OK, KaiserBill. You win. Kinda...
All of this completely tangential talk of volumetric efficiency got me thinking...
KaiserBill may be right, either by coincidence for the wrong reasons, or he could just be bad at putting together a coherent/consistent explanation. I really don't know.
The key to this could be a difference in volumetric efficiency at low rpm. The turbo system will have higher exhaust back pressure. KaiserBill did point this out (a few times I think), but never followed through with the connection that it could be reducing the engine's volumetric efficiency.
Ideally, during the intake/exhaust valve opening overlap, exhaust will completely flow out and be replaced with fresh air from the intake manifold before the exhaust valve closes. Then the intake stroke will continue pulling in more fresh air from the manifold. With more exhaust back pressure, it could resist that complete expulsion of exhaust, and leave some exhaust gasses trailing behind when the exhaust valve closes. That exhaust gas is now taking up space and reduces the amount of new fresh intake air that makes it into the cylinder. The total amount of gasses in the cylinder could end up being the same, but not all of it is new, combustible, fresh air from the intake manifold. This both means less fresh air for combustion (less power), AND less amount of air is consumed from the intake manifold. Less air consumed from the manifold means that air is flowing through the engine at a lower rate, which would be represented as a lower volumetric efficiency. And if the engine is consuming air from the manifold at a lower rate, then air would have to be flowing into the manifold (from the turbo) at a lower rate to maintain the same boost level as system with higher volumetric efficiency.
So there's an explanation of how a turbo and supercharger on the same engine could be operating at the same boost level, same engine speed, same temperature, etc., but the supercharger could be flowing air faster than the turbo and producing more power. I still maintain that it is not the flow capability (CFM rating) of the compressor that causes this difference, but the flow capability of the engine itself. I just previously failed to see that the engine's flow capability could be different between the supercharger and turbo.
I would be surprised to learn that exhaust back pressure from the turbo is enough to account for a 40% difference in power. I still suspect it has more to do with the tune.
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