OK, I started paying attention to my gauges and my driving to find out what's going on with my A/Fs. The first thing I've noticed is that when I'm cruising at part throttle (very lightly pressing the accelerator) at around 3000 rpm and under vacuum, my A/Fs hover in the 12.5:1 to 13:1. Is this a little bit rich? heck, it reads leaner than when the car goes to 0 vaccum and barely going into boost....

I will say that with the foot completely off the throttle, the A/Fs go lean and even off the scale. That makes sense to me since the car is under full vacuum. Shouldn't the A/Fs be near or at stoichometric under part throttle and vacuum?

Thanks

 

Have I missed something here? I thought the 'higher' the number is...ie, 12.5 is LEAN compared to say 9.0....

 

Well, stoichometric is 14.1, that's an ideal mixture for perfect combustion. Of course, that's not safe for acceleration under boost.
12.5 is "leaner" than 9, but anything richer than 11:1 is way too rich for almost any condition.
I believe that 11.5:1 is extremely safe for high boost conditions. Some folks use 12:1 for high boost and certainly, anything lower than 14.1:1 is technically "rich".
Still, I don't know if these A/Fs are appropriate for part throttle cruising......

 

You want the AFR to be in the 15-16 range when cruising at part throttle. If you are using boost comp, you get the fuel map so the AFR is a solid 11.8 at all RPM at WOT. Then you tune the part throttle AFR with the Throttle Inj Corr table. Then you can set up O2 Feedback to hit the target very consistently.

If you are not using Boost Comp, it is prolly easiest to use Automap and set the target AFR to 16 everywhere in the vacuum range except at idle where you want 14.7, or maybe closer to 13 if you have cams and it doesn't idle well at 14.7. After the Automap is done, manually smooth out the map until it looks right.

 

Well, the stoichiometric is 14.7:1.

Cruising around varies from car to car. Soem cars cruise at 15-16 some will backfire at that level, so they need to be richer.

 

Yeah I've had good luck with EVOs up to around 16.5. 15.5 works on crazy modded motors with cams, 16 works on a more stock setup.

 

You are right on the money there.... Thanks a lot for your help

 

Some cars do not take the 15.5 nicely

We have seen it on several Evos. The AFR goes to 9 at a very small thottle opening. If you tap the throttle above 10%, it goes back to the high 14s and 15s.
The way we have done it is with leaning out the map in the 1500 to 4000 and -14 to 0psi. With some throttle correction, it will be nice and smooth.

 

Yeah you will need to adjust the Accel fuel a bit so it doesn't go so rich at low throttle angles. The Accel fuel should bring it down into the low 12s for good throttle response without bogging. The easiest method is to play with the Accel Fuel max value. That will act as a hard limiter for that Accel Pump and keep the AFR from going too rich when just cracking the throttle open. Also, if you are using a boost comped map, the 0-5% Throttle Inj Corr table will be very lean, like -50% and then go up from there. This helps to keep the fuel under control at low throttle angles which have a big impact on the VE.