'm looking at plots showing the BSFC curve as very flat near peak EGT. Absolute best cruise power BSFC as only a wee bit LOP, like 20F max. It's also about where the power curve becomes a steep slope downward. The airspeed loss becomes significant.

This what I find too, but for the parallel valve and two mags, not EI.

My photo upload capability is crippled right now. I'll get the charts up when I can.

Re: your actual data plots, .405 -.410 is about as good as the charts show. It would be very interesting to see how much a high energy ignition system can push LOP temps w/o fall off in power (SFC plateau or rise).
 
So it is not ignition we need to extend LOP?

Sooo, our limitation for extending LOP is atomization/mixing? What volume and pressure of air might be needed? 1" hg, 1 psi? A pitot would be easy, but is more needed?

I did notice my FF wandered at cruise at 10K. It varied from 7.4 to 7.6 GPH. If I leaned more, then as it wandered to 7.3 or lower, it would run rough and I'd have to start the process over. It would take 5-8 min to sneak back down.

Is this part of the issue, or something else?
 
I would love and appreciate it if someone can explain the idea of a running pitot line in the cowl and how that will plumb and work with the injector system.
Is this some how gets connected to the spider to add pressure?!?!?!?

I doubt I will be doing such thing, considering that I can't even understand the concept, but would love to learn.
 
Carb enrichment circuit

Most of this thread is centered on fuel injection, but Dayton and I both have O360 Lycomings, carburated with C/S props.

I normally reduce from WOT in cruise to disable the enrichment circuit and promote better mixture distribution. Can anyone tell us if the mixture control can completely nullify the effects of the enrichment circuit? If so, WOT operation might be feasible if the mixture distribution was close to even.

Thanks, David
RV-6A KBTF
 
Easy peasy

I would love and appreciate it if someone can explain the idea of a running pitot line in the cowl and how that will plumb and work with the injector system.
Is this some how gets connected to the spider to add pressure?!?!?!?

I doubt I will be doing such thing, considering that I can't even understand the concept, but would love to learn.

No big deal - you have to understand that the fuel and air have to mix inside the injector body. Further, if the mixture inside the injector goes rich (as would be likely in an efficient inlet system, as opposed to relying on the upper cowling pressure supplied by the cooling system) due to MP being higher - or close to - upper cowling pressure, the engine will talk back when you try to run LOP above ~8000MSL. The engine manufacturers already make the parts for turbo engines - Tho you might be handy enough to make your own - AirFlow Performance has the required parts ?on the shelf?. Setting up the system is not Rocket Science.

In simpler words, the efficient inlets systems we make up reverse the air flow through the screens in the individual injector bodies due to MP being higher than the under-cowl pressure: the flow is outward instead of inward. The pitot system fixes the flow problem provided by efficient inlet system in the same manner that a turbocharged engine does using a shuttle valve - it is called an ?Upper deck reference system?.

How?m I doin??

If there is a process to put pictures in this forum, I could show you how easy it is, rather than trying to explain it. Those of you who see the engine get ?rough? as you go LOP can feel how we mess things up by not realizing that our super-duper inlets mess up the fuel system.

I have not tried it on my plane, but it COULD be that Dan Horton?s low velocity inlets would have enough pressure to overcome an efficient inlet? I could run that test question after Reno - I don?t want to mess with a working system!
 
Re damage, Lycoming has long recommended peak EGT for cruise. Gas temperature, valve temperature, and EGT mirror each other. An EGT 50 lower says the valve is 50 cooler. In the context of valve temps, 50F is less than 3%, which does not suggest 50 LOP (as compared to peak) offers a notable advantage in long term durability.

There would be difference between parallel and angle valve engines, based on the angle's typically lower CHT; valve seat temperature is probably lower, good for durability.

Hmmm....CHT is a larger factor in valve temperature than EGT, measured in the pipe. The valve spends more time closed than open and during that closed time it is moving heat into the seat and guide. The combustion chamber (including the valve faces) is subjected to its highest exposure to heat during the early part of power stroke. The exhaust valve is exposed to the exhaust gases while open, but this is a fairly short burst of less than peak combustion temperature and it gets right back to the business of cooling off as soon as it slams shut. The only way that valve temp could have a one to one ratio with EGT change would be if the CHT also changed at the same rate. 50* LOP should make for a cooler exhaust valve than 50* ROP because the head is cooler.

Peak EGT (or thereabouts) is going to be where the cylinder pressure is the highest and the CHT, too. To get the same CHT, the mixture needs to be further ROP than LOP (Deakin's red box). At altitude, it doesn't much matter if you run peak pressures because the CHTs aren't high enough to worry about, assuming good cooling. My guess is that CHTs under 400 (380 sounds good) is a bigger factor in longevity than ROP or LOP operation taken by itself.

