Here are 2 Dyno charts of my very own s2000 from a setup I had 10+ years ago. The only difference here in there 2 dynos is one is a log manifold and the other is a tubular manifold. Everything is the same. Same boost level same dyno, same exhaust setup.
Log manifold made 335whp on 91 octane at 12psi or boost and 400whp on E85 at 17psi of boost
tubular manifold made 395whp at 12psi on 91 octane and 501whp on e85 at 17psi

 

An an engine power output is directly related to how much air it can take in. The more air it takes in the more fuel can be burned and thus more power can be made. A bigger /more efficient turbo or supercharger with flow more air and do it more efficiently and at cooler air temps vs a smaller turbo/supercharger both giver the same exact boost level.

just look at the old comptech novi 1000 supercharger for the s2000 from 15+years ago. There lucky to make 300whp at 10psi. Compare that to the novi 1200 or even novi 1500 supercharger of today. You will see the bigger super charger makes a hell of a lot more power per pound of boost.

 

Here you changed the exhaust manifold, so you changed the engine flow ability. I completely understand that there can be more air flow (so more power) at the same boost level.

It's like upsizing the air pipings: it allows more flow with the same pressure. This is normal.
But this is not the case I wanted to understand in my first message.

 

Good article to read. May destroy some myths some believe are true.

Supercharger Efficiency Explained | Kenne Bell

-- Chuck

 

This article confirms the first answers of this thread:
For a same engine and exhaust (manifold included) and intake, the main reason why a brand of supercharger would make more or less power than another one at the same boost is it's efficiency (mostly: it's power consumption).

 

Part 1: Mass flow rate
Power is created by mass flow. We're combusting a fixed mass of air with a fixed mass of fuel.
Increased mass flow of air + proportionally increased mass flow of fuel = increased power.
Power adders work by increasing the mass of air (or oxygen) in the cylinder.

Part 2: Volumetric flow rate
Positive displacement compressors/engines, like a reciprocating engine, screw compressor, or rotary lobe/Roots blower, operate on intake volume flow.
Positive displacement means that for a single revolution, a fixed volume of air goes through it. This is roughly true whether that air is 1000 degrees, 10 degrees, 0 psig, or 100 psig. There is some impact of volumetric efficiency (filling of the cylinders), but this does not change this fundamental concept.
PV=nRT governs the basic principles.
Thus, to increase mass flow rate, increase the pressure into this compressor/engine.
10 psig results in a pressure ratio of PR = 24.7 psi /14.7 psia = 1.68.
Assuming that the inlet temperature remains the same and there are no additional losses from any accessories or significant change in engine VE, the theoretical engine power = 240 hp * 1.68 = 403 hp.

Different compressors/blowers have different power levels primarily because of one of the following reasons:
1. They're less efficient at compressing gas. This shows up in higher shaft power for the blower/compressor and increased discharge temperature from the blower/compressor.
2. They have significant accessory losses. For example, if the gearbox in a Paxton blower consumed 20 hp. (a gross example, but OEMs deal with these accessory losses all of the time).
3. They alter the VE of the engine. This may be via altering the tuning of the intake or exhaust. Turbos are notorious for this since they massively impact the filling/scavenging/exhaust cycle of the engine. Larger turbos on tubular manifolds result in much higher VE for the engine. Smaller turbos and log manifolds choke the engine and restrict the VE.

There may be a few things I missed- I was trying to keep this simple and relatively high level.

Tim

Edited: For part 2, what about a centrifugal compressor? Well, the centrifugal compressor changes the relationship between the engine and the compressor, but the engine is still a positive displacement pump. It doesn't care whether it's getting 10 psig from a turbo, centrifugal supercharger, Roots blower, or a leaf blower...

 

So not to confuse all the variables associated with turbocharging, but sticking to the relative question at hand between a Paxton vs Rotrex blower (since all else is the same) the Rotrex blower uses a different planetary gear system inside the blower which is more efficient to push 10psi then it is for a Paxton to push 10psi. There is pros and cons to this. Pros to the Rotrex is its not as big a parasitic loss to the engine, cons is it produces more heat overall with a lower rpm ceiling to produce high levels of boost. Both blowers have a sweet spot/max efficiency window and regardless of their pros/cons, both seem to yield similar power potential at their peak ability, Rotrex just does it at lower pressures. There is going to be some overlap in efficiency as well between the two blowers at different pressures/speeds. But the general nod goes to the Rotrex for overall efficiency. Durability and longevity does not. That goes to the Paxton style.

 

Thank you very much guys for your time and great answers. This is all clear now!

For my particular case: installing an old rotrex c30-94.
My car is ap1 and I just have a decat and hks silent high power dual exhaust. Intake is stock. Will run pump fuel "SP98" (France).

This c30 is rated by rotrex for 435bhp max.
Obviously, I won't reach that even if I am using it to its max rpm (pulleys 125 crank and 105mm charger, should produce approx 10psi, SC max rpm reached at 8800 engine rpm, 2% overspeed if I keep the limiter at 9000 which I am not sure to do...).
The reasons will be: fuel octane limiting timing advance, and oem intake and exhaust manifold limiting the engine flow ability.

 

With that blower at that pressure about 380whp is pretty typical here in the states.

 

380whp means 440+bhp?!
Isn't that a bit high?

The TTS kit I bought used (with the c30-94) made only 376bhp on the previous ap1 it was on. The limiter was at 8500rpm and I know the tune was quite bad: fuel table is like shark teeth et ignition seems like "default" (untuned?!) . I would love to get 400bhp but I doubt about it...

All of you in the US speak in whp and not in bhp. I understand the principle, but I often think the values given seam quite high.
For me, whp is near to 0,85 * bhp.
For stock ap1 that means 240bhp and 204 whp for example.