AFR's

raymond said

I am using 98 here in OZ, we leaned it down twice on the dyno and picked up around 30 HP but it was still in the high 11's on the AFR reading, I've gone leaner one more step but it has got soft on the hit and the 60ft. has dropped off, problem is I'm running an 850 TQ and it it doesn't like being that lean on the launch, I've slowed the secondary air door opening down but it's really not happy. I'm trying to source a decent size double pumper to get it to leave the line like it used to.

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The engine told you what it wants. Give it to it. There are some combinations that just won’t run at an AFR that everyone thinks is “the” number AND the O2 sensor can find to you.

So tune it from the spark plugs and when you are very close on the plugs then start looking at the O2 numbers. And, you have to remember you are getting the average O2 of 4 cylinders unless you have 8 O2’s, and if you do, you can drive yourself bat crap crazy trying get a;l the cylinders the same.

You have to tune to the leanest cylinder, and set your timing to the quickest cylinder. It’s a compromise.

You may have to work on cylinder to cylinder distribution or the O2 is fibbing. A lean miss will show rich AFR’s. Slow timing will skew O2 readings.

Plugs and time slips until you are close and then use the O2 and don’t tune to a number. Of course, if your O2’s are real wacky you should look to try and determine why the numbers are what they are.