Fun Engine Data

[Andy_RR]

...and rising oil temps and falling EGTs.

Also, given the CHT spread here, you'll see one cylinder go into knock before the others so you'll see EGT and CHT spread diverge.

[AndrewR]

As I said, it's about the available margin, not whether the engine was actually detonating at that point in time.

Do you have an alternative explanation for the limiting manifold pressure?

[Andy_RR]

You have to recognize that the margin you're talking about is there to cover operation of the engine with a barely competent operator and without any instrumentation at all - i.e. a properly fat margin.

You can read more here: https://www.faa.gov/documentLibrary/...AC_33_47-1.pdf

[skylor]

Originally Posted by AndrewR

It seems like a bit of a jump to infer that the limiting manifold pressure is intended to limit horsepower, not manifold pressure - particularly since the limiting manifold pressure line is not a constant horsepower.

In my (admittedly limited) experience reducing RPM is a very good way to induce knock/detonation. I suspect that the limiting manifold pressure is required for detonation margin at lower RPM.

• Low RPM is pro-detonation
• High manifold pressure is pro-detonation
• Slow burn (which you are deliberately targeting) is pro-detonation

It seems like a gamble to think you can run lean enough that detonation isn't a possibility, especially since lean mixtures are also generally pro-detonation. The line between lean enough to suppress detonation and too lean to ignite is pretty thin I think.

My opinion: engines run best and have longest life when run the way the designer intended.

The folks at GAMI have plenty of hard test data to show that mfg’s recommendations are not always the best for engine longevity.

Slow burn is not pro detonation...

Limiting manifold pressures are meant to limit peak internal cylinder pressures at certain RPMs. Actual horsepower output is one result of internal cylinder pressures, but limiting internal pressure can be achieved by means other than manifold pressure limits. Slowing the combustion rate has a large effect on reducing cylinder pressure by moving the peak pressure further past top dead center when the gas volume inside the cylinder is larger and expanding more rapidly than mixtures and charge densities that reach peak pressure closer to TDC.

Mixtures that aren’t lean enough, i.e. between 10 LOP and 50 ROP are really pro detonation. Much richer or a little leaner than that range buys you a lot of margin.

You would probably really cringe know that I race using nitrous oxide injection at only ~ 100 ROP and no ADI (but not at 1870 RPM )

Skylor

[F1Boss]

Lindberg went to the Pacific area during WW2 to help the pilots in their P-38s get more range - that is how they got Yamamoto. He also helped the Corsair pilots with the same setup. Extra range for the Americans was key to the Japanese loss in the Pacific: US planes could get to the targets that the Japanese thought were safe.

I am reading “The Aviators” by Winston Groom - he details Lindberg’s process and the very real difference it made for the war effort.

If I want to fly with my RV buds X-CY, I use this process to stretch fuel - generally I can burn 7GPH while my 4cyl buds burn closer to 9-10.

[DanH]

The FAA's published Swift fuel tests contain good detonation data for two engines, one of which is the TIO-540-J2BD, the angle valve turbo six from the Chieftain. None of the test points pull RPM into the 1800's, but it should be much the same combustion chamber. The closest test point is 2200 RPM and about 33" manifold pressure.

There does appear to be an interesting trend, in that detonation onset progressively moves toward leaner mixtures as RPM is reduced.

At 2575 and 42", detonation onset is found at the classic position, ROP, just leaner than best power mixture. With each power reduction it moves to a leaner position, with 2200/33" being slightly LOP.

Is it really relevant to an NA IO-360? I dunno, but I do note the manifold air temperature, 161F, well above the 100F standard I think is used for detonation certification with NA engines.

If you get curious about margin, push mixture richer from your very lean setting and watch the CHT trend. If the rise shows a knee, you're tickling the dragon, but light detonation won't hurt it.

[skylor]

Originally Posted by DanH

The FAA's published Swift fuel tests contain good detonation data for two engines, one of which is the TIO-540-J2BD, the angle valve turbo six from the Chieftain. None of the test points pull RPM into the 1800's, but it should be much the same combustion chamber. The closest test point is 2200 RPM and about 33" manifold pressure.

There does appear to be an interesting trend, in that detonation onset progressively moves toward leaner mixtures as RPM is reduced.

At 2575 and 42", detonation onset is found at the classic position, ROP, just leaner than best power mixture. With each power reduction it moves to a leaner position, with 2200/33" being slightly LOP.

Is it really relevant to an NA IO-360? I dunno, but I do note the manifold air temperature, 161F, well above the 100F standard I think is used for detonation certification with NA engines.

If you get curious about margin, push mixture richer from your very lean setting and watch the CHT trend. If the rise shows a knee, you're tickling the dragon, but light detonation won't hurt it.

That's good data! One thing they don't show is the CHT's at the onset of detonation during the test. My bet is that they were far higher than the 276 (max) that I had during my test. I would bet that they had to have them up in the high 300's or more to get the engine to detonate under those test conditions.

I've been told that it's nearly impossible to get a healthy NA Lycoming (with stock CR) to detonate at any condition on 100LL when the CHT's are below 400 degrees.

Incidentally, a few weeks ago I was doing some "edge of the envelope" tests to try to find the "backside of the power curve" at 3500', full power and gross weight. I was operating at ~27" Hg, 2700 RPM and ~ 16 gph (not quite full rich for me) and 50 - 55 kias. It was startling how quickly the engine heated up under these conditions. I don't think I reached detonation but in ~ 1 minute all cylinders went from ~340F to 430F (by far the hottest I've ever had this engine and completely unintentional) and oil went from ~195 to 238!

Skylor

[Andy_RR]

Originally Posted by DanH

The FAA's published Swift fuel tests contain good detonation data for two engines

Do you have a link to this Dan?

Originally Posted by DanH

At 2575 and 42", detonation onset is found at the classic position, ROP, just leaner than best power mixture. With each power reduction it moves to a leaner position, with 2200/33" being slightly LOP.

Is it really relevant to an NA IO-360? I dunno, but I do note the manifold air temperature, 161F, well above the 100F standard I think is used for detonation certification with NA engines.

The exhaust back pressure from the turbine and the large clearance volume will raise the exhaust gas residuals substantially over an NA, high-CR engine which will bring on knock earlier.

[DanH]

Originally Posted by Andy_RR

Do you have a link to this Dan?

Sure. I usually crop out the Swift fuel plots and look at the 100LL data posted for comparison. They used three different 100LL fuels, one specifically produced to barely meet minimum standards.

http://www.tc.faa.gov/its/worldpac/techrpt/ar0853.pdf

The exhaust back pressure from the turbine and the large clearance volume will raise the exhaust gas residuals substantially over an NA, high-CR engine which will bring on knock earlier.

Thanks, that makes sense.

[Andy_RR]

Thanks Dan!