It seems to me the goal of MAF modding is to remove a flow restriction in the intake -- and the "price" of making the MAF housing larger is the resulting off-calibration MAF sensor data. If so, why mess with the larger injectors? Wouldn't it be better to correct the "error" at the source by using a resistor or two to "recalibrate" the flowrate vs. voltage curve? This way the ECU never has to know that the MAF was switched.

Rusty? He drills holes in his maf housing. He's like the caveman doctor drilling holes in people's heads to release evil spirits! (BIG laugh, emoticons disabled)

There can be different goals of modding the maf housing, but usually it's to modify the maf sensor signal as seen by the ecu. This can either be to make the whole system work harder (more boost, longer) in an effort to get to the same target values which will be underestimated with the larger maf housing (my method) or to match up with big turbo programming which produce flow rates beyond the useful range of the stock sensor scaling. AEBs have a read limit of 187 g/sec, all DBW sensors go to 215 g/sec. BT's will go over 200 g/sec at peak pretty easily. Also, the signal curve is not linear and it goes pretty flat in the high flow range, making it tougher to tune a/f, so widening the scale is helpful in that way too. Programming can run with a maxed maf signal (187/215) but it throws a code and a/f is held flat which is only good so far.

The screens and straighteners really don't impair flow in a material way and increasing the diameter of the maf housing in an effort to get more flow won't accomplish much because the turbo inlet is still the same size, the funnel effect at work. So increasing the size of the maf housing and then tricking the signal back would be really complicated and not get any performance gain.

 

from my understanding, the main reason for using a bigger MAF housing is to keep the reading within an acceptable range to the ECU. with my stock MAF, i was hitting over 210g/s on stock software and boost settings. now, with the car tuned, i'm getting 180 g/s with the larger tt225 MAF. of course, you'd need software to adjust for the larger MAF.

just throwing on a larger MAF and injectors won't help. the ECU will attempt to adjust the fuel trims accordingly. when i had the larger maf and injectors with the stock ecu, i was only getting around 10ms on the injectors, yet the car was running lean because the stock ecu wasn't tuned to the larger MAF.

 

Wouldn't it be better to correct the "error" at the source by using a resistor or two to "recalibrate" the flowrate vs. voltage curve?

the simple answer is that the voltage curve is non-linear, adding a resistor would shift the curve, but not "recalibrate" the maf to read correctly.

 

from my understanding, the main reason for using a bigger MAF housing is to keep the reading within an acceptable range to the ECU. with my stock MAF, i was hitting over 210g/s on stock software and boost settings. now, with the car tuned, i'm getting 180 g/s with the larger tt225 MAF. of course, you'd need software to adjust for the larger MAF.

just throwing on a larger MAF and injectors won't help. the ECU will attempt to adjust the fuel trims accordingly. when i had the larger maf and injectors with the stock ecu, i was only getting around 10ms on the injectors, yet the car was running lean because the stock ecu wasn't tuned to the larger MAF.

Exactly. Your actual flows with your BT are still 210+, it's just the reading is scaled back to work with the stock sensor element. It's the easiest workaround. The ecu gives enough fuel to match 210 g/sec or whatever amount of flow and the 02 sensor read on the other end is kept on spec.

You can change to larger maf's and match with injectors on stock or chipped programs, you just can't make very dramatic changes or the results will be as you describe. I went from stock 2.44" ID to 2.62" ID and things are great with a slight fuel adjustment using the same program. If I tried to go to a 2.75" ID housing, it would be too much of a change, the trims could be adjusted for cruising, but the top and bottom would be way off.

 

You can change to larger maf's and match with injectors on stock or chipped programs, you just can't make very dramatic changes or the results will be as you describe. I went from stock 2.44" ID to 2.62" ID and things are great with a slight fuel adjustment using the same program. If I tried to go to a 2.75" ID housing, it would be too much of a change, the trims could be adjusted for cruising, but the top and bottom would be way off.

Guys this is great stuff -- I'm learning a lot. Thanks!

Is the injector adjustment as simple as keeping the MAF area proportional to the Injector size? (or injector size x SQRT(new fuel pressure/old fuel pressure) if you're going to mess with the FPR).

Why do the "top & bottom go way off" first? (I *guess* its because (1) WOT fuel trim is open loop & the error just scales along with the flowrate, and (2) idle is really hard to keep stable - small errors in flowrate measurement cause large effects when the throttle is barely cracked open.)

 

Guys this is great stuff -- I'm learning a lot. Thanks!

Is the injector adjustment as simple as keeping the MAF area proportional to the Injector size? (or injector size x SQRT(new fuel pressure/old fuel pressure) if you're going to mess with the FPR).

Why do the "top & bottom go way off" first? (I *guess* its because (1) WOT fuel trim is open loop & the error just scales along with the flowrate, and (2) idle is really hard to keep stable - small errors in flowrate measurement cause large effects when the throttle is barely cracked open.)

You get the general vicinity with the calculation based on area, but it gets skewed due to the non-linear scaling of the voltage AND the fact that mult fuel trims adapt in mostly low load conditions - cruising and part throttle. From my fiddling, the calculation gives too much fuel and results in mult trims in the "-" direction. Idle is tough because that is the part of the curve where the biggest signal modification occurs. Top can get lean if you leave the mult trim too far in the "-" direction and fuel gets pulled even in WOT when you really need it.

 

I really have nothing else to add. :salute:

Other than clarification that I didn't holey the MAF to get more flow. I holied it to measure less flow, so I could get around the AEB's "overboost" measuring scheme.

And I've never had an idle problem.

 

if you're tinkering with the MAF and such, i'd suggest that you try out lemmiwinks , assuming you have DBW.

 

^^Not tinkering yet... just curious.

Thanks for all the info, guys! :thumbup: