Can someone explain compression ratios?
Thread Starter
Lexus Fanatic
iTrader: (13)
Joined: Jul 2003
Posts: 10,681
Likes: 10
From: Tulsa, Oklahoma
Can someone explain compression ratios?
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
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
Instructor
Joined: Sep 2003
Posts: 1,237
Likes: 91
From: Canada
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.
Originally Posted by Indio
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.
I just lowered my compression ratio by the way.
Originally Posted by morris
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.
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.
Driver
Joined: Nov 2004
Posts: 193
Likes: 0
From: Bangkok,Thailand
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.
I think it hard to determind BEST compression for each engine. I say "the highest possible without detonation or pre inigtion.
Last edited by gowest; Aug 2, 2005 at 09:07 PM.
Thread
Thread Starter
Forum
Replies
Last Post