Ed Holyoke
 
Most of this thread is centered on fuel injection, but Dayton and I both have O360 Lycomings, carburated with C/S props.

I normally reduce from WOT in cruise to disable the enrichment circuit and promote better mixture distribution. Can anyone tell us if the mixture control can completely nullify the effects of the enrichment circuit? If so, WOT operation might be feasible if the mixture distribution was close to even.

Thanks, David
RV-6A KBTF

Sure, the mixture control can fully overcome the so called economizer circuit and a whole lot more. Leave it WOT and pull mixture a little at a time and let it settle out before you do it some more. You'll see an increase in EGTs and CHTs until you feel a fall off of power which tells you you've passed peak EGT. Keep pulling and it'll run rough and then quit. If that's not overcoming the enrichener, I don't know what is. The mixture control is the last word in how much fuel is delivered. Works for the idle circuit too. That's how we shut it down, after all.

When I was running a carbed 320, fixed pitch, I would lean until I felt the drop off and push back in a little looking for smooth running. At altitude, it seemed that pulling throttle just enough to see about a tenth of an inch MAP helped with smoothness while lean running.The theory here is that the slightly cocked throttle plate causes turbulence which promotes mixing of the fuel and air. Another trick that seemed to help was to put in a small measure of carb heat to aid vaporization and lean just a touch more to account for the slightly enriched mixture. The main thing I was always looking for was smooth running and CHTs under control.

I was just re-reading all the John Deakin articles and one thing that he said jumped out at me regarding WOT. I'll paraphrase: There's no functional difference between a partially closed throttle and a dirty air filter and who'd want that? He was talking about fuel injection, obviously. If you get better distribution and a smoother running engine by cocking the throttle a touch, I'd say that's worthwhile. I wouldn't do it as a way to lean, though. The mixture control works better for that.

Ed Holyoke
 
Most of this thread is centered on fuel injection, but Dayton and I both have O360 Lycomings, carburated with C/S props.

I normally reduce from WOT in cruise to disable the enrichment circuit and promote better mixture distribution. Can anyone tell us if the mixture control can completely nullify the effects of the enrichment circuit? If so, WOT operation might be feasible if the mixture distribution was close to even.

Thanks, David
RV-6A KBTF

The carb's mixture window is at the beginning of the fuel circuit and it meters total flow out of the carb's bowl. This includes the enrichment circuit, if you have one. I would guess that closing down the throttle plate introduces some turbulence and that is resulting in better distribution. Several others here report the same. Not sure, but if I remember correctly, the enrichment flow may not go through the main jet in the venturi and therefore that extra fuel would be less atomized.

Larry
 
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The most recent measurements were taken with turbo injectors and rails.

Lots of folks have tried it, but nobody actually measured anything. Turns out (when measured with rails) there is only one very short period in 720 crank degrees during which port pressure exceeds bleed air pressure, the old explanation for fuel staining. Without rails there were probably more random periods of low bleed pressure, due to ordinary turbulence in the upper plenum. I'm too lazy to take the rails off just to map randomness.

Feel suggests the rails let me run leaner while maintaining smoothness, but measurements are better.
.

Dan, can you clarify and expand on this a little? I have a direct (unfiltered) ram-air intake to a horizontal induction sump. I have roughly zero GAMI spread (certainly within 0.15 GPH) , and at my typical 9.5K-10.5K cruise altitudes, I can lean quite a lot (more than 50F LOP, often70--80F LOP) and stay smooth. I do get a lot of fuel staining. Would I benefit from a turbo-type pressure rail system?

BTW, I'm loving this conversation. Once again, a seemingly innocent question by someone (Dayton in this case) leads to a treasure-trove of rich insight (or lean insight? ;) )
 
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No big deal - you have to understand that the fuel and air have to mix inside the injector body. Further, if the mixture inside the injector goes rich (as would be likely in an efficient inlet system, as opposed to relying on the upper cowling pressure supplied by the cooling system) due to MP being higher - or close to - upper cowling pressure, the engine will talk back when you try to run LOP above ~8000MSL. .....

In simpler words, the efficient inlets systems we make up reverse the air flow through the screens in the individual injector bodies due to MP being higher than the under-cowl pressure: the flow is outward instead of inward. ....

While we are on this subject -- does everyone realize that with a good ram-air inlet, and the sniffle-valve outlet being plumbed to the cowl exit, it is surely the case that our sniffle valves are leaking manifold pressure out the cowl exit. The sniffle valve is nothing but a check-valve that opens when manifold pressure is equal or higher than ambient pressure at the outlet.

