The other day, I received a frightening phone call that went something like this:
Me: Hello, this is Dave.Scary Guy: Dave Coleman?
Me: Yes.Scary Guy: OK...um, if I take an LS block, a B16 head and Civic Type R pistons, what kind of compression will I get?
Me: Who is this?Scary Guy: Oh, this is Joe. I work at a shop. I'm building a motor for a customer and I have to order pistons today. So what's the compression ratio?
(Seriously, he was that vague, I'm not making this up.)
Me: I have no idea. You're gonna have to do some math.Scary Guy: Crap. I don't know how to do that. I just know how to order parts.
(No, seriously, I am not making this up.)
Scary Guy: Well, is it true that you can't run any more than 11.5:1 on pump gas?
I tried to explain that how much compression you could run depended on a lot more than the engine itself; like the local fuel supply, cam timing and, of course, how the fuel and timing curves are tuned. Mr. Scary was audibly dejected. He hung up disappointed, and I hung up scared. Somebody, somewhere, is paying this guy to build his engine.
OK, Joe. Let's talk about compression ratio.
The compression ratio is simply the ratio of total combustion chamber volume at its biggest point (when the piston is at the bottom of its stroke) to the volume when it's smallest (at the top of the stroke). The actual math involved is laughably simple. Remember division? All you do is take the volume at BDC and divide it by the volume at TDC.
What are those volumes? The combustion chamber is made up of a surprising number of shapes, the volumes of which are best calculated independently. The combustion chamber at BDC, for example, is made up of the following volumes: the cylinder, head gasket, piston dish (or dome), deck height, ring land crevice and the cylinder head.
Cylinder VolumeCylinder volume is the easiest to calculate. If you stayed awake in geometry class, you would remember the volume of a cylinder is:
Note that if you want the answer to come out in cubic centimeters, you need to convert the bore and stroke to centimeters first. For Joe's customer's LS VTEC engine, that's an 8.1 cm bore and an 8.9 cm stroke, a total volume of 458.6cc.
Head gasket volumeThis is equally simple to calculate if you know how thick the head gasket is. Typically, stock Honda head gaskets are about 0.6mm tall when fully compressed by the torque of the head bolts. The head gasket volume is just another cylinder.
With an 8.1 cm bore and a 0.06 cm thick gasket, Joe's customer's head gasket volume is 3.1cc.
Piston dish volumePiston dish can't easily be calculated. Instead, you have to look it up or try to measure it. If you have a piston in your hand and want to measure the dish, the easiest way is to fill it with water. Remember chemistry class? You'll have to drip the water into the dish from a finely calibrated pipette to achieve accuracy. You'll also want to put a sheet of glass or clear plastic across the top of the dish to ensure the top surface is flat and unaffected by surface tension. You will, of course, need a hole in the sheet so you can get the water in.
In the case of Joe's customer's Civic Type R piston, the piston is domed, so you can't fill it with water. Remember preschool? Try mashing the piston into a block of clay to make an imprint and then dripping the water into the imprint.
Or, if you're lucky, you can find the spec from a reliable source. In this case, the reliable source is King Motorsports and the piston dome is -6.8cc. The volume of the piston dome will be subtracted from the total volume (a dish would be added), so it's best to think of dome volume as negative.