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Here is a question for everyone... I have a std exhaust manifold off an A15 which is larger than the std A12... Should I use the larger one or stick with the A12 one for the purpose of the project??
There's a lot of theory around this, but if you picture this scenario, it might help you a bit. Any hot gas wants to expand, the moment you let it out into a larger area, that's exactly what it will do, and it will loose a lot of density. Now, if you keep it in the same size hole it was in before, the only place it can expand to is down and out of the manifold. You now keep the velocity and density of the gas, which will help you spin the turbo up much faster.
However, don't think by reducing the diameter you'll amplify the effect, that'll simply choke the engine, and you'll start having pumping loses again.
My advice would be to rather use the A12 manifold, but dowel match it to the head and smooth it out as much as possible.