Pops and I are building a pretty radical 383 for the street, we are going to used the stock ecm with a monoblade. We have a stock maf with Mallory billet ends. I have read bad things about tuning with a Z06 maf, what is the normally aspirated horsepower limit for the stock maf? Engine Analyzer Pro says we are using 910 CFM of air at peak HP. Will the stock maf handle it? This car will be tuned on a chassis dyno.


David

 

i would like to know this also. wonder if my 396 is suffocating

 

You've got two issues to address, particularly if you are using the stock PCM. First is the pressure loss as a result of air flow. As flow increases, pressure loss increases. Can't avoid that. Pressure drops, air density drops, mass of air in the combustion chamber drops.... eachy "CFM" contains less mass. So there is no "peg" point with regard to flow... just a point at which the pressure loss becomes a "significant" value and is costing you power. There have been some studies made of pressure loss vs. flow for the stock LT1 MAF sensor, and I believe that the numbers, or a link to the same have been posted here in the past.

The second issue is "calibration". The stock PCM calibration stops at 471gps, which is going to be roughly 842cfm, assuming a 60degF intake temperature and a 1"Hg loss in pressure before the MAF sensor. You would have to do an airflow study to see how the frequency output of the meter varied when you pushed a larger volume of air through it.... you could also evaluate pressure loss at the same time. The forced induction guys may already have done some of this work.

Just playing with numbers, you appear to be planning to run the 383 well over 8,000rpm, or push the VE above 100%. How do you plan to address the stock PCM rpm limits?

 

Well the cam gives up at 7000 rpm according to engine analyzer pro, EA Pro also says we are using 910 CFM at peak HP, yes VE is over 100% in places on the curve.
I have heard that there are problems tuning with a Z06 MAF, but may have to go that route anyway.


David

 

If I am correct the tables in the PCM read up through 510g/sec - that should be just over 910cfm (I think, real rough on that) I dont want to know what you're doing to get that much CFM on a 383 and only really pushing it to 7000rpm, but the heads must be rather extreme.

 

If you want to know the combo I will post it, this is probably the most radical 23* headed 383 normally aspirated LT1 on pump gas that has been built on this board. My Dad and I are shooting for mid to high 10's all motor on a drag radial with a fully optioned Z28 that will weigh in the neighborhood of 3650 lbs with me in it. Tall order I know....we will see.

David

 

Unless I missed a part of the table, my 94 stock programming stopped at 471.51 gps:

11,216 Hz - 471.51 gps

Now that I no longer run an MAF, the scanner reads the disconnected sensor as 471 gps.

 

You can run the stock PCM in speed density, no? Might get you what you need.

 

SO how many CFM is 471 gps, or can you convert it? Or does it rely on other factors such as density altitude, pressure drop etc.
Has anyone run a high powered car such as Mindgame with a FAST setup and put the meter inline unplugged just to se if it is a restriction?
At 471 gps does the ecm table stop reading incoming air and "stick" or go static like an injector? More air can come in but that is all it will read? ANd if that is so will a MAF translator or something similar fix the problem.
Why are so many people having problems tuning with a Z06 MAF? If you start at a baseline on a chassis dyno and know what you start with...why can't you tune with one?


David

 

Though it is set to 471, I remember looking somewhere and noticing that the tables went up to 510 (though not used, hence why it stops actually reading at 471) and some people had made it use the tables up to 510, but it didnt make much difference in the end.

When the PCM maxes out the maf and it "sticks" at 471, you can still compensate for air flow by adjusting the tune. OBDII cars when the maf maxes out tend to but fuel by about half (dont remember why though) which is another nightmare all together.

There is going to be a restriction no matter what you do with any maf sensor. I know the Z06 maf works on a Z06. That's easy. And you could sit there with a baseline tune and start from the ground up - but the issue is that the Z06 maf looses flow velocity on our cars because of the size of the rest of the intake tract. Flower velocity and more turbulent air gives the sensor a harder time reading an accurate count of exactly how much air is there. It does a decent job, but it has a hard time getting the precise values. It has the same problems as removing the screen from the stock maf.

 

The number was in my first post:

By definition, anything you put in the inlet track (other than a blower ) is a restriction. Is causes pressure loss, and the loss increases with the square of the air speed. Add 41% to the flow through the MAF, and the amount of pressure lost going through it doubles. The pressure loss is not a problem on a stock car.... but when you start to push 900cfm through it, its going to present a much larger pressure drop. Like I said... do a "search" for the link to flow vs. pressure loss for a number of popular devices. If the link shows the pressure drop at a particular CFM, its very easy to calculate the pressure drop at 900CFM. We can help you do that. I know it was posted here in the last year. If you lose 28"H2O in the MAF, you have lost 1psi out of the 14.7psi available to push air into the cylinders.... thats almost a 7% loss in air mass in the cylinder and 7% less HP at the flywheel.

I'll leave you with Geoff's info on that one... I haven't run with an MAF sensor for 5 years. It should be possible to put a powered MAF on a flow bench, vary the flow through the sensor, and develop a chart of output frequency vs. mass flow. You could even include the actual inlect duct configuration, to eliminate inaccuracies caused by the way it is actually installed in the car. At the same time, you could produce a plot of pressure loss vs. mass flow.

 

Have you looked into using a big ProM maf and the GM driver box they sell to convert the ford lodgic to GM?

There are some guys local here in detroit running it on bigcube NA ls1's fine but I've yet too see a boosted one local. Heres some info
http://www.ls1tech.com/forums/showth...ight=pro+m+maf

 

I am by no means an FI "expert" but I've had to skirt around stock MAF limitations on 3rd gen TPI cars which "peg" at only 255 g/sec due to programming limitations. Regardless, the main concern is that the MAF will start to lose "resolution" when you approach the upper limits of it's flow at a reasonably pressure drop. I would suspect that is why the factory stopped measuring around 471 g/sec on a 4th gen even though the system is capable of defining airflows up to 510 g/sec (I am taking the word of the others in this thread for those numbers- I have very limited 4th gen experience).

Kinda like trying to use a narrow-band O2 sensor to tune your WOT A/F ratio. It simply lacks the precision and "vocabulary" to give you a straight answer.

On a 3rd gen the "fix" was to get it on a dyno and start tweaking the "WOT fuel enrichment" tables in the upper RPMs at WOT until the A/F ratio was correct up in the way-way out there reaches of the power curve that a stock ECM was never really designed to handle.

 

That is easily done, which is a standard operation. Tune it in "Speed Density" and get it over with.

The issue I have, is I'd like to see guys with boost have a baseline to work with. In a turbo setup, a simple hose failure could mean 14psi jumps to 18psi. Without more fuel, you may nuke your engine. Only a PCM that can read boost can save the engine in such a case. Also it'd make tuning a lot easier...

So do a standalone? You could. I dont want to. I love the stock PCM for what it does- which a standalone has a hard time doing without extensive effort.

 

You can tune a 94-97 Factory ECM in speed density and remove the MAF sensor?

David