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What's a blow through MAF?

2183 Views 20 Replies 8 Participants Last post by  ATLSUPDAWG#2
Anybody know?

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It means to position the MAF such that pressurized air blows through it. In other words having it on the output side of the turbos/supercharger rather than on the input side (which is the stock location).

I don't know of any cars that come stock with the MAF as a blow through type, anyone???
I would think that oil would foul up the MAF if it was mounted there. Anybody running this set up?

I don't think anyone made it stock. Manufactures would have gone with speed density.

TYWRAP said:
I would think that oil would foul up the MAF if it was mounted there. Anybody running this set up?

I don't think anyone made it stock. Manufactures would have gone with speed density.

Oil is one of the downsides to this. It's been discussed here and on, and I thought I read something by Anarky(sp?) where he said he had done or was planning this.

One possible upside is speculation that the MAF will flow far more air when at higher then atmospheric pressure, and if you place it after the BOV you won't need to worry about venting metered air.
The question is: Do OEMs not use blow-thrus because the turbo seals spit out too much oil, or the PCV valve? Since removing the PCV hose to my intake, I have zero oil in my pipes. My car occasionally spits out blue smoke, but the compression is fine. So I'm betting the oil seals are leaking on the exhaust side.
Sounds like a good idea if the PCV valves were not routed back to the intake. Would the MAf be able to measure the air though if it was over 500 RWHP worth (not just flow more).?

Oil, turbulence and higher operating temperature and pressures may reduce the accuracy and longevity of the MAF, some or all of this is probably why they are not an OEM aplication.

I don't see how the MAF could actually meter any additional air beyond the upper limit of its output signal (which seems to occur from what we've all heard around 500rwhp).

I still think it's a damn shame that we can't go out and buy a larger ProM MAF like any Mustang owner can do.
Actually, i have heard that the new Greddy E-manage will allow people to use different MAF's on the car. I'm not exactly sure how its gonna work, you might want to contact Greddy for more info.

TYWRAP, Lagtime,
I believe the the Audi/VW 1.8T motor uses a blow through MAF.

As far as a restriction AnArKey is correct that "When on the pressure side, the MAF offers no restriction to HP, at least up to 800RWHP" (probably more like 700rwhp), but Lagtime is also partially correct when he said "I don't see how the MAF could actually meter any additional air beyond the upper limit of its output signal (which seems to occur from what we've all heard around 500rwhp)."

I have made 526rwhp and my MAF was at 97% of its abitlity to read the incoming air. This would theoretically mean that the MAF could meter 542rwhp, but it would be a restiction to air-flow.

You could conclude that a blow through MAF would allow you to run up to about 540rwhp without a restriction to airflow, but the reliabilty and accuracy could be in question. The decision of risk vs. benefit is up to everyone personally.

It would appear that I am a good candidate for a blow through MAF. If you consider that I am almost maxing out my MAF and it is definitely a restiction. The way I see it my injectors are already at 91% dutycycle (@ 526rwhp) and you shouldn't exceed 85%. Removing the restiction and adding 20rwhp (just a guess) would only mean that I would have to turn the boost down to stay within the limits of my injectors. Now lets say I add a fuel system and turn up the boost. I would still have to replave the blow through MAF because now I am limited to 542rwhp because of the MAF's ability to read the incoming air. The MAF may actually be the best for a BPU car, but at BPU I don't know how much of a restriction the MAF really is. Someone like Andi may be able to answer this.

It still seems that the dual MAF setup is still ideal. It will probably not be a restiction until over 700rwhp and the signal should be good for almost 1100rwhp. You would have all the advantages of the MAF without the negatives of the blow-through MAF. It is also cheaper than a VPC when you consider a used MAF sells for about $75. You could have a very high capacity air metering system for about $225. You will also need a S-AFC but you should have some sort of fuel computer for these scenarios anyway.

Just my 2 cents, Steve
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Its a very simple thing to use a voltage divider circuit to support larger injectors. To see the relationship of MAF voltage to airflow go here:

For our purposes we can probably ingnore the variable 'a', since its probably really small, and the ECU will ajust for small errors anyways.

I think I will probably try this, since I see I have no oil in my IC pipes. I know the ECU spikes voltage to the Pt wire during startup to clean stuff off of it, so it might be able to burn off a small amount of oil.
Couple of things...

