Compression Ratio

[mattfrox]

HI all i an\m new to this forum.

I bought a 12:1 wiseco on ebay the other day thinking i could simply replace my stock piston in my 200m. I can simply switch them but i want to run on pump gas. Whats the highest CR for pump gas? Wiseco makes a 10:1 right? I did the math and to reduce the compresion ratio from 12:1 to about 11:1 all that is needed is an addition of about 0.3mm to the cylinder height.

So i guess what i am asking is; has anyone has played around with different thickness head gaskets to vary the compression ratio?

Thanks, Matt

[mattfrox]

sorry guys...i did the math wrong

for stock 200...bore is 6.5cm...area of circle is pi*(r*r) so:

3.14*(3.25*3.25) = 33 cm squared..so about 0.1 cm (1mm) will increase total volume of cylinder about 3.3 cc. This is enough to change CR from 12:1 to about 11:1.

Any thoughts?

Thanks again, Matt

[HaggLE]

I would be changing the base gasket to a thicker one (or use two) rather then messing with the head gasket.

By my calculations the cc's in the head will be 17.5 before you modify anything. If you add 3.318cc's (recalculated it) to the head you end up with a compression ratio of 10.2:1

This is low enough to run regular but you may as well run a 10:1 piston...

[Tri-ZNate]

If you want to get exact its 33.183 but you probably knew that. So you want to get a thicker head gasket to raise the head up some, so the compression is lessened? It sounds like it would work but can you just go and get a thicker gasket or do you have to double up? oh and :welcome:

[Tri-ZNate]

Haggel you must have just beat me to the punch.

[mattfrox]

Good point i would add on the bottom gasket rather than the head.. this would likely prevent any seal problems plus its less critical in terms of performance.

I dont know if i would double up or not. Can you buy gasket material in different thicknesses? If so i would buy the thickness i needed and and make a gasket for the bottom end.

The reason i didnt but a 10:1 is because there was a really cheap 12:1 on ebay so i just bought it. 10:1 is the highest CR on pump gas...what if i use supreme like 93 octane?

Thanks again..i feel welcomed at the forum already., matt

[trikezilla]

How do you exactaly figure out the compression ratio?? i mean, how do you set the formula up????????

[mattfrox]

check this link for a general overview:

http://e30m3performance.com/myths/more_myths1/comp_ratio/comp_ratio.htm

[mattfrox]

Haggle..how did you calculate the head volume for a stock 200..just curious?

[Mobular]

You can also kill off some of the compression by putting the piston in a lathe, and milling off some of the dome. Just a wee bit tho!