Compression Ratio and Boost Pressure Relationship
The compression ratio of your engine has a direct relationship to how much boost you can run. If you have a high compression ratio, such 9.5:1 or 10:1, you will only be able to run a small amount of boost.
The compression ratio that is built into your engine is called “static compression.” When you combine the boost you are running in conjunction with your compression ratio, the result is known as the “Effective Compression Ratio.” Formulas have been developed that convert your static compression and supercharger boost to the effective compression ratio. Table 1 provides this information.
You can find your static compression ratio on the left side of the chart. Then read across to the right under the boost you want to run and the number in the box will be your “effective” compression ratio. Experience has shown that if you attempt to run more than about a 12:1 effective compression ratio on a street engine with 92 octane pump gas, you will have detonation problems. To some degree, this can be controlled with boost retard devices, but we do not recommend that you set up your engine and supercharger to provide more than a 12:1 effective compression ratio. Please note that all engines differ in their tolerance to detonation. You can build what appear to be two identical engines and one will detonate and the other one won’t, so the numbers given in this chart are not absolute hard and fast figures. However, if you follow this chart, you will be close enough that if you do experience some detonation, you should have no trouble controlling it with one of the aftermarket boost retard ignition systems (such as the such as the Holley Ignition P/N 800-450).
Table 1 shows that you obviously can’t try to run 10 pounds of boost on a 9.0:1 compression ratio engine. This gives you an effective compression ratio of 15.1:1, way beyond our 12:1 figure. If you are building your engine from scratch, it is a good idea to try to build it with a relatively low compression ratio, such as 7.5 or 8.0:1. It is fairly easy to change the boost to get the best combination of performance and power, but it is extremely difficult to change the compression ratio, especially if you want to lower it. Additionally, you will make more total power with a low compression, high boost engine than you will with a high compression, low boost engine.
Intercooled Boost Relationships
What type of fuel do I need with a supercharged automotive or truck engine? The primary issues that determine the type of fuel needed are whether the engine is fuel-injected or carbureted, the compression ratio of the engine, and whether or not the supercharger system is intercooled. For Intercooled ProCharger EFI/TPI applications with compression ratios less than 9.5:1, boost levels of 14-17 psi can be safely run with full timing on pump gas, and will produce horsepower gains of 75-100% (depending upon the boost level and the motor specifications). For 9.5:1 EFI/TPI applications running without an intercooler, boost levels above 5 psi will require the use of ignition/timing retard on pump gas, and will produce horsepower gains of 35-45%. Boost levels above 12 psi should generally be avoided even with racing fuel on a 9.5:1 motor. Of course, lower compression motors will be able to run more boost, and higher compression motors should run less boost, everything else being equal. All Intercooled ProCharger systems for street applications are designed to allow the use of pump gas with full timing and will not affect daily drivability. For carbureted motors, the rules are slightly different. Carburetors deliver the vast majority of fuel in a liquid state, and as this raw fuel atomizes from liquid to gas, a chemical state change actually occurs. Due to this endothermic reaction, which draws heat and cools the incoming air, a carbureted motor can safely handle more boost than a comparable EFI/TPI motor. For carbureted engines with compression ratios of 9:1 or less and boost levels in the 8-14 psi range, pump gasoline works very well. Compression ratios of 10:1 and higher require lower boost levels, higher octane fuel, intercooling, or some combination of the above. Compression ratios in the 7or 8:1 range can usually handle 12-20 psi on pump gasoline.
