944J

Turbo vs Supercharger

I have the books on this stuff by Bell and it seems like you should be able to get almost equal performance from an supercharger.

But can anyone tell me from experience why you can't get the huge HP that you get from a turbo on a 951 engine by using a supercharger on the same 951 engine?

It seems like the SC kits around are good but they are way off on the final rear wheel torque and hp.

I'm talking about using the same engine for each setup: 951 2.5l, low compression, intercooler, etc.

IMB951

I've never heard of a supercharger being used on a 951 engine. Care to show me one?

Fishey

Turbo will have more HP and TQ...

Geo

OK, first of all, turbos are much more efficient. They will produce more boost with less increase in temperatures at the discharge. And while a supercharger will produce some positive boost off idle, a properly sized turbo will build boost much faster.

Dave951M

Turbo is more efficient, sc has parasitic drag. Turbo is technologically simpler than the mechanically driven sc, but both have to be set up properly to keep the engine together. Either can make big hp numbers, but like I said, it's all in the setup.

Jakerx

i'm impressed. another turbo vs. supercharger thread and no one's flamed the starter

but yeah if i were to do something with any car i would go turbo over a supercharger. yeah there's the issue of lag and what not, but i like the way dave put it for the sc "parasitic drag." it's all in the way it's set up and tuned.

Danno

Another way to say it is for the same boost, say 15psi, the turbo car will have more power than the supercharged car.

BruceWard

All of the supercharger kits I have seen are designed to be bolted on without changing the internal components or compression ratio to be fitted to a normally aspirated engine.

If you were to fit a supercharger to a 951 engine and it produced the same amount of boost as the 951 turbo then you should get a similar amount of power.

The SFR supercharger for the 968 is only running aroudn 6 PSI of boost to produce ~ 350 BHP.

Danno

"If you were to fit a supercharger to a 951 engine and it produced the same amount of boost as the 951 turbo then you should get a similar amount of power. "

Well, the supercharged 951 will get less power at the same boost as a turbo 951 for two reasons. One, the outlet air-temps will be higher on the supercharger. AND about 20hp will be consumed in driving the supercharger. So to get equivablent HP as a turbo 951, the supercharged one will have to run 10-15% more boost.

BC

I disagree on the outlet temp statements.

ANyone care to prove the outlet temp statements with factual figures? I'd say that the outlet temps on a CENTRIFUGAL SC will be lower then out of a turbo.

Danno

Here's an article on the testing guidelines for superchargers: CAPA Performance-On the Subject of Supercharger Testing and Compressor Efficiency

Superchargers, like the very popular centrifugal Vortech, typically reach a maximum of 70-72% efficiency in their sweet-spot. And they can only maintain that efficiency at very, very low boost levels of up to 1.6 bar (8.7psi). At the more common 15psi of boost used, they drop to 65% efficiency.


Compare to the popular Garrett TO4E turbos, which are in the 78% efficiency range. At 15psi, it is still in the 78% efficiency range. Actually, it can go all the way up to 21.8psi and still be 78% efficient. And the new GT-series GT35R is about 79% efficient. And it can hold that efficiency up to 2.5-bar or 21.8psi of boost! At that boost-level, the Vortech has fallen completely off it's compressor-map!

Z

It really depends on the specific application and intended use of the vehicle. While not as high as the efficiency map of the turbo posted above, there are superchargers that are more efficient at higher boost levels than the map of the Vortech posted above.

The maps are generated on a test stand of some type, and don't take into account some factors present in the actual operation on the engine in a vehicle. The drag of the supercharger would be measured on the test stand, as would be the drag of the turbo. The loss due to the exhaust backpressure to the engine with the turbo would not be taken into account on the test stand though. Every turbo owner knows their boost pressure, but how many know their exhaust backpressure that the engine is seeing and has to work against to pump exhaust out?

The potential effects of added heat from the plumbing routing and turbo being confined in the engine compartment wouldn't be considered in the maps either. I'm sure everybody's seen pictures of turbos glowing red hot. Anybody ever seen a supercharger like that?

The maps also doesn't take into account lag, but how important that is again depends on the specific application, what the specific goals are, and how the vehicle will be used. With real estate it's location, location, location. With cars it's application, application, application. An awful lot of people don't consider that. To get the best setup, you have to realisticly decide exactly what you want the car to do, and how and where you're going to drive it before you make your decision as to how to set it up.

944S2NUT

Here is my buddies old 944 S/C. This car no longer exsists . Couldn't get the S/C to work right so it was scrapped.

http://gallery.rennlist.com/gallery/album216

Danno

A roots supercharger is completely different from a centrifugal one. Positive-displacement superchargers do have the advantage down low. But that article is taking it to an absurd extreme to prove a point. Where is it from anyway? Who ever drives their car starting at 1100rpm, except in 1st gear anyway? Also if you take the derivative (slope) of the 2nd chart to get acceleration, you'll see that at 2000rpm+, the turbo is accelerating faster than the supercharger. It would be more representative of real-world driving situations to start there than at 1100rpm.

You're right about the specific application and one has to judge the intended usage of the car. If you want modest gains down low, like a 300-350rwhp boosted 944, or a 400-450rwhp 928, then a roots SC would probably be better for all-around daily-driver. Centrifugal superchargers on 944 hasn't shown much promise, but that probably has more to do with the configuration and tuning. I'm actually investing a roots charger at the moment.

However, if you're into higher HP figures for road-racing, then a turbo would be better. Like a 450rwhp+ 944 or 600rhp+ 928. You'll get the highest power-output where you'll be spending most of the time, near redline.

Geo

Danno is 100% correct. A properly sized turbo is much more efficient than a properly sized supercharger. All that other stuff about backpressure, etc. is a red herring. You have backpressure with a supercharger as well. Properly sized, you will always have more power potential with a turbo.

As for the discharge temps, it's a FACT that discharge temps are higher, boost for boost with a supercharger. Efficiency of a turbo or supercharger is defined by how much the compressor heats the air in compressing it.

Also, that data from Porsche is probably ancient. I didn't see a date on it. Modern turbos perform significantly better. A Roots type supercharger is the least efficient.