I have 9.5:1 compression, oversized valves, headers, Holley 390 4V, 2" duels all the way back to bumper with 340 square tips. When I first got the car it had a pair of old turbo mufflers in place which were unable to control a nasty drone at 2300 to 2700 rpm, and nasty highs only a straight six can make. I would stuff foam ear protection plugs in my head for any trip over a quarter of an hour; the noise was unbearable, couldn't hear the radio at idle never mind while under way.

After a year or two I replaced those old turbos with OEM stile mufflers in the stock location just ahead of the rear axel, and still there was drone, and nasty tones emitted, but not nearly as bad.

I than cut in two Flowmaster HP 2 mufflers 12" long ahead of the OEM units, and those babies quieted the nasty high tones, and all drone down to a nice barely audible engine sound at previous drone rpm, and no noise at highway speed. This car is a rag top, so there is a lot of wind noise, and its easy to hear what is going on out back.

Two thumbs up for HP 2's.

I don't think there is much back pressure with a 2" duel system with OEM muffler when just three cylinders are dumping into each side. Probably loosing those two square 340 tips, and installing a pair of down turns would quiet low rpm, no wind noise conditions when idling around a parking lot.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC