SPD Tuning Service offers an exhaust kit for use with Strömung dual or single tip stainless mufflers or the Prodrive Oval tip stainless muffler. Of the three muffler choices the Prodrive is the quite one of the bunch with a deep mellow tone that just lets you hear to shift the transmission. The Strömung units are not by any means loud, as they are still quite legal sound levels, but give you a deeper, fuller sound the cockpit as you run through the gears. These are quite popular! Just the right sound.

With Strömung units, the rear section kit alone will provide approximately 20 ft-lbs torque improvement from 3200 rpm, about 3-400 rpm earlier spool-up than the stock exhaust. The horse power gain is about 10-12hp all the way to 7000 rpm. These are significant numbers, especially considering the motor will not code or otherwise run improperly. It really brightens up the motor.

We use a 2.5 inch diameter pipe that is capable of flowing 300hp. We could use three inch, but it is not wanted a normal road car setup.  We found that the earlier spool-up of the turbo was lost with 3 inch pipe without any gain on the top end. This is a serious flaw to using 3 inch pipe. We give away too much power below 4500 rpm for no additional gain on the top end.

Just like when we choose to run the RS 2.5 motor on 2.25 inch pipe, we have selected 2.5 inch as a  practical pipe size for the US WRX engine that is known to do the job. We offer smooth mandrel bent mid and front pipe sections, the latter eliminating the third cat for track sessions. Combined with the higher muffler flow, it also aids the top end power. It is a very simple solution. By lowering the pressure after the turbocharger we are able to get a larger pressure delta over the turbine blades giving more power. The front cat pipe improves power from 6000-7000 range, but it is keeping the 2.5 diameter and the change of the rear pipe that provides the low end torque. 

The main CAT that you can see off the turbo is not a restriction under 300hp, so we ask everyone a) leave it on, and 2) do not lust after a larger unit as it is not a problem.  The same CAT we have is on the 280hp STI Japan cars. Leave it there. If we had 100 octane gas I would be selling larger cats all day long! But our gas flow at 270hp is not critical with the turbo main CAT in place. We do offer a larger capacity main Cat down pipe if needed, but its primary function is to withstand the higher EGT (exhaust gas tempreture) we see at 270-280hp.

We have removed the front pre-cat and found the motor makes slightly more power, but needs to have a new ECU setup for the engine to run properly. When combined with an all stainless main cat and Prodrive ECU kit these parts take the engine as far as we like to see it go with cast pistons. This issue is discussed in greater detail elsewhere in the site (Big Power) (WRX Engine)

Again - we are discussing modifications for which we expect to use 91/93 octane gas that will limit the results of further modification to the engine and turbo system. We are tuning for complete reliability, yet are committed to noticeable improvements only.