Gasoline Octane Question

[Wayne 962]

Someone recently asked me this question:

To: pelicanparts@csi.com
Subject: Octane Q & As


Great Web Site! Getting better all the time. My Question is: I can get 100
octane fuel at my 76 station. I have been mixing it with 92 octane in a stock
car I run. ( 1981 Monte Carlo, 305 ci.). No problems and higher RPM ability.
Can I run pure 100 Octane in my stock 1972 1.7 914 w/FI ? Can I run pure 100
octane in my 1975 3.0 911 (FI) ? Can I cause damage by using this fuel? By
the way, this fuel cost $ 4.00 per gallon. No Problem ! Thanks,
Gizmonaz@aol.com

Hi there. There is a FAQ about gasoline on the site; you can find it by doing a search for it, or it's in the tech articles section of both the 914 and 911. The FAQ has much more info that I could ever know.

As far as I do know, running the 100 octane fuel in your car will not hurt it at all, but even more important, you won't see any performance increase. The reason why the racers run the high octane fuel is because they have high compression engines. The high octane fuel reduces knocking, which increases as the compression increases. So using the fuel with your street cars would be a bit wasteful.

On the other hand, running low-octane fuel tends to make the fuel burn quicker, and thus hotter. Running about 93 in the older cars is what our race machine shop recommends, primarily to keep the heads (especially important on the air cooled cars) cooler

Thanks,

Wayne

[John Rogers]

When we do an autocross with the 2.0l 914 or the 1987 Turbo I add 1/2 tank of Trick 101 octane gas to prevent any chance of detonation. Both engines have been modified and this is important in southern CA in the summer as temps can get over 100 degrees. The Trick gas has additives that clean the system really well and the inside of the tail pipe will be real clean after a day of full throttle use.

[Joe Browne]

Running higher octane allows you to run more timing without detonation. I would increase my timing 6 points at the track and out 101 octane in the tank. This allowed for some modest performance gains.

[Dave at Pelican Parts]

I don't know that I agree with your machine shop guy. My *guess* is that the reduction in head temps from running higher-octane gasoline would be on the same order as the reduction in power from running that same gasoline (when not required). That is, not really measurable.

I talked with my grandfather last year about octane and aircraft engines. (He's been a certified airframe & A/C engine mech for longer than my *mom* has been alive, and an FAA inspector for many many years.) He recalled that there was a bulletin passed around blaming high-octane fuel for causing premature exhaust valve failure on some engines.

These were low-compression engines designed for 80-octane gasoline. The owners were putting 100-octane (or was it 110?) fuel in, mostly due to better availability. The 100-octane stuff burns slower than the 80-octane, and so the mixture was still burning as it exited through the exhaust port. This resulted in hotter exhaust valves, which eventually failed.

This is not to say that you can't use premium-grade gas in a 914--remember, most 914s were designed to use 87 pump octane (some up to 92), quite a bit higher than in the example above.

However, it does make me think that putting VERY high octane gasoline in your 914 may not be the best idea in the world.

--DD