Have a odd situation, changed my auto-lites #65 mainly to replace the tube seals and put fresh plugs in to replace the ones i installed 2 yrs ago. Upon inspection the 65's looked almost new but went ahead and installed a set of 64's i had laying around. As long as I have owned the Dart have run 91 octane, felt stronger after the plug change so on a whim put a tank of 89 octane in and the results are incredible, increased mileage and a noticeable perfomance increase and the miss is no longer present. Strange indeed.

Interesting find. It could be that your timing was too far advanced. The colder plugs will effectively retard your timing a little. On a stock Slant with stock tuning, you will probably get best performance with 87 or 89.

Lou

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Home of Slant6-powered fun machines since 1988

A motor will make the most power with the lowest octane it will run on without detonation. The higher the octane, the less energy per gallon of gas. Whenever there is a spec fuel in a racing series, the engine builders build the motors to 'fit' the fuel.

Cecil

What is meant by "without detonation"?

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There's a difference between ignorance and stupidity.
Ignorance is not knowing any better.
Stupidity is knowing, yet doing it anyway.

Detonation is a self ignition condition, kind of like desieling. The fuel self ignites to where there is not a controlled flame. This can be before or after the spark plug fires. The result is a power loss and sometimes engine damage. Many things cause detonation; hot spots in the chamber, hot plugs, too much ignition advance, too much compression for the octane.
Cures for detonation can be colder plugs, polished combustion chambers, smoothed piston tops, lower compression, less or slower ign advance, fuel with a higher octane rating.

Cecil

Thanks guys, you just confirmed what i suspected. TIME TO REBUILD AND ADD EFI! Amazed I can tune at all, considering it was last overhauled in the Nixon admistration! Think i might have some chain slack along with the Exxon Valdez situation, but still runs strong with decent mileage. Wonder what the oil pump strainer looks like?

H'm. Not that I'm aware of. A colder plug can reduce the tendency for autoignition (a type of spark knock), and can therefore allow levels of timing advance that would not work with a plug that runs hot enough to light the mixture before the spark, but a colder plug cannot effectively retard the timing.