Checking your own cylinder...
Checking your own cylinder volumes allows you to determine your exact compression ratio. It is also relatively easy to do and only requires a few tools.
Stock car racing engines are all about efficiency. That's because all the simple solutions for making power have either been maximized or eliminated by the rule book. You aren't allowed power adders like nitrous or a blower (lucky drag racers) and the easy answer of simply upping the displacement (in most cases) isn't allowed.
So you have to make the most of what you are given, and since every motor on the racetrack is so similar, the game becomes one of who can squeeze the most power out of the same Chevy small-block everybody else is using. And one of the best ways to do that is to make sure you are getting every last bit of compression you are allowed by the rules.
The concept of efficiency when it comes to compression is simple: A droplet of fuel only has a specific amount of energy, but if that droplet can be squeezed into a tighter space before being ignited it is capable of doing more work with that same amount of energy.
You will also need some method...
You will also need some method to hold your heads level. A set of head stands, like this one from Powerhouse Products, is invaluable for this and any other time you need to work on a set of heads. They keep the heads up off your work bench so that your freshly machined decks won't get damaged or scratched up.
Of course, that's the reason many racing series limit compression. They know that's one of the surest ways to limit engine power. Others limit compression without expressly stating it. This can be done by putting minimum limits on combustion chamber size, stroke, and even the shape of the piston top. Either way, to make the most power you have to make sure you are right up against the limits of what's allowed when it comes to making compression. And to do that you have to know how to determine exactly what your compression is.
Can't Somebody Else Do It?
With buying a new engine, or having your old engine sent out to be rebuilt, your engine builder should normally check the chamber volumes and determine the compression ratio. But there are often instances when you may want to do this yourself. For example, you may be purchasing a set of used heads from another racer or off the internet and need to know that they haven't been reworked so extensively that they are illegal at your track. Or you may be building your own engine.
This is the scenario we faced while preparing for an upcoming story using Circle Track's dyno test mule Chevy 350. So far, we've tested rocker arms, oil pans, and exhaust systems with a standard Chevy crate motor short block. But for future tests we want a stronger bottom end that can handle more power. Along the way, we thought we'd also test to see just how compression affects engine power and to do that we need to see just what we've got.
We're working on a set of...
We're working on a set of heads that have just been pulled off a motor, and we're hoping to use them again without any machine work. They'll need to be cleaned up a bit first, though. Resist the temptation to scrub the deck of the head-even with a Scotchbrite pad-as it can warp the deck and cause a head gasket failure. Instead, clean the deck carefully with a razorblade.
The first step in determining your compression ratio is understanding how to calculate total engine displacement. Displacement basically is the area swept (or displaced) by the piston as it moves from the bottom of the cylinder bore to the top. This does not include the combustion chamber in the cylinder heads or any of the cylinder bore above the piston when it reaches top dead center (TDC). The formula for determining displacement is:
Bore X Bore X Stroke X 0.7854 X Number of Cylinders = Displacement
For this formula, both bore and stroke must be measured in cubic inches because the fourth number (0.7854) is a constant used to convert to cubic inches. As an example, let's consider standard Chevrolet 350 dimensions-the bore is 4 inches in diameter and the stroke is 3.48 inches. That makes the formula look like this:
4 X 4 X 3.48 X 0.7854 X 8 = 349.849
No big surprise since we said it was a Chevy 350 to start with, but this gives you an idea how the formula works. The bore can be measured with a dial bore gauge, but you can even get by with a dial caliper in a pinch. And the stroke can normally be pulled right off the crankshaft spec sheet. For calculating the compression ratio you will need to know only the displacement for a single cylinder, so to determine that all you have to do is divide by the number of cylinders-or just leave that multiple out of your original equation.