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.

[airsix]

lean is cool too

have a look at this article.... from the AVWeb site...These guys know a wee bit about air cooled egnines...

What F4i and lapuwali said...

You can learn things from this data, but you can't make a 1:1 comparison between this data and our engines. There are too many differences. First, they operate at a max of around 2,700rpm. They also have static-timed mageto ignition (dual plug) that has higher spark intensity as rpm increases vs our Ketering ignition that loses intensity as rpm increases. They run higher octane leaded fuel. Basically there are more differences than similarities.

I can tell you first hand that yes, you can dramatically drop EGT by leaning out the mixture. However by the time you've leaned it out enough to significantly drop the EGT the power is dropping off like Niagra Falls. The power is less than when the EGT is at the same level in an over-rich condition. In otherwords you can get the same EGT by being too lean or too rich, but you'll have more power when it's at the same temp and rich. Peak power is very near peak EGT temp (on an aircraft engine). I know this from sitting on the ramp and tuning the mixture for max power using the tach and EGT guage as a guide. (Airplanes have a built-in dyno we call a 'prop' (IMG:style_emoticons/default/wink.gif) ). We typically lean out for max rpm and max EGT, then richen it back just a hair. Remeber, that procedure works great on a static timed aircraft engine turning 2,750rpm with dual-plug mageto ignition running on high-octane leaded fuel. The same technique might be 'less effective' on a typical typeIV.

-Ben M.