Leaner can mean cooler running, mixture experiments Options

[lapuwali]

In another thread, TimT posted a graph produced by Pratt & Whitney showing the relationship of CGT, EGT, mixture, and power in air-cooled aero engines. Despite the often held assertion here and elsewhere that "our" engines need to run rich to stay cool, the chart showed that CHT fell as the mixture got leaner from "best power" (about 13:1). It also fell as it got richer from best power. There was a question in my mind how well this generalized to the Type IV.

My 1.7 w/ Webers had been, I knew, running rich on the idle circuits, and I was waiting on a set of leaner idle jets. These showed up, and I installed them this morning (50s down from 55s). Crisper running, and I hope much better gas mileage (was below 20mpg). My CHT gauge held the real surprise, however. It's installed on the hottest cylinder (No. 3), and previously warmed up very quickly to 300dF after just 30 seconds or so after a cold start. It would climb to 350dF as soon as any load was put on the engine, and up to 400dF when pulling up a two-mile long 6% grade near my house. With the leaner idle jets and no other changes, it was much slower to warm up, and never exceeded 275dF idling and puttering around the neighborhood at 25mph. Up that long hill, it just barely touched 350dF.

I don't have a wideband mixture meter (yet), so I have no idea if I'm still rich or somewhat lean. Drivability is still good, so it can't be all that lean. I haven't yet tried to do a plug check (used to be sooty black). In any case, it's one data point that seems to support the P&W chart.

[DNHunt]

First let's make sure everyone is on the same page. Reading these posts 3 reference points were referred to 1) Stoich 2) best power and 3) lowest EGT, CHT.
That gets kind of confusing. I suspect we should be considering best power or best temp as a starting point. That gets Geoff, Jake and even PW on board with the same reference point.

Stoich as a tuning point is a random number important in only in thermodynamics and no more important than any other number you could pick. Nice to shoot for but maybe not attainable.

The other numbers are something that can be worked toward.

I have a few observations gained from tuning 1 engine so take it for what it's worth. My engine needed to be tuned to run around 13.5 and richen to 12.5 on accel.

When trying to tune to leaner running conditions and very light load the duty cycles of the injectors are so short tuning becomes impossible. A/F ratios would bounce between 13 and 17 and the car would buck slightly. Say good bye to lean cruise. Coming off such a lean cruise condition the car would stumble.

Accel enrichment was easy to over cook. Very little went a long way so I ended up with a small increase over a very short interval and the car can run pretty well without any at all.

The dyno helped to smooth the variations out in the A/F ratio. I'm not sure I would have ever been able to do that with the dyno.

I suspect that there is not much room for fuel savings on the lean side max power. No doubt it will run and maybe it will stay cool but with the factory injectors it will not run well. If you use smaller injectors or meter fuel with an variable fuel pressure regulator you might be able to tune in a lean cruise but, I couldn't.

Dave