First off I know that the heads are different for different years, buy Clymer list an 8:1 compresstion ratio for 87-92. Has anybody CCed the combustion chamber on or off the engine. I'm interested in finding out because I CCed mine, (piston TDC, used head gasket, small amount of sealing grease) and came up with a total combustion volume of around 22 cc's, and 254[.040 over]/22 = 11.54. Okay so I'm thinking my head has been milled, but when I check the dome height measurement I get 17.5 (87 head) which is minus 1.1 millimeters from stock. This however is not enough to make the compression jump from stock listed 8:1 to 11.54:1, but only 9.5:1.

So what kind of compression psi are you guys reading with regard to compression ratio, (I realize porting, engine condition, etc, may have some effect)

Also if anyone has imput on squish angles I would like to hear it.

 

I believe the compression ratio listed by manufacturers for two strokes is always the corrected ratio. The height of the exhaust port is taken into account since some charge escapes and the rest doesn't get compressed until the piston passes that port. In other words, the entire piston stroke "doesn't count".

Stock LT250's run 175psi of compression. This sounds very high (the highest stock compression I have heard of) but I know this as fact. Make sure you have a gauge you can trust. Too many people make incorrect diagnosis based on faulty compression readings.

 

how low do most people usually go on compression before a rebuild?? i got 150 on out tester after about 3-5 kicks with no carb on it.

 

thanks for the info, my head has some scarring from a roller bearing on it and I was thinking about having the dome opened up to get rid of it. I'll look more into the corrected compression ratio.

 

i had 158-160 after new rebuild so your good.

 

what psi is a good point to start looking at a rebuild?mine is fresh now but it will help in the future (preventive maintence) thanks alot

 

I've always gone my the no more than 20% rule, take a measurement once the engine has started to break in, if it ever goes lower than 20% of the orginal it is time for a rebuild.

As far as compression ratio goes I found this on MacDizzy

CR = ( V1 + V2 ) / V2
CR is compression ratio
V1 is cylinder volume at exhaust closing
V2 is combustion chamber volume

Using this, my compression ratio is 7.6:1

If anybody goes by anything different for corrected compression post it up.