by GDII » Wed Jan 20, 2016 6:18 pm
It depends on the car. 98 is more stable and less prone to detonation than 91 and 95 so if the car has an ECU it can run more timing and inject less fuel to get more power and fuel efficiency. However new cars have better technology so you can run 91 on a high compression engine and it will run fine, because it's designed for that fuel. If it's a new car and it's not designed to run on lower octane fuel, don't or you will do damage and possibly get terrible fuel economy.
Im running my EP81 with a carbied 2E on 95 solely due to the fact that it gets detonation running on 91 even though it's designed to run 91, it's not about economy.
I had a friend running his AE101 BZT with a black top 4AGE on 91, it was getting terrible detonation and fuel economy wasn't great. I suggested he run it on 95 and it got far better economy and way less detonation on hill starts. Obviously this engine was not designed to run on low octane fuel so it would have been pulling the timing and dumping fuel in to compensate for the knock readings it was getting.
1990 SW20 MR2
Previous 1990 EE90 Corolla