My weekend driver is 1972 Charger Rallye 340 automatic with 3.23 gears. I'm looking for more low and mid-range torque as it's more a cruiser than a racer. The car has a set of exhaust manifolds from a 1970 340 and 2" full length pipes with no crossover. I plan to install a crossover, but would I see an improvement going with 2 1/4" or would I be best staying with what I have. My car rarely see's anything above 4000 rpm.

I would say yes you would see an improvement with dual 2 1/4 vs dual 2 inch. Have you ever considered hedders? Hedders with dual 2 1/4 on a 340 would be perfect in my opinion, and I also think that your idea to have an h pipe (crossover) is another good idea.
I have a 300hp crate 360 magnum and I put it in a 1991 dodge power ram 150 4x4 truck, The previous owner had just gotten new dual exhaust for the 318 that was previously in the truck and he got dual 2 inch pipe, I hooked up my hedders to this dual 2 inch exhaust and ran it that way for about a year. Then when I had the money I took it to an exhaust shop and I had them re-do the exhaust with 2 1/4 pipe. I had new dual 2 1/4 inch exhaust put on along with an H pipe and then I had them put some 3 inch inside core diameter glass packs that are 18 inches long on for mufflers. When I drove the truck for the first time with the dual 2 1/4 inch exhaust I could tell a difference in power and sound, my torque seemed a little better off the line and it sounded more like a mopar too.

Those manifolds are a 2&1/4" outlet so yes you'd see an improvement (tho I ain't sure how much) but as you know the ex is the most restricted part of the overall air pump (the eng). Muffler selection is important & I'd suggest some free flowing (noisy) ones with some glaspacks as resonators to quiet it back to a reasonable level. I would likely go to 2&1/2 if I was going to get a new ex system.I would do extensive dist timing mods & if it is an original Carter AVS then it is extremely lean & I'd convert to Eddy 2 step primary rods/short pri jets to richen it up. EDIT missed that it was a 72 so it would not be an OE Carter AVS. I saw the "70" for the manifolds

Forgot to mention it has as set of unknown make turbo mufflers. I'm just optimizing my stock setup. When I got the car it's performance was enemic and it had a bad vacuum leak. I made significant gains by performing my version of a super tune with the limited equipment I have on hand. Richened up the original Thermoquad which was way lean, blocked the crossover in the intake and installed an oil valley baffle. Also bumped the timing and installed lighter springs in the electronic distributor. Just looking at other area's were I might free up a few horsepower.

Call me crazy but I just removed the LD340 intake that came with the car and replaced it with the factory cast iron piece. I'm going to do some experimentation as I suspect the LD340 with a notched plenum may not be doing me any favor's in my normal range operating range. It's a driver that see's a lot of time in the 1500-2500 range. I'm willing to sacrifice some top end to improve my low and midrange.

Actually the OE iron manifold is a very good piece. What about advancing the cam 4 deg?

X2 on advancing the cam
to get more torque in the 1500 to 2500 RPM range

Exhaust pipe diameter and muffler changes primarily affect high rpm full throttle operation

I would put the largest true dual pipes you can fit and run restrictive reverse flow mufflers. I mention "restrictive" because you need some backpressure to maintain torque. It's a similar setup that I had on my '86 Ram 4x4 318 2bbl with 3.23's and there was a noticible increase. If I remember correct it was 2 1/2" to the mufflers and 2 3/4" out the back. Smaller displacement and heavier vehicle from what you have........

Otherwise, if you are really adventurous, a better solution would be a 360. There's no substitute for cubic inches and if you use the Magnum barrel intake, it builds alot of torque in the 1500 to 1800 range. Sure, it would cost more money but I venture a guess that a 1/4" difference in pipe isn't going to make any noticible increase and could be a waste of money.

Thanks for the replies. I thought about advancing the cam but I have never done more than install a cam and line up the dots. I'll have to get smart on it. The idea of running wide pipes with more restrictive mufflers is interesting, but what is the difference between smaller pipes or a more restrictive muffler since they will both produce back pressure?

I wouldn't want any back pressure. Open and free flowing is the key. Tune them to your rpm range by changing diameter. You want velocity in the rpm range where you want power. If you want low rpm, then a smaller diameter will do that. Downside is they don't have the capacity to flow the volume of high RPMs. This creates back pressure, but only above the RPM you are looking for. A larger diameter will handle the volume of higher RPMs, but velocity will be lower at low RPM, costing you low rpm power.

Bottom line... Back pressure is bad. Velocity is good

The difference is you want to support volume in the pipes by using a bigger inside diameter but have a pressure wave further down the line coming from the muffler to get your torque.

With smaller pipe size you lose the volume and have the restriction too close to the combustion.

That's my understanding of it, it worked in my application, and if anyone thinks it's wrong please point it out and why.

2.25" with a 2" crossover would be the ticket. Run some good flowing mufflers, not some unknown mufflers with unknown internal design. The 2.25" pipes will give you good flow velocity.

If you want to improve the exhaust manifolds, find some mid-90's manifolds from a 318 or a 360 - NICE design! I put these on my '73 Charger 340 Rallye - all you have to do is grind the center webbing on the manifolds so there is no obstruction coming out of the heads. I had to have a custom trans dipstick tube made for the RH side - there's a guy here locally that makes these.

Anyone have any experience with TTI headpipes that allow you to use 2 1/2" or 3" pipes directly from the 2" factory exhaust manifolds on a 340? I'm talking about 1971 - 1974 manifolds.

I always get a kick out of hearing someone say,you need some back pressure for whatever reason.

diameter/length are critical in header primary tubes for pressure wave tuning/scavenging & all that good engineering stuff but after the collector flange you want NO restriction/back pressure. With manifolds same deal you want NO backpressure. I firmly believe that people who claim to have slowed down after adding a larger ex/tail pipes/muff system also changed something else at the same time which actually is what caused the slowing down.

..i'd re-think the cross over unless you want to reduce noise.
..they add no power,and stock 340 was 2 1/4"