**Engine 1: XR600R**

bored out to 101mm

stroke is 80mm

stock compression is 9:1

new compression with JE piston is 10.6:1

**Engine 2: XR650L**

bored out to 101mm

stroke is 82mm

stock compression is 8.3:1

new piston is same JE piston from above

Given that these two pistons are interchangeable, what would the new compression ratio be in the XR650L engine with the above mentioned high comp piston for the XR600R?

Hey all, I remember this from college, but had to look it up on the internet to refresh myself since I haven't used it since. I'm getting higher calculated result at 10.9:1. I am thinking that this number is probably a bit high and estimating that the actual compression ratio will be near 10.5:1. See below for the explanation.

Compression Ratio = (Displacement + Static Volume)/Static Volume

Static Volume is the volume left in the cylinder at TDC (piston relief and/or dish, Deck height to Piston volume, Head gasket volume, and combustion chamber volume minus spark plug volume)

Now reworking the formula for the information we have, you get the following:

Static Volume = Displacement / (Compression Ratio - 1)

Now to do some math...

Here are the XR600R calculations:

XR600R 101mm Piston, 80mm Stroke, 10.6:1 Compression

yields <<59.4667cc>> static volume. (I used

Excel for the calculations and came up with 640.9477 cc Displacement)

XR600R 97mm Piston, 80mm Stroke, 8.3:1 Compression

yields <<64.6872cc>> static volume (591.1849 cc displacement)

There is a 5.22cc static volume difference.

Here is some guess work...I estimate that the head gaskets are both 1.5mm thick (torqued head) and am using the piston diameter so...

600R stock head gasket =11cc

600R Big Bore gasket = 12cc

This makes a 1cc hit for the big bore

On the 650L, I estimate the Static Volume reduction will be 4.22cc with the high compression piston.

Now for the 650L computations...

XR650L Stock 100mm Piston, 82mm Stroke, 8.3:1 Compression

yields <<70.55cc>> static volume (Using

Excel, I came up with 644.026CC Displacement)

Assuming that the static volume is now 66.34cc (from 70.55cc above) when changing to 101mm bore, the calculation for the 650L with 101mm bore, 82mm stroke, 66.34cc static volume is:

**10.9:1 **using the first compression formula listed above. **Note that the static volume difference is where the calculation uses an assumption that is probably inaccurate.**

This is probably 90% close but not accurate for the following reasons:

-The Cylinder bore is always larger than the Piston (that is why your bike is actually 649cc versus 641)

-The static volume changes when a different head gasket is used (diameter and thickness differences).

-The stock 650L and 600R pistons are not identical and therefore we cannot accurately calculate the static volume difference as I did above.

-If the wrist pin centerline to piston crown distances for the stock XR600R and XR650L are different then this is inaccurate.

Note: I had to edit this after rechecking my work.