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Originally Posted by RarlyL8 View Post
I see various ranges of acceptable AFRs on boost with CIS. JBL likes 12.0 as the upper limit, you give a range of 12.2-12.6 while I shoot for 12.0-12.2 as the target.
What engine parameters are you basing your figures on? How do these figures change with increased boost pressure, say 0.7 to 1.0 bar on a typical 350WHP (@ 1bar) CIS 930 engine?
From dyno data you have done what is the difference in power when leaning from say 11.8 to 12.6 AFR? I have not done enough runs to see statistically significant results.

It's hard to generalize when it comes to what AFR gives most power. It varies from engine model to engine model. Also, the AFR that gives most power might not be the "safest" AFR.

CIS being mechanical and maybe flowing unevenly, it might be a good idea to run it richer than it should.

But most "real-world" engines give best power between 12.5-12.8. It all depends on plethora of details. Squish, ignition timing, position of ignition plug, cam timing etc.

Here is an interesting article, written by inventor of LM1:

Also, most EFI systems don't measure lambda on per-cylinder basis. You just assume that every injector flows the same and tune for correct lambda.

As I said before, It might be a good idea on this particular case (930 almost stock) to go for 12.2 to gain some margin to protect from failing parts but IMHO, the biggest inherent 930 CIS problems is that it leans towards the end.

On older cars, it often runs like this: Pig-rich on the onset of boost (like 10-11:1), then gradually leaning towards the some cases reaching 13.5:1 around 6000 RPM. I believe that engine would go better if this non-linear AFR behaviour could be corrected. With other words, less rich in the midle and more rich up top.
Thank you for your time,
Old 04-09-2008, 07:00 AM
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