[stownsen914]

To the OP, are you just trying to quiet the car a little for your own benefit, or do you also need to make sound at noise-restricted tracks? If you just want to quiet the car a little, Phase 9's do a good job as noted above without killing power. But they won't be quiet enough to make some noise restricted tracks like Laguna or Lime Rock. Lime Rock is 88 db these days, which is not easy to make. I've managed to do so with my 914/6 racecar with a 2.7L race motor using a custom muffler that I fabbed up. I know of at least one car with a 3L race motor that has made Lime Rock's limit using the full Quiet Horsepower setup pictured above. If you're trying to make a restrictive limit like 88 db, you may need to address intake noise as well, as Dave suggests.

Scott