Hey guys, I've been thinking, I know thats a bad thing, but here it is. I was reading on some probe forum stuff(2.2L turbocharged) and they seem to be able to just turn up the boost and let the MAF handle all the fuel calculations until the thing runs out of fuel.

Do we do this on the MAF vette's?? I'm stuck with a '93 speed density machine and I have switched over to an aftermarket ecm that will read a 2bar map so I'm really not very well in tuned with this MAF stuff.

Can we just start adding more air and letting the computer see what is going on? And just mess with the ecm when we upgrade injectors or want to change timing?

And why do some people suggest putting the maf at the inlet of the blower on our vettes instead of in the intake piping? Post BOV would seem to be the best place for it, but I remember people having issues when it was on the boost side and I'm not sure why.

 

good question

 

Yeah, supercharged SD vettes need more tuning than MAF cars..

Sinxe you switched to an aftermarket EFI you'll have to tune your car with that MAP setup.. Good luck chief..

Putting the MAF BEFORE the supercharge rintake is useless., and you are correct, will mess your A/F mikx up bigtime when your BOV/surge valve opens..

 

ummm, ok, I was looking for a little more indepth response than that. I'm obviously not worried about tuning my car, I'm just asking for general knowledge and all that theoretical vs practical stuff.

 

Brian,

the maf will make finite adjustments, but you still will need to get the tune close.. ie.. the Power Enrichment tables under boost will need to be mapped accordingly.. the maf can make adjustments but not massive changes that quickly.. if we were to rely soley on the maf to add fuel to a boosted car, you would be buying pistons long before it corrected itself all the way. kind of like on the DFI, you have a learn function.. with a percent limit as to how far it will correct itself.. very much like the stock ecu in closed loop, but it will not tune itself as well as the wide setup. I believe the MAF is good to around 428 g/sec and can be tweaked with an offset to a little over 500g/sec if I remember correctly from a while back of reading it numbers are vague in memory so don't nail me to them.. so once you max the maf your finite adjustment to your BASE tune is gone and you are tuning solely with the PE tables to feed fuel beyond that. in essence the MAF works very well with a solid base tune to adjust for atmospheric changes and other smallish circumstances. now when you are reffering to the probes, they are a factory turboed car? i know some were but which are you referring too? if it has the baility to sense boost from the factory and has a MAF like the Buick GN, then it will be able to calibrate itself with some mods, but large changes, example 1 bar, will need to be adjusted further in the computer. mainly for timing and or to work with additional larger injectors.

I know you are already aware of alot of what I just wrote but was throwing it out for some of the others.. this may or may not be all correct, so take it with a grain of salt.. I am going by what I read so long ago I could be explaining it bass ackwards.

Chris

 

Since the Maf will measure Mass flow(by weight not volume), it does indeed properly meter fuel with the addition of boost. More grams/sec airflow, proportionally more fuel. But you still have to retune.
As others have stated above you need to go thru the PE tables and bring them down from 12.5 (stock tune) to more like 11.8 a/f. The stock limit for the Pcm on 94-96 cars is 512grams/sec. But you can get around that at the cost of some resolution.
On my own car I cut all tables for the Maf in half, along with cly volume,injector flow constant and everything else that has to do with the fuel calcs. Then I retuned. Now it can read up to 1024grams/sec without the ecm resetting, Or at Least I think so as I havnt maxed it yet.
You definitly want the Maf after the BOV or at least use a recirculating one. Otherwise it stalls when you come to a stop after whomping on it, and belches a nasty black cloud between shifts.
I know in several factory turbo efforts the maf is used for all fuel calcualtions and they reference the MAP for ignition, boost control, boostcut etc. subies for sure.

 

Yes some of the Probe GT's were factory turboed, I think it was '89-91 or something like that. But I think those cars didn't even have a MAP sensor, just a MAF.

Ok so basically on OUR cars it takes the map/rpm reading comes up with a fuel calculation and then uses the MAF reading to "adjust" it to something more favorable.

I guess I'm curious to know why can't I get a pcm that reads only a MAF sensor and an ECT sensor, and have it calculate exactly how much fuel it needs to be put in the car. And program the a/f ratio for the by how much air is going into the engine or if the car needs to be warmed up.

Obviously you'd need something for IAC. But I would think throttle tip in and what not should be handled by the MAF. I'm sure you'd have to mess with it a bunch with finding the right equation to cacluate in the number if cylinders and injector size. But once you got those right I would think it would be a constant calculation.

Am I way off on my thinking here or does any of this make sense??

 

Some great stuff ice and mn_vette...
I log my fuel trims and adjust the maf to get the trims to about +/- 3 % the I rely on the PE to get the AFR correct under boost masuring with a WB...

I was using Gregs superpumper for a while to add fuel before I changed to my 60# injectors. Now I just have the two walbros and have been working on getting the fuel dialed in. A majority of the time just getting miles on the engine getting it broke in... I also agree I had to increasemy MAF to almost 512 as well... I havent really played with the VE tables at all...

I have noted that there is something weird since i have gone to the 60#'s that the computer calulating my mileage is way off it's like it is calculating the fuel for a 30 # injecotr it was saying i was getting about 34 mpg and i only getting about 17mpg...

Tuning to the 396 has been somewhat painful as I pretty much seem to breaking my own ground so a lot a data logs and tweeking to get things close but I am making steady progress... And other weird phenomenon with anynew build fixing quirks along the way HEH

My experience has been that the maf is pretty quick if and when I am close on the fuel trims the throttle response is very crisp... I love tuning to the MAF.. Mine is before the S/C but I have a return so the metering reamins correct as some mentioned above, if you vent to the atmosphere it will definately run the trims full rich

BTF, ( Baldturbofreak) I'd be interested to see what you are doing in terms of your injector and cyl settings etc.. if you are willing to share... I'm using LT1 edit to tune my ecm...

Some excellent thoughts guys...

Mo