With each exhaust port inlet to the manifold, the main chamber continuously gets bigger, allowing for the added volume, but also to keep thermal velocity up as well. The expansion of the exhaust gasses through the manifold and up-pipe is what drives the turbine side of the turbo charger. The faster you can keep the velocity of the gas, the easier it is to spool the turbocharger. Thermal barriers play a part in this as well.
Due to the material having a high ability to retain heat, the heat remains inside the manifold instead of being expelled. The hot exhaust gasses are still expanding as they exit the engine, and it is ideal to get those gasses into the turbocharger while theyre still expanding as this will create more action on the turbine wheel & result in a faster spool-up. This is one huge advantage to using a set of well-engineered exhaust manifolds over headers, which increase the time it takes the exhaust to reach the turbocharger due to the extra length added to the tubes in order to create equal-length pipes; the result of is the gasses are cooling and therefore contracting by the time they reach the turbine wheel, reducing the action created.
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