Am sort of a Mustang guy so... Recalibrated MAF sensors for larger injectors on Fords is a bad way to go - yes I know everyone does it. Doing this on a Supra would also not be a good idea IMO. Just running a larger diamter one migh tnot be so bad though so long as it's stock calibration but what about when you go larger with injectors which it sounds like you might have to do? The problem with a recall'ed MAF is that it mucks with the ignition curve. Your ECU think it's seeing X amount of air but is really seeing Y - the difference being the % of difference between the stock injector size and the new one. That could put you in a completely different part of the ignition maps. Follow? Hard to explain but the reduced voltage sent to the ECU to get the lower pulsewidth moves the timing around too - not cool on a forced induction motor.

Dual MAFs... That requires a multiplier, yes? For some reason I thought divider:hammer: Does anyone make this off the shelf? This ought to avoid the problems I mentioned above but what about bigger injectors? How do you compensate? (still learning) Does that compensation run into the problems I mentioned above?

Have you guys considered the AEM computer? Can supposedly run MAF but they seem to be reccomending MAP :( Under $2K which would seem to be somewhat offset by the price of some of the add-on hardware you guys are considering. This is th epath I'm considering taking as it seems to give full control over the computer to the driver\tuner....

P.S. Speaking of oil fouling MAFs and blow throughs. Pro-M sells MAFs specifically for blow through. That would seem to say that calibration changes some. Heated air might cause it to be off too since it uses a heated wire to measure airflow (Mustangs anyway, what about Supra?). Be aware that oil from say a K&N CAN foul the sensor! They're supposed to clean themselves (the better ones anyway) but this has supposedly caused problems for owners who "recharged" their filters and got a little nutz with the oil :rolleyes: So oil should certainly be a concern in a blow-through app IMO. Also, if you really want to see if the MAF is a restriction when drawn through put a vac gauge behind it and if you see vaccum at WOT it's a restriction. :)
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An Apexi S-AFC fuel computer will combine the two MAF signals into one for the ECU. It will also let you adjust for larger injectors.

Later, Steve
One note about the Dual MAF setup. I believe i discussed this with you via e-mail one time. With the AFC setup to average the signals, my car would not idle. I'm guessing the reason was due to the fact that the ECU is reading a certain amount of air coming in, but the air is actually 2 times that amount therefor the ECU is not adding in enough fuel, so the car stalls.

When i changed the AFC to add the 2 signals, the car idles perfectly. But when i took it for a ride, the AFL% of the MAF's went to 100% around 20 percent throttle. Not good IMO.. So, thats where i got stuck.

Anyone have opinions on the above?

Steve- When are you planning on doing the dual MAF setup? I'd be interested to find if you get the same results.
Wow, that's a complicated bugger! I saw mods for 300ZX and others to go twin MAF but not Supra. Do you simply tell it to double the signal across the board or does it have a specific function for twins on the Supra? Looks pretty powerful but also possibly a handful to tune properly. I think I'll be waiting to see how well the AEM computer handles things. I prefer MAF metering but AEM seems to like MAP better - maybe because of maxing out the MAF? A straight divider circuit rather than the more complicated Apex piece might be desireable if that's the case. Thanks for the quick response and refence to the Apex piece - as I said before - lot's to learn about the Supra and not everything I knew before easily applies :(
The Apexi S-AFC allows the use of 2 MAF's. I believe it basically take the two signals and adds them togther before pasing them to the ECU.

Later, Steve
We discussed this on the list, you can't just add or average the singals, depending on your injectors. Look at the link I posted before, airflow vs. voltage is not linear.

Assuming half the airflow goes through each MAF, then each MAF has an output voltage equal to one over the square root of 2 times what the voltage normally would be with one MAF ( 1 / 2^.5 ). So to get it to work with stock injectors, you need a 41.4% voltage amplifier (easy to do). Clints setup works because his injectors (850s right?) richen it up enough, the ECU can account for some error.
Thanks for the info. I guess that explains why the AFL% goes to 100% so early in the throttle % with Dual MAF's and having it setup to add the signals. Now with my car, i have 720's, T66 single turbo, fuel system etc, so i would think it would be close to what CLints setup was, just that i would have to add more fuel than him, correct?

Do you know what the ECU does when it sees 100% MAF air flow? If i achieve 100% air flow of the maf's at 20% throttle, what will the ECU do above that 20% throttle? Any idea?

Good info, I will need to read up on it more. Can you give me some more specifics about the 41.4% voltage amplifier.

Thanks, Steve
Steve, its a simple Op-Amp multiplier circuit. Vout = Vin * ( 1+ R2/R1) where R1 and 2 are resistors (obviously you can replace one with a pot for ajustability). You can see the specific wiring diragram in a small book they sell at radio shack.

I attemped to draw an Ascii diagram but it got destroyed in the post. Its a really simple circuit anyway.

You might want to angle the BOV return line away from the MAF, some people have had stalling problems even with their BOVs vented back in, they say because some air is blown out the MAF.
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