Displacement Effects And Strokers
Stroker engines greatly increase displacement, but generally the heads that top them were designed for much smaller displacement engines. With a stroker, the clearance volume at TDC remains about the same as with a smaller engine, but the total volume goes way up as a result of the far greater swept area. Essentially, you now have a far greater volume of gas compressing into the same amount of space at TDC, and the result is a much higher compression ratio. With more cubes, higher compression ratios become easier to achieve. In fact, often the problem with strokers is to select components that prevent the compression ratio from ending up too high. For example, a 416 small-block stroker with a zero-deck piston, topped with an Edelbrock head and Fel-Pro gasket, will have a ratio of 12.2:1. The exact same combo of a zero-deck flat-top in a 340 will have a ratio of 10.25:1. As can be seen, sometimes with large-cube strokers, the challenge is getting compression ratio out of the engine.
There are several ways to get a stroker's ratio down to a pump-gas level of around 10:1. Dropping the pistons in the hole, using dished pistons, running thick gaskets or big, open chamber heads will all accomplish a lower ratio, but will preclude building an effective quench. The best solution is to use a piston with D-cup, which has a dish over half of the surface to reduce the ratio, while the far side remains at zero deck for an effective quench.
Cams and Effective Compression
What does a cam have to do with compression ratio? Nothing really, but most guys have heard that the cam and compression ratio need to be considered together when working out an engine-build combo. While the camshaft doesn't change the engine's compression ratio, it does have a dramatic effect on the compression pressures the engine will see. Why? Some guys think it has to do with overlap, but actually it is the intake-valve closing point. We all know that a piston goes up from BDC toward TDC on the compression stroke, compressing the air/fuel mix. The intake valve, even with a stock cam, closes after the piston is on its way up from BDC. Obviously, there is very little compressing going on while the intake valve is still open. As the cam's duration is increased, the valves stay open longer, and the intake valve will close even later in the compression stroke. Remember, compression doesn't begin until the valves are shut.
Interestingly, the piston's motion from BDC is accelerating, while we tend to think of increases in cam timing as a constant in terms of (crank) degrees. Remember that the piston is accelerating as it comes off the bottom of its stroke, so the actual change in piston position for each degree of later intake closing becomes more pronounced. In other words, as cams get bigger, the loss in trapping efficiency accelerates rapidly. Adding mechanical ratio compensates for the loss of pressure, so typically the bigger the cam, the higher the compression ratio should be.
Duration is only part of the equation as far as when a given cam's intake lobe will close the valve. The other factors are the lobe separation angle and the installed centerline. The chart below shows the effects of different cam characteristics on trapping efficiency or cylinder compression pressure.