This will be a write up explaining the issues with using Individual Throttle Bodies (I.T.B.’s) with a Positive Displacement Supercharger (PDS) such as a Roots or Twin Screw.
As the name implies, A PDS Moves specific amount of air per rotation. A 2 liter supercharger Will move two liters of air for every 360 degrees of rotation. As long as the inlet is open. 3L engine at 1000 RPMs in theory consumes 1.5 L Of air. A 2L supercharger, with a drive ratio of 3 to 1 over driven will produce 6 liters of air. at the same 1000 engine RPM.
Neither the engine or the supercharger are 100% efficient at doing their job. The engine has restrictions in the intake, intake ports valves and so on. The supercharger also has restrictions on the inlet and output sides as well as internal leakage. The rotors are not 100% sealed to each other or to the housing.
The next problem that comes In is thermodynamics. Anytime you Compress air, it will heat it up. The more you try to squeeze the air molecules together the more heat they release. The Rotors Of the supercharger also increase the heat of the air by friction. At wide open throttle and maximum power, this works just fine. The supercharger is moving as much air as it can forcing it into the engine with the ITB’s wide open. But when you close the throttle at higher RPM’s, the supercharger is still going to try and force air into the engine. Some would think using a very large bypass valve would solve this problem. And to a certain degree it does.
But now you are just recirculating a very high volume of air through the supercharger and through the rotors. Which will cause the air to heat up even hotter. That will cause the supercharger temperature to rise, and have a continuous cycle of adding more and more heat. The way a lot of OEM’s have alleviated this problem, is to have the throttle body on the inlet of the supercharger. What this does is restricts the air going into the supercharger so that it is in a vacuum. This limits the amount of Air in the supercharger so that the heat will not increase, and with the rotors
being in a vacuum, they spin very freely and have very low Friction and low parasitic drag.
Now in conclusion you can use ITB’s. But you would have to add a main throttle body to the inlet of the supercharger. To prevent the supercharger from building boost when you don’t want it or need it. This will help keep charge temperatures down and increase efficiency. You don’t want the supercharger trying to make 16 pounds of boost Or more while you’re just cruising on the highway at 60 mph.
Article written by our Chief Engineer, Keith Caldwell of K.C. Design Concepts.
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