Because all manner of forced induction use energy to make HP. The turbo uses exhaust and HEAT to spin the turbine. This causes backpressure. A supercharger is driven off of the crankshaft. This causes drag on the engine.

Some of the energy they help create goes to making them do their thing.

 

Good question. I emailed Peter (RICE RACING) about this awhile ago, because I thought how cool would it be to have a quick spooling turbine section, that does not sacrifice VE up top! Here is what he wrote on that subject:

"It is REALLY hard to design the WG to relieve allot of the restriction caused by the turbine and turbine a/r. You need to (to make it work well) have the WG and the entry to the turbine at a "Y" junction if you can follow what I mean?

It cannot be a "T" junction, to overcome a small a/r You need 2X45mm WG HKS are good, Or one large WG 60mm Though these are hard to plumb well to let the exhaust flow efficiently with out causing back pressure to the engine. Cause remember you can measure say 1 bar on the intake manifold but it may be 2 to 4 bar in the exhaust manifold ! As the WG opens based on Intake pressure, NOT exhaust pressure. This is common on cars with small a/r rear housings and Hybrid turbo. Not so critical on cars with small exhaust ports and street port but they still do suffer."

So in theory it could work if the manifold was designed right. Another thing I just thought about, if you do any sort of extreme porting, where there is large overlap and torque peak shifted way up, even if you do get good boost low down, it will hardly be noticeable, because the engine is not even efficient at this point. Look at peters LARGE street port car, it makes 10 psi (I think), by 3k RPMs, but it does not even start getting into it's powerband until around 4500rpms.

Hope that helps, and I hope Peter doesn't mind me quoting him.

 

Thanks 2DoritosOnAStick!!! that certainly makes sense!!!

so it seems that there will be a bit of a trade off due to the difficulty of designing a good enough W/G system...mainly i guess due to space limitations.

 

True (nothing's free in this world!), but a lot of the energy a turbo utilises is waste heat energy, and they require considerably less power than a supercharger to get the same output.