Chris B

How accurate that formula is depends on the quality of the inputs.

If your injectors are flowing at all times exactly what they should (FP is constant, etc.) and your MAF is metering in EXACTLY the amount of air coming into the engine, your A/F charge quality is the same as GM *assumes* for a stock vehcile - then you are fine.

The injector flow is probably pretty consistent, esp. if you have flow matched injectors and used those values, and have a quality fuel system.


The MAF will most definitely be off - if any tuning has been done to the tables (say to tweak BLM's), or if any changes have been made around the maf (intake path, etc.) - it will be off. Not a tremendous amount, and it should still be precise, but accuracy will suffer.

Finally there is charge quality - just because I spray mass equivalents of 12:1 doesn't mean that is what a wideband will read - the wideband will pick up how much oxygen is left over, and from that determine your a/f ratio - this is going to be the "effective" ratio - that takes into account imperfect atomization, etc. You may have to actually spray in a 12:1 real ratio to observe a 13:1 effective ratio if you have poor atomization, clumping, etc - since if atomization is poor you will need to put in more fool to combust with the available oxygen. Changing camshafts, cylinder heads, headers, etc. will all have an effect on this.


Now that said if the MAF is working properly the commanded AFR should be withing the ballpark of the measured AFR, or you have a calibration problem somewhere. But do you really want to bet your engine on that?



Chris

LT4ultraZ

Now would this work with a supercharged car.I max out my MAF at 5300 rpm in cell 15 with 128 blm. Sorry if this is a stupid question i am Still learning. I havent had my car on the dyno in about a year with some new parts and some programing sence then.

96TurboTA

Using the BLMs to tune your car, assumes that the computer is getting accurate data from the sensors, correct? If you have an exhaust leak or a bad O2 that you don't know about, won't the computer change the BLMs to adjust for what it perceives as a rich or lean condition?

95Blackhawk

96TurboTA,

You are correct and this goes along with what Chris said about having basically a correct running car or this whole process will be flawed.


Chris,

As for your statement about MAF and calibration problems, that is exactly what I had and so we could talk in circles about "MAF is wrong so adjust, but we don't know its wrong so don't adjust." Only way to be sure is follow up with a wideband which I did. This showed the results of my recalibration to be correct.

Now again, this is for everyone to decide for themselves whether they want to do this or not.

Regarding the concern about engine damage, well I am finding not many people at all know when damage occurs with a "lean" condition (IF YOUR THE ONE THAT KNOWS, POST PLEASE). But what is lean? What AFR is it? Yes it is variable based upon conditions and engine, but then the AFR where we theoretically make the most HP would be all over the board, but yet it is around 12 - 13 AFR (seen graphs where they pinpoint it at 12.6:1 AFR).

Ben

Mikey97Z

I have one question. Wouldn't it be better to tune tune your BLM's ABOVE 128 in cell 15 so that they stay locked at that value when PE kicks in? Instead of below 128 where some fuel will be added ( an extra unknown variable ), plus PE too?

JSK333

Mikey, probably a good idea... but there is also a program that will lock the BLM at 128 when at WOT, which if you're tuning for 128, would end up working ok.

Jon A

Exactly. Maybe there's something I'm missing, but I wonder why more people don't do that? It will take away an extra margin of safety GM gave us, but if one can verify the computer isn't being fooled into pulling fuel by a false rich condition, I would think this would make the PE tables' formula as accurate as it's going to be.

My car (stock, headers/CAI) ran pig rich on the dyno--even richer than the PE tables told it to. That was before I was set up to scan/program. Now that I am, I know this was because my BL's were all in the low 120's. When they jump to 128 at WOT it's naturally going to be richer than the PE tables predict.

Since I have this verified via wideband, why not do something simple--like bump the injector constants slightly, putting the BL's at 128 or slightly above which should restore accuracy to the PE tables' formula?

JSK333

The ideal in this case, given our limit with the stock PCM, is to tune via the MAF sensor tables by disabling PE mode and trying to get your BLMs to 128 at WOT RPMs/loads.

This way, you know that the MAF sensor is correctly reading airflow at your usual WOT areas, because it will be trying and correcting for 14.7:1 AFR. Then, once you get the BLMs close to or slightly above 128, you can reenable PE mode.

At that point, your PE tables should be as close to accurate as is possible.

At least one person has verified this method as working while on a dyno.

Those who say that the VE tables are used in MAF sensor mode could also try tuning with those instead of the MAF tables, in the WOT areas with PE disabled. If you can get your BLMs to 128 this way, then reenable PE mode, you should get the same effect as above.

96ZRDR

In a speed density car, this would work the same with the only difference being that changes would be done in the VE tables right?

