Lean Of Peak -O-Meter

[Steveden]

I'm going to try this again
I have Cub with an ECI Titan 340, not injected. Its basically a 320 that has been stroked. I always try to fly 60-80 degrees ROP with this carbureted engine because I understand without injectors to control the fuel flow to all the cylinders you can't run LOP, correct? If that is true, my question is: what is the best ROP setting for my 340? At 2300-2400rpm the CHTs are around 360 to 370, EGTs are 1290-1340, fuel flow below above 5000' with some leaning is 6-7 gph with 60-80 degrees ROP on the first cylinder to hit peak. I'm just wondering if this the best way to run the engine or if I have this all wrong

 

[Raymo]

Lots of carburated engines run LOP just fine. You won't know until you try. A little carb heat can sometimes help even the air/fuel mixture, too. Download your data, and upload it to the free Savvy site and send a link to the flights. If you perform the flight test profile, that would be helpful.

https://resources.savvyaviation.com/resources/other-documents/flight-test-profile/

I suggest you read John Deakin's Pelican Perch articles on the subject and watch Mike Busch's videos, available on YouTube.

 

[Steveden]

Ok, I will check it out, thx

 

[DBRV10]

Most of the presentations I have seen appear to give too much concern about red box for normally aspirated engines. IIRC Walt at one time made a comment that it was very hard to induce detonation on 100LL with a normally aspirated engine and CHTs below 400.
I normally cruise at around 65% with EGTs 0-20 degrees lean of peak and haven't found any issues doing that with my RV10. If closer to 75% I might go to 20-30 LOP, but don't see any benefit of going leaner unless running a turbo. My peak EGTs run around 1490-1520.
Would love your thoughts.
Kelly

Kelly, that would make a great talking point indeed. And you are correct, Walter Atkinson would have said, as we teach and demonstrate in class, a normally aspirated "Conforming Engine" running on "Conforming fuel" (avgas) cannot have detonation supported. Different in an engine with advanced timing (mags or EI's set up badly) or high IAT, high oil temp, and high CHT. But that is not conforming of course.

Turbo engines, you can induce it even on a conforming engine.

With all the avgas vs unleaded avgas detonation testing on the GAMI dyno, we have never seen detonation once past peak EGT. It is all concentrated around the 35-40 through 75 or so dF ROP range.

If you want long life and a good engine for overhaul, stick to the APS methods. As you are.

I am sure that Walter right now would have been smiling reading your post. He is missed.

 

[DBRV10]

Articles #63 to #66

"Pelican's Perch" Index

A complete index to John Deakin's popular Pelican's Perch column on AVweb.

www.avweb.com

 

[kellym]

Kelly, that would make a great talking point indeed. And you are correct, Walter Atkinson would have said, as we teach and demonstrate in class, a normally aspirated "Conforming Engine" running on "Conforming fuel" (avgas) cannot have detonation supported. Different in an engine with advanced timing (mags or EI's set up badly) or high IAT, high oil temp, and high CHT. But that is not conforming of course.

Turbo engines, you can induce it even on a conforming engine.

With all the avgas vs unleaded avgas detonation testing on the GAMI dyno, we have never seen detonation once past peak EGT. It is all concentrated around the 35-40 through 75 or so dF ROP range.

If you want long life and a good engine for overhaul, stick to the APS methods. As you are.

I am sure that Walter right now would have been smiling reading your post. He is missed.

Thanks David. My IO-540 is about as conforming as it can get, with 8.5 compression and Bendix S-1200 mags. I recall that it was originally certified on 91/96 Avgas, which of course disappeared a few years after Lycoming certified the engine. So I know it has more detonation margin than the big injected Continentals that run the same compression, but were only certified on either 100/130 or 100LL.
Of course I have to watch CHTs carefully when flying in the summer in AZ. Not an issue once OAT drops below 80F.
Kelly

 

[hoyden]

I have an RV6 with an IO-360 that I've been flying since 2001 and always running LOP. At the time I read John's articles at Pelican's Perch. Most of the time I'm not on a mission and simply enjoying the scenery and the flight. I typically lean to 2000 rpm and 20" with fuel flow 4.8-5.0 gph. CHTs run 280-310F. I read recently that running with CHTs less than 350F can lead to lead fouling the exhaust valves. I haven't had any problems but I wonder if running at low CHTs has that risk.

