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.