86 IROCZ28

Just wanted to ask this question. BLM's above 128 mean a lean condition and below 128 mean rich condition.

Since the ECM compensates and your scan tool reads say 145 then it means that the ECM added fuel. So does this realy mean that the car is not running LEAN?

If the ECM compensated the the air/fuel mixture was correct. So what benefits are there from tuning the VE tables to run 128/128 BLM's?

Is it just a case that the ECM does not compensate precisely enough and having adjusted the VE tables to run 128 is just better?

If say the BLM's were at 160 then the ECM cannot compensate after 160 so then the tuning would be required. But if the BLM's are below 160 or above 108 then the ECM should prevent the car running LEAN or RICH.

Correct?

Chevero

You are correct for non-PE running. When the ECM gets enough thhrottle (usually about 70%), it goes into PE. When in PE, the ECM uses the non-BLM altered VE table to determine fuel input. The PE table then alters the fuel for performance purposes. BUT, it starts with the basic VE table, so if it isn't right, then the performance (PE) mixture will be wrong.
Again, the PE area does not benefit from BLM mixture correction, so beware--

PaulD

Thanks, I was wondering the same thing.

RBob

iTrader: (1)

Not sure whch ECMs (maskid's) you are talking about. The ones I've dealt with: $8D, $42, $61, $88, $58, among others all handle the BLM in PE mode as such:

If the curent BLM value is < 128 (removing fuel) it is then either not used or set to 128 and then used.

If the current BLM value is > 128 (adding fuel) it is then used within the BPW calculation.

Whenever the BLM is adding fuel it is used to add fuel during PE mode. When removing fuel it is ignored. The $88 mask will even use the integrator in PE mode if it is adding fuel.

RBob.

Chevero

By golly, you are correct RBob!
I had not noticed that before, but when I looked at several of my $8D DataMaster runs, I saw what you describe. So GM has sort of protected us from lean running during PE (and perhaps HiWay, but have not fully checked that yet).

kevm14

Also, don't forget that VE influences your open-loop AFR (like when warming the engine up after a cold start). So, after dialing in your VE, you could, theoretically, improve your open-loop performance, or at least, the drivability.

TKD89RS

so is it better for the blms to be above or below 128. below 128 right which means its removing fuel?

86 IROCZ28

I would rather have my car running rich so BLMs below 128. I think running rich is safer for the car??

91L98Z28

well ideally right at 128, or as close as possible +/-.

You don't want it so rich that you wash down the cylinder walls with fuel, or too lean either. Too far to either extreme and you're damaging parts. you don't really want to aim for a low blm (less than 128) just to "err on the side of being safe".

kevm14

But you DO want to aim for, say, 125. This will make consistant your WOT fueling. The ECM makes no changes to the PE AFR if the BLM is 128 or below. Above 128, and it scales the PE AFR, so if your BLMs are all random, you won't get a consistant WOT AFR...non-consistant is no good for anything.

Theoretically, if your BLMs are ALWAYS 128 across the board, and no higher, you're fine. But you'll be riding right at the edge. So from what I've read, 125 is a good number to assure consistant WOT fueling...

HTH

D's89IROCZ

So..... what if I optimize the VE tables and keep it in an open loop state ? Would this ( in theory) be the way to makeing the car run the BEST it can?

Am I on the right track?

kevm14

Then your AFR is determined by the open-loop AFR (non-PE) tables. You could theoretically dial it in pretty damn close to 14.7:1, but that's pointless because the O2 is really good at keeping it there. If you wanted to run the car more like a carb, but with better fuel atomization, you could dial in the VE with the O2, then run in open loop and setup the AFR however you want. Just use a wideband to make sure what you see in the tables is close to what you get at the exhaust. For those two to match, you'll need to have the proper injector constant and VE tables.

Even then, I don't know for SURE if it is guaranteed. It just seems logical that it would be. If the ECM knows the injector flow rate, and the volumetric efficiency of the engine, it should be able to calculate the AFR (that's what I assume it does when everything is dialed in perfectly).

I think you should just let it use 14.7:1 closed loop mode all the time, though. Open-loop is for warm-up, WOT and highway mode only, imo (and idle, on some ECMs).

87400tpi

No need to force open loop for tuning.That is what locking the blms is for(fine tuning ve).Then you tune using the integrator.Imo open loop tuning should be done by a w/b o2 for race applications.If you tune you car in open loop then drive it in closed loop it will most likely run bad.Race apps don't require the tables/code for driveability.Those tables get enabled in closed loop.So locking the blms will get the fine 128s everywhere.Glenn the moderator is the godfather of this.