Lean of peak in a carbureted O-360

Today I went up to 13,500' to be well above the 75% engine power area and tried to operate the engine lean of peak. This is a rebuilt O-360 using Superior cylinders, 9.5:1 pistons and the cylinders ported and flow matched to very close tolerances by Ly Con.

I have been running EGT around 1330 degrees F. Continue to lean and watch the EGT increase and then around 1400 degrees F it actually started decreasing with further leaning while still running smoothly.

I have never been able to do this.

Around 1340 degrees on the lean side of peak I noticed a power drop (apparent) then the normal roughness. Fuel flow at this time was around 7.3 gph (full throttle) and 8.5 gpg around 1330 degrees rich of peak.

Since I had one Lightspeed EI before the engine rebuild I am surmising that the flow matching process is solely responsible for now being able to run lean of peak in a carbureted O-360.

I forgot to see if the cylinder head temp dropped but will try it again to better understand any issues. Comments and suggestions welcome.

fyi at APS in Ada, OK where they teach LOP operations on typically fuel injected engines they have found that by using a bit of carb heat on non-FI engines it helps the combustion and you can get smoother LOP operations.

LOP operation is definitely possible with a carbureted engine, but it will depend on the individual engine, they each flow differently. Unless you have 4 cylinder EGT indication though you really don't know if you're LOP on each cylinder and that is important to know. Especially with your higher compression, you really want to avoid the "red zone" (operating near peak EGT) when above 75% power.

I know the carbureted O-360 on my RV-8 was a bit of a freak and just happened to have an EGT spread of only 25° at normal cruise power settings. I have several witnesses who could look at my UBG-16 analyzer and see it from the back seat for themselves. Most carbureted engines however will have a 75-100° spread so you could be at 75% LOP on the one cylinder you're monitoring then at peak or ROP on others and not even be aware of it.

randylervold said:

I know the carbureted O-360 on my RV-8 was a bit of a freak and just happened to have an EGT spread of only 25? at normal cruise power settings. I have several witnesses who could look at my UBG-16 analyzer and see it from the back seat for themselves. Most carbureted engines however will have a 75-100? spread so you could be at 75% LOP on the one cylinder you're monitoring then at peak or ROP on others and not even be aware of it.

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I have a 80+F EGT spread, but the "gami spread" is zero...and that's what counts. Anyway, just want to advocate disregarding EGT spread and focussing instead on gami spread. That's the delta in fuel flow from the first to last cyls to reach peak EGT.

Gami Spread?

dan said:

I have a 80+F EGT spread, but the "gami spread" is zero...and that's what counts. Anyway, just want to advocate disregarding EGT spread and focusing instead on gami spread. That's the delta in fuel flow from the first to last cyls to reach peak EGT.

Click to expand...

Got it so LOP operations requires 4 channel EGT.

I think Ron Lee is rocking the single channel.

But its Gami spread - delta in FF from first to last cyl to peak. So what does that tell me? The smaller the delta FF the better, but does that just mean the EGT spread between first & last to peak is smaller? I thought that was the goodness factor, EGT. spread. I'm not sure what to do with FF/EGT spread? I am sure it makes sense, but I'm just not practiced in it or have the right framing behind the principle. Bottom line if you have roughness and all cylinders are not LOP in the safe area you can't do LOP, or at least properly, unless all jugs are "balanced" FF wise or EGT wise depending on your fame of reference?

So does that mean to do LOP ops properly, you need 4 x channel EGT and FF gauge?

I see why Lyc kept it simple with lean to rough and enrichen a little.

gmcjetpilot said:

Of course that means to do LOP properly you need 4xchennel EGT and FF gauge? You can see why Lyc kept it simple with lean to rough and enrichen a little.

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Well...to KNOW what you're up against, yes, you should be looking at all cylinders' EGTs and CHTs. If you don't want or need to know all that data, decades of alternate techniques have sufficed. Only if you want to eek out the absolute most from your engine do you really care about the details.

Thanks Dan my bad I could have looked it up

dan said:

Well...to KNOW what you're up against, yes, you should be looking at all cylinders' EGTs and CHTs. If you don't want or need to know all that data, decades of alternate techniques have sufficed. Only if you want to eek out the absolute most from your engine do you really care about the details.

