Information on Superchargers

There are three types of superchargers out there.

Screw
Centrifugal
Roots

And Example of a roots type, like the one in the Bonneville or in the '03-'04 Cobra. It was Patented by the Roots brothers in in the mid 1800'*. It is a positive displacement supercharger. It has an external compression system, so... it acts like an air pump that takes the inlet air charge and compresses it in the cylinder and manifold. It maintains constant boost across all rpm ranges. It only creates boost when the throttle is opened substantially or most likely under a load. And, intake air volume is in direct proportion to engine/blower speed.

The rotors inside typically have three lobes positioned approximately 120 degrees apart centered on each rotor shaft in a helical pattern. This increases the number of pumping cycles, reduces air flow surges, in turn increasing its efficiency.

The rear end plate of the casing houses bearings that support the rotors. the front houses another set of support bearing, houses rotor gear train drive that maintains precise rotor timing also known as rotor phase. these gears are similar to a transmissions as they are helical gears in order to make the gears have a greater contact surface. the gears are operated in an oil bath.

There is actually a safety device in these things, its called a backfire valve. This spring loaded pop off valve is located on the blower housing or in the intake manifold. It opens at a specified pressure to prevent damage to the unit during engine backfire.



As opposed to the roots type supercharger, the screw type is an internal compression blower. It compresses air inside the case as it passes along the rotors. So, the faster the unit spins, the more boost the unit creates. this makes the unit more efficient than the roots type since it creates less heat.



Now, the centrifugal supercharger is also an internal compression supercharger, but... its design is very very similar to a turbocharger except, that it is driven by a belt, not your exhaust. it uses a centrally mounted impeller to compress the air. the faster the impeller spins, the more boost the unit creates...henceforth, it will make maximum boost at maximum engine rpm and minimal boost at minimum engine rpm.. for this reason, this design is popular in traction limited or stock applications where large amounts of low end torque are not needed or desired. This style supercharger has an internal drive unit to provide approximately a 4.5:1 ratio to accomplish high impeller speeds.


Now, blower drive ratio and boost...

the blower may be an under drive, 1:1, or an overdrive ratio Most drive systems incorporate pulley sizes that allow the drive ratio to be altered in steps of about 3%. Typically, overdrive ratios will, increase boost, decrease belt life and draw more power. Under drive ratios typically have the complete opposite effect.

Drive ratio is only one thing that determines boost. the other factors are; engine displacement, clearance inside the unit, valve timing, compression ratio, belt slippage and exhaust design. the best intake system starts with a better exhaust system. The amount of boost that can be safely ran is determined by compression ratio of the engine and gasoline type that will be used. example; to control detonation, an effective compression ratio of 12.5:1 or lower must be obtained when used with 91 octane pump gas.
Effective compression ratio= combines static(mechanical) compression ratio with the amount of supercharger boost. The formula to find this number is as follows....

(boost divided by 14.7)+1)xStatic compression.


For example...a 9:1 compression engine with 10 pounds of boost has an effective compression ratio of 16.0

Other contributing factors to detonation are as follows;

ignition timing
coolant temp.
cooling efficiency of cylinder heads.
Air fuel ratio
intake air temp.
Spark plug heat range
Combustion chamber design
and exhaust back pressure.



The reason why some superchargers are more efficient than others is because of a thing called Adiabatic Efficiency. This means how well a compressor uses the energy put into its drive shaft without heating the air or the unit excessively.

Roots style are about 50% efficient.
Screw style 65%
Centrifugal style 75%
And turbos are as high as 85%

OK... So... What i found is that GM decided to use the M62 on the Bonneville Because the Supercharger was designed for engines from a 2.5 to a 4.0. This is actually the 5th gen. of this supercharger. Then for the series two of the bonnie, GM decided to use the the new M90, which was designed to be used in engines from a 3.0 to a 5.0. The difference in these two is that the M90 has a a bigger air flow capacity than the M62.

Now, I also found out that Eaton superchargers are roots type, but special roots type. the rotors are actually twisted in a 60 degree helical pattern and a special geometry inlet and outlet ports to assist in reducing pressure variations that allows a smooth transition for the air to exit the blower reducing operation noise. This improves its efficiency. And with the helical pattern of the rotors and an axial inlet, the supercharger can be spun up to 14,000 RPM while reducing package size.




So, I hope i kept you entertained and helped you understand superchargers a little better.


Thank you for reading,
Eric.