Just last week I tried the BMP for the first time and it worked without issue; much simpler operation than my old way of pulling throttle, then the prop, and finally the mixture. And then tweaking all of them again to get to my desired setting. I did the BMP at about 1500' AGL and from there brought my throttle and prop to my cruise or slow climb setting.

A side note: When I updated the ignition with two E-Mags I found I could lean as low as 4.2 gph before one of the cylinder would stop firing. The engine ran fairly smoothly on 3 cylinders.

I wonder if running at that reduced power and CHTs can lead to problems.

 

[DBRV10]

No problem with your CHTs, more OWT's being created. And yes the BMP works. Remember this is exactly what they did with the big Wright 3350's ....a 10% BMEP drop it was called.

 

[airguy]

Kelly, that would make a great talking point indeed. And you are correct, Walter Atkinson would have said, as we teach and demonstrate in class, a normally aspirated "Conforming Engine" running on "Conforming fuel" (avgas) cannot have detonation supported. Different in an engine with advanced timing (mags or EI's set up badly) or high IAT, high oil temp, and high CHT. But that is not conforming of course.

And one more "non-conforming" engine datapoint (and my engine/fuel system is most definitely "non-conforming" in a variety of ways) - on my IO360 with 8.7 compression, I can easily induce detonation in the 25-75 ROP area while running on 91E10 automotive pump gas. My initial climbs are usually done full power and well rich of this area, or pulled back lean of peak for cruise-climb, either will keep the engine happy. The "red-box" is very real when operating on lower quality fuels, not so much on good 100LL.

 

[kellym]

And one more "non-conforming" engine datapoint (and my engine/fuel system is most definitely "non-conforming" in a variety of ways) - on my IO360 with 8.7 compression, I can easily induce detonation in the 25-75 ROP area while running on 91E10 automotive pump gas. My initial climbs are usually done full power and well rich of this area, or pulled back lean of peak for cruise-climb, either will keep the engine happy. The "red-box" is very real when operating on lower quality fuels, not so much on good 100LL.

Of course. Your engine was never certified on less than 100 octane fuel, either 100/130 that was available when your engine was designed and certified, or 100LL today. 91 octane mogas is at least 10 octane points lower, probably more. Using a really sub-par fuel is going to create issues, probably anything above 65% and leaner than 150 ROP. Not that different from running mogas in a turbocharged engine. The 200 hp angle valve IO-360 is somewhat marginal at its original 25 degree timing on 100 octane, which is why Lycoming changed the timing to 20 degrees.

 

[DBRV10]

Kelly, yes the octane number for car gas ion the USA is AKI or R+M/2 and in terms of MON it is about 85-86. 100LL is typically 101.5-102 MON, so 15-17 points of MON number.

The biggest threat from mogas is the RVP (Reid Vapour Pressure) which can be quite variable throughout the year.

 

[airguy]

The biggest threat from mogas is the RVP (Reid Vapour Pressure) which can be quite variable throughout the year.

Bingo, yes. I run electronic injectors on a common fuel rail at 45 psig, that solves the problem nicely. That's the way the automotive world handles it - because it works.

 

[DBRV10]

I assume you have boost pumps at the tank end and not a pump up front sucking fuel all the way to the engine bay? I know you are smarter than the average bear so I guess my assumption is correct.

For anyone reading, this is critical.

 

[kellym]

Kelly, yes the octane number for car gas ion the USA is AKI or R+M/2 and in terms of MON it is about 85-86. 100LL is typically 101.5-102 MON, so 15-17 points of MON number.

