why would Mazda design the exhaust manifold to cut off half of the flow when the engine gets hot... or does this flapper shut off half the exhaust when the engine is cold?

so my question is why is this flapper valve in the manifold? what good does it do my car? would I be better off with it removed like i see on aftermarket manifolds? doe this flapper assist with cold starting? emmissions? etc...

thanks for your help,
TR

 

belay my last..
Called the twin scroll.
Helps low end for quicker spool.

 

ok... so is that flapper open at lower rpm and then closes at higher rpm? or does it have to do with engine heat?

would it help me or hurt me to remove it, and why?

why do all of the aftermarket manifolds I see NOT have this flapper valve even if they are a crappy ssac mani made for the stock turbo?

thank you for your help,
TR

 

The turbo is a dual scroll unit as previously mentioned. At low engine speeds the flap covers the less restrictive secondary scroll (smaller, I know it is counter intuitive) forcing all the exhaust through the more restrictive primary scroll for faster turbo spool.

At higher engine speeds when the turbo is already at full spool the secondary scroll to ease restriction and allow more high rpm power.

In 1989 they changed the design. It was still dual scroll, but exhaust flowed through both scrolls always. They kept each rotors exhaust pulse divided up to the turbo exhuaust wheel and that helped to regain some of the superior boost response and low end torque that the 1987-88 TII had while providing better high end power where the rotary engines naturally make power better anyways.

The 1989+ turbos also have a better dual wastegate design that stops the boost creep with modifications that the 1987-88 TII experiences.

Some people remove the dual scroll flapper and port the secondary scroll inlet larger for better high end power at the expense of low end power.

Works OK, but it is much better to get a '89+ turbo or just 89+ manifold, oil/water fittings and exhaust housing to bolt the identical S4 compressor and rotating assembly to if you are after more power.