compression/stroke/bore calculation question for gurus

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?

My calculations say 9:1

Seriously? I would have thought it was higher than that.

Anyone else able to do these calcs?

I'm not sure you can without the head CCs.

maybe this way, lemme see:

stock XRL:

Bore and Stroke: 100.0mm x 82.0mm

(10cm * 8.2 cm)

Compression Ratio: 8.3:1

Displacement is pi * r * r * stroke or

3.1415926 * 5cm * 5cm * 8.2cm or

644.026 cc

OK, so the ratio of (piston down volume) to (piston up volume) is 644 + head volume : head volume, or 8.3:1

make an eqn:

644 + Hvol = 8.3 x Hvol

7.3 x Hvol = 644, head volume = 88.22cc (sounds big to me)

Do the same for the 600 stock specs to get the 600 head volume, then run the numbers the same way but with the larger bore but with a 'stock' 600 piston, then see how much extra volume the high-comp piston must take up, then apply that info to the 650L numbers (with the larger bore).

maybe I'll have time to crunch those numbers later...

Dave

XR600R:

97 × 80 mm, 9.0:1

displacement is 591cc, 8x Hvol = 591, head volume is

73.875cc

Take the stock piston and bore it out to 101mm-

new displacement = 641cc

641 + 73.875 : 73.875 = 9.67:1

Now, the higher comp piston brings that CR to 10.6:1

again, 641 + Hv = 10.61Hv, head vol = 66.7cc

73.875 - 66.7 = 7.715, so the high-comp piston takes up 7.715cc of head volume.

Dave

OK, 650L with the big bore:

101mm x 82 mm gives a 657cc displacement.

657 + 88.22 : 88.22 is 8.45 :1 with the stock piston

sub 7.715 off that head volume and you get a 80.505 effective head volume.

657 + 80.505 : 80.505 = 9.161 : 1

So, 9.161 : 1 should be the comp ratio if you stuff that piston into an XRL. (If I'm not fulla malarky!)

BTW, there's an easier way to do some of that if you look at the % diff in the values, you just have to remember to div. by 2 and square in the right places.

Dave

My 9:1 ratio used similar calculations, not sure why mine came out slighly lower.

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.

The wrist pin centerline to piston crown distances for the Je XR600R and XR650L are different by .060 with the 600r being the smaller of the 2 so if the 650l piston is 10.5-1 then with .060 more room in the cylinder it would have to be in the 9s.

You guys are unbelievable. I can't thank-you enough. Absolutely my deepest thanks for the answer and the tutorial. Not only do I have an answer, I know how to get my own next time!

9.2:1

I'm not sure if you noticed, but you can alter the compression ration through changing the head gasket thickness. A difference of 0.5 MM (or 0.020") can probably give you 1 point on the compression.

I've seen "thick" head gasket kits for older high-compression muscle cars that allow you to run current pump gas in them. Of course, they lose power doing this.

I've also seen "thin" JDM gasket kits for Supra Turbo engines that raise compression.

I'm not sure if you noticed, but you can alter the compression ration through changing the head gasket thickness. A difference of 0.5 MM (or 0.020") can probably give you 1 point on the compression.

I've seen "thick" head gasket kits for older high-compression muscle cars that allow you to run current pump gas in them. Of course, they lose power doing this.

I've also seen "thin" JDM gasket kits for Supra Turbo engines that raise compression.

You can do the same with the base gasket- but how much can you do this without affecting the valve timing?

Moving the cam closer to the crank will retard the valve timing.

Dave

Moving the head .020" up or down is probably 5 times less effect than your cam chain stretching over time and changning your cam timing. You cam chain probably stretches 1/8" or more.

Moving the head .020" up or down is probably 5 times less effect than your cam chain stretching over time and changning your cam timing. You cam chain probably stretches 1/8" or more.

Yeah, I didn't mean to infer big changes.

Well, since this is a math thread already-

- assume the crank sprocket has a radius of one inch.

dist. around an arc is the radius x the angle (in radians), write as

S = R x angle, angle = S/R

angle (radians) = 0.020 inches / 1 inch = 0.020 radians

pi radians = 180 deg., so angle (deg) = 180 x 0.020 / pi

angle = 1.14 degrees of valve timing retardation.

Bigger than I thought.

Anyone have an XRL crank out so we can actually measure the radius of the sprocket?

Dave

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