Click to expand...

Thanks dan I should have looked it up, Gami has all kinds of info they published. I read it last year, now I recall. To bad my old Lyco has a gami spread of more than than desired 0.5 gph, at least with smooth operations on an O-360. I'm encouraged some carb guys have got reliable smooth LOP ops. May be I need Lycon to balance by jugs. Probably cheaper to buy a FI system.

I had a O-470 powered Skylane and ran it lean of peak. I belive it was Mike Busch who discovered the carb heat trick a long time ago. He used to own one himself. For whatever reason, that particular engine ran very smoothly and was well-balanced. A friend's high compression O-470-U had a EGT and gami spreads comparable to what you'll find in injected engines with balanced fuel injectors. In my experience, carbureted TCM engines tend to run lean of peak a little better than Lycomings. Not by design, though, I'm sure.

gmcjetpilot said:

Thanks dan I should have looked it up, Gami has all kinds of info they published. I read it last year, now I recall. To bad my old Lyco has a gami spread of more than than desired 0.5 gph, at least with smooth operations on an O-360. I'm encouraged some carb guys have got reliable smooth LOP ops. May be I need Lycon to balance by jugs. Probably cheaper to buy a FI system.

Click to expand...

Two different things here - flow balancing balances the amount of AIR that goes into each cylinder. Flow balancing should be thought of as icing on the cake, but the fuel/air mixture is far and away the biggest rascal. Having equal flows of air into each cylinder does not mean that the fuel/air mixtures will be equal (gami spread). FI can balance the FUEL/AIR mixtures in each cylinder (as can certain combinations of throttle/carb heat in some carb'd engines). The Achilles heel of the carb system in this regard is that the fuel is in the form of droplets, traveling through the induction system. Droplets which are not going to evenly distribute themselves into each of the four (or six) pathways to the cylinders. Our work locally on several O360/O320's would indicate that gami spreads of 1.5 gph are not unusual. The upshot is that one cylinder will go too lean (and run rough), while the others are at peak or well rich of peak. This also explains large cht's (yes, cht's) variations that many RV's suffer from.

With fuel prices at 4 bucks and counting, one needs to seriously consider FI (and EI, for that matter) for fuel economy purposes alone. I regularly fly alongside airplanes which are burning 1 to 1.5 gph more than I. This is 8 to 12 grand over a 2000 hour life, a real attention getter!

Vaporization

Alex Said:
[The Achilles heel of the carb system in this regard is that the fuel is in the form of droplets, traveling through the induction system. Droplets which are not going to evenly distribute themselves into each of the four (or six) pathways to the cylinders.]

Alex - dead on here. Per replies from the APS guys, the advantage of using carb heat is to encourage vaporization of the fuel droplets to get a more even mixture distribution. I show a carb throat temp of 113-117 degF when I am running smoothly LOP. This makes sense, but I can only guess at what is going on inside the intake runners.

As you also said, EI is a big factor here as well. I don't think I could run LOP smoothly without it.

dan said:

I have a 80+F EGT spread, but the "gami spread" is zero...and that's what counts. Anyway, just want to advocate disregarding EGT spread and focussing instead on gami spread. That's the delta in fuel flow from the first to last cyls to reach peak EGT.

Click to expand...

Indeed, didn't mean to imply otherwise. Sadly I never checked the peak point (gami) spread on my RV-8. Unfortunately I don't think the concept is widely understood. To explain it a different way in terms of every day flying, you want your cylinders to hit peak EGT at same exact point as you slowly pull the mixture back to lean it out. Otherwise, you'll have one cylinder LOP and others could well be ROP. Measuring EGTs via incremental fuel flows is a data collection technique used to determine the spread, not a way of leaning your engine.

Yes, you need 4 channel EGT to make the adjustments, but IMHO 4-cylinder EGT and CHT are so available and affordable why would you not have it installed? As Alex pointed out, in these days of high and higher fuel prices it makes a very real difference in fuel consumption. I'm in the process right now of tweaking the nozzles in the IO-320 in my RV-3B, if anyone is interested info can be found here. I'm down to a .4 gph spread now and heading for the goal of .2 gph. Dan, you got to zero?

randylervold said:

Dan, you got to zero?

