At just under 3,300 pounds and with an honest 330 hp under the hood, the Z4 M Coupe is already a fast machine. In parts two through four, we improved handling and braking, but now it's time to add more power to this highly competent chassis. In keeping with the original premise, I endeavored to do this without voiding the factory warranty and to back up power improvement claims with further track testing and thorough dyno work.

Considering the M Coupe was launched in 2006, the selection of aftermarket engine performance pieces is limited. Such are the problems of a low-volume car. While working on this part of the project, no software was available, two intakes were known to be in development, there were a couple of exhaust systems just released, the power pulleys from the E46 M3 would likely fit, and a lightweight flywheel-and-clutch combination was on the market. The pulleys and flywheel/clutch combo were ruled out due to potential warranty issues with bolt-on aftermarket parts that rotate with the engine.

So the choice was: which exhaust? Past experience with cat-back systems has shown little or no gain; in some cases, even a loss of power on the dyno. Fortunately, I'd seen excellent gains on an E46 M3 using a full Supersprint exhaust and the company had just released a similar system for the Z4 M Coupe and M Roadster. A discussion with the local BMW dealer regarding the complete replacement of an exhaust system revealed that my new-car warranty would remain intact-excluding, of course, the new exhaust. The full system was therefore ordered, selecting the Powerloop street muffler option to keep sound levels legal.

To fully document the upgrade, I performed some baseline runs. RCTS Canada R&D Inc., in Calgary (a local tuner and Dinan dealer), volunteered its 248C Dynojet for testing. All runs shown here were done with the same climate-controlled dyno, same operator, gear (fourth), fuel (91-octane), new dyno-dedicated air filter (except for the last run described), rear brake setup (Brembo 13.6-inch two-piece rotors) and the same air pressure in the rear tires (36 psi).

By Doug Neilson
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