max compression?

[bug killa]

Well I don't know if this question is worded correctly, but how does one compute the maximum compression ratio one would be able to run on a certain octane rating of gas? Basically I wanan know why everyone says 13:1 wont' work on pump gas (91 or 93 octane). And also how is the octane rating in Japan different from the US scale? I know that they have way less sulfur in their gas, but they have way higher octane?

[Phil C]

Can't answer all of your questions but I can give it a shot...

Maximum compression without detonation is not just a linear curve based on the octane of the gas. You can't say simply that a 11.5:1 ratio will work with 92 octane gas because it has a lot to do with the design of the head, the shape of the piston the size of the piston and any number of other factors I forgot to mention. 10 years ago I think you would have gotten an answer that 11.5:1 wasn't possible on pump gas. Not positive but I think some of the new sport bikes are running as high as 12.5:1 and they run on pump gas, it's just a matter of how the engine is designed.

US gasoline is rated by the RON+MON/2 method. It is the average of the RON and MON ratings which are two different methods of measuring octane. In Europe and Japan they use only the higher of the two ratings, can't remember which is which. Basically if you see 97 octane in Paris or Tokyo it's not going to be too significantly better than the 92 we get in America.

[bug killa]

cool... thanks for the info! We got some smart fellers on the board. :)

[Griffin]

To answer your question very un-scientifically - you basically have to increase the octane till it stops detonating... the trick is to make sure you run it rich and take it east when testing . Also if you have a tunable ECM changing/tweaking the spark advance can help a lot.

Griffin