GregPac

The voltage output of the MAF is calibrated given that the sample port (the hole in the sensor itself) is a fixed fraction of the total air flow. If the body is bored and the sensor remains the same then the ratio is changed and the calibration is thus wrong. For example, if the original bore is 2" and the sensor port is 3/8" then the sensor is measuring 3.5% if the total air flow. If the body is bored to 2.75" then the port is measuring 1.86% of the total. Thus for the same actual air flow the sensor would be sending about 1/2 the voltage, which would cause the engine to run extremely lean.

Now, that being said, UrS4 is making these, and they're working well. So what am I missing????


Greg

VAP

<center><img src="http://pictureposter.audiworld.com/711/boredmaf.jpg"></center><p>
But first, the sampling tube is 10mm on our cars (.3937"). So it's a teensie bit larger than you gave it credit for. Also I havent had the greatest of luck using the older MAF sensor or housings with this modification. The Pre-96 MAFs in my experience are prone to CEL-throwing. The only workaround I've found to-date for the 95 &amp; earlier MAFs is complete replacement with the later "A" (1996 &amp; up) version sensors and housings. Well that and/or an ARC-2 or similar MAF calibrator. Another thing to consider is that the later "A" designation MAF sensor stems incorporate an integrated 10mm calibrated orifice that completely surrounds the sensor element just as if the sampling tube were still in place.

But the output voltages arent as extreme as the math or your formula would suggest for bored vs stock housings. I can't remember specifics but I was getting slightly lower voltage outputs once bored but they were *very* slight. Things like before at idle 3.22v/after boring 3.12v. At full (under load) throttle before: 4.89v/ after: 4.69v. For the most part an adjustable FPR can compensate across the board for those differences. But with air/fuel monitors in place and all other induction modifications on, the car never once approached dangerously lean conditions even without benefit of an adj FPR. And it started, idled and ran as smoothly as it ever did albeit with greater acceleration. It may well turn out to be that the 10mm calibrated orifice surrounding the later model year MAFs sensor wires is the culprit/savior in all this since it seems capable of accomplishing by itself what all that aluminum mass sampling tube was required to do before.

Least thats the track I'm taking for now.

100avantQ

Is it possible to put a 96+ maf sensor on a 92 100? isn't one four wire and one three?

VAP

upgrade to the later model #078 133 471A without any additional work as its a "tit-for-tat" swap item. And the "A" supercession model MAF housing can be easily bored without incident of throwing CEL's. The reason the older MAF housing can't be easily bored is due to it's o-ring seat being milled away when boring which causes sealing issues unless it's sealed with silicone sealer when installing it to the MAF housing. And that in-turn causes it to be very difficult to remove if you need to replace it. And it not having a calibrated orifice for it's sensor wires seems to make it more prone to throwing CEL's if it's not recalibrated or externally controlled with a MAF calibration unit like the ARC-2 or similar.

MFZERO

and I just chnged the new 4 pin sensor(in larger MAF) to the old 3 pin one.