TobyZ28

I've gotta say this make s a lot of sense to me too, it may not be 100% bang on but should allow for the MAF to be recalibrated correctly..
The best case way of calibrating the maf of course would be to use a machine thats built for getting the maf readings for each of the table entries. Then you know the MAF's accurate.... and if you applied your little equation you could even find out how close it worked out to be.

I belive this would apply to any car (stock or non-stock) as long as its using the MAF, if the MAF is not as accurate as the computer tables, then the normal mix and PE will actually be off...

97Z-M6

first off how do you lock the cell in place, say cell 15 for ex.


and i dont understand all this talk about get the blms right in cell 15 for wot tuning, it makes no sence. pcm doesnt care what cell 15 is blm wise, it only cares what the blm was the instant you go wot. if it was above 128 then it will use cell 15 and adds the fuel correction amount to the final afr in the pe tables becuase it percieved a lean condition, but is correcting for it, and uses that to protect the engine from running lean up top. if the last blm seen is below 128 the pcm uses cell 18, and adds no fuel becuase it knows its safe becuase it was corecting for a rich condition.

cell 18 is a open loop cell for high load and high rpm, but while not in open loop (before the o2s are used, like after cranking the car) this cell is not used in pe mode becuase the car is still in open loop, it is using the open loop afr tables, but after the car has entered closed loop this cell is used it pe mode if the blms are below 128. this is the cell you want to use.

the reason i cant understand the reason for getting cell 15 right at 128, is because when your racing on the street say from a role the pcm uses the last blm to determine the cell it will use for wot. but if your siting (not moving)at the staging lights at 2500 rpms if your not past 66% tps then your not in pe mode so your blms in cell 18 better be below 128 becuase if not soon as you move your car will go to cell 15 and add fuel.

you have to say how often are you going to be in cell 15 at the staging lights, never becuase the car is going to be in cell 16 at a idle sitting still and if you rev the engine up it will go to 18 not 15.

oh and if you dont let your car go into closed loop before you stage then you will not even use the pe tables for fueling it will use the open loop afr tables.

and i have proved this to myself with data master at the track.

jonota

We know that BLM's go to 128 when PE engages, what the issue is here is that the MAF sensor in your car may be reading an incorrect air consumption rate in the upper rpms. Since this is what the computer will use to try and achieve that target AFR, if it is incorrect, you're definitely not going to be getting the AFR you are searching for, and consequently, probably not the power. Lets imagine for an instant that you are running at 120 or so BLM's in cell 15 without PE mode engaged. If so, your MAF is probably registering too high, therefore when the computer calculates the injector pulsewidth, it will be rich, the O2s will read this, then your BLM's will go down and integrators as well until the situation is correct (if your closed loop).
Now if you look at the Power Enriched version of that same scenario: Your WOT with a target AFR of 12.6:1, but your MAF is saying your getting more air than you actually are... in which case your car will run RICHER... making less power in some cases.
Also realize (this goes for the people worried about running stoich at high rpms) that if the opposite is true (your BLM's are HIGHER than 128 in the upper rpms), your MAF isn't reading ALL of the air, so you will run LEANER than you thought. This may make more power, or you could be running EXCESSIVELY lean and get knock, or worse. So the question you have to ask yourself is this: Is it worth a couple of WOT pulls with PE disabled to ensure your not running stoich or worse EVERY time you punch the throttle

because guess what:

I WAS

Jonota

Dan K

Guys,
I've learned quite a bit since Ben and I were tossing this around. Having a wideband now makes it easier.
When I tried this on my Z, I changed all of the maf tables so that I had the cell 15 blm as close to 128 as I could and assumed that I had achieved a 14.7:1 a/f ratio. I then added the correct amount of fuel to get the a/f ratio in pe to 12.8:1. No reason why I picked that number, just thought it was a good place to shoot for.
Problem was, I was getting knock all over the place. No matter what I did I was getting knock at wide open throttle. No amount of adding fuel or pulling timing would get rid of it.
Went back to stock maf tables, used the same timing, and made my guess as to what to have in the pe table and guess what...no more knock. Injector pulsewidths were nearly identical as with the "corrected" maf tables, so I assume I had roughly the same amount of fuel in it.
I just wish I would have had the wideband when I did this experimenting.
Now that I do have it, and this topic got brought back up, I wish I still had the Z so that I could go do some more testing.

jonota

Under what circumstances were you getting knock? With PE enabled or no? Without PE enabled, I expected knock, and I got it (although it didn't pull max out). With PE enabled, after the MAF tables are corrected, I don't. I am finishing the corrections now, just did my WOT runs without PE enabled. I will report wether I get knock afterwards when I am done.

Jonota

Dan K

After I changed the maf tables I would get knock. And yes it was in pe mode. If it wasn't it wouldn't have bothered me as much.
For some reason, once I modded the maf tables I would get knock at wot. Never did figure out why.