S4 turbo or S5 turbo rotors = more power? every other variable remains constant
 

Just wondering what you guys think. If you were to run the same amount of boost, and use the EXACT same engine setup (and boost levels) what would make more power?

I have a pretty good understanding and I think that the S4 turbo rotors would make more power due to more air/fuel (boost) being able to go through the engine at any given time due to the larger rotor dishes.

On the other hand the S5 turbo rotors are lighter and have a slightly higher compression which may make a bit more power as well.

So would the S5 rotors just make more low end power, and the S4 rotors make more when it hits boost?

Thanks for the input.
Dan_s_young

I made more power with the S5 rotors. Partly due to the fact that they can extract more energy from every combustion cycle. You can run more boost to compensate for the loss with the S4 rotors. BUT you said all else being equal.

Higher compression on the S5 rotors would mean more power.

S5 turbo builds better boost due to the turbine design...both low end and high end, although the maximum power would still be very close.
This theory is just due to the fact that the S5 turbo turbine has two (large) equal runners versus the S4 which has one large and one small runner...

Spark plug locations are different on the rotor housings, but I dunno how much of a difference this makes, cause you need to compare the ignition timing on both (stock ECU?).


-Ted

I don't understand how you figure there is more energy from every combustion cycle with a higher compression engine. Since the S4 rotors have a larger dish you should be able to put more air and fuel through the engine at any given time therefore producing more power.

Yea Ted I know that there are other items that change the power levels. What I mean is say both setups are a complete S5 block, s5 intake, s5 turbo, just changing the internals from S5 to S4.

Just from my understanding of forced induction you always want a lower compression setup for forced induction. More air, more fuel = more boom. Still wondering if the 0.5:1 compression difference between the S4 TII and S5 TII rotors is enough to make a substantial difference or not.

You said everything else remaining constant. :) The higher the compression ratio, the more heat, pressure, and energy is created per stroke.

Why? Why always want a lower compression setup for FI? The main reason people run lower comp rotors is because it yields a safety factor, all else remaining the same, when running heavy loads. But, the dirty little secret to this has to do with the volatility of the fuel that's being used. Use a fuel that's designed for heavier loads (slower burn rate, higher auto-ignition temperature, colder combustion temperature) then you can use a higher compression ratio.

There is a substantial enough difference with the 1/2pt increase in compression ratio.

B

What BDC said. All else equal, the higher compression will yield more power and quicker spool along with crisper off boost throttle response, the lower comprerssion will yield a safty margin. It depends on your goals as for power, power delivery, intended car use, and a ton of other factors such as turbo size and manifolds. Either CR rotor could yeild simular results, its all in the tuning and supporting mods.

~Mike.............

Right, but it's the type of fuel that's used as the primary determining factor. Tuning, support mods, etc. are secondary.

B

thats when alky comes into play right BDC ;)

are you wishing you put S5 rotors in now Dan?