The split in the middle is for the twin scroll system.
Originally produced to limit the lag on larger diesel turbos, now being adapted for smaller turbos.
To Quote AMS "Separate exhaust gas pulses from each cylinder drive the turbocharger's turbine with the maximum exhaust gas energy possible. This results in quicker turbocharger response and higher horsepower than an equivalent non-twin scroll design."
Best way to describe it is to imagine a hose pipe. Pinch the end and the jet is stronger but flow is reduced. Double the hose ends, same force produced by the jets and double the flow.
On a turbo this would drive the turbine harder and still flow the same gasses.
This type of divided chamber or twin scroll turbine only works if you have the corresponding manifold that separates flow from cylinders 1,2,3 from 4,5, & 6 (and if you look up inside a stock GTS-T manifold you will see that divider, with a small balance cut out, but the principle is there!!)
Allowing the exhaust gasses to come up against a relatively large flat area like this is never a good idea...
If you are brave get out the grinder and radius the top of the divider, then use a normal T4 gasket.
Don't use the specified T4 divided/twin scroll gasket, as you will have a thin piece of metal completely exposed to the full wrath of the hot exhaust fust above your turbine wheel. Even the highest quality gasket is at risk in this exposed way...
But as has been said there is no reason why you cannot use divided chamber turbine housings on ordinary manifolds (with the caveat above), you are just not getting the best out of the design.
thanks for explaining in detail, i'll probably leave it for now then until i can put the T78 that it fits, the Greddy exhaust manifold is just a work of art and i would have loved to see it on the car but on a subtle turbo
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