Note: Alamo Autosports is recommended to those in the North Texas area for dyno testing. $60 buys you 3 runs on a Dynojet Dynamometer, worth it just for the experience of seeing and hearing your car dynoed. Contact Brice, Steve Pak, or Steve Webb at Alamo Autosports 1218 Colorado Ln. Arlington, TX 76015 817-860-4300

There is a lot of discussion over whether you can improve a car's performance by replacing the stock exhaust with a "free-flow" aftermarket exhaust. How best to come up with a quantitative answer? With before and after dyno runs, of course.

A day on the dyno at Alamo Autosports in Arlington, TX was scheduled.

Three stock dyno runs were done. They were all close, but the best and worst were thrown out for the purposes of this article. The M roadster, with stock exhaust and no performance modifications, reached a peak rear-wheel horsepower of 217 between 6150 and 6250 RPM. Peak rear-wheel torque was measured at 217 ft./lbs. between 4000 and 4150 RPM. At the bottom of this section of the article is a chart with the full numbers, and the full-size graph of the stock HP and torque curves may be seen by clicking on the small graph at right.

How might an aftermarket exhaust improve performance? By freeing the exhaust flow. This picture shows one way the Supersprint exhaust improves over the stock exhaust. The pipes shown go between the connection at the rear of the catalytic converter and the muffler. The Supersprint exhaust is on top, and the stock exhaust is on bottom. Notice how the Supersprint exhaust pipe takes a straighter path. Also notice how the stock exhaust is somewhat crimped in the middle (to clear a chassis cross-member, which the Supersprint avoids by routing the pipe slightly lower).

After the three "before" runs were completed, the car was driven off of the dyno and allowed to cool. After cooldown, the car was put back on the dyno (used as a lift), and the Supersprint exhaust was installed. After installation was complete, three "after" dyno runs were conducted. Peak torque gain was 6 ft./lbs., and peak HP gain was 5 HP. Since the dyno runs were conducted immediately after the exhaust installation, the numbers reported are for a non-broken-in exhaust. A follow-up set of dyno runs is planned to acquire HP and torque curves for the exhaust after break-in.

Click on the left picture below to hear and see one of the dyno runs after the Supersprint exhaust was installed. The video is of the Supersprint run that produced the highest HP value. Please note that the numbers below are from the middle stock dyno and the middle Supersprint dyno, so the peak HP below is 1 less than the peak HP mentioned in the video.

Click on the right graph below to see the full-size comparison of before and after torque curves. As you can see from the graph, there is a definite increase in torque (important for acceleration) in the entire midrange.

Sound