Good idea Monte. I'm sending the ECM to be soldered on Monday. I don't feel confident doing it myself, so Craig Moates will be doing it. So when I get it back we will get started.

 

Sorry about sticking the stuff in here, it was teh only thread available taht was somewhat related. I would have sent a PM but I figured having more then one set of eyes looking at some of the data would be more informative.

 

Yep, lets just make sure this info makes it into the other thread, that way every one who reads it will be up to speed.

I have a feeling there are going to be some major gains when AZ gets this figured out.

 

I'm confused now. I thought the ecm had no input from the AF ratio at wot. If that is the case, why is it set for 13:1 AFR at WOT? Isn't the oxygen sensor useless then?

 

The ecm has plenty of input as to what AFR the motor runs at. Your right the stock O2 is just used to trim your part throttle to 14.7 to 1. At wot the ecm calculates your WOT AFR from your VE curve. So once you get your VE curve tuned perfectly you can just name the actuall AFR you want to run at WOT.

 

But what good is naming your AFR ratio you want to run if the ecm has no idea if it is at that AFR or not?

 

There is one big ASSumption here. The ecm assumes that the VE curve is correct for 14.7 to 1 which is what your narrow band O2 sensor is good at seeing, and what the GM engineers tunes your stock VE curve for. If you make changes to the motor then yes the ecm is not going to know its not at the desired AFR. Thats why a WBO2 is so important for WOT tuning. You can crutch it and just tune your VE the best you can to the 14.7 then rely on the ecm's calculations. The problem here is that you really really don't want to run your engine at high loads and high rpm's at 14.7. What I did when I was tuing without a WB is tune my VE the best I could up to 3800 to 4000 rpm at part throttle or not in PE mode which is where the WOT AFR table comes into play, then I guessed at the cells I wasn't able to tune based on the cells I was able to get data at. When I finally got my WB I found out I was runing 14-15 to 1 at 3500 rpm, 12 to 1 between 4000 and 4800 and when way rich above that almost to 10 to 1. So I wasn't very good at guessing. My tune is starting to look very good now.

 

In a speed density system, the VE curve, cylinder volume, and density of air give an estimate of the mass of air in one of the cylinders that needs to be fueled. The code assumes that the VE table is correct and so is the estimate of air in the cylinder. The air/fuel ratio is needed to give the needed mass of fuel to the mass of air in the cylinder. Once this is done, using the mass flow rate of the injector and some other stuff can then give a time based calc for how long to run the fuel injectors.

In a tbi system, the code really doesnt look like this. Its more a bunch of fudge factor tossed in with some map readings, something that kinda looks like an AFR term, and the BPW. Either its a severe case of lazyness on the programmers part, or its for some other reason. A bit of conjecture on my part, but I suspect that reason is the fact that tbi is a wetflow system. The fuel vapors displace the actual air in the manifold, which renders a standard SD calc a mere approximation. All the fuel dynamics mean they probably had to have correction factors built in.

 

What I was getting at was that the ECM makes no changes to the AFR at WOT based on O2 readings correct? It will only make corrections under part throttle to change the AFR to the desired 14.7:1, and under WOT the computer does not use the O2 at all right? Instead it just goes by the preset programed tables regardless of how far off they are, or am I completely wrong here?

 

Oh, no, the ecm doesnt use the O2. The learn is disabled when in PE.

 

Alright good, I saw the WOT AFR and thought I must be all wrong, lol. Still don't fully understand the WOT AFR role though?

 

The AFR is the keystone in the fueling. In your ecm, the air fuel ratio is used to get the ratio of fuel to the ratio of air thats estimated to be in the cylinder. AFRs of some sort are used at all times, whether in open loop, closed loop, or WOT.

 

Forgive my ignorance as I haven't even begun to get a grasp on prom tuning, but if the computer recieves no input from the oxygen sensor at WOT, how can it determine the AFR at WOT?

 

Alright, theres the lowdown

A commanded AFR is used in the computer. This is the air fuel ratio that the computer wants to produce. This is needed in the fueling like said above. However, the computer has no way of varifying the actual AFR. So, while the commanded AFR may be 12.5:1, it could be 13:1, 10.5:1, 15:1, etc. If the tables are right, the actual and desired AFRs will be the similar. The only AFR that the computer can really target is 14.7:1 with the O2 sensor. So, the AFR is needed to produce the correct fueling, but it cant be varified at anything other then 14.7:1. This is why the fuel tables need to be correct, so your actual AFR and the commanded AFR are close to one another.

 

Thank you for explaining that! So really the only way to know for sure is to experiment with the VE tables while using a wb O2 sensor?

 

Yep... Theres no other (reasonable) way to determine what the fueling should really be.

 

bravo !!!!! Good explanation Dimented, I some times can't quite explain what I'm thinking, but that makes alot more sense.

 

Ok, the computer went out Fed Ex today and Craig should have it tomorrow morning. I'll be tuning before ya know it.