XR4Ti Archived Tech Articles
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 them. 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.Back to the Tech Articles main page
Published by Chris Anglin.