View Full Version : Altitude vs Overboost cutout

05-29-2006, 05:05 PM
Is there any correlation between altitude and the overboost cutoff point in a stock TII logic module?

I recently went up to the mountains (~10000 ft) in my Shelby Lancer and it seemed to hit overboost easier. The gauge was still showing 15 psi at WOT, thanks to the grainger valve regulating the boost level, but it almost seemed as if the overboost cutoff point was lower.

I have an adjustable zener diode (MAP clamp) installed and maximum MAP voltage is 4.36v at WOT. I never hit overboost anymore at ~4500 ft, but I was hitting overboost when over ~8000 ft. I've read that the overboost cutoff voltage is 4.65 for TII computers, but I could not dial the MAP clamp any higher than about 4.40v without hitting the overboost protection (at ~4500ft). Does the overboost threshold change with altitude? It seems like it.

05-29-2006, 05:15 PM
I have a hunch the overboost threshold is dependent on ambient pressure that the computer sees via the MAP solenoid. Can anybody verify this? Less ambient pressure (higher elevation) = lowers overboost threshold. Lower elevation = higher overboost threshold. I'm betting if my car were at sea level I could dial the MAP clamp a little higher.

05-30-2006, 09:09 AM
You are correct.

05-30-2006, 01:29 PM
Good to know for sure, thanks.