Can I install a bigger supercharger for more HP?
Not really. The size of your supercharger is not what’s holding you back. For those that just asked the question, we’re done. If you want to know why, read on:
The roots blower found on our cars are very good at moving air, but not very good at producing pressure. In fact the higher the pressure, the less efficient your supercharger becomes. In turbo or centrifugal power adders, the unit can actually become more efficient with boost, but more on that later.
As the pressure differential from the blower inlet to the blower outlet increases, more air begins to seep around the rotors, causing a loss in efficiency. At some point, it actually costs you more power to produce the boost than the HP you gain by the increase in airflow to the engine. For the older Eaton blowers, this point wasn’t much over 10psi. Once Eaton added the Teflon on the rotors around 1995, the efficiency of the blower increased and the maximum usable boost pressure also increased. In 2004 some of the blowers were redone with a new epoxy rotor coating and tighter lobe to case tolerances. This further increased efficiency and became known as the Gen 5blower. This version of the blower is found in the M62 on the 2.0 liter Ecotec. Gains with this latest M62 can be had with boost pressures up to around 17 pounds. Since the M62 is capable of producing over 17 pounds with a small pulley, there isn’t much reason to switch to an M90 blower because the increased boost isn’t going to make more HP.
Are there any gains to be made with a larger blower?
Yes. The larger M90 blower can produce the same boost level and airflow as the M62 while spinning slower. The slower spinning lobes create less heat, which means more efficiency. At the same boost level the M90 should make around 5% more power from that increased efficiency. Whether a 5% gain in HP is worth the upgrade cost, is up to you. We don’t feel it is because of the other alternatives.
What about running two blowers?
If both blowers were attached to the same intake plenum, then you could expect the same type of situation as described above. Two blowers would simply behave like a larger blower and give gains as such. High boost levels would hurt both blowers in the same way that hurts one. There is an alternative that no one has tried yet and that is to feed one blowers outlet to the inlet of the other blower. This would result in possible gains well over 30psi. This would also solve the belt slip issue and give you the opportunity to dual intercool. Of course you’d have to be crazy to go through all the work involved with custom plumbing and mounting but it would be a pretty cool project.
So what about a twin screw blower?
Whipple and Kenne Bell blowers use intermeshing ‘screws’ in a male/female pattern to move air. This design, unlike a roots blower, compresses the air inside of the blower. The design produces a steady flow of air, rather than pulses of air created by an Eaton blower. This lends itself to more efficient operation, especially at the higher boost levels. However, this style of supercharger is not without its down sides. At low boost levels (1-6 psi) the charger doesn’t work quite as well as the roots blower. While under normal driving conditions the twin screw blowers rob slightly more HP than the Eaton blowers. The tolerances of the twin screw are much tighter and more critical. This makes the blower itself heat soak quicker. It also means a much higher failure rate than the nearly indestructible Eaton blowers. The twin screw does offer some HP gains over the Eaton blowers at the same boost levels, but the real magic happens when you want to run boost levels at 15psi or higher. Unfortunately this means belt slippage issues. Most users will find that they have to run wider belts and/or custom tensioners. Depending on boost levels, a twin screw can make 5-20% more HP on Ecotech than a M62.
So what do you do for big power?
In short, run a turbo. Turbos have virtually no limit on boost level or HP level. Running off of the exhaust, they don’t have any belt slip issues either. Not that a turbo doesn’t have any downsides, it’s just that this article was written to answer the larger blower question.