Octane Options

[Bulldog9]

Lets talk Octane..... What is the consensus on grade of fuel? I'm running 96 x 71 bore & stroke with .060" deck height 58.8cc chambers in the heads, and flat top pistons with no valve reliefs, and figure my CR is around 8.6:1. I'm thinking I can run regular, but no idea.

I've been running Chevron 92 so far, but dont want to wast $ and leave unburnt fuel residue. The higher the octane, the more resistant the fuel is to combustion, so unnecessary use of 'hitest' is dirtier combustion wise than regular.

I haven't done any experimenting yet, but wanted to get some experience before I do. I plan to run the current tank down to close to empty, then put in 5 gallons of regular and see how that goes, but just wondering.

[Dave_Darling]

If you're running a stock mild, mild, mild cam I would suggest sticking with premium. Porsche specified 87 octane for the US-spec 2.0 with 7.6:1 compression, and the equivalent of 91-92 octane for the Euro-spec 2.0 with 8.0:1 compression. They do try to err on the conservative side, so you can probably run higher compression on ~92 by just giving up some of the safety margin.

If you have an aggressive cam, you will effectively "leak out" some of the compression. How much depends on more factors than I can calculate--it's usually best to talk to the people who designed your cam grind to find out what they think you should run.

--DD