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I have a 62 235 engine. I put the clifford intake and headers on with a holley 390. I am going to replace the jets and discharge nozzles on the carb as recommended by clifford. So I'm going to run the exhaust. I originally thought that the more flow the better? So I got the collectors for the headers at 2 1/2". Then a friend told me that too much flow can rob you in torque. I never heard that before. But should I reduce the duel exhaust to 2"? I plan on putting in a X pipe also.

Six cylinder engines really don't respond like a V8 does when X pipes are installed, a simple H pipe will do the same if you want to save some money.
As far as pipe size goes, keep it small since you are running two of them. Your engine will not produce any more exhaust then a stock engine does as long as you drive it like a stock truck. Higher rpm is the only way to make more exhaust, the intake and headers will only let air in and out easer, not produce more.

With that said, 2 2" pipes will be plenty, smaller pipe will hold heat in which lets the exhaust flow faster, it slows as it cools.

Joe

I agree. 2" is all you need. Any more is bulky & unnessesary.

Seems to me like 1 1/2 or 1 3/4 would be plenty for a 235.I am running 2 1/4 on a built 350,and most guys are running 2 1/2 on big blocks with good results.Just my 2 cts.

Take a look-see at my exhaust on MariAnne, my wife's 53' 3100. I made the collector and used a single in/two out muffler. Plenty of back pressure (what your friend was referring to) with a nice mellow sound. All pipes are std size for the model.

Dave,

If you want a single tail pipe, Dave (53moneypit) has one of the best solutions. If not, all you need is a simple H-pipe to run duals. The cross over need not be much larger than 1.5". X-pipe would be a waste here as the velocity of the exhaust is not there. I would also run 1 3/4" to the mufflers and 2" the rest of the way.

If you have the motor spinning to 5-6 grand, then I would worry about running a larger exhaust than that

Davidn,
Scott and other have it right. Nothing larger than 1+3/4 or 1+5/8" is max if you are looking for gas mileage. Add H-pipe if you want smooth sound. Individual pipes if you want the rap sound. Move mufflers back as far as you can to move the sound back. Mufflers are your call. Many like glass packs, go for stainless pipes and mufflers to do it once. good luck