Compression and octane

[oldgoat2]

Oval port open chamber head with 140 thou. off. Zero deck block and flat top pistons.How much compression would this be and what is the lowest octane to use without having problems? Thanks in advance

[dougskewl]

I have a similar setup I am putting in. Machinist sayd it would be around 11.5 or 12 to one on compression. He recommended premium at the least but would be better on avgas.

[Dan32]

Oval port open chamber head with 140 thou. off. Zero deck block and flat top pistons.How much compression would this be and what is the lowest octane to use without having problems? Thanks in advance

run atleast 3of 5 gal 110 racing fuel and 2 gal good 93 octane not that it exists anymore ! i would run straight race gas myself but its up to you , less carb problems with less ethonal!

[Mike Croley]

There are some other factors to consider other than just static compression . For instance a big bore engine requires more octane than a small bore engine , and steel cylinder heads require a bit more octane than aluminum heads . But a general rule of thumb is ,
up to 11.8 compression - 92/93 octane pump gas will work
11.8 to 13.0 compression - 98 or 110 race gas or a mixture of 93/110
13.0 to 14.0 compression - 112 or 114 race gas depending on application
14.0 to 15.0 compression - 116 race gas
above 15.0 compression - 118 race gas or higher ( Renegade offers a 120 and 123 )
These numbers are strictly rule of thumb , but they are a good starting point . A little more octane is ALWAYS better than not quite enough octane . Always check all 8 plugs to tell you what the engine wants and needs .