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The Extra Inch...

bjdecker

Well Known Member
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I've been tinkering with various shapes of ducts trying to squeeze a bit more manifold pressure out of (into?) my IO-360-A1B6 equipped RV-7.

My intake manifold is the "standard" VA-132-2 S-duct, pulling intake air from the left inlet ramp. At wide open throttle and various altitudes I am getting roughly ambient pressure downstream of the fuel injection servo (#3 Cylinder port) -- Some examples:

6500ft -> Field altimeter of 30.02" -> 23.6" MAP...
8500ft -> Field Altimeter of 29.95" -> 21.8" MAP...

But I want more; It seems like I could fabricate a scoop of some stripe to convert that hurricane of propeller wash and incoming air into something more meaningful pressure-wise (and mass).

I fabricated a "hood scoop" to sit on top of the air filter...and it didn't work; only gained .1" of MAP and as expected all the CHT's & Oil Temp increased... Uncomfortably...

Back to the TLAR drawing board. I converted the hood scoop into a splitter of sorts, still attached to the top of the air filter, but only occupying the bottom half of the left inlet. It also didn't work; minimal gain in MAP" ~.1", CHT and Oil Temp were unchanged. Until you factor in the reduction in OAT we had yesterday (-10°F), so the efficiency suffered a bit, but well within the operating margin of my plenum/baffle system.

What's next? I think I'll remove the air filter from the system and see what the MAP does...
 

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I remember my 1968 Mooney had a filter bypass that added approximately 1" of manifold pressure.
 
I remember my 1968 Mooney had a filter bypass that added approximately 1" of manifold pressure.

So did my '86 205SE and the F1R intake on my last -7. I just don't want to cut holes in the bottom cowl...again...
 
my 1968 Mooney had a filter bypass that added approximately 1" of manifold pressure

Thanks Mel.
The only way to do it... just looking at those snorkel and FAB convoluting routes gimme, and the intake air, some quite severe headache ;)

Filter on for take-off alright, 1000' or outta dust whichever first, ram air... the only way for any intake to properly breathe.
 
There is an RV-8 up here in the PNW that has a similar 'hood scoop'. Don't know who the owner is though. I saw the airplane back in 2005, and saw it again recently at Chehalis, WA. Still had the 'hood scoop'.

i-HHcHhxZ-L.jpg
 
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Use a larger filter for almost no loss.

What Dan said!

I suggest a scoop is not the best approach to get a bump in MP.

I did a beta test of a new snorkel before Oshkosh. I did the same data runs with the orginal snorkel and the new one. While I suspect the reason for the snorkel update was far more applicable to the IO-390-119EXP (using the FM-200) than my IO-360-M1B I did measure a MP bump of 0.4” to 0.5”. The stock snorkel provided a bump as well, just not quite as good.

Side note - I’m using the new AFP FM-150C instead of the stock Precision unit. This is a bolt on replacement - and I recommend all IO-360-M1B engine buyers specify the 150C.

I understand Van’s took the beta test snorkel (that I reluctantly sent back to them) to scan to create production molds. Considering all the stuff on Van’s plate of late I would not expect this to be on the front burner. I do note that this new snorkle and the FM-150C was discussed (with photos) by Van’s at Oshkosh this year.

Carl

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Filter Loss?

Confused
I don’t know if it’s right but Looking at the 1 sec log files from my IO360 with the stock filter with roughly 50 hours since cleaning the manifold pressure is never more than 0.4” lower during the takeoff roll than it is with the engine shut down at the end of the flight.

Is there enough dynamic pressure available in the cowl to get another inch of pressure?
 
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<snip>
Is there enough dynamic pressure available in the cowl to get another inch of pressure?

That is a really good question. I took air intake data at various times by the Garmin flight data for temperature and baro altitude at WOT. Then, used an on-line atmo calculator to make the comparison to the MAP. It seems my MAP is just slightly higher than the ambient pressure - .5 in-hg IIRC. Air filter drop was quite low, (almost DanH territory) but have no idea for the servo pressure drop.

I need the new Hartzell Composite prop.

https://aerospaceweb.org/design/scripts/atmosphere/

Brian - I think your data just says that you are getting nearly all the benefits of dynamic pressure already.
 
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Brian, there's data out there which says the available static pressure at the snorkel filter location is very similar to the pressure measured with a typical horizontal "ram" inlet. The observation matches theory, as well as measurements from popular research papers.

There are differences between individual configurations of course, but in general numbers, a good cooling system will see a static rise equal to 80~85% of available dynamic pressure. Take a look at the best pressure coefficients in CR3405.

We must slow the flow to convert available dynamic pressure to increased static pressure. The rise for a straight "ram" intake is never 100% of available dynamic, as it's not a closed end system; i.e. like the cooling system, we cannot slow the velocity to zero. However, like the cooling inlet, we can design for internal diffusion or external diffusion and get a static pressure rise. How much depends on details. There is no guarantee it will be better than the intake located in the cooling inlet. However, it is very likely to exhibit less duct and filter loss on the way to the servo.

Specific to Mike's question, yes, the static pressure rise in the upper cowl can easily exceed 1" Hg. As above, the trick is piping it to the servo without filter and duct loss.

