NC Helmholtz for CARB/"Street" Autocross

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Joined: Thu Oct 15, 2015 8:50 am
Location: Los Angeles, CA

NC Helmholtz for CARB/"Street" Autocross

Post by ianwhthse »

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. :D

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. :D

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?
Ian Whitehouse
2010 PRHT GT, Red/Tan
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Re: NC Helmholtz for CARB/"Street" Autocross

Post by Brian »

Front and rear have to be made together in the JIG, thus we don't have just rear to offer.

I think what you are seeking to do could be done by GOOD exhaust shop. Indeed adapter will be needed to step up from about 2 inch stock to 2.5 inch with this midpipe. Alignment in the rear cavity of the car is critical, getting it right will depend on talents of the welder at the exhaust shop.
Brian Goodwin
Good-Win Racing
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