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View Full Version : COMPRESSION RATIO / OCTANE NEEDED?



Honda Guy
01-19-2002, 01:56 PM
What is the highest compression piston you can run on pump gas(93 octane)?

beerock
01-19-2002, 02:48 PM
it depends on the compression once the piston is installed.

they vary a little bit.


best way to make sure of it is use a compression tester.

after that youll know if you need to be running straight 94 or 50/50 mix of 93/108-(vpc12) or straight race gas

im running a 50/50 mix with 204 psi.

thorspapa
01-20-2002, 01:40 AM
Rule of thumb for octane on different compression setups?

Air-cooled engines 125cc - 450cc - up to 10.5:1, premium pump fuel. 10.75:1 to 11.75:1, mix race and pump 1/2 and 1/2. 13:1 and higher, race fuel or alcohol.

Water-cooled 125cc - 450cc - up to 11.5:1 (12:1 on some engines), pump fuel. Anything higher, race gas.

Smaller engines can deal with higher compression without detonation (less surface area). Really big bore engines (like an XR600) won't deal with compression well at all, and will require higher octane fuel than the compression ratio would seem to necessitate.

Hope this helps. Don't sue me if it doesn't work for ya.:D

Red Rooster
01-20-2002, 07:00 AM
Put this in the F.A.Q.