I understand how to adjust my long term and short term fuel trims, but I don't wholly understand why.

I've read Jeff Hartman's book, [u]Fuel Injection[/i], read AZScoobie's guide to adjusting it, and Mick the Ginge's UTEC guide. After reading, I understand that somehow, tuning your 0 column stabilizes the 10-100% columns, but I don't understand how it affects those columns. It would seem that tuning that column would just amount to tuning AFRs for zero TPS-to-crossover point.

I also don't fully understand why adding a larger MAF tube (ie. Godspeed or Perrin) is necessary for a larger turbo. I guess it makes sense that our MAF has an upper voltage limit, but what tubing then has to be larger in addition to the MAF tube? The turbo inlet? What are the ramifications if, say, the MAF maxes at 4.5 volts which I don't know what that would correspond to, but let's just say 300 cfm (am I going in the right direction?); and the turbo pulls in 400 cfm and the MAF is still registering 4.5 volts? Obviously leanness, but how would a larger MAF tubing solve the problem. The MAF max is still 4.5 volts. Does the larger MAF tube slow down the velocity of the incoming air?

Thanks.

 

pat, if your MAF reaches peak voltage and you keep pulling more air, then you are right, leaness will occur. But not a little, you will lose that power.
A larger MAF has the capability of reading a larger range of air flow.

EXAMPLE: with made up figures

stock (again these are made up numbers)

cfm range 0- 250cfm
voltage range 0-5 volts
therefore at 125 cfm you are getting 2.5 volts.

Aftermarket big MAF
cfm range 0 -500 cfm
voltage range 0-5 volts
therefore at 250 cfm you are getting 2.5 volts.

stock= 125cfm @ 2.5volts
BIG= 250cfm @ 2.5 volts

What does that mean?
Well if this was the only thing you changed, you'd need bigger injectors.. theoretically, TWICE as big. Because for the same voltage you car will only supply enough fuel for the 125cfm, when you are actually flowing 250. Therefore if you double the fuel injector size, your airflow increase will equal your fuel increase.

Does this make sense?

This is how I put 550cc injectors on my car with the stock ECU. I put a 2 bar MAP sensor on in place of my 1 bar MAP sensor.

Basically I told the computer I was flowing less air than I really was, but compensated for it with the extra fuel.

 

it's easier to explain in person where we can draw graphs.
Call me if your bored.
I'm not doing a darn thing.
Except tonight is Mardi Gras and I'm going out to look at BOOBIES!!

oh yeah,
if you really are trying to draw in a lot more air over stock you will restrict your flow some. But you really gotta be drawing in a lot of air.

You should investigate what MAF the STi is running. That MAF should work for you because you have the same displacement.