The best dished piston design (left) retains the quench portion of the piston to create be
One way to squeeze every last ounce of power out of your engine is by carefully selecting the parts and blueprinting the engine. Compression ratio is a great place to enhance power while still using pump gas. The variables that come into play include piston top configuration (flat, dished, or domed), piston deck height, head gasket thickness, and combustion chamber volume. Two other variables are bore and stroke. Let's assume we're building a typical 4.030-inch-bore, 3.75-inch-stroke 383. We'll run through several classic combinations to generate a pump-gas-friendly compression ratio. We won't get into camshaft selection here, but basically with a longer-duration cam with more overlap, you can afford to pump the compression slightly to offset the loss of cylinder pressure at lower engine speeds.
An important point worth mentioning is to keep the piston-to-head clearance as tight as possible. If you can get that clearance to anywhere near 0.040 inch, that's great, especially because most head gaskets come in around 0.042 inch thick. This tight clearance increases mixture activity in the chamber, which improves power and efficiency. We're not going to go through the clumsy math of computing compression. Instead, we'll refer you to the Performance Trends Web site, where you can download a free compression ratio program that is far quicker than doing the math longhand. To keep things simple, we'll limit the combos in these examples to a 4.030-inch bore and a 3.75-inch stroke.
Combo A: Let's do a typical flat-top piston with four valve reliefs (roughly 6 cc for volume) along with a 0.042-inch-thick head gasket and a 76cc chamber, which is typical for a stock iron production head. For piston deck height, let's use 0.005 inch. As the chart shows, the safest package with a mild hydraulic cam is going to be the 9.54:1 version, while the 10.13:1 version would probably make the best power on pump gas but would be sensitive to things like hot intake air temperatures, since hot air can easily make the engine prone to detonation.
Combo B: Let's look at an 18cc dished piston with many of the same variables as above, such as a 0.005-inch deck height, a 0.042-inch-thick gasket, and a smaller combustion chamber. By including other variables, such as a smaller dish or a larger chamber, you can see how the numbers change.
Combo C: Now let's try something different. Let's keep the piston 0.020 inch below the deck and use a 0.015-inch-thick Fel-Pro embossed steel head gasket with a rubber coating (PN 10094, $19.88, summitracing.com). This gasket can be used on aluminum heads. Our goal here is to reduce the piston-to-head clearance to 0.035 inch. Note how this affects compression. This requires that we minimize piston rock with a long rod and tight piston-to-wall clearance. This option does not require decking the block, and the gasket is also less expensive.
Clearly, there are dozens more combinations that we just don't have space to detail here, such as what happens to compression when changing bore and/or stroke. As an example, adding stroke bumps compression because the piston is moving farther down in the bore, creating more volume to compress. The beauty of the Performance Trends program is that you can play around with different variables very quickly to come up with an optimized combination.
|ENGINE PACKAGE ||COMPRESSION RATIO |
|Combo A: |
|Flat-top piston, 76cc chamber, 0.042 gasket, 0.005 deck ||9.54:1 |
|Same as above with 70cc combustion chamber ||10.13:1 |
|Same as above with 64cc combustion chamber ||10.82:1 |
|Combo B: |
|Dished (18cc) piston, 64cc chamber, 0.042 gasket, 0.005 deck ||9.54:1 |
|Dished (12cc) piston, 64cc chamber, 0.042 gasket, 0.005 deck ||10.13:1 |
|Dished (12cc) piston, 70cc chamber, 0.042 gasket, 0.005 deck ||9.54:1 |
|Combo C: |
|Dished (12cc) piston, 70cc chamber, 0.015 gasket, 0.020 deck ||9.78:1 |
|Dished (12cc) piston, 64cc chamber, 0.015 gasket, 0.020 deck ||10.41:1 |
|Flat-top (6cc) piston, 70cc chamber, 0.015 gasket, 0.020 deck ||10.41:1 |
|Flat-top (6cc) piston, 64cc chamber, 0.015 gasket, 0.020 deck ||11.14:1 |