How much rise? The plot below is a standard RV-8 cowl (red) compared to a revised RV-8 (blue). The upper plenum values are roughly 12 and 14 inches of water at about 165 knots. That's 0.88" and 1.00" Hg above freestream static. There would be more if the aircraft were flying at max velocity.

I went back to original data for more precision. The reported upper pressure for the stock cowl was 12.7" H20 at 163.4 true, or 0.934" Hg. At 3500 PA and 50F, the available dynamic pressure was 1.144"Hg, so the plenum rise for this particular stock RV-8 was 81.6%.
 

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I did a beta test of a new snorkel before Oshkosh. I did the same data runs with the orginal snorkel and the new one.

Hey Carl,

Do you happen to know the part number for the new snorkel? Is it VA-132-3? or VA-271-1? Or...something else??

Cheers,

B
 
I have been thinking on this for quite a while and here was my approach. Put a 2” slide valve on the rear vertical baffle above #3 cylinder operated from cockpit. Route a 2” piece of scat tubing under cylinder 1 & 3 to the starboard side of the snorkel with a 2” flange attached over a 2” hole in the snorkel. Bend and fiberglass into place with no air leaks. Then above 1000’ pull the slide valve open to augment air coming thru the filter and increase MP by at least the pressure loss thru the filter. Since I have no alternate air valve, I figured this would also do double duty and get me air to the engine if the filter got plugged in flight. Not full power with it only 2” but partial power is better than none. Have not built any of this yet. Opinions.?
 
Opinions.?

I doubt you'll see a manifold pressure rise for three reasons.

The flange entry and long hose will exhibit pressure loss just like the filter, probably more.

The Cp plots in 3405 suggest slightly lower pressure available in the rear baffle zone, as compared to just inside the inlet.

The legacy snorkel flows poorly around the sharp bend at the lower neck, and you're still stuck with it. The neck is a significant change on the new snorkel.
 
The Extra Inch...Continued...

Thanks DanH, Carl, Carl, Bill, others...

I spent some quality time with Savvy this afternoon and analyzed a dozen or so flights since the Condition Inspection. What jumped out at me was the fact that my MAP has been steadily decreasing since last October. Why is that significant? It happens to be the last time I cleaned & recharged my K&N air filter.

The Data, gathered at 6500ft - 30.02"Baro, 2500RPM, WOT, 50°F ROP.:

Fresh Filter -> 23.9" MAP
11 month filter (<60 hours) -> 23.4" MAP

So there's .5" of MAP lost sucking through a dirty filter, who'd a thunk it...
 
I just put a new filter and I saw no change in MAP.. I looked back 6 months and compared with similar flight a week ago and they both show 20.0 inches at 11,500ft.. so for me, brand new filter did not seem to make any difference. Old filter was of unknown "history" (unknown to me.. no idea when it was cleaned last time)
 
I just put a new filter and I saw no change in MAP.. I looked back 6 months and compared with similar flight a week ago and they both show 20.0 inches at 11,500ft.. so for me, brand new filter did not seem to make any difference. Old filter was of unknown "history" (unknown to me.. no idea when it was cleaned last time)

Assuming a local baro of 29.92" (std.day), your 20" MAP is .6" above "ambient" or "free stream" air pressure. Not too shabby...
 
To the OP: Your filter is too close to your scoop, and you have a bad transition of shapes. The key to air flow is to keep the airflow laminar. There are many engineering rules of thumb for this, like the number of diamaters between transitions, and the air-to-cloth ratio of the filter size. I look at your design and I see turbulance. A few sugestions if you are going to stay with your current pick-up location:
1. Reloacte the filter to an area where you can put a good transition in and out of it.
2. Make your scoop round, like the old ship air inlets.
3. put an air flow straightener after your pick-up. You are capturing turbulant air there and need to settel it down.

The Sam James cowl has a very good design. I forget my numbers, but get good MP with mine. I had to rework the filter box to get better tranitions. and I installed airflow straighteners after my filter.

Having said all of that: your current performance seems fine. You are not going to get a noticable preformance increase with this work. Unles you are racing, I would spend the extra time flying.
 
Assuming a local baro of 29.92" (std.day), your 20" MAP is .6" above "ambient" or "free stream" air pressure. Not too shabby...
I did a careful calculations of actual MP on the ground with the engine off (correcting for field elevation and local barometric pressure). I ended up inserting a correction factor of -0.4” into the SkyView EMS to get a more accurate MP readout (as in before the correction the MP pressure was reading 0.4” higher than actual).

Assume the whatever you are reading for MP is off unless you calibrate the sensor.

Carl
 
I did a careful calculations of actual MP on the ground with the engine off (correcting for field elevation and local barometric pressure). I ended up inserting a correction factor of -0.4” into the SkyView EMS to get a more accurate MP readout (as in before the correction the MP pressure was reading 0.4” higher than actual).

Assume the whatever you are reading for MP is off unless you calibrate the sensor.

Carl

Carl,

Yup - If I had a Dynon system (D180, Skyview EMS, etc.) I could easily adjust MAP readings. Unfortunately, the Garmin G3X suite doesn't have a simple means (read: none that I can find) available to do this...

However, doing the same math on the ground yields a number what's within .01" of what the Garmin G3X reports...so, no adjustment needed.

B
 
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