It always seemed to me that, Delphi 85mm MAF being "rich"
(high freq per flow), there ought to be a point where porting
one would produce a roughly-stock (F-body) cal curve without
the resistor trick. I finally got around to putting this to the test.

I hogged out one of my spares (actually, my old SLP "screwdriver"
MAF) removing the center vane and everything but the sense
bridge (first pic). I put this MAF in tandem with my factory stock
one and used a dust collector blower to feed them both the same
airflow. I used a fat old Variac to dial up and down the airflow. I
cobbled up a pair of 5V loads and read frequency with a pair of
DMMs w/ frequency function.

First pass I found the two running pretty close but the ported
still a tad higher than stock. Reconnecting the resistor ala SLP
could still push it to match. I dressed it a tiny bit more with the
file and went past matched-frequency, to where the ported 85mm
reads slightly lower frequency than the stocker. This means I will
hit the 511 g/sec limit (I wish) before the MAF hits 12kHz limit. So
full useful range.

The frequencies are pretty close across the airflow band I was
able to produce, though this is well short of the frequency I see
at WOT; only 8kHz max with this blower (Harbor Freight, alleged
900CFM, lying phuquers). Still in 4th pic you can see that the
frequency factor appears to be settling in nicely to an asymptote
so I think at higher flows it will be well behaved and no problem
to dial in (and get roughly-right beforehand).

So bottom line is that with judicious hacking you can get an 85mm
cheapo MAF to fall about wherever you want, and get full PCM
air mass flow range out of one (not the 440-460g/sec that the
stock piece & PCM limit at, for 12kHz).

 

great thread!! Quick question. How could we read frequency output of the MAF on our cars, say when its idling? Is it as simple as using a multi meter while its running. I'm just looking to know exactly where my ported MAf is at idle. Cool test nonetheless!

 

Just log MAF freq.

Nice work Jimmy, I was on the fence of setting up a similar test with my PCM bench harness and scanner.

 

Did that affect any part-throtte driving? Was it harder to tune after that? Part-throttle/WOT?

 

I understand why you did this test, but can the same thing be done by just altering the maf table in the PCM? I have HPtuners, and I picked up an 85mm delphi maf and setup the maf table as per a few different online sources. I'm still in speed density mode tuning my VE table, but one day I plan on plugging back in the maf and dialing in the maf calibration.

Thanks for your efforts.

 

The thing is that as-built the 85mm MAFs hit 12000Hz
before they hit 511 g/sec airflow. The stock 85mm
tables run up to 459g/sec, that I've seen. This is
leaving about 10% airflow range on the table. If you
don't use all that then you don't really care, just use
a Z06 MAF and table or something. That's what I've
done for years (descreened truck). I just had to see
whether even more would be possible.

 

You are right Jimmy, I tuned an 02ss the other day that made 440rwhp through a 9" and an 85mm MAF, and he was 2 cells from the end of the table!

 

So, basically, to get the most out of an 85mm MAF and for your pcm to "register" to its fullest, you have to port it out??