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I need help in choosing the right fuel to feed my high-compression 428/360 hp Pontiac that's stuffed into a ‘67 Goat. I’ve heard a million opinions on what I can do to protect my engine against detonation. A friend recommended Super 104+ additive. Others recommend different additives, either “octane boosters” or “lead substitutes.” Still others say additives don’t work. High Performance Pontiac Magazine says there are no tricks: “the octane rating of the fuel must match the compression ratio of the engine”. With today’s premium fuel rated at 93 octane, HPP says the engine compression ratio should be somewhere near 9-9.5:1 for a street engine.

So what's your opinion? My ‘69 428 has a compression ratio of 10.5:1. What octane rating should be used for this compression ratio? Do additives work, if so, which ones work better than others? Do lead substitutes raise the octane level, and are they better than “boosters?” What about aviation fuel? I thought I could retard the spark but someone told me that it would raise the cylinder temperature and negate the effect of the retardation. The same person recommended I use the lowest restriction air filter that I could find to allow more cold air to enter the combustion chambers. I wouldn’t mind paying more for high-octane gas if it were readily available.

The last thing I want to do is lower the compression ratio, but I certainly don’t want to ruin my engine. Can you help?

Thanks

Chris Dupre

Texas

Your octane question has been discussed to death, without a definitive answer. My experience with "boosters" has been less than spectacular, but they seem to work for some people. The engine type, state of tune, conditions, load, etc, all affect whether the "booster" is useful. When considering additives, stick to major brands, follow the directions and let the car tell you if its needs are being met (or not).

As far as lead is concerned, true tetraethyl lead additives are what gasoline companies once used to increase octane. Most lead substitutes these days are designed to replace the lubricating properties lost in today's unleaded fuels and don't claim to raise octane ratings.

One thing is for sure, 93 octane unleaded gas will not allow a 10.5:1 compression Pontiac engine to perform to its potential. There are certain tuning tricks you can do to help limit pre-ignition: these include retarded timing, cooler heat range spark plugs, richer fuel mixture, lower engine temperature, slower distributor advance. However, the engine still won’t perform like it was 1969 all over again.

Racing or aviation fuels have plenty of octane, but are not readily available and the engine would have to be retuned to receive the maximum benefit. Also, you might wind up in a “tight-spot”, needing gas with no proper fuel available.

Marty