XR4Ti Archived Tech Articles

    Big VAM info - a short description of why you need the appropriate ECM (Electronic Control Module, aka computer) when you install the big VAM (Vane Air Meter).

Post Date:    Thurs, 07 Jun 2001
Subject:      Re: Big Vam Question
List:         none

Some are putting in the big VAM without putting in an appropriate computer.  
I think that's a really bad idea.  Here's why.  The voltage signal output 
from the VAM is based on how far open the VAM is.  Say the small VAM is 25% 
open, it's going to put out 2.7 volts.  If the large VAM is open 25% it's 
going to also put out 2.7 volts.  Now here's the problem.  The same 25% 
opening with both VAMs have a different amount of airflow going through 

The stock computer knows what voltage equals what amount of air coming into 
the engine with the stock VAM.  If you change to the big VAM and leave the 
stock computer, it assumes that a certain voltage output means a certain 
amount of air coming in.  The problem is, there is now more air coming in 
than the computer thinks, thus the computer will inject a smaller amount of 
fuel, resulting in a lean condition.

Note that this is ignoring other factors such as the ability of the O2 
sensor to compensate for some of the lean condition.  But at wide open 
throttle, when the big VAM really matters, the computer is ignoring the O2 
and running on a set map.  This means that the engine is going to be lean.  
Bad news.

So you want to run a big VAM.  You'll need one of the following; an 86 SVO 
computer (PE calibration), a 87-88 TurboCoupe computer (LA3 calibration, 
ACT sensor installed in the lower intake, and some slight rewiring), a 
Superchip with the PE airtables built in to compensate for the big VAM, or 
an EEC Tuner where you can tune with custom calibration.

Remember that most of the above options require an intercooler to run the 
big VAM.  So you'll need an intercooler before hand.

I am running the 86 SVO PE calibration ECA in my racer.  My intention is 
to run an EEC Tuner in the future.  Here's the bottom line; do exhaust work 
first, like a well-bent 2.5" or 3" system front to back, then put an 
intercooler on, then you'll be ready for the big VAM.

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Published by Chris Anglin.