I've heard about twin chamber scroll, and how that's the type of turbochargers the TII's have. What is it, though? What does it do?

 

A twin-scroll turbo has two seperate exhaust paths that aim the exhaust gases at the turbine wheel at two different angles. I don't know the exact science behind it, but it improves the turbo's responsiveness. Many modern turbo'd cars use twin-scroll turbos, but the FC was one of the first.

The S4 uses collected manifold that has a flap that can shut off one of the scrolls at low revs. This forces all of the engine's exhaust gases down one scroll, increasing it's velocity significantly and hence spinning the turbo up faster. At 2700rpm the flap opens and the manifold feeds both scrolls.

The S5 uses a fully divided manifold instead, so each rotor is dedicated to one scroll only.

 

that's really cool.

 

Sort of. In a stock setup, it works fine. But once you start to mod, the twin scroll becomes a restriction.

 

You mean the S4's flap becomes a restriction, not the twin-scroll arrangement itself.

 

probably not the best way to go about it though. The S5's didn't use it, and I really haven't heard of any other cars using a twin-scroll system.
My MR2 uses a similar system to the S5's, only instead of each rotor firing through a separate hole, Cylinders 1+4 fire through one, and Cylinders 2+3 fire through the other. I guess this is the "Twin Entry Turbo" system stamped on the intake manifold.

 

Read my post a bove, the S5 turbo is twin-scroll! Twin-scroll turbos are more common than you realise.

 

This is the description from the book RX8 by Jack Yamaguchi, p146.

"13B Independent Twin-scroll Turbo (1989-1991). A new turbocharger, called independent twin-scroll turbocharger, replaced the twin-scroll instrument. The new turbocharger retained two separate scroll areas, but minus a trap-door valve. Each of the independent scrolls received exhaust gas from one of the two rotor housings. Mazda's rotary engineers discovered that by completely separating the exhaust gas passages all the way to the turbocharger's entry area, no energy loss was incurred due to exhaut gas pressure interference between the two rotor chambers, thus exhaust gases would strike the turbine blades with more force."

 

Yes. The twin scroll (aka split scroll) design is nothing new. The main concept is to separate the exhaust pulses for the reasons listed above, and also to allow engines with few exhaust pulses per revolution (2-rotor, 4-cylinder, etc.) to maintain a maximum exhaust gas velocity to the turbine which results in faster spooling time at lower rpm's. The advantage of the twin scroll design is not as significant at higher rpm's and on engines with many exhaust pulses per revolution (V8, V12, etc.). Think of it as having two scrolls with a low AR ratio as opposed to one scroll with a large AR ratio.

The 87-88 twin scroll design was unique in that it had the flap that shut off one scroll at lower rpm's. Unfortunately, it didn't work as well as a conventional twin scroll design with a divided manifold, which Mazda used on the 89-91 cars.

 

Is the "no-flap" design on the s5 responsible for the extra 14hp difference?

 

The difference is 20hp on the turbo models, and I think the answer to that is no it is not responsible for the increase. The more important question is why it is only 20hp increase. Boost raised from 5.5 to 7.5, Higher compression rotors, lighter rotors, Better intake??? and hp went fromm 182-202. Seems like it should have been 210-215 with those increases.

 

arent the S5's ports bigger too?

 

Absolute manifold pressure only increased 10%, but the actual resulting power increase would be less than that.
8.5:1 to 9.0:1 is a 6% increase, but again the power increase you get from that is less.
That does't increse power, it's just like adding a lighter flywheel (S5 got that too).
You mean the AFM or manifold? The AFM has been proven to make practically zero difference, but the manifold runners are bigger. I've never seen any back-to-back comparisons of the intake manifolds but I doubt it would be huge improvement.

All up maybe it sounds like it could've been a bigger increase, but not much. The S5 might've had a slightly milder state of tune (fuel and ignition), so that there were improvements in economy and emissions as well as power.

 

On the S4 system, when removing the rats nest, is it problematic to remove this system and the solenoid that goes with it, or should it be kept by some means? Not that it would be a major issue to keep, but I just want to get a bit of feedback as to what the actual, real world drawbacks and/or benefits would be to getting rid of the system...

 

I plan to keep it when I remove the other solenoids (easy to do like you said), but if you want to know what it's like without it working simply unplug the solenoid and drive about for a while. The actuator will hold the flap open and you'll be able to decide if the increase in lag is acceptable to you.

Personally I feel it's pointless to remove the solenoid if you're not going to remove the flap too. It's like wiring NA aux ports open. Low-end losses, but no gains.

 

OK, so it keeps the port open without the solenoid...yeah, I guess it's worth keeping since I'm not yet up for pulling the exhaust mani and getting rid of the whole mechanism...