With the affect of the AWE-Tuning tubular exhaust manifolds on boost onset measured I moved on to recording the pre-turbine exhaust pressure.
These readings confirmed what I suspected, that the pressure near the turbine housing inlet would be greater with the AWE manifolds installed.
The reason the pressure is increasing is that the AWE manifolds create less of a restriction in the exhaust path.
Using the picture above to illustrate what is taking place, the pressure in the cylinder following the combustion process is high (P1), and unaffected by the exhaust manifold. As the gasses travel through the manifold friction and changes in direction cause the pressure to drop, so that when the gas reaches the inlet to the turbine housing, the gas pressure is lower (P2).
When the stock exhaust manifolds are used there is a greater loss of pressure, and since the pressure at P1 is the same regardless of the exhaust manifold, then the pressure at P2 will be lower with the stock exhaust manifold than with the AWE manifold.
Looking at it from a different angle, if the pressure a P2 is some value with the stock manifold, when the AWE manifold is installed the value of the pressure, P2, should be greater, which is what is shown by the chart.
What remains to be seen is how much, if any, affect this reduction in exhaust manifold pressure loss has on the engine’s volumetric efficiency. Hopefully the DynoJet will be able to answer that question.