Re: Type of gas and whether to use lead substitute?

Posted by Redhexagon On 2022/12/7 1:28:26

A cast iron thumper of an engine such as ours is a lot more prone to detonation than modern engines, given the same compression ratio. There is no comparison.

Most engines like the Packard V8 are fine on 87 AKI gas if their compression ratio is under 9.0:1 and the ignition timing is set correctly. You might need to put 89 AKI in them if you are at low altitude or hot conditions.

I consider 89 AKI the minimum for engines of 9.0:1 to 9.5:1. 91 AKI makes me more comfortable.

Above 9.5:1 I'm grabbing 91 AKI. I like 93 AKI if I'm going through the desert in summer with them.

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I have never once in ten years of daily driving old carbureted cars had any sort of problem what so ever from using plain old pump gasoline with up to 10% ethanol.

I've had to jet some of my 1970's and newer engines richer since ethanol burns leaner and those cars were jetted on the edge of lean originally anyway, but cars prior to the 1970's are usually jetted rich enough from the factory to not have that problem.

Unleaded gas will accelerate valve seat wear if your engine is running on plain iron seats and you run it under heavy loads or high speeds. The effect at lower speeds and loads is greatly reduced. Lead substitute additives are just upper cylinder lubricants, and there are many brands of those. Even the old Marvel Mystery Oil and Seafoam are upper cylinder lubricants. Lucas and AMSOIL also make specialty upper cylinder lubricants. Adding them to your gasoline will help the valve seats, among other things. Modern gasoline doesn't have much lubricity to it, and Packards do not have PCV to recycle some engine oil through the intake for you, so adding some more lubrication property to the fuel is not a bad idea.