Can you ever have too much of a good thing?
In the case of boost, the answer is a resounding yes.
As we told you in Blower Basics (Part 1), boost is the amount of air pressure created by your supercharger and is the result of a combination of factors: engine displacement, blower displacement, and blower drive speed. A proper amount of boost is good—very good. A supercharger creating the optimum amount of boost can produce substantial power gains and/or improved starting and low-end performance, depending on the supercharger style.
Too much boost can be bad—very bad. It can lead to detonation, which can be catastrophic in forced induction engines. With assistance from the supercharger experts at Weiand, we’ll help you zero in on the safe, optimal amount of boost for your street application.
Compression Ratio is Key
When deciding on what boost level to run, you must consider your engine’s compression ratio—static compression and effective compression. Static compression ratio is the compression ratio that is built into your engine. When you combine your engine’s static compression with boost pressure, you get the “effective compression ratio.” This effective compression factor was established to help determine your engine’s safe boost level.
So what is safe boost?
For street engines running on 92 octane gasoline, you should keep your effective compression to 12.0:1 or below, according to Weiand. Once you exceed the 12.0:1 effective compression mark, detonation begins to become more likely. Naturally, you can attempt to control detonation with boost retards and other modifications, but Weiand recommends you stay below or near the 12.0:1 range to make any detonation problems as manageable as possible.
Use the handy chart below to determine your safe boost level based on your engine’s static compression ratio. Again, if the combination of static compression and boost take you over the 12.0:1 effective compression ratio, you may find yourself dealing with detonation issues. Also, remember that low compression, high boost engines will make more power than high compression, low boost setups.
Can you run higher boost levels than those recommended in the chart? Sure. But you’ll need to make modifications (covered in our next installment) to your engine to do it reduce the chances of detonation or mechanical failure. To bring safe, practical performance increases to your street-driven vehicle, follow Weiand’s basic recommendations above.
(Supercharger) Size Matters
If you’re opting for a Roots-style supercharger, such as the ones offered by Weiand, size does matter. According to Weiand, you typically want to run larger blowers on larger engines. Assuming a constant speed ratio between the engine and the blower, a larger blower will make more boost than a smaller one on the same size engine. As engine size goes up, the supercharger will begin to make less boost; therefore, it’s ideal to use a larger blower on larger cubic-inch engines.
Keep in mind, you can make boost adjustments by running larger or smaller drive pulley sizes. You can use smaller pulleys (overdrive) to run a blower faster for more boost; you can add larger pulleys (underdrive) to run the blower more slowly for less boost. Using the popular 6-71 supercharger, the chart below shows you how pulley size can effect the boost pressure of a given blower:
It is perfectly fine to run a larger blower on a small engine as long as you use the proper underdrive pulleys to create safe boost levels. However, it is not practical to run a small, overdriven supercharger on a large engine, because you would have to run the blower very fast to create a reasonable amount of boost. At some point, the blower would become inefficient and could actually heat up the air, causing loss of boost.
Bottom line: larger blowers are for larger engines.
Boost Ranges & Variables
When shopping for a supercharger, you’ll notice boost levels are noted in ranges—as in 6 to 8 psi. This is because there are outside factors, including carburetor size, camshaft profile, and valve size, that can cause boost to vary. For example, at full throttle, your engine is going to need about 50 percent more air than it did before the blower was installed. If your carburetor is incapable of flowing the necessary airflow, you will have lower boost.
Boost readings are typically observed or measured at the intake manifold. If your engine has restricted cylinder head ports, small valves, or a non-performance camshaft, you will actually see artificially high boost readings in the higher-rpm range. This is because the boost pressure can’t get into the engine cylinders efficiently and begins to build up within the intake manifold. Although you’re observing high boost levels, the actual amount of boost pressure reaching your engine is lower and your engine’s power gains have maxed out. Conversely, lower readings on your boost gauge can mean your supercharger is making more power.
In part three of our Blower Basics series, we’ll talk about engine and ignition upgrades that can maximize the performance of your supercharger. We’ll also share some recommended modifications for running higher boost levels—all courtesy of our friends from Weiand.