|CC - know it, or blow it!
CC-ing is one of those things that very few racers have ever done or for that matter, have even seen done. For the racer who has to meet compression rules it is a must for keeping yourself from beening disqualified. For the rest of us it has to be done to make sure you don't have a grenade under the hood ready to explode! This month I'm going to show you how cheap and easy it is to do this procedure yourself and keep all those expensive engine parts inside your motor. As with all my tech tips, we're going to keep it simple and straight forward, by cc-ing a combustion chamber and figuring the compression ratio for a basic flat top type piston motor. Enough jabbering, let's get into it!!!
To calculate your engines compression ratio you must know the volume of your head's combustion chambers, or the cubic centimeters. Once you have obtained this measurement, figuring the compression ratio is a snap.
To get started there are a few things you'll need.
From left to right are an acrylic sealing plate, a burette, some white grease, and some fluid. You can make a plate and buy a burette or you can buy them as a kit from many places for right around $20.00. I know in some ways that is ridiculous for what you get, but when you figure the cost of a new engine it seems pretty cost effective to me.
Let's get started. Get the head up on the bench, chamber side up. You'll want the valves and spark plug installed that you plan on running. Take some white grease and smear it around the valves and the plug. Remove any extra grease, leaving just enough to keep the fluid from leaking out of the chamber. Next smear some grease around the outside edges of the chamber. You want enough to make a tight seal when the plate is put down but you don't want so much that it gets into the chamber because this will throw off your measurements. A little trick here is to give the plate a slight twist both clockwise and counter clockwise to get the plate to make a nice tight seal.
Now it's time to mix up the fluid. I suggest a 50/50 mix of water and rubbing alcohol. This is not only cheap but creates fewer bubbles and will not rust the head as much as straight water, add some food coloring to the mix, this makes it easier to see in both the burette and the chamber. Pour the fluid into the burette, most kits come with a 100cc burette, fill it to the 100cc mark. Slowly start to pour the fluid through the hole in the sealing plate, continuing until the fluid reaches the opening. If at anytime you spill some of the fluid or over fill the chamber you'll have to start all over, don't do the "Oh I think I spilled this much thing" and make a guess.You also don't want any bubbles under the plate, this will throw your numbers off. To get rid of any bubbles tip the head to allow the air to escape, for those stubborn bubbles I've found that gently tapping the plate with the handle of a screwdriver works great but be careful, if you move the plate and break the seal you'll be starting all over.
Once the chamber is full see how much fluid is left in the burette, subtract this from the 100cc's you started, you now have the total cubic centimeters of the combustion chamber. Right this number down, and on to the next step we go.
Next you need to measure the distance from the top of the piston to the deck of the block, write this number down.
The only thing left to do is figure the thickness of the head gasket after it has been squeezed. If you don't have an old gasket to measure call your head gasket manufacturer and ask them what their gasket will squeeze down to after properly beening torqued. You will also need the bore and stroke numbers for your particular engine.
With all of these measurements gathered you are ready to figure your compression ratio.
Here is the formula that you need to plug your particular numbers into. Believe me, you don't need to be a rocket scientist to do this. If I can do it anyone can!!!
compression ratio = V1+V2+V3+V4 divided by V2+V3+V4
V1 = calculated swept volume (bore x bore x stroke x .7854)
V2 = deck height volume (bore x bore x .7854 x measured deck height)
V3 = head gasket volume (bore x bore x .7854 x compressed gasket thickness)
V4 = combustion chamber converted to cubic inches (1 cc = .061 ci) e.g. a 76 cc chamber is 76 x .061 = 4.636 ci
For an example here are the numbers for Dad's 300 Ford 6 cylinder we just put together for the 2001 racing season.
bore = 4.040
stroke = 3.98
head gasket thickness = .050
combustion chamber = 68 cc (This is the cc's for a 240 Ford with the combustion chambers cleaned up, which is what we run on our 300's)
V1 = (4.040 x 4.040 x 3.98 x .7854) or 51.019
V2 = (4.040 x 4.040 x .7854 x . 020) or .256
V3 = (4.040 x 4.040 x .7854 x .050) or .640
V4 = (68 x .061) or 4.148
So here we go!
51.019 + .256 + .640 + 4.148 divided by .256 + .640 + 4.148
56.063 divided by 5.044 = 11.1
So the compression ratio is 11.1 to 1 or time for racing fuel and not that crap you burn in the every day transportation.