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Yep. Out of the box, AeroSport Power IO-360-A1B6 w/Airflow Performance injection with .028" restrictors all around. Gami spread was 0.0 gph at wide open 24 squared (climbed to an altitude that yielded 24.0" MAP when wide open).

At other power settings the gami spread varies, but the worst I've seen is about 0.2 gph.

I got some .025" restrictors from Don at Airflow Performance, who mentioned I might be able to eek out a bit more economy with better atomization via the smaller nozzles. One of these days I definitely have to play with them...been meaning to do that but haven't found the time (YET!).

I (HEART) FUEL INJECTION. ;-) Sorry, this is a carb thread...

My friend's Lancair 235 with an O-320 had great balance on EGTs at WOT, but the front two cylinders would run richer and the rear two cylinders leaner at part-throttle.. If you look up in the manifold above the carb, you will see that the intake runners are evenly spaced out around the periphery. When at part throttle, the throttle-valve tilts forward which will favor fuel-air flow to the two front cylinders, and tend to starve the rear two cylinders.

dan said:

Yep. Out of the box, AeroSport Power IO-360-A1B6 w/Airflow Performance injection with .028" restrictors all around. Gami spread was 0.0 gph at wide open 24 squared (climbed to an altitude that yielded 24.0" MAP when wide open).

At other power settings the gami spread varies, but the worst I've seen is about 0.2 gph.

I got some .025" restrictors from Don at Airflow Performance, who mentioned I might be able to eek out a bit more economy with better atomization via the smaller nozzles. One of these days I definitely have to play with them...been meaning to do that but haven't found the time (YET!).

I (HEART) FUEL INJECTION. ;-) Sorry, this is a carb thread...

Click to expand...

I guess I'm lucky as well.. My Mattituck IO-360 with Precision Silver Hawk injection has a Gami spread of near zero right out of the box. I've seen as high as 0.4 gal/hr, but it's usually 0.0-0.1 gal/hr.

I have noticed that when running WOT and leaning, my EGT spread averages about 17F. However, when running less than WOT and leaning, my EGT spread averages about 40F. At full rich, the EGT spread is near zero.

Are your numbers similar when not running at WOT? I read the stuff about having induction differences between the cylinders when the throttle valve isn't full open, but I'm surprised it is so dramatic. Three of the cylinders are very close to each other, and the fourth is the +40F offender. Not a big deal since the Gami spread is near zero, but it still annoys me...

-Geoff

Injector Nozzle Tuning

As stated before the EGT temperature number is not a factor in getting the engine balanced. Peak EGT corresponds to a specific fuel air ratio. The point at which the EGT peaks the EGT number may be different from cylinder to cylinder. This is due to a number of things driving the temperature, the main one being exhaust backpressure. So your engine can be perfectly balanced fuel air ratio wise (all cylinders peak EGT at the same fuel flow) but at that point there can be differences in the EGT number being recorded. A common misconception is to have all the EGT?s read the same. THIS IS INCORRECT and can damage the engine.

Don

Don at Airflow said:

As stated before the EGT temperature number is not a factor in getting the engine balanced. Peak EGT corresponds to a specific fuel air ratio. The point at which the EGT peaks the EGT number may be different from cylinder to cylinder. This is due to a number of things driving the temperature, the main one being exhaust backpressure. So your engine can be perfectly balanced fuel air ratio wise (all cylinders peak EGT at the same fuel flow) but at that point there can be differences in the EGT number being recorded. A common misconception is to have all the EGT’s read the same. THIS IS INCORRECT and can damage the engine.

Don

Click to expand...

Understood...

Having nothing to do with ROP, LOP or peak anything, I was just more curious as to why one of my four EGTs would be +40F (compared to the others) under all conditions except mixture full rich or wide-open throttle. I probably wouldn't have even been curious if all four had been different, but the fact that three are almost identical and the fourth is different piqued my curiosity.

I've read about less than WOT and/or exhaust backpressure causing it, but I'm unclear as to why it happens. Alas, there are probably so many variables that it's beyond the scope of this lecture...

-Geoff