Here we have an Edelbrock small-block head, which has been modified with some porting and
How To Measure
No compression-ratio calculation is worth spit if the numbers used to calculate it are wrong. In the old days, a lot of the chamber volumes published for stock heads were NHRA minimum-legal blueprint specification figures for Stock and Super Stock racing classes, which were miles away from production reality. These days, aftermarket head manufacturers are fairly honest about chamber volumes. For the most part, using published catalog numbers will get you close in calculating compression ratio-unless, of course, the heads have been altered by milling, porting the chambers, different valves, or the like. To find the true chamber volume, the heads need to be cc'd. Here's how to do it.
Any gearhead knows that pistons affect compression ratio. Two factors come into play when determining the ratio-compression height and crown configuration.
Compression height is the distance from the centerline of the wristpin to the top deck of the piston. A piston with a short compression height will sit lower in the bore as the crank reaches TDC. Estimating the piston deck height is fairly simple. Add half the engine's stroke to the rod length and the piston's compression height. After that, subtract the deck height to get the piston deck, or how far down in the hole the piston should end up at TDC. For example, a 360 has a stroke of 3.58 inches. Half of that is 1.79 inches, which is the up-stroke from the crank centerline. Add the length of a standard small-block rod at 6.123 inches and you get 7.913 inches. Now, if we are considering a Speed-Pro H116CP piston, which has a compression height of 1.670 inches, we add that in to get 9.583 inches. Since an uncut Mopar small-block cylinder measures 9.600 inches from the crank centerline to the deck, and our up-stroke/rod/piston numbers add up to 9.583, the pistons should be .017 inch down from the top of the block at TDC (9.583-9.600 = -.017).
Crown configuration is fairly self-explanatory. Valve notches and dish designs add clearance volume and cost compression, while domes take up clearance volume and add ratio.
When building an engine, one of the most important choices is selecting the correct piston for the job. For the typical stock-stroke, pump-gas small-block, a zero-deck piston is usually the best choice. Typical low-cost, stock-replacement, flat-top or dished pistons may look like a bargain, but the price you pay in compression ratio is anything but cheap. Take, for instance, a stock-replacement 318 cast piston. These have a compression height of 1.720 inches, putting the piston a good .100 inch in the hole at TDC. The valve clearance notches cost another 1.5 cc. With standard 64cc 318 heads, you're looking at 8.0:1 compression, while a set of higher-flowing 360 heads drops the ratio to 7.75:1. There's no way to make power at that ratio. Milling the 360 heads .060 inch and using a thin, .025-inch-thick MP gasket will just score an 8.6:1 ratio, but now the pushrod geometry is messed up, the intake face will need to be milled, and you've spent more money than you would have by getting higher compression pistons to begin with. The same 318 with a set of Keith Black PN 167 pistons can be set to zero deck with a little block milling. This will net a 9.3:1 compression with higher-flowing 360 heads, or over 10.2:1 with late-model 302 or Magnum heads using a standard .039-inch-thick gasket-bagging an ideal quench clearance (see below) in the bargain.