883-to-1200: Part V: Determining My New Compression Ratio & Squish the Hard Way
At this point the cylinders, heads, and pistons have been removed from the cases. The tappets (lifters) are sitting in a cup of fresh 20-50wt oil waiting to be reinstalled.
Now, being of the scientific ilk, I decided I'd buerette test the heads and the forged dished pistons to determine my actual hot and cold mechanical compression ratio before I re-installed the new pistons.
Compression ratio (CR) 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 or [bore/2]^2*pi*stroke is not the only calculation that needs to be done.
There are three additional volumetric areas that need to determined; (1) the VERY small cylindrical area that is occupied by the thickness of the head gasket; (2) the volume of the dome of the head where the valves open and close; (3) and the depressed space in the top of the Wiseco piston, called the piston's dish area. (See Figure 1 below.)
There are two ways to figure 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 and describe how I did it below.
My good buddy 'Zoom' Rushing, a pharmacist by trade, brought me a 10cc syringe to use. By filling it with water then squirting a known quantity of water into the area I want to measure I could get an accurately measurement of say, the head's volume.
First I inserted the spark plug into the head, next I filled the syringe with water and squirted it into the head chamber repeatedly until it filled.
After four 10cc squirts I only had room enough to squirt 8cc's more into the head chamber.
Next I, laid a flat piece of glass from a 5x7 photo picture frame on top of the head. I looked 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 compression chamber perfectly (without a bubble) confirming Denish was right; 49.5cc's of volume in an 883 head.
If you have your heads shaved at a machine shop, this volume would be a bit less; the effect of which would be an increase in your CR and HP.
Using the same method described above I filled the dished space atop the Wiesco piston and found its volume to be 10.8cc's
Now we've got two of our three calculations out of the way: 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.
The last calculation we need to figure out is the volume of that little slice of space that is created by the head gasket which will separate the top of the cylinder (cylinder deck) from the head by the compressed thickness of the gasket. This area is commonly referred to as the "squish" area.
Using a thicker head gasket will lower your CR and HP whereas using a thinner gasket will increase it.
Figuring out the volume of that squish area requires you to know two mor things, (1) the thickness of the head gasket when compressed and (2) how much the top surface of the piston extends above the cylinder deck at TDC; or how far below the deck it is at TDC. Got that? If not read it again and again. Draw a picture. Do whatever it takes to understand visually what I'm talking about.
Time for 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 and lower the it to the base gasket. Holding the cylinder firmly down against the base gasket rotate the engine (by putting the tranny 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 in 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 and use your finger at the edge of the cylinder bore to feel when the piston reaches absolute TDC.
With the piston 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 cylinder deck, even with it or slightly higher. If either the first or third case, you'll need a feeler gauge to determine how much.
If the piston is inside the bore, whatever you measure will be a negative number in the next formula. If its above, it'll be positive. In my case I found the piston to be .0025' [i]above[/i] the cylinder bore (+.0025").
Remembering we used an UNCOMPRESSED paper base gasket to arrive at the above measurement means we have to account for the .0050" of compression it will experience after the headbolts are torqued down. With that in mind I'll add the .0050" of compression of a H-D paper head gasket (found in Denish's book) to the +.0025", and arrive at .0075". This is the actual height the piston will extend above the deck of the cylinder after the head is bolted down and the base gasket compresses.
Now if you lay a .027" head gasket on top the cylinder, that will effectivly raise the head .027" above the cylinder deck and raise it.027"-.0075" inches above the piston. In my case that would result in that little sliver of space from the top of the piston to the the top of the headgasket to be +.0195 inches. Denish calls for a safe range between .0200 and .0300 inches. Since I was so close to .0200" I decide to proceed.
From the information above we can now figure out the volume of that slice of .0195 inches of space.
If, Bore = 3.498" pi = 3.14 height= .0195" Then [(3.498/2)^2*3.14*.0195] = .187 CUBIC INCHES. Almost two-tenths of a cubic inch!
Now convert .187 CI's to cc's: [.187*16.38706] = 3.1cc's.
Finally, we've got all the info we need to calculate the cold mechanical compression ratio of our new engine. Here is that formula:
Cold mechanical CR = (cylinder volume+dish volume+squish volume+head compression chamber volume)/(dish volume+squish volume+head compression chamber volume)
Applying that to our example we get: (600.321+10.8+3.1+49.5)/(10.8+3.1+49.5)= 10.477 or approximately 10.5:1 Mechanical Compression ratio in a cold engine.
Others have researched and found that a cylinder and head will expand the compression chamber approximately .040 cubic inches when an engine is heated to operating temperatures. With that in mind, if you add the increase in expansion (.040 CI or 3.2 cc's) to the cold squish volume of 3.1 cc's we calculated 3.2+3.1.=6.3cc's, then replace 3.1 with 6.3 in our formula, it 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, the engine's compression ratio at operating temperature.
From this example you can conclude, if you narrow the gap (the squish) between your cylinder and head by using a thinner head gasket or shaving your heads your compression ratio will increase and so will your horsepower.
And don't forget, as your engine reaches operating temperature the compression ratio will lower because of the heat induced expansion experienced in the combustion chamber.
In summary, I found my final choice of gaskets (H-D base paper base gaskets and Bartel's .027 copper head gaskets) netted me the following results:
| Cold | Hot | |
|---|---|---|
| Front Cylinder | 10.477 | 9.620 |
| Rear Cylinder | 10.418 | 9.571 |
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