I'm not entirely sure on this one, but I think it has to do with the pressure within the housing. If you move to a larger MAF housing it takes more air to pressurize the MAF to overload its voltage. Again, I could be way off base with this one and if anyone has more knowledge on this subject, please add to it or correct me.

 

I'm pretty sure it's not the pressure in the intake.

 

Not pressure.

It has to do with how much air the turbo sucks in through the MAF. The more air, the more MAF voltage. If you peg the MAF sensor, your screwed. The air speed and mass increases so much, that the MAF looses its sensitivity/resolution and thus, you cant tune any further.

:lol: I guess that is pressure.

 

keep the info comming please. I'd like more details

 

Its physics and the limitations of the stock maf housing. When air is drawn through a small orrifice, it speeds up, it's that simple.

 

So If i had a smaller Intake The air would speed up and increase my turbo Spool time lol?

what you need is a turbo xs u tec and run speed density with a 5 bar map. blow thru maf and use the maf to measure ambient temp's and such..

 

this is what I've learned so far and I think that I'm on the right track.


Ok, I understand the aspect of why we need to go with a bigger MAF but what aspect goes into tuning for this new big MAF. I understand that the MAF reads air flow based on how a much a heated element is cooled. If you use a bigger MAF housing, you can allow the same amount of air to pass at a slower rate therefore not allowing the heated MAF element to cool as much and keeping the MAF voltage lower. I am assuming that when you retune you are going to now have to adjust the g/s in the MAF table based on having lower MAF voltage compared to the same air flow. So, do you adjust the new g/s by figuring out the % increase in area of the Big MAF compared to the old MAF housing and adjusting upward acordingly? I think I'm starting understand this but let me know if I'm wrong.

 

honestly the MAP sensor is running out of resolution before the MAF sensor.

 

more fuel = larger intake.
it's to compensate for more fuel being delivered.
the stock sizing is limited to a certain amount, once it's maxed you're stuck.
the big maf's diameter is larger to allow the maf to read MORE air for the increase of fuel being delivered.

 

Think about a given diameter. Let's take the stock 65mm diameter. As air flows past the MAF the voltage changes. The voltage corresponds to an axis on a lookup table in the ECU. You look up the voltage and then it reports the airflow in grams / second.

Now let's look at a bigger diameter MAF. Doesn't matter the size. As air flows past the MAF the voltage changes. This change in voltage will be the exact same as with the 65mm MAF because the hot wire is affected in the same exact way! Obviously, more air is entering the motor due to the larger diameter, so something must be done. This 'something' is a full recalibration of the MAF lookup table (which few tuners actually do).

Why go to a larger MAF? 2 reasons.

1) Less restriction. The bigger the straw the more you can suck up.
2) More resolution. The MAF registers only up to 5V. The 5V setting on a stock MAF corresponds to ~350g/s. This is equivalent to about 350whp on our cars. If you have a car that is pulling in 400g/s then you have an issue because the MAF lookup table only goes up to 350g/s. Here is an example of the MAF lookup table: stock compared to an older APS 70mm I did ...

So how do tuners get their stuff to work with a stock MAF? Hacks. Simple as that. They trick the ECU. This approach is the ghey.

t

 

Let me take a stab at explaining this...

The MAF sensor reads from 0-5v. It can't register anything above 5v. This voltage range corresponds to an air flow range. Approximately 0-350g/s as Wolf mentioned above. Once your turbo starts flowing near the limit of this flow range, the MAF voltages approach 5v. Once you exceed this flow limit, the MAF voltage cannot rise any more and therefore the ECU has no way of measuring any more air flow. MAF voltage will just peg at 5v and the ECU has no flow reference for anything above that.
Now consider increasing the diameter of the MAF housing. You can think of this as if you're exposing the MAF sensor to only a fraction of the airflow that's actually going through the housing. Thus your voltage to airflow relationship changes. For a given airflow level the MAF sensor will now register a lower voltage. So the result of this is that your set 0-5v range can now span a significantly wider airflow range. How wide depends on the diameter of the housing as well as the placement of the sensor within the housing.

The general tuning strategy for a big MAF is to measure the diameter of the new housing and the stock housing, derive a ratio between the two and use it to scale the entire MAF table. This only gets you to a rough starting point and you have to fine tune from based on fuel trims and observed a/f ratios. This is the way most tuners go about it and its the simplest way to do it. That being said, the real proper way to do it would be to use a flow bench to span the flow range of the new housing and measure the corresponding sensor voltages. Of course most tuners don't have a nice flow bench to play with .

Thanks
-- Ed

 

Im not a tuner, but I do intrument work.
I agree with above except for resolution.
0-5V= 0-350g/s
0-5V=0-500g/s
depending how fine the ECU or the MAFS cuts up the Voltage you will have bigger (not as fine) jumps from a change in air flow.
So, well the sensor may not sense real well at slower RPMs because the airflow is alot slower not affecting it as much. Change in flow may take longer to detect. You may have idle issues or part throttle hesitation. This can be tuned out, but you cant get as fine of A/F control.
We dont care we just want more power. Manufactures need tighter control for emissions and mileage.
Remember you could make this engine run good with a carb. so how close to a perfect A/F mixture do you need.
Neat idea someone came up with.

Tony

 

Yep I beleive you have it now. You just neeed to recalibrate it to be able to read correct air flow with a bigger diameter tube