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Originally Posted by oab_au
Mat I think you have answered your own question, that you said here:
While I had my car apart for the 5MT install, I fiddled with the exhaust a little bit. In some of the exhaust related discussions we've had here, there was some discussion as to whether or not there would be any benifit to gutting the pre-cats and using them as an expansion chamber. I never saw any truly conclusive results from those forums, so I did a little experiment
I think the expansion chambers that you replaced the front cats with, have killed the sonic action of the header pipes. The wave is being terminated at these chambers, which have shortened the effective tuned length of the header pipes. This would raise the resonate peak up out of your rev range, so the torque that it provides would disappear.
Harvey.  |
Thanks Harvey. I had considered that. In actuality, I've been without the cats for a while. I had them cut out about 3 years ago, but the guy who did it did a horrible job putting the pipe back in. All kinds of pipe overlap at the joints, so there were some pretty serious restrictions there. Thus, I figured I might as well give it a try
I can't say whether or not I noticed a difference from when I had the cats to when I had them removed.
So, in reality, I guess I've kind of done the opposite of what has been said to be effective for the SVX (moving the cat/y-pipe/1st expansion chamber toward the rear of the car).
Would that also explain the exhaust sound at high RPM?
One more question for you, Harvey. In other exhaust threads I read on this forum, you and Mychailo sort of concluded that the 3rd cat and resonator were both in locations that would provide torque peaks based upon sonic tuning/pressure tuning. Is that based upon the assumption that the cat is expansion for the sonic waves while the resonator is expansion for the actual pulse of air? In doing my expansion chamber project I thought I would be creating a new torque peak for high RPM, but it sounds like I just replaced the OE sweet spot into a range where I can no longer access it.