Octane... proof?! (or a dumb question?)

[gwittman]

I actually used Avgas in my race car when I first started racing in the late 80s. I don't think it was the 100LL, I think they also had a slightly higher octane Avgas that had more lead in it. The compression ratio I was running then was about 12.5:1 and the Avgas worked just fine. The Avgas was readily available at a local regional airport and it cost much less than race fuel. After a few people told me it was not good for my engine and I started bumping the compression up a little more, I started using race fuel but never did see that much difference between the two fuels.

I did try a different race fuel once at Riverside Raceway that was suppose to have more lead in it and be better for the engine. The lead was so high it fouled the spark plugs. I was not very impressed with that and never used that brand again.

 

[gwittman]

I have experience related to high octane fuel in my Coupe. I use to attend High Speed Driving Schools, with my Coupe, that was put on by the BMW club at various race tracks. One time while attending an event at Riverside, a friend had some race fuel that was old enough he did not want to run it in his race car. He gave it to me to add to my tank. I was about three gallons of 110 octane fuel. It boosted the octane enough that I was able to bump the timing some to get a little more performance.
The higher octane had another effect that I was not aware of until it was my turn to work one of the corners. One of the corner workers, that had been working corners all day, ask me if I was the one that was driving the blue Coupe. I said yes and she said we call that the flame car. Being bewildered, I ask why. She told me that as I was approaching certain corners, a big ball of flame would come out the back of the car. I thought about it and realized that as I was down shifting I was over revving the engine and the rev limiter was cutting off the spark. The carbs would then fill the exhaust with a nice mixture of unburned fuel and air at a very good burn ratio. As the revs came back down the rev limiter would activated the spark and send out this big ball of flame out the exhaust pipe. Fortunately, the octane was high enough that is was a rapid burn rather than an explosion that would probably have taken out my exhaust system. I did not have any idea this was happening but did pay more attention to my downshift points after that. My car was probably less entertaining then but the muffler may have gotten its life extended a little.