Thought this was worthwhile feedback and a good read on the low compression 340 courtesy of a member from another forum I follow (tricky lugnuts) from about 13 years ago;
Just rebuilt my 1972 340 over the summer and it runs great now -- meaning I have been happy with its performance, economy, and street manners. I am no expert, but here are some of the things I've learned.
The compression ratio was dropped from 10.5 to 1 in 1968-71 340s, I believe, to 8.5 to 1 in 1972. Quite a significant drop that a head gasket change alone will not recover. Do you like buying new pistons or milling heads? All compression ratios were dropping at the time, though. (The 426 was no longer available, so what does that tell you?) Mopar held on longer than most of the others.
Most 1972 340s also have a cast crankshaft and cast pistons, instead of the forged ones used in 1968 to 1971 340s. The heads on 1972-73 340s do have smaller intake valves also, 1.88 inch as opposed to 2.02 inch, though the 1972 heads ('J') are essentially the same and can be machined to easily accept the larger valves. Valve size is a compromise, at best, between larger valves for better high end operation, i.e. maximum flow at top speeds, and smaller valves for better throttle response, i.e. low-end torque and acceleration.
My good buddy is a mechanic and a mopar nut -- has been for at least two decades -- and his biggest gripe against the 1972 and later 340s is the cast crank and cast pistons. But, theoretically, at least, they should be lighter than forged ones and thus rotate quicker, but just not be as strong -- at least that's what I've read.
Also, 1972 and later 340s came with electronic ignition. My buddy likes forged cranks and pistons, adjustable rockers and solid lifters, and high compression ratios, and also
dual breaker points -- but he doesn't drive his car everyday like I do in the appropriate months of the year. My 340 got my Charger to California and back, from Ohio, without a single tune up.
I rebuilt mine with Sealed Power pistons and rings (I think that was the brand, off the top of my head) and kept the stock compression ratio of 8.5 to 1. It runs just fine up any hill on 87 octane with the timing set conservatively, and with 93 octane I can advance the timing and get it to squawk the tires when it hits second gear, though I like to richen the mixture as I advance the timing.
I also kept the smaller intake valves (but had hardened seats installed) and stock exhaust and intake manifolds and reinstalled the stock windage tray. I Put in a reproduction camshaft for the 1968-71 340, and it has good throttle response at all RPMs, especially with the kickdown properly adjusted, and will easily cruise well above 100 mph with my 3.23 gears. Has a great idle and plenty of vaccuum for the power breaks and the heater box controls. And it gets almost 17 mpg on the highway with the right driving conditions. I also put in a new torque converter, essentially stock, and that is a critical piece as the 1972-73 340s with a cast crankshaft are externally balanced with the converter and damper.
Wish I had some drag strip numbers to pass on to you, or some dynamometer numbers -- but I am not a rich fellow. Speed costs, how fast do you want to go? And of course, going fast then breaks things. The performance of a 340 will also depend on what you put it in, i.e. is it an A, B, or E body at stock weight, or has it been lightened. Mine is in a stock weight B body -- not a good combination for racing, because of the weight, according to Mopar Performance. But it sounds good, runs good, burns rubber, and scares most of the stoplight racers around here from trying anything and surprises a good number of those that do. It's also a hell of a lot lighter and less nose heavy than my buddy's 1971 with a 440.
As the other fellows have said, tuning makes a big difference (in any carbureted car), and I am confident that my 1972 340, even in essentially stock condition, will outrun a 1971 or earlier 340 so long as mine is tuned better than his, despite the smaller intake valves and lower compression ratio.
The 1972 340 was factory rated at 240 net horsepower (at 4,800 RPM) and 290 foot pounds of torque (at 3,600 RPM), as opposed to 275 gross horsepower for the 1968-71 340s (at 5,000 RPM) and 340 foot pounds of torque (at 3,200 RPM) and 290 gross horsepower for the six-pack engines.
In general:
Gross (theoretical output) horsepower minus frictional losses within the engine equals
Net horsepower (measured at crankshaft) which, minus frictional losses in the transmission equals Shaft horsepower, which, minus frictional losses in the driveshaft, differential, axles, and wheel bearings equals True horsepower.
If I were you I'd throw some quality headers on it and play with the tuning of the carb and timing, if anything, first. Make sure your ignition system is good. I didn't go the header route because they can be such a freaking nuisance. Play with the top end next -- i.e. intake manifold, headers, cam -- or pull your cylinder heads and have them milled slightly to raise the compression and gone through with the larger intake valves installed. Spend lesser amounts of money on things you'll be able to and want to reuse (headers, intake manifold, distributor) should you tear into the lower end and raise the compression ratio with new pistons. I assume you're like me and not made of money and can't just drop it off at the machine shop and tell them to build you a high compression forged piston and crank 450 horse small block -- otherwise you'd have already done that. I was slightly disappointed with the performance of my 340 until I got the kickdown linkage properly adjusted. That three-- one downshift can be fun, fun, fun. If you're feeling really evil (so far as the health of your 340 is concerned), take advantage of that lower compression ration and install a nitrous kit, or supercharge it if you've got the big bucks. I'd still take the time to put in some quality cylinder head studs if I were to do that, though. A 150 horse nitrous kit will more than compensate for the 35 horsepower difference in the factory rating -- which, as I hope you noticed, is also different from gross to net. Just don't rev it up over 6,000 RPM in stock form. That's my advice.
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Mostly true information there. Still depends on what kind of performance you are looking for. Stop light to stoplight ?
