See how a set of aftermarket exhaust manifolds from a reputable manufacturer would fare against the Audi B5 S4 stock exhaust manifolds.
Discounting all of the advertising hype that AWE heaps upon their tubular exhaust manifolds, I was still interested in seeing how the pressure drop through the AWE manifolds would compare with the stockers.
The AWE manifolds take an interesting approach, the primary runners are only about 4 mm larger in diameter than stock, about 15% more cross sectional area.
The inlet to the primaries is larger than stock, which should make them closer to being ‘gasket’ matched to the exhaust manifold gaskets.
The secondary runner is about 14mm larger, which results in about a 50% greater cross sectional area than stock.
The stock exhaust manifolds maintain roughly the same cross sectional area through the entire exhaust manifold, except for the last few millimeters at the outlet.
The turbine inlet for the BorgWarner K04 turbocharger is 40.3 mm across, a very close match to the stock exhaust manifold.
Even though the stock exhaust manifold outlet matches the K04 turbine housing inlet more closely than the AWE exhaust manifolds, the rapid increase in cross section out of the stock manifold, followed by the necking down inside the turbine housing, is probably less than ideal.
The AWE outlet has roughly 3 mm less cross section at the outlet edge versus the diameter of the turbine inlet, so that the exhaust gas is traveling into a slightly larger cross sectional area, the preferred transition.
The AWE exhaust manifold was attached to the flowbench in the same manner as the stock exhaust manifold had been, via an adapter that simulates the air exiting the cylinder head. The runners that were not being tested were blocked off to prevent air from escaping out of them.
Testing of the article was done up to a depression of 28″ of H2O.
Shown below are the results of the testing of each individual runner for the stock Audi B5 S4 and the AWE tubular exhaust manifolds.
The AWE-Tuning tubular exhaust manifold clearly flows a significant amount more air through each primary runner for the same pressure drop as compared to the stock exhaust manifold.
What this means for performance on the engine is not able to be determined from these results.