Sorry this may seem like a tangent to the engine sweet spot discussion, but it bears directly on the idea that our induction system is efficient enough to create higher MAP than cowl pressure, either above or below the engine.
 
Agreed!

While we are on this subject -- does everyone realize that with a good ram-air inlet, and the sniffle-valve outlet being plumbed to the cowl exit, it is surely the case that our sniffle valves are leaking manifold pressure out the cowl exit. The sniffle valve is nothing but a check-valve that opens when manifold pressure is equal or higher than ambient pressure at the outlet.

Sorry this may seem like a tangent to the engine sweet spot discussion, but it bears directly on the idea that our induction system is efficient enough to create higher MAP than cowl pressure, either above or below the engine.

I look at that drain every time I?m under the plane and think the same thing.

I wonder if this is a real problem - if so, put the drain in clear air and point it forward. I would think it would ?leak? less that way?

I am tweaking that exact area now - maybe I can do something to let that outlet see clear air...
 
Sooo, our limitation for extending LOP is atomization/mixing?

That is my conclusion so far.

What volume and pressure of air might be needed? 1" hg, 1 psi? A pitot would be easy, but is more needed?

Any bleed air pressure source based on airspeed has the same limitation; in general terms, it can't be a lot higher than manifold pressure at WOT. Still, we can attempt to gather all there is with careful design. A dedicated pitot supply of sufficient diameter feeding bleed air to turbo rails would be the most effective. Higher bleed air pressure would require a compressor of some kind. Potentially, a modified nozzle might boost atomization.

That said, to what end? Far LOP = slow airplane. If really lean (high MPG) is the primary goal, install EFI. It's one of the things EFI does really well.

did notice my FF wandered at cruise at 10K. It varied from 7.4 to 7.6 GPH. If I leaned more, then as it wandered to 7.3 or lower, it would run rough and I'd have to start the process over. It would take 5-8 min to sneak back down. Is this part of the issue, or something else?

Something else.

I would love and appreciate it if someone can explain the idea of a running pitot line in the cowl and how that will plumb and work with the injector system.

Look up the difference between standard Bendix-style nozzles and turbo nozzles. Short version...

As Mark noted, a small amount of air is entrained in the fuel flow inside the nozzle. The air comes from a small hole in the side of the nozzle body, under a shroud and screen.

Standard nozzles take bleed air from the surrounding atmosphere, meaning from the upper cooling plenum. The deltaP across the nozzle is thus subject to cooling air pressure recovery, an area where some folks do better than others (see CR 3405).

Add a turbocharger, and MP becomes higher than upper plenum pressure. So, turbo injectors plumb all the nozzle air bleeds to an air supply rail (generally a steel tube) which is fed by a pressure tap just downstream of the compressor outlet. That makes bleed pressure again higher than MP at the intake port, as the induction system suffers some pressure loss between the compressor and the intake valves.

In our little tweak, we install a set of turbo nozzles and feed rails, but pressurize it with ram air from some kind of pitot source. The result is (in theory) slightly higher bleed pressure, and/or more consistent bleed pressure.

In simpler words, the efficient inlets systems we make up reverse the air flow through the screens in the individual injector bodies due to MP being higher than the under-cowl pressure: the flow is outward instead of inward.

I did make pressure measurements prior to installing the turbo rails, although those methods were manual, with a manometer, not the digital capture of port and rail pressure differential done later. Those measurements say that on average, bleed pressure is higher than manifold pressure, even with a low loss induction system capturing a high percentage of available dynamic pressure. Note "on average". There are short periods in the 4 stroke cycle where pressures reverse to a small degree, and upper plenum turbulence could induce further periods.

Dan, can you clarify and expand on this a little? I have a direct (unfiltered) ram-air intake to a horizontal induction sump. I have roughly zero GAMI spread (certainly within 0.15 GPH) , and at my typical 9.5K-10.5K cruise altitudes, I can lean quite a lot (more than 50F LOP, often70--80F LOP) and stay smooth. I do get a lot of fuel staining. Would I benefit from a turbo-type pressure rail system?

I think you would, but the difference will be small. A real factor here is not easily measurable....perception of smoothness, and the point at which it is unacceptable. Perception and acceptance seem to vary a lot.

While we are on this subject -- does everyone realize that with a good ram-air inlet, and the sniffle-valve outlet being plumbed to the cowl exit, it is surely the case that our sniffle valves are leaking manifold pressure out the cowl exit.

Gave my sniffle valve a flotation test about 8 years ago. It failed ;)
 
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