MAF lesson

[Warped]

For what its worth here is something we discovered while at the nats.

Two nearly stock GN's (86 & 87). Both performed as expected for their mods. The only odd thing was too much knock on pump gas at only 12-13 pounds of boost, attributed to our current crappy gas. This was even with the power plate. The 86 was run at the nats with 5 gallons of 105 and still knocked. Attached Direct Scan and the only clue was the MAF numbers maxed at 200. Put on a known good MAF and the numbers maxed at 255, knock went away, even at 17 pounds of boost.

Max MAF on my 87 has always been 230. Even with the power plate I've had to dial down the boost to 12 to keep it from knocking. Put on the LT1 MAF and now boost is back to 14 with no knock.

Both cars ran well, idled well, got great mileage but both had bad MAF's.

 

[Fred 86 GN]

I thought the stock MAF would max out at 255 if the engine was pulling enough air to make 350hp or so. Are your cars making that kind of power? That would be about 107mph in the quarter with a normal weight car.

My MAF has yet to be maxed out, but is getting really close since it is around 250+ and the car is running almost 108 mph.

I have heard that running a cold air induction setup changes the air flow pattern through the MAF to the point that it becomes difficult to max it out...

If my statements here are refutted, then I think it is time for me to get a different MAF and see if my troublesome knock retard gremlin goes away.

Thanks for the information!

 

[TurboDave]

Not always true Fred. Depends on whether the MAF is original, rebuilt or what, and also age comes into play.
Generally speaking a healthy stock original MAF should peak out at or near the shift points (high 4,000 rpm range), at about 14-15# of boost. That starts falling off with extreme age (more than 10 years).

It's almost impossible to get a remanufactured unit to get much over 200 at those same conditions, and damn hard to get one to hit 255 even at much greater boost levels. There are exceptions of course, but as a rule that's how they work.

So yes, those low reading, reman's are hurting fuel deliver, and quite badly in some cases.

 

[bruce]

Originally posted by Warped
For what its worth here is something we discovered while at the nats.
Both cars ran well, idled well, got great mileage but both had bad MAF's.

If you want a real thrill,
build an ecm bench, and put an accurate frequency counter on the MAF input. Then you'll see why there are so many MAF problems. The ecms are designed to work from 30-150 Hz. Starting at 153 Hz, the code breaks down, some call it MAF drop out. The other thing is all the higher flow numbers are really crunched up into the top end of the frequencies, so any error and thing fall apart. Things like taking the screens out, only allow you to just hammer the calibration so it runs. If you MAF doesn't limit it's out put to slightly less then 150 Hz, you'll have problems, and they include a momentary injector turn off, and less then 255 MAF numbers. So even at 20x a sec you can miss capturing what's happening.

You can have all kinds of weirdness, the code has to match the MAF.

Two cures, I see working, one a MAF that doesn't exceed 150 Hz, or a MAF you can't seemingly peg, ie Translator.