As a precursor to the official baseline that will be used for later comparisons, I ran the M Coupe with its stock rear wheels. The OEM rear wheel/tire combination (9x18 double five-spokes and 255/40 tires) weighed about four pounds more than the upsized aftermarket set fitted in part two, so a measurable difference was expected. This pre-baseline run is shown on the graph as a black line with a peak of 253 wheel-hp and 215 lb-ft of torque. The aftermarket Forgeline ZX3-R wheels and Nitto Invo tires (10x18 and 275/35) were then installed and run for the baseline proper. The difference (plotted in green) was much larger than expected. Peak measured values for the official baseline were 266 wheel-hp and 225 lb-ft of torque, a gain of 13 hp and 10 lb-ft.

Note that the aftermarket wheel/tire setup has an inflated diameter of 25.55 inches compared to 25.8 for the 255/40 tires. This lends a slight mechanical advantage, but the bulk of the gains are due to the significant difference in rotational mass of the Forgeline ZX3-R competition wheels. These forged rims provide added strength and durability, significant weight savings (that allow increased wheel and tire widths with no penalty), great looks, and more power to the pavement. This is an upgrade that really works.

Next, the Supersprint exhaust was installed in two sections (to test the cat-back portion separately). Calgary BMW completed the installs on two separate visits: two-hours' shop time for the cat-back and a further eight for the step headers and metallic cats (which required removal of the front cross member). The X-pipe and street muffler cat-back tests revealed only negligible gains of 2 wheel-hp and 2 lb-ft of torque (this data is not shown on the graph). A cat-back test with the Supersprint race mufflers would likely provide slightly better gains.

With the full system installed, the dyno revealed excellent results (plotted in blue): peak values of 290 wheel-hp and 236 lb-ft of torque, up by a fairly significant 24 hp and 11 lb-ft. The step headers performed exactly as advertised by providing improved low-end torque and strong horsepower gains up top. Note that the baseline torque curve's dip has strengthened and flattened out nicely, showing gains in the 20 to 25 lb-ft range from 3000 to 4500 rpm, right where it's needed for everyday driving.

By Doug Neilson
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