If you take a large supercharged engine and make it smaller, but all other components remain the same(heads, supercharger,etc), will the reduction in power be relative to a n.a. engine? Or because of the constant displacement supercharger, will the increase in cylinder pressure compensate for the lack of cubes?
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supercharger relativity?
- Thread starter jstein
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patprimmer
New member
In theory the increase in cylinder pressure will compensate, as the same mass of air is being pumped if it is truely a 100% efficient positive displacement pump, but as the smaller engine will accept less volume of air at the same rpm, the manifold pressure will go up, which will cause more leakage past the blower, so a little less air (depending on clearances) will be pumped.
Presumeing it's a Roots blower, the air leaking past the rotors will be heated by shear, and by conduction from the rotors which will be heated by friction, so the new charge will contain some preheated air as well as ambient, further increasing pressure and/or reducing density.
Also, to pump the same mass of air to a higher pressure will absorb more power to drive the blower, so the sacrificial losses will increase.
On the other end, depending on how you changed the capacity, piston speeds might be reduced, so there might be less losses in the inertia of the reciprocating parts, and less drag from the rings. If the bore is decreased, the rings will be shorter and the pistons lighter again, thereby also reducung inertia and friction.
Bottom line, it's a whole lot of compromises, and all we can do is try to pick the optimum compromise. The knowledge gained by observing successfull examples is a good guide as to how the various pros and cons ballance out
Regards
pat
Presumeing it's a Roots blower, the air leaking past the rotors will be heated by shear, and by conduction from the rotors which will be heated by friction, so the new charge will contain some preheated air as well as ambient, further increasing pressure and/or reducing density.
Also, to pump the same mass of air to a higher pressure will absorb more power to drive the blower, so the sacrificial losses will increase.
On the other end, depending on how you changed the capacity, piston speeds might be reduced, so there might be less losses in the inertia of the reciprocating parts, and less drag from the rings. If the bore is decreased, the rings will be shorter and the pistons lighter again, thereby also reducung inertia and friction.
Bottom line, it's a whole lot of compromises, and all we can do is try to pick the optimum compromise. The knowledge gained by observing successfull examples is a good guide as to how the various pros and cons ballance out
Regards
pat
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