The biggest threat from mogas is the RVP (Reid Vapour Pressure) which can be quite variable throughout the year.

The amount RVP varies depends a LOT on both temperature and EPA. Here in Phoenix it is 7.0 for the warmest 6 months, and 7.9 for rest of year, but at least 5% and half of the year 10% ethanol. In Alaska, it was 14-15 in the winter, and around 8-9 in the summer.
Unfortunately the highest mogas rated fuel is 91 octane, some areas go a couple points higher, mountain regions typically only offer 89 octane, since engines can't develop much more than 70% power.

 

[airguy]

I assume you have boost pumps at the tank end and not a pump up front sucking fuel all the way to the engine bay? I know you are smarter than the average bear so I guess my assumption is correct.

For anyone reading, this is critical.

Yes - I'm running the full SDS system by Ross Farnham. I have no engine-driven mechanical fuel pump, and I have a pair of dual electric pumps in the standard boost-pump area just immediately forward of the fuel selector. These pumps pull fuel directly from the tank through the standard 3/8" line with a LARGE inline fuel filter, and push it forward under pressure through the fuel rail and right back to the pump module where it hits a fuel pressure regulator. The regulator holds a constant backpressure on the fuel coming to it, and dumps the rest back to the tank through a return line. This keeps the fuel cool, under constant pressure, and in constant circulation. The pumps are plumbed in parallel for a pump failure, and the regulator pressure is referenced to MAP so that there is always a constant pressure delta across the injector face. The injectors only know duty-cycle - how many milliseconds they should be open - they rely on the pressure across them, from the fuel side to the manifold side, to be the same all the time so that the fuel flow expected is the fuel flow achieved. Running a reference line from MAP to the fuel pressure regulator lets the actual fuel rail pressure rise and fall as you pull the throttle back and forth to keep that delta the same.

For others reading - this is NOT a standard installation - there is a lot of RTFM required to do this, please don't experiment with it until you understand it. Modifying your fuel system can kill you very dead if you don't understand it.

 

[kellym]

Bingo, yes. I run electronic injectors on a common fuel rail at 45 psig, that solves the problem nicely. That's the way the automotive world handles it - because it works.

The automotive world does not need to handle issues with RVP, because it is adjusted through the refineries and distribution points for the expected temperatures and altitude. The only reason the automotive world went to very high pressure injection (hundreds of psi or more) was to enable direct cylinder injection instead of the older technology port fuel injection. For the timed port fuel injection they use pressures more like you are operating with to get consistent mixture for economy and emissions purposes. Still has little to do with RVP. For cars/trucks RVP is only an issue if you go from low to high altitude or cold to warm temperatures on one tank of gas.
RVP matters for aircraft because of the wide altitude and temperature range you may encounter in a flight that can generate vapor in the fuel system.
Depending on how much suction is needed to pull fuel from the tanks, that can be a problem, regardless of how the pressure is handled downstream of the pump(s). Some designs, like the RV-10 are not very good in this regard, having a significant rise in the fuel lines from the tanks to the fuel selector that might present an issue with mogas, where it does not with avgas. If you are using fuel with ethanol, as it sounds you are, that is even more of a problem, because ethanol in the 5-10% blends raises the RVP by a full point, and the octane of the base fuel is lowered to account for the ethanol octane boost.....not to mention the issues with ethanol absorbing moisture into the fuel.

 

[airguy]

You're preaching to the choir, but maybe others will read it.

Even before I installed the SDS and the fuel rail, when I was still running mechanical injection on a modified Bendix RSA5 setup, I was running 91E10 in west Texas summertime heat with zero issues (including hot starts), and at altitudes up to FL210, with my normal cruise being in the high teens. I didn't do that by accident, and it wasn't with a stock system. It requires a hydraulically correct fuel system as you alluded to, and sadly those are not ubiquitous in aircraft.