Question about AFR, Boost and RPM

[Jorge T]

I posted this on another forum ..
I have been tinkering with my Xede since I fubared my custom map by adding a tubular header and changing the exhaust system. I run 93 oct.

I always hear tune to 11:1 or 11.5 etc. but in my interpretation this value is at full boost (20-21psi tapering to17-18 psi). I’ve read raising EGTs by leaning out AFR improves spool up in the lower rpm range but how lean is safe? Another question that comes in mind is, what is the best AFR at peak Boost, at 5K rpm and so on?


I have made these plots from actual data logs in third gear and would like to hear comments by those with experience.



Thanks in advance for the input

[turbolarry]

Originally Posted by Jorge T

I always hear tune to 11:1 or 11.5 etc. but in my interpretation this value is at full boost (20-21psi tapering to17-18 psi)...

I’ve read raising EGTs by leaning out AFR improves spool up in the lower rpm range but how lean is safe?

Another question that comes in mind is, what is the best AFR at peak Boost, at 5K rpm and so on?

First off I've never used Xede.
Your first statement is a little confusing. Can you clearify?

In regards to EGT's;
Leaning things out anywhere is not a good idea. Leaning things out to purposely drive up EGT's is dangerous. Lean is not safe... ever.
If your really into drag racing, really know what your doing, and have complete control of timing, you could retard timing at a specific RPM range to have a late ignition. Basically some cumbustion takes place in the exhaust manifold at low RPM's to spool a really big turbo, and that is what drives up EGT's. Not because it's lean. This is at a very high level of competiton too.

In regards to AFR; Don't put too much trust in other people saying, "this setting is safe," and "tune for these numbers." Every car is different. What's safe for one guy might get you aluminum on your plugs. You need to look at the entire picture; timing, knock, and AFR, not just one number. Maybe try shooting for a specifc timing number, at a specific RPM, 1-2 counts of knock, and then see what the AFR is. If you get no knock, bump the timing, or raise the boost, or take out some fuel. Go slow, might slight adjustments, monitor what your changes do.

If you could update your car's profile it will help too.

[krisj144]

larry is correct. you can't just tune for say 11.3:1 and call it a day otherwise the autotune feature in the ems would be worth a damn. timing is way more important than afr and it is more difficult to get correct as well. for a street tuned 4g63 by a novice tuner, the most I would shoot for would be 11:1 even with zero knock and that's with zero ignition advance because I'm cautious when it comes to tuning. basically, when I tune, it's for a safe conservative afr with zero knock and then I play with timing slowly and gradually. furthermore, timing will help spool more than afr as a rule of thumb.

leaning it out in the lower range to promote spool is rather silly too becuase it's very bad for the turbo (similar to antilag ports in the manifold) and you simply can't change the characteristics of the turbo with an overly agressive tune. if the turbo runs out of efficiency above 25 psi, then running race fuel and clamping the wastegate line just isn't the same thing as running a proper turbo. thus, the hard parts dictate your tuning to a large extent.