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Originally Posted by oab_au
There seemed to be a few different exhaust mods that had that resonance. The ones where the two header pipes are joined with a Y pipe, instead of joining in the low pressure area in the front of the secondary cat, will have gas pressure problems besides any sonic problems, as the unexpanded gas pressure can travel up the other header pipe to enter the open exhaust valve on that side. While the two header pipes join in the front of the secondary cat, the gas pressure and the sonic wave that enters the other pipe will be at a low intensity.
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As I mentioned to Trevor (who, by the way, replied faster than you to my original post
), using a Y-pipe instead of the main cat (or suitable resonator) seems like the "best" way to excite the unwanted "tinny" resonance; I have a (less and less confident) recollection that people who kept the main cat intact and removed the pre-cats also had problems.
(NOTE: things get long and detailed here; bail out if you're not in the mood...)
Actually, as I type this, I'm picturing a second effect that could contribute to resonance at the same frequency. A positive pressure wave from an exhaust port hits the expanded volume of the main cat. From this interface, a negative pressure wave reflects back up the same pipe. This is the desired effect. But at 3k RPM, I'm guessing the exhausting cylinder is coming close to TDC, but not there yet, and the intake valve hasn't opened yet. (i.e., the system is tuned for higher RPM) So that cylinder is gonna look like a rather small. closed volume. And the other two exhaust valves on that side are closed. So, for the most part, this bank will act like a closed pipe, and the negative pulse will reflect (as a negative pulse) and travel back to the main cat. This time, the negative pulse will reflect off of the large volume in the main cat and travel back as a positive pressure wave. What's interesting is that this reflection will coincide exactly with the arrival of the strong exhaust pulse from the other side of the engine.
So, from the point of view of the main cat, every time a strong positive pulse arrives from the LHS of the engine, some (small) portion of it will pass though to the RHS. At the same moment, a negative pulse will arrive from the RHS and reflect back (positive pressure) to the RHS. Both of these will contribute to a positive pressure wave travelling back to the RHS just in time for a new exhaust valve opening. This is assuming we're at the magic RPM around 3k.
Anyway, both effects are weak, but it's interesting that they combine exactly in phase. This is true if the collector pipes are equal in length; making them unequal would break up the critical RPM into 3 different RPMs for the left-only, right-only, and side-to-side mode. Also interesting is that altering the volume of the main cat shifts the importance from one mode to another. If the volume is negligible (i.e., Y-pipe) the side-to-side mode dominates and the one-side-only mode will be tiny.
(There's always
some reflection off a discontinuity, though.) If the volume is huge, the side-to-side mode is non-existent, but the one-side-only will be stronger.
Anyway, if anybody has read this far
, I'd be interested in hearing some comments.