Proud new owner of a 2010 Copper Red GT PRHT, here.
I love the Helmholtz midpipe, but need to stay CARB and "Street" autocross legal. So I was thinking about cutting the stock midpipe at an appropriate length and frankensteining it to the rear piece of the Helmholtz midpipe. Then stick on the Roadstersport street single.
I figure the stock midpipe and Helmholtz won't line up perfectly (and aren't the same size, regardless), so an exhaust pipe expander/step-up and a pipe with a couple kinks to get it all lined up, welded all together, would work out okay. Or a flexpipe. In any case, it wouldn't be too hard for an exhaust shop to piece it all together.
It's a Grand Touring PRHT, and I'm not a national level driver, so I'm not so much interested in saving every ounce of weight and making every last horsepower, just want all the fun sounds.
So a couple Qs:
I know the Helmholtz chamber is carefully tuned to cut down the specific frequencies; would having the OE cat, etc. mess with the frequencies being attenuated? At least, in a meaningful amount.
What is the OD of the stock pipe?
Can the Helmholtz be purchased without the catalyst piece?
What am I not thinking about?