Lean of peak questions.

[fscott]

My MS'd motorcycle is down for an engine rebuild right now, and I'm stuck waiting on parts. So I'm doing a lot of bench racing lately.

Here's my latest brainstorm/fart.

I'll be adding an LC-1 to the bike, and want to see about running lean of peak at cruise. The bike is turbocharged, so naturally I'd go rich again once boost comes on.

At 65mph cruise in top gear, the turbo's putting out 2-3psi of boost. Of course, I'm at partial throttle, so MAP may well be less than 100kpa still. I think. I haven't checked this because I haven't ever gotten the bike tuned enough to go for a long ride with the MS. This is all based on when the bike was still running on the stock ECU. In any case, I don't have info on the MAP readings at cruise.

So if the turbo's making boost, but the throttle is closed enough that MAP is less than 100KPA, is it safe to run lean of peak?

[Bernard Fife]

is it safe to run lean of peak?

fscott,

It depends what you mean by lean of peak. I assume you mean lean of stoichiometric air/fuel ratios (~14.7:1 with gasoline)?

The AFR you can safely run depends on a lot of things, like ignition timing, compression, volumetric efficiency, cam timing, operating temperature. In generally, though, the lower the MAP, the higher the AFR can be.

In addition, the lower the MAP, the safer it is to 'experiment' with higher AFRs. At anything over 90-100 kPa, running leaner than stoich is 'iffy' for an engine with decent volumetric efficiency.

Below 50 kPa, up to 17:1 is safe to try, however, you may see a lean surge.

Between 50 kPa and 90 kPa, the AFR you can experiment with safely without potentially damaging your engine decreases from ~17:1 to ~14.7:1.

Only the MAP matters, not any boost before the throttle plate(s), as the MAP is what the valves/cylinders see, and this is what determines the cylinder pressures.

Lance.