AFR tuning question

[Sy#26]

I seem to be getting some odd results when trying to tune to a specific AFR via a wideband oxy sensor. Same boost levels, same RPMs give different AFRs - is this normal, or is something FUBAR'ed w/ my setup? For example, I'm after an AFR of 12.2 by adjusting F30 & F77 tables:

3500 RPMs, 3rd gear, kickdown from cruise: 3500, 19psi gives 12.7 AFR
3500 RPMs, 2nd gear, WOT run: 3500, 19psi gives 12.2 AFR
3500 RPMs, 3rd gear, WOT run: 3500, 19psi gives 11.8 AFR

These numbers are taken after a second or so, allowing any AE 'pump shot' effect to fade away (on kickdown).

So, if this is normal behavior, how DO you use the AFR values, which seem to vary greatly depending on other factors?? After switching from 40# injectors to 50#, I've lost almost 3/4 second which I can't seem to get back.

Comments greatly appreciated.

 

[jwaller]

my only guess would be IAT's changin things somewhat and somehow maybe your timing isnt exactly the same... need some more info..

 

[Sy#26]

MATs on the runs above were, respectively, 84°F, 95° and 102°. Timing was 12° for all three.

What do you typically see from gear to gear, John?

Thanks - Barry

 

[BigAl...]

the biggest variable in your test is load! then your mat's hotter air = less air = richer.

BTW that's pretty low timing!

BigAl...

 

[Sy#26]

MAT change in F31M is about 3% over that temp range; AFRs differ by a lot more than that.

Timing-wise, that's about the max w/o knock on 93 pump gas I can get; knock thresshold is around 13° right there, retarded 1° for safety's sake.

 

[ItsMyTy]

the biggest variable in your test is load!
BigAl...

what he said

 

[Sy#26]

Well, someone needs to 'splain to me how that works. If you mix 'x' fuel w/ 'y' air - squeeze it in the cylinder (same rpms, temp & MAP) & pop it w/ a spark - how does it 'know' how much 'load' its under? Seems to me, it'll push on that piston the same amount, regardless of what's going on *after* the piston, i.e., gear ratio, weight, etc. By that reasoning, if I put 500# in the truck bed, AFRs will change also - doesn't make sense to me. I can change AFRs by adding or subtracting weight from the truck = changing load?

 

[bigtime]

Maybe this will help, It's from tech edge. (where I bought mine from).


"The sensor reads the partial pressure of gases in the exhaust and infers the AFR, rather than by measuring some magical AFR directly. This may be an issue on forced induction, and in particular, on turbo-charged engines.

AFRs will indicate richer than they are, causing you to run leaner than you think.
Lean AFR's will be richer (or less lean) than indicted.
A solution is to ensure you locate your sensor away from the turbo......"

http://techedge.com.au/vehicle/wbo2/default.htm