I have an '87 trx 250r with a Wiseco high comp piston, Pro Design cool head with an 18cc dome, and the bore is stock. I am trying to figure out my compression ratio. Any input would be appreciated.

 

They make a tool you screw into your head where the spark plug goes, you kick the machine over like 5 times and it gives you a reading on the dial. Should be able to get one at an automotive shop?

 

There is a difference between compression and compression ratio.
Compression ratio is the amount of volumn at BDC divided by the amount of volumn at TDC.
If you didn't increase the stroke of your engine, the compression ratio stays the same as stock, 7:1 for an 86, 7:5 for 87-89.
Now, with the head mods, you have increased/decreased compression. Use a compression tester to find out what you have.

 

It does not matter what year. All years 85-89 had the same 72mm stroke. The change in compression between years was due to the thickness of head gaskets. The ratio will change with a smaller cc dome, shaved head, or thinner head gasket.

Look at it this way, you are compressing the same amount of mixture with a stock stroke (no matter what year)to a different volume above the piston.

Now a compression check could vary if the rings are bad, you would get a lower reading, while the ratio would stay the same.

I'll use my 250R engine for an example and just throw in some numbers.
My bore is .080 over which would make it about 262cc's. I know I have a 21cc cool head. So take 262/21= 12.48:1
This isn't adding the cc's of the gasket and spark plug either.
ERIC

 

The correct formula for calculating uncorrected compression ratio is: (displacement + head volume)/(head volume). So for a 262 cylinder with a 21cc head compression ratio is 283/21 = 13.48. That formula doesn't take into account the volume of the head gasket, but that's a negligible amount.

 

Yea, that too. Actually I think there is more than 21 cc's in the head but I know the dome is 21 cc's. 12.5:1 still seems kind of high considering stock it like 8:1.

I didn't know there was such thing as a high compression piston for an R?
ERIC