When it comes to centrifugal superchargers, both Vortech and ProCharger offer a number of large compressors to suit high-powered street engines and dedicated racing combinations.
Getting the most from a supercharger, regardless of the compressor design, is dependent on flowing enough air to satisfy the airflow capability of the engine. A blower’s maximum boost will not be realized on a large-displacement engine that isn’t matched with a commensurately sized compressor.
Positive-Displacement Blowers: Calculating Boost and Increased Boost with a Pulley Change
Changing the supercharger drive and/or the crankshaft pulley/damper is the common method for increasing the boost output of the blower. Generally speaking, reducing the size (diameter) of the supercharger drive pulley will spin the rotors faster to produce more boost. Increased temperature in the boosted air charge is an inevitable byproduct as well. Depending on the engine combination, there can be a point of diminishing return with such a change, but it’s the most effective way to increase the output of the supercharger.
Determining the approximate amount of boost a positive-displacement blower such as an Eaton TVS or Whipple will produce, as well as how much more it will produce with a pulley change, is determined with a few simple calculations. First, start with the theoretical max boost of the combination. It’s determined with this formula:
PR x 14.7 x SV / EV (½) − 14.7 = max boost (psi)
Air flowing through the heat exchangers of an intercooling system will reduce the maximum pressure of the boosted air charge before it enters the engine. This cutaway shows the integrated charge-cooling brick on an Edelbrock E-Force supercharger with Eaton TVS 2650 rotors.
PR is the pulley ratio, which is the size of the crankshaft pulley divided by the supercharger pulley. For example: An 8.2-inch-diameter (208-mm) crankshaft pulley and a 3.1-inch supercharger pulley (78 mm) delivers a pulley ratio of 2.64 (8.2 / 3.1 = 2.64).
14.7 is the normal air pressure: 14.7 psi (1 bar).
SV is the supercharger volume in liters. For an Eaton 2300 TVS-type blower, that would be 2.3 liters.
EV is half of the engine volume. The total engine volume is divided in two because one rotation on a four-cycle engine is only half of a complete cycle. For an LS3 6.2L engine, the engine volume number for the equation would be 3.1L.
14.7, again, is atmospheric pressure.
Putting it all together for an LS3 with a 2300 supercharger and a 2.64 pulley ratio lands at 14.09 pounds of max boost: 2.64 (PR) x 14.7 x 2.3 (SV) / 3.1 (EV ½) − 14.7 = 14.09 pounds of boost (0.97 bar).
Changing the pulley sizes changes the pulley ratio, thereby affecting the maximum boost capability of the compressor. With the example above, changing only the blower drive pulley from 3.1 inches to 2.8 inches (approximately a 10-percent reduction) changes the pulley ratio to 2.93. When plugged into the boost calculation formula, the maximum boost increases to 17.25 psi (1.2 bar).
The comparatively minor change in pulley size makes a significant change in the speed of the supercharger and its output. The caveat here is that a significant increase in boost comes with a significant increase in heat that can lead to detonation.
It is also important to note that the boost calculation formula does not take into account a couple of important factors that will reduce the maximum boost pressure that actually enters the engine. The first is the overlap factor. All engines, even those with blower-friendly camshafts, have a measure of valve overlap, where the intake valve opens before the exhaust valve closes. The amount of overlap determines how much boost is siphoned off; there is approximately 5-percent loss for every 10 degrees of overlap. On a combination with 8 pounds of calculated boost (0.55 bar) and 10 degrees of overlap, the loss is 0.4 psi (0.27 bar), for a total of 7.6 pounds of boost (0.52 bar).
The other factor is the intercooling system and other airflow restrictions. The boosted air charge will lose some of its maximum pressure as it travels through the intercooling circuit’s heat exchangers. The bottom line is the maximum theoretical boost will not be the pressure of the air that enters the engine. The formulas given, however, provide guidelines for determining the output of a blower and the expected results of pulley changes.
Centrifugal Superchargers: Calculating Maximum Boost
The airflow output of a centrifugal supercharger increases with the square of its impeller speed. That generally means it makes very low boost at low engine speeds and increases with engine speed.
Calculating max boost for a centrifugal blower, at a given RPM level, starts with determining the engine airflow requirement:
D x RPM / 3,456 x 0.9
D is the engine displacement in cubic inches.
RPM is the engine speed.
3,456 is a calculation factor.
0.9 is the estimated volumetric efficiency of the engine without boost.
Let’s assume the calculation is for an LS3 engine, which has a displacement of 376 ci, and we’re calculating for the engine’s performance at 6,000 rpm. The formula works out like this: 376 (D) x 6,000 (RPM) / 3,456 x 0.9 = 587.5. That means the airflow requirement for the engine is 587.5 cfm at 6,000 rpm.
Maximum supercharger boost depends on the airflow capability of the compressor and the displacement of the engine. Because it takes more air to fill the cylinders, the same compressor will produce less boost on a larger-displacement engine.
Next, the max airflow of the supercharger (let’s say 1,000 cfm) is divided by the engine’s airflow requirement, multiplied by 14.7 (atmospheric pressure) and, finally, one “atmosphere” (14.7) is subtracted from the total to arrive at the theoretical max boost for the given engine speed. It looks like this: 1,000 / 587.5 x 14.7 − 14.7 = 10.32 pounds of max boost (0.71 bar) at 6,000 rpm.
Increasing the supercharger’s airflow, moving up from the 1,000-cfm ProCharger C-2 compressor to the 1,500 P-1SC, for example, increases boost at 6,000 rpm to 22.83 pounds of boost (1.57 bar), a 220-percent increase in boost for a 50-percent increase in supercharger airflow.
As with positive-displacement superchargers, the max boost of a centrifugal system is affected by valve overlap and the restriction of the charge-cooling system, as well as other factors, such as ambient air temperature. Similarly, more boost brings more heat, which can lead to detonation without proper tuning considerations.
Music to the Ears?
For many contemplating a supercharger, the sound, or lack thereof, is an important consideration. Whether it’s the whir of a centrifugal’s impeller or the meshing of a set of rotors, superchargers generate sound during operation. Some think it’s noise, while others think it’s music to their ears.
Generally speaking, centrifugal superchargers are noisier. At least, they make more sound than Roots and screw-type blowers at idle and low RPM. The Roots/screw-type compressors are, for the most part, silent at idle.