I did that for a while.
2-1/4" Feather Duster head pipe, and all the way back.
If you run the car for long periods of time, what I mean is: Over an hour every time you fire it up, cool. No problem. You won't believe how much water comes out of the tail pipe when you take off for the first couple times.
Velocity drops as the gasses cool. By the time it hits the muffler, your going to have pretty cool, low velocity exhaust gas. This, if allowed to cool wet, will make some pretty caustic stuff in the muffler and tail pipe.
If you run across town for ten minutes, shut down for thirty, then fire up again, it's bad news. Muffler and tail pipe rot. The track officials don't like to see a pint of water blown out when you stand on it either.
It costs nothing at the track to go back to 1-7/8" for the tail pipe. If you want to play at the track with a full 2-1/4 system, just put a cut out somewhere around your butt. That way you can tell the engine is running during test and tune when you stage next to a super-pro! :-D
The volume drops radically as the gasses cool. That's going to happen pretty quick as they pass back under the car. If you go to 2-1/4" for the head pipe, remember that your going from the stock manifold to a larger diameter pipe. Even with the truck manifold, I had to grind the crud out of mine to match it up to a 2-1/4" head pipe. That's the magic number till you hit the muffler. Then drop it back to a noodle to keep it dry for a driver!
CJ
OH! Edit: PS! :: I use a turn down to keep from attracting attention. I don't want to state my intentions to the local constabulary. I don't like to hear other folks junk. I could hazard a guess they don't want to hear mine. With my current setup, Unless i push the R's up over 4500, you wouldn't know anything was abnormal about my car. Quiet! At WOT it kicks 105db above 4000. I make a point of avoiding that sort of thing when passing peoples houses and such. I sure don't want to listen to a bunch of middle aged idiots with noisy machines when I'm in my backyard grilling something. It's cool at the track, not out in the world.
My two centavos
