EastSideRacer

Whats the difference between high / standard compression?
What influence the compression level?
Can someone target a ratio, or is it circumstantial?

Top Ramen

Compression usually refers to 'static compression ratio' which is the ratio of the volume in a cylinder when the piston is at the bottom of its stroke to the volume in the cylinder when the piston is at its highest point. To use a simple example, if a cylinder has a volume of 100cc when the cylinder is at the bottom and only 10cc when the piston is at the top, then its static compression ratio would be 10:1. If the piston moved up far enough to result in 8cc volume at the top of the stroke, the compression would be (100/8):1 or 12.5:1.

Standard compression is whatever the motor has in stock form. This can range from as low as 8:1, in an older muscle car or a turbo car, to over 10:1 in a modern Japanese or European car.

Anything that influences the volume of the cylinder either at the start of the stroke or at the end of the stroke, will affect compression. For example, if you increase the length of the stroke while holding everything else constant, the piston moves farther up and down in the cylinder, you get a higher compression ratio.

If you increase the dome of the piston or the volume of the valves, you decrease the volume at both the top and the bottom of the stroke, but the percentage change is much higher at the top of the stroke, so it increases compression. Increasing the combustion chamber volume would lower compression for the same reason.

If you know the measurements of the parts you are using, you can get very close to a particular compression ratio. To explore the effects of some of the variables, try playing with the factors in this on-line calculator:

http://www.zealautowerks.com/bseries.html

Hope this helps.

EastSideRacer

Thanks a lot that was a very good and logical explanation. I will be sure to post my k24 components shopping list before I commit. As for right now I am on my way to get a k24a2 long block, hope it is nice and clean.

Texas rocks!

k20coupe

How much you paid for the k24?

k20lover

i always wanted to ask this question................good info guys

C_Rock77

Well put Top Ramen.

Simply put, it's the amount of "Squish" the Air-Fuel goes through.

The Compression Ratio is = The total volume of the cylinder / total volume of the combustion chamber.

Think about it this way: Increasing/Decreasing the first space will do the SAME to the CR. Increasing/Decreasing the volume in the combustion chamber will do the OPPOSITE to the CR.

Increase total volume of cylinder: increase CR
Decrease total volume of cylinder: decrease CR
Increase total volume of combustion chamber: Decrease CR
Decrease volume of combustion chamber: Increase CR

Usually, most changes in CR are made by changing the combustion chamber volume. Anything removing material in the chamber makes it bigger. Anything adding material, makes it smaller.

Thicker Headgasket effectively makes the chamber bigger. Thinner makes it smaller

Just another way of saying same things Top Ramen said....people's minds work differently. Might help someone else understand the principles involved.

pandaevo

so just to make sure that I am doing this correctly. I have a bone stock USDM B18C1, my oil rings are bad so i ordered a set of RS Machine 81mm PR3 (B16A) pistons, going to use a stock B18C1 head. Nether block or head have been decked/milled. I will be using stock B18C1 Rods, and Crank and my CR should be 11.3:1??? I hope to pass CA smog with the above on a stock P72 ECU or chipped P28. the rest of my mods at this time is Skunk2 Pro intake manifold, AEM short ram intake, stock exhaust, DC 4-2-1 header. After I fix the burning oil issue I will put in Skunk2 Pro 1 cams, supertech valves/retainers, custom 2.5 inch catback using an RSR axleback, and SMSP header. thoughts??? Opinions???