I'd like to keep the dual exhaust. It almost looks like the exhaust was stock and the hollowed out the cat and replaced the old mufflers with some ebay "tuner" mufflers. Very unpleasant to drive on the highway.

What's a good size piping to keep the sound lower, but a nice deep tone?

 

2.5" seems to be the point where the piping is too big to help muffle the sound unless it's a turbo car. Even bonestock, the 12A in my '84 RX-7 is one loud machine with its RB dual tip. The guy that did the exhaust work suggested that size even though I was thinking just 2" since the stock exhaust manifold's outlet looked like 2" when we got off the old exhaust system. The other problem is that the actual exhaust system tubing just isn't as thick to help contain the sound energy until it hits the mufflers.

 

if u need something quieter, racingbeat will be ur best bet. but i think their systems r like 2".

 

Piping size isn't going to be as critical as a proper set of resonators and mufflers.

On the NA, a 2.5" midpipe that splits into 2" - 2 1/4" Y pipes are sufficient.

You can replace the cat which will cut down the noise a lot. Also, add a resonator or two (I like the Vibrant Ultra Quiet resonators). A set of non-straight through mufflers helps greatly as well.

It's hard to go wrong with any of the Racing Beat streetable exhausts.