Mods to an actual supercharger

[z-ya]

I think you are correct in saying that a centrifugal supercharger is an easier install, especially from an intercooler point of view. I think that centrifugal superchargers are less popular mainly because of how boost builds with engine RPM. A centrifugal supercharger builds boost at a very linear rate, so full boost is no possible until the engine approaches redline. This is where the peak torque is also. A roots type, or turbocharger for that matter build boost at a very low RPM. Peak torque is achieved at a lower RPM, resulting a nice seat of the pants feel.

 

I've tuned a bunch of 350Zs with centrifugal superchargers, and they make big HP numbers, but don't really set you back in the seat until you reach 6000RPM or so.

[ww]

z-ya, can't that be adjusted by the type and vane angle of the centrifugal supercharger?

 

I assume the high rpm requirement for the 350Z's is the result of them being sized for the 3.5L motor and the desire to maintain efficiency up through the rpm band.

 

By sizing the centrifugal supercharger for low rpm boost and maximum efficiency under 3000 rpm, where the turbo would take over, should minimize the effects of the power delivery.

[z-ya]

It has to do more with the RPM of the centrifugal supercharger at low engine RPMs. Since the centrifugal supercharger is driven by the crankshaft, it's RPM has a fixed relationship to it. The centrifugal supercharger has to be geared so that at redline, its max RPM is not exceeded.

 

With a turbo, you can hit max turbine RPM at low engine RPMs. The waste gate keeps the turbine from spinning at to high an RPM. This is how max boost is achieved at lower RPMs.

 

If a centrifugal supercharger had some way to freewheel (disconnect itself from the crankshaft) when a preset boost level is achieved, a higher crank to supercharger ratio could be used. In this scenario it would build boost more like a turbo.

[RTz]

Another way to say it... the centrifugal SC's boost goes up with the square of its speed. Hence, double the RPM is 4 times the boost. So, if you want max. boost to be 10 psi and max. operating RPM is 7000, then at 3500 you're only getting 2.5 psi. Furthermore, you still have all the parasitic drag at low RPM... without the boost to go with it.

 

I'm with Pete... I don't fancy centrifugal SC's.

[stealthimageZ]

so would that mean then that a centrifugal S/C would be geared toward the higher RPMS as a turbo is? And if the last sentence is true, then it's only reason for it being called a S/C would be because it's driven by a pulley, and it doesn't have the low-end power that a roots style supercharger has?

 

(sorry if I'm asking stupid questions, I'm just tryin 2 get a better understand about it...)

[z-ya]

A centrifugal S/C is like a turbo, but it is driven by a crank pulley rather than exhaust pressure. A centrifugal S/C compressor is very similar in design to a turbochargers compressor. The main difference is that a turbo will produce much more torque at low RPMs because it freewheels, and is not directly coupled to the crankshaft. So peak boost can be made at much lower RPMs than a centrifugal S/C.

[RTz]

so would that mean then that a centrifugal S/C would be geared toward the higher RPMS as a turbo is?

 

As Pete mentioned, a turbo is not mechanically connected. In theory at least, a turbo can make peak boost at a low RPM, and then bleed off excess at higher RPM's. That is the function of the wastegate. At higher RPM, the wastegate reduces the exhaust volume getting to the turbine by bypassing the appropriate amount for the circumstances.

 

And if the last sentence is true, then it's only reason for it being called a S/C would be because it's driven by a pulley,

 

Not entirely. They are ALL "superchargers". Super, in this case, means 'more than'. 'More than' meaning... more air is processed than would be without the 'device'. The term "turbo" implies that the supercharger is driven by a turbine.

[stealthimageZ]

K. Got it! Thnx for answerin the questions. I'm just tryin 2 figure out what I want to use for my future project. So far I've got a few plans and I'm just tryin 2 figure out which one I want to go with in the end.

[(goldfish)]

If only I could find a cheap twin screw. I'd be a happy camper. My limited googling found one that should push 30psi The $4-7K made me sad.