Ok there are a few thing that are not being said here that I feel should be.
There are three different kinds of superchargers and two different kinds of turbochargers.
Two of them have been covered. The roots type and the centrifugal. Roots do NOT compress air but they do give a constant pressure to the engine. A centrifugal basically is just like a belt driven turbocharger. There are two types of turbochargers. One is the average with a max fixed PSI and one is a variable pressure turbo. The variable pressure turbo is better the a regular turbo, but for instant horsepower nothing beats a roots type super charger or the third type that has not been mentioned yet. This is the twin-screw supercharger. A twin-screw supercharger improves on the roots supercharger by putting "fins" on top of the rotors. This will compress the air giving you boost. Twin-screw superchargers offer extremely high amounts of volumetric efficiency, as high as 95%. It also gives a very high amount of heat efficiency for a supercharger, as high as 80%.
If you add a supercharger or turbocharger to an engine, there will be a noticeable response after there is an additional 3 psi going into the engine. This will give u about 1/5 horsepower increase. For a turbocharger to do this it will take .4-.5 seconds. A supercharger will take .1-.2 seconds to reach 3-4 psi.
All in all, it depends if you have a long revving large displacement engine or a higher revving, usually smaller displacement, engine as to what will affect your horsepower and torque more to your taste. I am biased toward large V-8's. I drive a 1989 5.0 Lincoln Towncar. One other thing I think I should mention here is the fact there is an equation for how about much horsepower a person can get by adding a supercharger or turbocharger.
The equation is as follows
original horsepower times pressure ratio times corrected thermal efficiency times volumetric efficiency. From this subtract the power needed to drive the mechanism. to figure out the pressure ratio, take 14.7 and add the number of psi the turbo gives. Then divide this by 14.7. The ratio for the purpose here will be 1.05. A turbo, on a redline of 7000 rpm, usually takes 2 horsepower and can take as much as 10 horsepower, depending how efficient the exhaust is. At 7000 RPM the average supercharger takes 11 horsepower.
A roots corrected efficiency is 85%, so it would be .85 in the equation
A twin-screw has an efficiency is 88%, so it will be .88 in the equation.
A turbo has 89% efficiency, so it would be .89 in the equation.
I do not know what it is for a centrifugal supercharger.
I will give an example for all three types.
They will all have a 6 psi boost and 140 horsepower base
Roots:
(140)x(1.41)x(.85)x(1.05)-11=165
Twin-screw:
(140)x(1.41)x(.88)x(1.05)-11=172
Turbo:
(140)x(1.41)x(.89)x(1.05)-2=184
I must give credit to the magazine Hot Compacts and Imports for most of this information. Some of this information is from Whipple's web site. I have a few opinions which I expressed but nothing to affect the info. Talk to me later if you want more info than this because I probably can get it.
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