supercharger (idea)

Historical note: Turbos used to be called turbosuperchargers.

I notice a lot of people seem to be heavily biased against superchargers. There are plenty of writeups extolling the virtues of turbochargers. This writeup, then, will be in defense of the oft-maligned supercharger.

That writeup about turbochargers making more top-end power than superchargers is somewhat misleading. There is a lot of prejudice in the import market against superchargers.

Positive-displacement superchargers, such as the twin-screw Eaton models, will usually make full boost by the time they're somewhere between 1,000 and 1,500RPM, and make constant boost all the way up to redline. This "breaks" at very low and very high RPMs due to volumetric efficiency. The fastest-spooling twin turbo systems (two small turbos rather than one large one; this is done to reduce turbo lag) are lucky to make full boost by 1,800RPM. To illustrate why this is a good thing, take a look at this chart.

Left curve: Roots/Twin-Screw Supercharger
Right curve: Turbocharger or Centrifugal Supercharger
PSI    9|        ______________________
       8|       /             |
       7|      |             /
       6|     /             |  
       5|    | <= Diff =>  /
       4|    |            |
       3|    |          ./
       2|    |       ../
       1|   /    .../
 Ambient|  /  ../
----------------------------------------
RPM     0K  1K  2K  3K  4K  5K  6K  7k

In this example, a supercharger and turbocharger (or a centrifugal supercharger, which has a similar power curve) have been set for a maximum boost of 9PSI. For the supercharger, this is determined by pulley size: smaller pulleys = faster rotation = higher boost. For a turbocharger, this is accomplished by a wastegate. They both achieve the same maximum boost level. However, the turbocharger does NOTHING for low-end torque. If you are looking for a mod that will improve driveability, and you have a four-cylinder engine, and both supercharger and turbocharger kits are available for your engine, you're cheating yourself if you don't evaluate the supercharger. The reason? Four-bangers are notoriously lacking in torque, especially at low revs. Superchargers do a lot for engine torque down low when it's really needed. Turbos don't.

The big argument a turbo snob will use is that turbos use "wasted" power from an engine, whereas superchargers have to tap the crank for their motive force. What they don't mention is that turbochargers raise the backpressure inside the cylinders. A *little* backpressure is good, as it helps with the scavenging effect that helps suck exhaust out of the tailpipe, but turbochargers contribute more than is actually helpful. The hot wheel turbine of a turbocharger slows down the exhaust gases (you didn't think kinetic energy was free, did you?), meaning that the engine has to work all the harder to complete its breathing cycle.

What they should really say is, "I like how it sounds when it spools up, and I like the way the blow-off valve sounds when I shift." Superchargers are usually a bit quieter than turbochargers, and don't usually need a blow-off valve. Instead, they use a bypass valve to zero out boost when the engine is at cruise or idle.

Turbo snobs often say, "Turbos make more peak power than superchargers." Refer to the chart above. What good is that if your car is still gutless below 3,000RPM? This argument is also silly because you can tune an Eaton supercharger to run up to 30PSI. Parasitic power loss in an Eaton supercharger is usually around one horsepower. BFD.

Speaking of 30PSI, that's what the top-fuel dragsters run. And it comes from a supercharger, not a turbocharger. Know why? Top-fuel dragsters can't afford to have extra-restrictive exhaust systems.

Detonation, or pre-ignition, can be a problem with both superchargers and turbochargers. The solution is to either make a blower that doesn't add as much heat (like the twin-screw Eaton models), or use an intercooler. It adds some cost, but it is well-proven. Another way to handle detonation is to use higher-octane gas, but that may require that the spark timing be advanced.

Last, superchargers don't have to be "warmed up" or "cooled down," and many of them feature self-contained lubrication systems with oil that doesn't have to be changed for up to 100,000 miles of use. Turbochargers generally have to share oil with the engine, and many of them will die prematurely if they aren't given some time to warm up (engine exhaust gas is HOT!) and some time to cool down (rapid contraction is hard on the components).