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XLH 883 to 1200 Upgrade
by Rick 'Rickko' Eliopoulos

Determining Compression Ratio & Squish

At this point the cylinders, heads, and pistons have been removed from the cases. The tappets are sitting in a cup of fresh 20-50 wt. oil waiting to be reinstalled. Now, being of the scientific ilk, I decided I'd burette test the heads and the forged dished pistons to determine my actual mechanical compression ratio before I re-installed the new pistons.

Compression ratio is the ratio of the volume of the cylinder when the piston is at BDC to the volume when its at TDC. The formula for finding the volume of a cylinder is written in every grade school primer. Besides that, if I'm building a 1200cc engine, then obviously one jug is going to be 600cc's. But using the formula to find a cylinder's volume: radius^2*pi*height, is not the only calculation that needs to be done. There are three additional volumetric areas that need to determined; the VERY small cylindrical area that is occupied by the thickness of the head gasket; the volume of the dome of the head where the valves open and close and the depressed space in the top of the Wiseco piston, called the dish area.

There are two ways to find the head's volume, either look in The V-Twin Tuner's Handbook by D. William Denish who calculated it to be 49.5cc's or find out for yourself if he is correct. I chose the later path.

My good buddy Zoom Rushing, a pharmacist by trade, brought me a 10cc syringe that I could use by filling with water and accurately measure the head's volume. First I inserted the spark plug and tightened it into the head, next I filled and refilled the syringe after squirting the water from it into the head chamber. On the forth refill I carefully squirted 8cc's into the head chamber. Then, laying a flat piece of glass from a 5x7 photo picture frame on top of the head I started to see if the water filled the chamber. It didn't. I added another cc of water. Still didn't fill it. I add 1/2 cc more and this time, the glass sealed the water filled chamber perfectly and without a bubble. Confirmation that Denish was right; 49.5cc's of volume in an 883 head.

Using the same method we filled the dished space atop the piston. It worked out to be 10.8cc's

Now I got two of the three calculations done: Head volume: 49.5cc's and dish volume: 10.8cc's. The last one is easy to calculate but takes a little research to get the figures to plug into the formula.

Figuring out the volume of that little slice of space between the top of the piston and the base of the head, sometimes referred to as the 'squish area', is a matter of determining how high the piston is at TDC relative to the cylinder. That means you need to know a few more facts. One, you'll need to know the UNCOMPRESSED and COMPRESSED thickness of base gasket you'll be using (That thickness will raise your cylinders that high above the case); two, you'll need to know the same dimensions for the head gasket you chose and; three, you'll need to know the relative difference between the top of the cylinder and the top of the piston at TDC. Got that? If not read it again and again. Draw a picture. Do whatever it takes to understand how to figure this out.

OK, a real life example. My uncompressed H-D paper base gasket is .017" thick (compressed its .012" thick) and my Bartel's copper head gasket compressed is .027" thick (copper doesn't squish with 35 ft-lbs of torque on the head bolts). Now, place that base gasket over the studs on your cases, then attach the piston to the rod (don't clip it in), then place the cylinder over the piston. Now lower the cylinder to the gasket, then rotate the engine (by putting the transmission in 5th and turning the back wheel) until the piston is at TDC. If you are doing this to the front cylinder, TDC can be found by pulling the huge timing plug allen screw found on the case just below and between the base of the two jugs on the right side of the engine and centering the vertical line on the flywheel in the viewing hole. If you are doing this to the rear cylinder, you'll just have to eyeball it.

With the piston is at TDC run your finger over edge of the piston and cylinder wall's bored edge. You'll notice one of three things. Either the piston is slightly lower than the bore, even with it, or slightly higher. If either the first or third case, you'll need a feeler gauge to determine how much. If its inside the bore, whatever you measure will be a negative number in the formula. If its above, it'll be positive. In my case I found the piston to be .0025" above the cylinder bore (+.0025").

Now remembering we put in an UNCOMPRESSED base gasket which raised the cylinder up .017" means that if it were under compression (i.e. heads bolted down), the piston would be .another .005" above the cylinder in addition to the .0025" it measured uncompressed making the piston extend above the cylinder height, +.0075" . Now if you lay a .027" head gasket on top the cylinder, that effectively raises the head deck height .027" above the cylinder top or .027-.0075" above the piston. This case would result in a deck height of +.0195". Denish calls for a safe range between .0200 and .0300. Since I was so close I decide to leave it be. From this information we can now figure out the volume of that slice of space remaining (.0195") created by the inclusion of a compressed base gasket and the head gasket.

We said that the head gasket is .027" thick. Since we know the piston extended above the top of the cylinder by .0025", our slice volume, or squish area will equal to (bore/2)^2*pi*height or (bore/2)^2*pi*(the thickness of the head gasket-the .0025" the piston is occupying at TDC of that head gasket's thickness). [(3.498/2)^2*3.14*.0195]= .187 CUBIC INCHES. Now convert that to cc's. [.187*16.38706]=3.1cc's.

Now we've got all the info we need to calculate the cold mechanical compression ratio of our new engine. Here's the formula:

(cylinder volume + dish volume + squish volume + head compression chamber volume)/(dish volume + squish volume + head compression chamber volume)

For our example: (600.321 + 10.8 + 3.1+4 9.5)/(10.8 + 3.1 + 49.5) = 10.477 or 10.5:1 Mechanical compression ratio with a cold engine. Others have researched and found that a hot head and cylinder will expand adding about .040 cu. in. to the squish area’s volume in the combustion chamber. With that in mind, if you increase the squish volume by that .040" or (6.3cc's) our formula would change to: (600.321 + 10.8 + 6.2 + 49.5)/(10.8 + 6.3 + 49.5)= 9.62; or 9.620:1 net mechanical compression ratio.

From all this you can conclude, if you narrow the gap between your cylinder and head (use a thinner head gasket) your compression ratio will increase. But at the same time, as your engine gets up to operating temperature, the compression ratio will lower because the heat will cause expansion within the combustion chamber.

With that example behind us, let me say, based on my final choice of gaskets (H-D base gaskets and Bartel's .027 copper head gaskets) my compression ratios ended up to be:

Compression Ratios Cold Hot
Front Cylinder 10.477 9.620
Rear Cylinder 10.418 9.571

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