I'm using TunerPro RT to work on the tuning of a '91 L03 engine in a '91 Caprice. I've done a little logging and data tracing just to get a feel for the system and its all working pretty well now, thanks to some help from Mark Mangus to get the .ads file for the Caprice working right.

I'm trying to get an understanding of all the tables in the bin, and I was surprised that there are a couple of tables based on engine vacuum rather than MAP, for example the table:

TCC Locked Spark Correction Vs. RPM Vs. Vacuum

Vacuum is not a variable sent back in the logging, so I'm assuming I have to calculate it as:

Vacuum = 100 - MAP

Correct?

Anybody know why they had these tables based on Vacuum rather than just basing them directly on the MAP?

Thanks,

Paul T.

 

Vacuum = current BARO reading (BARO) - MAP.

I am going to take a stab at the fact that Vacuum is fairly constant, where MAP will vary some due to weather, altitude, etc.

 

Fast355, what you said makes sense, but on this ECM it doesn't have a baro sensor (I don't think so anyway, '91 Caprice with L03 and 16136965 ECM) so I guess it must estimate the vacuum using the MAP value.

If thats what they are doing it still seems like it would have been less of a hack for the tables just to directly use the MAP value instead of this "fabricated" vacuum amount.

Paul T.

 

The MAP and the BARO sensor are the SAME thing. A BARO reading is taken at key on, prior to startup. While driving, the BARO reading is taken as well. Lets say you are in OD, at 1,800-2,000 rpm and go to 1/2 throttle to pull a moderate hill or accelerate. The MAP may easily be 98-100 KPA when that happens, the ECM re-calculates the BARO reading during that time. Most stock calculations that I have seen will recalculate the BARO reading above 1/2 throttle and below 3,500 rpm.

 

Ok, that makes sense, thanks.

Paul T.