What does it all mean?
I know this sounds stupid.......but what are the pros and cons of raising or lowering your compression?

 

I will preface this by saying that I am not a mechanic, but I can tell you a little. The compression ratio has to do with the volume of the cylinder swept by the piston. It is a ratio of the volume when the piston is at the bottom of its travel to the volume at the top of it travel. In a car with 9:1 ratio, the fuel-air mixture is "compressed" to 1/9th of the volume it was before the piston hit the top of its travel. The higher the compression ratio, the harder it is on the engine. Winston Cup cars used to run ratios above 13:1, but to cut down on cost and mechanical failures, they started restricting them. They were going to bring them all the way down to 9.5:1 over several years, but for some reason they got down to 10:1 then they went back up to 11:1.
As far as the specific pros and cons of raising the compression ratio, I am sure there are some people on here that could tell you a lot more than I can.

 

thats a pretty good explination of CR. another way to think of it is: if you have 100cc when the piston is at BDC, and 10cc whenit is at TDC you have a 10 to 1 CR divide the volume at BDC by the volume at TDC

as for what the pros and cons are, high compression will yeild more power, but also increases the likelyhood of detonation, the higher you go with it the higher the Octane you will need. comperssion will limit your ability to add boost safely if you are going forced induction. the more compression yo have the less boost you can add. more compression will restore some low end power lost from a larger cam with a good deal of overlap.

low compression will yield less power, but have less tendency to detonate, and be easier to boost to higher levels. compression too low will hurt low end power with a big cam with lots of overlap

Keep in mind these are generalizations, that show you trends with respect to compression, best compression will depend on the specific combination.

 

As was stated above, to run boost, it's better to have an engine with a lower CR (often times 8:1 - 8.5:1). You can put a blower onto an engine with 9:1 or 10:1, but you can't run as much boost pressure (ie: 6 lbs instead of 10 lbs), unless you're running a motor that's specifically for racing, that's been built to take the abuse (such as Nascar engines running 13:1 --- they've obviously got super strong bottom ends).

Also, the level of octane fuel you run is totally dependent on your CR. An engine with a ~9:1 CR will easily run on "regular" 87 octane gas. An LS1 with a ~10.2:1 CR can run on 87 octane, but it's "recommended" to use higher (91 octane). If you bump the CR upto 11:1, you'll have to run at least 91 octane, but then 93/94 octane would be better. Once you go above 11:1, even with high octane gas, you may need octane booster (or higher octane racing fuel) to eliminate "pinging" (detonation?).

HTH.

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2002 Z28 - A4, 2.73's, Blk/blk, leather, T-tops, sport appearance package - just bought on June 26, 2002! - Pics

1981 Z28 - 383, TH350 trans., 3.42 posi - sold August, 2000 - Pics

1973 Camaro - Dad's & my project in the works - to be built: 355 Chevy, 11.5:1 pop-ups, "camel hump" heads (w/2.02 valves), solid cam, 4-speed.....it should be quick! - Pics

 

Excellent explanations.

COMPRESSION = POWER

But if you go too high on the CR and don't have adequate timing and fuel the engine can detonate.

 

low compression ratio's limit cam choices.
jeremy

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'93 z28 m6 (Purple Pearl Metallic)
stock shortblock, 230/236 cam, mildly ported heads, hooker longtubes, csi waterpump, all the usual bolton stuff.....
7.98 @ 88.7mph 1.84 60' (1/8mi)