Octane

[Jim Staruk]

I have a 1965 (1964 carry-over) series seventy-five limousine with a 429ci engine.  I have had the engine completely rebuilt (Jasper) and well as the transmission and carburetor.  What octane would you recommend for my car.  I usually put in the highest octane available but with today's prices, am I wasting my money?  Thanks!  Jim

[3ft7bqmb7dryo]

Jim, the compression is 10:1 so the highest you can get is recommended.  The head gaskets are thicker than the originals, so that will lower the compression down just a bit. With that, you can advance the timing ait as well. This will help with the engine responsiveness. 

[Jason Edge]

These are 10.5:1 compression. 93 octane, or as close as you can get to 93, should be fine. Doing the octane math, Cadillac recommended a minimum of about 94.5 using the current R+M/2 pump rating method, but 93 should run fine. There is more on Octane on our help page.

[Jim Staruk]

Thank you Russ and Jason.  I just wasn't sure if rebuilding the engine reduced the original octane recommendations.  Jim

[Jason Edge]

Just to add, if you bored the engine out any (i.e. 10, 20 or 30 over) you will slightly increase compression, which will usually offset or exceed any lowering compression from thicker head gaskets.  Bottom, line you still have a relatively high compression engine that likes a bit higher octane gasoline. 

[Michael Vogler]

Amen, Tony.