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Old 04-05-2005, 02:47 AM   #6
Scooby Specialist
Member#: 56468
Join Date: Mar 2004
Chapter/Region: NWIC
Location: Seattle, WA
04 Improved STI
Dirty White


The best documented tuning lesson I've seen with something so simple as half an AFR unit and one degree of timing.

I've been mucking around alot with leaner AFR on my dinky VF39, and while I save a bit on gas, I'm not getting much more power going from a general AFR of 10.9 to 11.4, the same 0.5 AFR gain you are talking about. I have found that I can maintain about 3-4 degrees more timing with an AFR of 10.9 than with 11.4. And my super high tech dyno (timing acceleration in 4 gear with the Hydra data logger) says I'm getting a little more power.

I could split the difference, but for now the richer AFR seems safest and I'm sure there's little to be gained from a 0.25 AFR enleanment, a 1-2 degree more of advance, but I'll check it out.

I'vw been told that are no general rule when it come to tuning. I just never believe anybody. I've been trying to experimentally try to figure out the relative contributions of leaner AFR vs. more spark advance. The power peak for gasoline is leveling out as you approach the 12-13:1 AFR mark. How sharp is that peak. Jblaine had a great thread on this awhile back. My sense is that it's still pretty steep in the high 10s to mid 11s of AFR. Once you at or above 11.5:1 your gains in HP/TRQ by raising AFR are not that great--that's when timing is your best friend.

In my range of high 10s to mid 11s AFR, timing and fuel are really flipping back and forth as to which is going to give you more power.

What really throws a wrench into my tuning is what I think is VE. In the peak torque area around 4-5000, the engine is very sensitive to almost any change. A little less fuel here, a little more advance hear, and boom boom I can hear the det start. You graph is convincing me to try leaning out the upper RPM range little more and calling it a day on in the 4-5000 range.

Another beautiful post Jeff, thanks. I'll have to post my $27 detonation listening device on the boards soon and get my web site up so I can post pics and graphs too. I bow to your greatness, some day I'll get down to PDX to meet you.
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