CCing Your Heads
In order to properly calculate and eventually set your compression ratio, you must know the volume of your combustion chambers. Not only should you know the correct volume of each, but they should all be of identical volume. If each combustion chamber is the same volume (Among other things), you can ensure that each cylinder is carrying an equal load, resulting in a smoother (and properly) running engine.
Before you can do this, you must either buy or make a combustion chamber CCing kit. Since they are pretty easy to make, that's what I did. How I did it was I used a new 60cc medical catheter syringe (NO NEEDLE REQUIRED OR DESIRED). I carefully measured and cut a circle of the correct size out of Plexiglas. I then drilled and countersunk a hole in the center big enough to just fit the end of the syringe into. See below photo for my "home brewed" CC kit.
If you are like me and have just completed P&Ping your heads, you will your heads are probably still disassembled. You will need to at least have the valves laying in place in order to CC your heads. Since you don't have the spring pressure to hold the valves tight as well as clean mating surfaces to seal the valves, you will need to add a thin film of grease to the seating surface prior to sliding them into place. Don't forget to install a spark plug in the plug boss or all your water will run out.
First thing you will need to do to your fully assembled heads is to apply a VERY THIN film of grease to the cylinder seating area of your plastic disk. This will act as a seal so the water doesn't escape the combustion chamber and also so air won't get in.
You're ready to place the round Plexiglas plate onto the cylinder seating area . Give it a good wiggle with your fingers to ensure the grease seals against the head.
Now fill your syringe to EXACTLY 60cc's.
Slowly inject the water into the hole being careful not to cause the water to bubble or foam on the chamber. Allow it to fill completely with NO BUBBLES present. Once the combustion chamber is completely full, write down the amount of water left in the syringe.
Then subtract that number from 60. the remaining number will be your combustion chamber volume FOR THAT PARTICULAR CYLINDER. This must me accomplished for EACH OF THE REMAINING THREE COMBUSTION CHAMBERS in order to properly CC the heads.
Once you have gotten the combustion chamber volume for ALL the chambers, now it is time to get them all even. On my heads, here are my preliminary numbers:
Cylinder #1 - 47cc; #2 - 50cc; #3 - 48cc; #4 - 48cc.
I will need to add 3cc to #1 and also add 2cc to both #3 & #4 in order to bring them all up with #2 @ 50cc.
Some will say that if you're checking a matching set of stock (un-modified) heads, volumes are usually pretty close. Some mechanics will only check ONE combustion chamber and set CR based off that reading to save time. I personally don't care to assume volume. As crusty old Navy Chief I once worked for used to say, ". . . to ass-u-me something usually ends up making an *SS out of U & ME!!" So the moral of the story, NEVER ASSUME ANYTHING. Especially if you have done ANY reshaping & polishing of the combustion chambers. Chances are, you will need to make some adjustments.
Make in these adjustments is similar to engine balancing only that when you CC heads you will try and make the remaining 3 chamber volumes match the LARGEST volume combustion chamber. This can be accomplished by grinding *small* amounts of material away and re-checking the volume until they all match. Once they all match, you now know the exact volume of each and can properly calculate and adjust your engines compression ratio.
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Revised: August 29, 2003 .