12200411 PCM swap for 1996 to 2000 GMT400 trucks

[L31MaxExpress]

If I have time, I will throw together a HPT file for a 350 later with the correct MAF curve and injector data for the MPFI upgrade as well as throw a decent timing curve in it. If I have the time might even segment swap one for a 4L60E truck too.

 

[1998_K1500_Sub]

As I remembered the are HUGE discrepencies in the MAF flow curves between an 01-02 van and a 96-2000 truck or van. When GM changed the airbox on the vans to the 01+ newer style the MAF curves changed dramatically. They all 3 use the same MAF part number.

So "mass" isn't "mass" it depends on "other factors".

This disparity seems to suggest it's not only the mass flow through the sensor but the flow's trajectory through the sensor, i.e., the flow can be non-uniform flow (non-uniform velocity and/or non-uniform density) across the aperture of the MAF sensor, and the sensor is actually only measuring the flow in a certain subset of its aperture.

Am I surprised? I guess not, there's only a few wires in the sensor and if the flow's non-uniform then... "of course".

Did I expect this? No. I guess I hadn't thought about it.

So anybody who sticks a non-OE style of intake / air box in front of the MAF, or relocates the MAF, may require a "retuning" of the OE MAF flow curves.

 

[Supercharged111]

Well looking around I had a HPT file for a 1998 Black Box and a 2003 P59 L35 Astro. The MAF curves fit practically over the top of each other. The couple of places the frequencies match are exact as well. The 1998 Blackbox curve stops at 11,216 hz with 357.70 gms/sec and the P59 is 362 gm/sec @ 11,250 hz and GM flat lined the curve at 362 gms/sec at higher frequencies. Not like a 4.3L is getting anywhere close to that anyway. That knowledge however tells me that yes, the MAF curve needs to be reworked in an 0411 van file to run in a GMT400 correctly. The MAF curve in the 0411 in the 01-02 vans is specific to the goofy air box they ran.

Oh absolutely. My 1500 MAF curve may be helpful (assuming I got IFR correct) but the dually has a completely not-stock air box. Both needed a significant rework to get it to compute the correct amount of fuel without relying so heavily on trims.

 

[1998_K1500_Sub]

Oh absolutely. My 1500 MAF curve may be helpful (assuming I got IFR correct) but the dually has a completely not-stock air box. Both needed a significant rework to get it to compute the correct amount of fuel without relying so heavily on trims.

D@mn... this was an eye-opener.

 

[L31MaxExpress]

This should be an eye opener., merely pulling an air filter, instantly maxes out short terms.

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[L31MaxExpress]

So "mass" isn't "mass" it depends on "other factors".

This disparity seems to suggest it's not only the mass flow through the sensor but the flow's trajectory through the sensor, i.e., the flow can be non-uniform flow across the aperture of the MAF sensor and the sensor is actually only measuring the flow in a certain subset of its aperture.

Am I surprised? I guess not, there's only a few wires in the sensor and if the flow's non-uniform then... "of course".

Did I expect this? No. I guess I hadn't thought about it.

So anybody who sticks a non-OE style of intake / air box in front of the MAF, or relocates the MAF, may require a "retuning" of the OE MAF flow curves.

That is why the OE intake couplers and MAF are also indexed for alignment.

 

[L31MaxExpress]

Oh absolutely. My 1500 MAF curve may be helpful (assuming I got IFR correct) but the dually has a completely not-stock air box. Both needed a significant rework to get it to compute the correct amount of fuel without relying so heavily on trims.

It would be interesting to compare with what I come up with using the OE curve and frequency values, and graphing to create the curve for the revised frequencies.

 

[Schurkey]

I remembered the are HUGE discrepencies in the MAF flow curves between an 01-02 van and a 96-2000 truck or van. When GM changed the airbox on the vans to the 01+ newer style the MAF curves changed dramatically...

So "mass" isn't "mass" it depends on "other factors".

This disparity seems to suggest it's not only the mass flow through the sensor but the flow's trajectory through the sensor...

...Did I expect this? No. I guess I hadn't thought about it.

So anybody who sticks a non-OE style of intake / air box in front of the MAF, or relocates the MAF, may require a "retuning" of the OE MAF flow curves.

Another in a long line of reasons to be entirely suspicious of slapping-on a "Cold Air Intake".

I had NO idea the MAF output signal varied, depending on the plumbing ahead of it; although I probably should have.

Nice reminder that on these engines, EVERYTHING is related to EVERYTHING else.

 

[L31MaxExpress]

Another in a long line of reasons to be entirely suspicious of slapping-on a "Cold Air Intake".

I had NO idea the MAF output signal varied, depending on the plumbing ahead of it; although I probably should have.

Nice reminder that on these engines, EVERYTHING is related to EVERYTHING else.

One of the reasons I like Volant. I have tested their intake on GMT400s, GMT800s and currently run one on my 2011 Nissan Pathfinder. They put the R&D in to make sure it ran correctly. I also had a good result with an Airaid MXP 4" intake on my old Titan, but that was a purpose built intake for that truck, not some slapped together tubing. That intake had a massive filter and a venturi reducer that was sized for the MAF to sit it. If you can get a cartridge MAF to work properly in a revised intake tube on stock tune it definitely had the R&D done.

 

[Supercharged111]

Here's the 1500 MAF cal.

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And the dually.

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You'll notice bumps in both. It's where the new cal is merging with the old cal. Takes about a jillion hits to really smooth it out. They were bigger before. If you're a glutton for punishment I can post the values, but it would blow up the page pretty bad. I will say for the stuff I couldn't reach I just extrapolated the last corrective value all the way to the top. The dually was done down at 2500' elevation and the 1500 here at 6500'+. Both have Whipples, but the 1500 still uses a stock airbox whereas the dually got its own airbox and intake setup from Whipple. That one bypasses the fender but still ultimately draws from the same spot at the core support.