Superchargers

I understand that the recommended size for a supercharger on a 5.0L engine, running 92 octane fuel is a 6/71 blower. I also understand that superchargers are inefficient at relatively high rpms, so:

If you were to stick a supercharger that is bigger than the one recommended, and underdrove it (by using a bigger pulley wheel) to get the same amount of boost that you would from the smaller sc, am I right in thinking that the larger, under-driven unit would work more efficiently at higher rpms? (As it is under-driven, it would be running slower at any given engine rpm than the smaller one giving the same amount of boost)

So noone has a clue? I'm pretty certain what I think is correct, just would like some reassurance.

Yeah, the supercharger will turn slower, but something you have to remember is you will make a lot less power that way. You will move a lot less volume of air at lower RPMs.

Actually your thinking is wrong, although I certainly understand your logic. What you are missing is the volumetric efficiency of such an air pump (which is really what a roots supercharger is, it does not internally compress air), and that higher blower RPM for the most part equals higher volumetric efficiency. Now I'm not all that well versed in the blower design you are talking about, since X-71 roots blowers are about as low tech as you can get on the modern forced induction scale. But with a derivative of this design such as the much more modern Eaton units, higher blower RPM directly equates to higher volumetric and adiabatic (read: thermal) efficiency. This is only true up until a point though, at which time the functional efficiency of the design begins to drop off rapidly and temperatures soar as volumetric efficiency plummets. Even blowers have redlines, and you'd better respect 'em.

So the thing to take away from this is that every air pump and compressor is designed to operate efficiently within a certain rate of flow and RPM. A 6-71 blower, which is theoretically capable of flowing 7 liters of air per revolution (hence the numbering; 6 x 71ci = 426ci, which is 7 liters) can pressurize a 302ci motor (5.0 Ford) to at least 6 psi. As the blower increases in RPM, up to a point it becomes more and more efficient at flowing air. That's why even a large positive displacement air pump can't pressurize the intake manifold much near idle; it's RPM is too low for it to work efficiently. But to complicate matters the engine also has it's own efficiency curve for ingesting and expelling air, and between the two you'll get a given amount of manifold boost that varies with blower and engine RPM. The rub in this is that those very old style X-71 blowers are terribly inefficient at very high RPM, so at first they boost very well but towards the top end require more and more power from the crank for less and less efficient flow, which shows up badly in the engine's power curve.


In the end, the most efficient blower for a 302ci Ford most certainly isn't a 6-71. If you limit yourself to the least efficient possible design perhaps, but if you consider what's available in the aftermarket today, much better blower alternatives are out there. If you're principally looking for high RPM power you should be interested in Vortech or Paxton units, as they are the best possible compressor design for high RPM flow (though they suffer from a serious lack of low RPM boost ability). Otherwise you can look into Eaton units, which are fantastic when looking at power over the entire RPM range so long as you plan on keeping boost below say 10-12 psi. Beyond that you have the Kenne Bell/Whipple alternatives, which are twin screw units (truly named Lyhsolm compressors) that combine most of the advantages of both previously mentioned designs with few drawbacks (this of course comes at a price premium). They will make great boost at virtually any RPM, and will also function very efficiently all the way to redline.

Hope this extremely long post helps explain things, peace.

Originally posted by texan
Actually your thinking is wrong, although I certainly understand your logic. What you are missing is the volumetric efficiency of such an air pump (which is really what a roots supercharger is, it does not internally compress air), and that higher blower RPM for the most part equals higher volumetric efficiency. Now I'm not all that well versed in the blower design you are talking about, since X-71 roots blowers are about as low tech as you can get on the modern forced induction scale. But with a derivative of this design such as the much more modern Eaton units, higher blower RPM directly equates to higher volumetric and adiabatic (read: thermal) efficiency. This is only true up until a point though, at which time the functional efficiency of the design begins to drop off rapidly and temperatures soar as volumetric efficiency plummets. Even blowers have redlines, and you'd better respect 'em.

So the thing to take away from this is that every air pump and compressor is designed to operate efficiently within a certain rate of flow and RPM. A 6-71 blower, which is theoretically capable of flowing 7 liters of air per revolution (hence the numbering; 6 x 71ci = 426ci, which is 7 liters) can pressurize a 302ci motor (5.0 Ford) to at least 6 psi. As the blower increases in RPM, up to a point it becomes more and more efficient at flowing air. That's why even a large positive displacement air pump can't pressurize the intake manifold much near idle; it's RPM is too low for it to work efficiently. But to complicate matters the engine also has it's own efficiency curve for ingesting and expelling air, and between the two you'll get a given amount of manifold boost that varies with blower and engine RPM. The rub in this is that those very old style X-71 blowers are terribly inefficient at very high RPM, so at first they boost very well but towards the top end require more and more power from the crank for less and less efficient flow, which shows up badly in the engine's power curve.


In the end, the most efficient blower for a 302ci Ford most certainly isn't a 6-71. If you limit yourself to the least efficient possible design perhaps, but if you consider what's available in the aftermarket today, much better blower alternatives are out there. If you're principally looking for high RPM power you should be interested in Vortech or Paxton units, as they are the best possible compressor design for high RPM flow (though they suffer from a serious lack of low RPM boost ability). Otherwise you can look into Eaton units, which are fantastic when looking at power over the entire RPM range so long as you plan on keeping boost below say 10-12 psi. Beyond that you have the Kenne Bell/Whipple alternatives, which are twin screw units (truly named Lyhsolm compressors) that combine most of the advantages of both previously mentioned designs with few drawbacks (this of course comes at a price premium). They will make great boost at virtually any RPM, and will also function very efficiently all the way to redline.

Hope this extremely long post helps explain things, peace.

^ Resident Tech Guru ^

:)