GreyStang

If I gave my Stang a 91 octane predator tune, but one day I needed to use a lower octane.. would it be OK as long as I didn't punch the throttle with the lower octane?

jishead

No. Kinda. Not really. There is probably some safteway margin. You are most likely to detonate when under load like at low RPM with the A/C on going up hill at low RPM's. There are too many variables to know for sure. EVEN ALTITUDE will affect when/how much you detonate. I have been told by numerous sources that our 2V engines DO NOT have ping detectors meaning you EEC will not pull timing to protect your engine. Some computers like Toyota will pull timing like forever or until the computer is reset. I think that the 4v (Mach 1, Cobra) engines do have the knock sensors.

jishead

Your engine if more efficient with the baseline DIABLO tune. This is due mostly to the timing curve that running higher octane has allowed you to agree to use. If you are running at like 55 or70 MPH on the freeway you will notice how easy your engine pulls hills with A/C going uphill even in O/D with nasty stock gears with the Diablo tune. Your timing is advanced, your EGR is opened to further fight detonation and your setup is CAPABLE of making better MPG. HOWEVER, if you love the extra 5-10 HP, you will be tempted to gas it all the time and you will not see one single MPG different. With my diablo, before my 4.10's, I was able to get 28+ MPG on a long trip. In the city, when getting on it all the time, I was down to like 22-24 (b4 gears).