Comparison of turbo to supercharger

[Barge]

With all this talk of "boost" and very little talk of CFM I would say that it's a pretty fundamental difference between a centrifigal supercharger and a turbo.

With a turbocharger in a way you are trying to control how much pressure it creates which in tern defines how much cfm the engine will try to use.

With a supercharge you are saying that at this RPM i'm going to spin the supercharger which will produce this much cfm of air. The boost pressure itself is really an aftereffect.

So you're kind of doing the same thing in 2 different ways.

Also note that most turbochargers can build boost pressure (whatever) earlier in the powerband because most of them have turbine much smaller than ideal.. and by ideal I mean making max power at max rpm. Hence the need for a wastegate.

You could make a turbocharger act like a centrifigal supercharger by simply putting on a bigger and bigger hot side until it produced the boost/cfm you are looking for at max rpm w/o diverting any exhaust gas w/ the wastegate.

With a supercharge w/o some crazy gearchanger system or something you have to pick one point where you want the supercharger to excel at because otherwise you have no way of changing the supercharger RPM independent of the engine RPM.

Now if you developed a centrifigal supercharger with a cvd type transmission on it then you could be all sorts of cool. Seems like someone would have done that by now.

I would say that at a singular point in time with the supercharger and turbocharger setup to make max power at the same RPM the supercharger theoretically might actually be better because you would have a much freer flowing exhaust... i guess at that point you'd then have a study of exhaust backpressure vs. mechanical losses.

Just to comment on the bleeding of supercharger boost: You would have to either be using a blow through system or you would have to recirculate that air. The other consideration is that by doing so you are basically making the supercharger work harder for nothing.

 

[warmonger]

Of course the two are two sides to the same coin However we refer to boost to keep it simpler because everything is calibrated that way, not by cfm. cfm changes with air density whereas the pressure is something we are trying to hold constant. Pressure is really the only variable we can hold constant in this equation.
Starting with a pressure ratio gives us a good idea of what kind of compression ratio we get in the cylinder, therefore it tells us the max pressures we might see in the cylinders and we can use that to determine maximums. Especially when you have temperature changes introduced by different ambient temps. Cold day your cfm is way up, hot day it is way down yet pressure is the same on both days.

So I don't think it is really wrong for us to talk in terms of boost from one system to another.

On the other hand when we start talking about intake piping changes, manifolds, cams, or displacement changes then we need to talk about changes in CFM. When we talk about turbo sizing we must refer to the pressure first, then the cfm we can deliver at that pressure must match the engine.

Last, your comments about turbos only needing wastegates because they are not sized 'ideally', is really a bit of a stretch to me. I see where you are coming from but I don't think it is worded for 'everyone'. I already identified earlier on that a turbo can be sized for linear increases in power or anywhere in between. I think 'ideal' has to be determined by the criterion you set when you begin to size your unit.
It is ONLY related to what rpm you want to achieve max pressure by, and how wide you want that power band with a constant pressure to be. There will be compromises but essentially the turbo/compressor are ideally sized when the power band you are after is achieved, and the backpressure gained by maximum rpm is within acceptable levels.

 

[TourDeForce]

Been a great discussion, thanks to everybody. I was aprehensive initially, but ultimately I'm glad we all got into this thread. Bottom line for me: I have much more learning to do, and this thread is a very good springboard into the direction I need to take. I just hope I haven't been too much of a drag on things...

 

[muntus]

As far as both systems are concerned, I have a question about the UIM.

The SVT UIM(as opposed to the non-SVT) flows better without boost. I assume, of course that it flows better under boost. Would a supercharger out-perform a turbocharger with a restrictive intake(or vice versa)?

What would be the power increase with the SVT intake over the non-SVT on both systems(if there is a difference with either one)?

 

[BurritaSVT]

Well muntus I will put this in terms of CFM versus PSI. The non-svt will have smaller channels thus making the torque down low and mid range higher than the SVT along with the SE cams too but choke down some at the top end. The CFM will be the same at low psi but as the psi increases the restriction will increase thus making the turbo or supercharger work harder to maintain the PSI. The supercharger is driven by belt and the turbo driven by wasted gas. So the only big difference is that the turbo can be adjusted easier by turning up a boost controller to get more gains to overcome the restriction. The supercharger will not have that ease of adjustment. They both will take power from the motor but the turbo will see less loss since the turbo requires little effort to turn the turbine. If you have any restriction like FMIC then the SC will drop PSI the motor sees and will need a resized pulley and the turbo can just increase the psi by wasteegate once again maintaing the manifold pressure. Joey

 

[warmonger]

Yes. But lets be fair about it and point out that raising the boost on a turbo car hits a wall, a point of diminishing returns. Assuming your car can handle 20psi and you are running into serious pressure drop across the intake at 15psi, you may have trouble just raising the pressure up to gain cfm. This is where Barge's comments come in. Just because you raise the pressure doesn't mean you are getting any more cfm into the engine and it also causes issues.
As you raise it the exhaust pressure goes up along with the heat. This causes the pressure to back up into the cylinders, meaning less intake charge goes in and making the combustion temps run hotter. The compressor is now fighting the intake pressure restrictions plus the excess hot exhaust still left in the cylinder and it will eventually stall out, or become so inefficient that intake temps skyrocket and you basically blow your motor.

With the supercharger you don't have any of those issues by raising the boost. The exhaust stays pretty much free flowing...up to a point but like anything the more intake volume then the more exhaust volume so it will have its limits.
As pointed out by Joey, the SC must change the pulley ratio to increase the boost pressure/provide more cfm but there is also a point when the SC compressor will stall out due to restriction. That is on top of the belt grip issues discussed earlier, and of course the solid mechanical coupling sucking more power from the engine the faster it spins and the more pressure it tries to build.

Neither one is a "free lunch", but IMO when sized correctly the turbo is the closest thing to "low-income lunch."