Once again I come to the mountain seeking knowledge and wisdom.:worthy:

How do you calculate the actual compression ration and what the compression reading should be based on ratio.

this is an xr650l with the wiseco 8.6 compression ration (I think that is the real number not the 10.5 claimed when purchased.)

I measured the distance from the top of the piston installed to the top of the cylinder and it was 1.003 mm.

The compression readings are 92-95 with the decompression. By totally backing off the adjuster it jumps up to 110-115. I used two different gauges.

The bike has not been started or run yet still working on that think it is a timing issue ordered a new key for the crank, as trying to set the valve lash kept changing.

From reading the post I can find have seen any where from 130 to 175 on the compression reading. Dose this normally increase once the rings have set?

I put oil in the cylinder and had zero affect on the reading so the rings should be ok. I put carb cleaner in the ports on the head and did not have any liquid seep out of the valves. The head was done by shop last year but sat for 7 months before assembly.

My thoughts are it should be higher than it is.

comp ratio is the volume of the cylinder, head gasket (squashed thickness), and head volume. Then with piston at TDC you get the leftover of cylinder volume and gasket and head are the same, so once you know total volume at BDC and TDC you can calculate the ratio.

Mx760 said:

1.) You have to calculate the cylinder swept volume, head gasket bore compressed volume, and at least know the head chamber volume. Once you have that, cranking compression is still at the mercy of cam timing events.

2.) Look in the manual and you will see that 92psi is the factory spec with the compression release active. The Wiseco piston is not going to improve upon that very much.

3.) I don't quite understand where this is going.

I think this is much ado about nothing since the engine hasn't even been fired yet.

Head Trauma

I saw the 92 in the manual with out any reference to a number with out the decomp in use. The search's in the forum I saw numbers from 130 to 175 and was questioning is 110-115 good? is it anything to worry about?

Was also wondering what the actual compression ratio would be. Then the analytical part kicks in wondering if you can take the compression ratio and determine what the compression reading should be? The goal here is to see how health or close to what it should be.

I found a couple of formulas on the internet and plugged in the following numbers

Stroke 82 mm
Bore 101 mm
head gasket .35
Gasket bore(if correct term ) 102.5 mm
Head Volume 58cc

It came up with 11.5 to one and I know there is no way that is anywhere near right. it was on the internet so it must be true:p

Mx760 said:

1.) Did you do the compression test with the throttle closed or wide open?

2.) The numbers above do not account for the piston being 1mm below deck.

I tried to do a compression test on my 600, but it is difficult with 10.5:1 compression and no e-start. With a rolling start, I wasn't able to get any more than 150psi on the gauge before the rear tire locked up...in 5th gear. The calculators I have used show that with my cam, bore, head volume, etc, that the cranking pressure should actually be up around 240psi at my elevation. At sea level they predict more like 270psi.

compression test was done with throttle wide open and once with the carb off the numbers were the same.

Those are some huge numbers. Your bike must fly!

I would have thought the numbers would have been higher. I am more interested in getting this right than making extra power. I would have thought (no data to back this up ) it would have been 130 to 145.

Dose compression normally go up once the rings seat?

Mx760 said:

1.) Good.

2.) It would go better if it had a cam right for the combination. It's very octane sensitive because it doesn't.

3.) Often times, yes.