Feb 8, 2007 update to the Mazda STS2 Miata project car

STS2 Miata - Some Exhausting Dyno Work

For the New Year, its “out with the old, in with the new”, so they say. Well, there’s a new dyno in town and its right where the old one was. In fact, it looks a whole lot like the old one but is lots shinier. DP Miata competitor Steve Hudson has purchased a Dyno Dynamics> 2WD chassis dyno and is now sharing shop space with our friends at Applied Racing Technology. His business is called Power Curve Dyno and we have a hunch we’ll be spending a bunch of time there.

Steve invited us to help shake down the new unit and we used that opportunity to do some exhaust system testing. We wanted to answer these questions: 1) What kind of relative power losses result from running our Supertrapp at various plate counts? 2) How much better is our Supertrapp than the OE system? and 3) How does a good full-length aftermarket system perform relative to our Supertrapp?

To help answer our last question, the good folks at Moss Motors supplied us with a very nice Borla dual-exhaust cat-back system. This thing is of the absolute highest quality. The materials are strong, the welding is impeccable, the fit is perfect and the sound is sweet.

We those things in mind we develop a plan. We’ll baseline with the Supertrapp in the “street-choke” configuration of 6 plates. This is a reasonable setup for running the car around town and avoiding that boominess that results from more plates. From there, we go to 9 plates which is typical of what we have to run in competition at autocross sites with noise controls in place, and then on to 12 and 18. At some point, we expect to see no additional power gains. Finally, we completely uncork the Supertrapp to our normal competition setting. After that, we install the Borla for a couple of passes and then the OE system.

The testing goes pretty much according to plan. We take a couple of passes to warm things up and get our consistency going, then take real runs for data purposes with the 6, 9, and 12 plate configs. As expected, the 6-plate setup is choking us significantly, being down between 3-8 lb-ft across the board. More interesting, though, is that the car responds better to 9 plates than 12. 12 makes more noise and slightly less torque across most of the range. Only above 6400 does it do better and just by a little.

We move on to 18 plates and find the same results as 12, but even more noise. Finally, we uncork the thing completely, which just about doubles the noise. This setup is the best so far, delivering the same low and mid-range as 9 plates, but a few more ponies above 5200. Given this, we’ll be running the thing uncorked during competition for most national events, but use 9 plates for anything with noise control. With the OE cat in place, that should be sufficiently quiet anywhere.

Next, we put the car up on the lift and swap in the Borla. The install goes very quickly and easily as everything drops right into place. Since its only a test, we angle the second exhaust outlet downwards to avoid the bumper. Permanent installation involves cutting the bumper cover for clearance using the supplied template.

Back on the dyno, the car produces almost the same torque curve as the uncorked Supertrapp. There is a small deficit between 4400 and 5800 RPM of 1-2 lb-ft, but its quite minor. The good news is that the car produces a much more tolerable exhaust tone. Very authoritative, but not loud or boomy at all. Borla has done a great job of maximizing power across the range without producing noise. In short, this is an excellent dual purpose street/track exhaust

As a final comparison, we put the OE exhaust on the car to see how we are doing with our performance exhaust systems and we see that the stock system makes 1-6 lb-ft less over the RPM range.

We also notice that the stock system makes more torque than our Supertrapp did when it was choked down to 6 plates to be tolerable on the street. With this in mind, we decide to leave the OE system in place for off-season street driving duties.

This dyno testing gave some interesting results, including one big surprise. We did not expect the Supertrapp to stop making more power at 9 plates. But this is good news for our ears. The Borla did perform as expected, producing almost as much power as the best (and loudest) system but doing do with a very streetable tone.

As a “PS” for those that are interested in weight reduction, the Borla was 23 lbs, OE was 21 and the Supertrapp was 6. Borla also makes a single-tip version of this unit which would be a tad lighter.

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