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.

The chart below compares the aggregate wheel torque from dyno charts of (4) BW RS4 K04 equipped B5 S4’s against that of (6) TiAL 770 equipped B5 S4’s.

The intent of the chart is to show how one aspect of the vehicle performance compares between cars equipped with significantly different turbocharger hardware.

The specific attribute this chart addresses is how peaky the engine torque curve is, measured at the wheels on a dyno.  That is, how rapidly torque builds and how rapidly it then drops off.  Between these two turbochargers the 770 is the larger, this normally results in a torque curve that is shifted to the right, and is greater, than a vehicle equipped with a smaller turbocharger, such as the BorgWarner K04.

That trend holds for these two turbochargers whose baseline values are shown with solid lines.

To get an idea of how peaky one turbo is versus the other I have shifted the BW K04 torque curve to the right, and up, transposing the values so that the shape of the curve is retained but the location on the chart is different.

As shown by the dashed red line and solid blue line, the torque curves for the BW K04 and TiAL 770 are quite similar.  Whether one chooses to define this shape as ‘peaky’ or not is something I won’t attempt to answer.

It’s important to emphasize that the two turbochargers would feel quite different from the driver’s seat on account of the location of the curves on the chart.