Guys,

I don't fully understand what the best compression ratio's are for different cars. Boost/no boost etc etc. Someone care to explain?

Thx,
Jonny

 

It's difficult to explain what the "best" is. Ultimately, the best compression ratio is one that makes power without detonation. This is a simple concept and represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). For example, if a hypothetical cylinder had a displacement of 500cc and a 50cc combustion chamber (plus volume over the piston crown to the head) the CR would be 500/50, or 10:1. Traditionally, a ratio above 10:1 or so is considered a high compression engine for the street. Forced induction will usually have a compression ration in the 8's..like 8.5:1. to prevent detonation...

 

If you mean compression you don't have a choice unless you rebuild your engine, as a rule most modern engines will have 9.5:1 or less because anything higher will not run on todays pump gas, at the track the guys with 11:1 or higher compression will be lined up for racing fuel that's in the 110-115 octane range and usually costs 6-7 dollars per gallon, if you mean boost on a FI car that's completely different, please clarify.

 

Many cars have a compression ratio higher than 9.5:1 in fact the GS400 is one of them the 98 model is running at 10.5:1. I am pretty sure that the 3 generation is significantly higher, I want to say somewhere around 14 but I cant recall with any certainty.

I just lowered my compression ratio by the way.

 

If the compression ratio is 10.5:1 this would be the reason premium fuel is required, but Morris I'm sure you will never see a mainstream production car with 14:1 compression, 93 octane won't get the performance it's capable of, that's Pro Stock motor territory and they run on race fuel.

 

Yup you made me get up and look.

The new GS looks like it goes up to 11.5:1

 

Some Saab engine have varialble compression which can change from 8:1 to 14:1 depend on condition. I think it hard to determind BEST compression for each engine. I say "the highest possible without detonation or pre inigtion.