Just pulled the 340 from a 73 Roadrunner I bought (I know it's a B body, but there's more advice here) ..... anyway, I knew the 73 motor was a wheezer but, lo and behold, I got a '70 block (and hoping internals) , but rest of the assembly (727, heads, exhaust man., intake man., all 73 correct) If I luck out, and it is a 70 rotating assembly and 10.5:1 - seems like a waste to run the 1.88 heads - or am I wrong ? Would I fell a difference in a street B body? I know comp ratio is a combination of the 2 , and I know a 73 needs all the help it can get - any advice ? I'll probably pull the heads to confirm - here's hoping.
Waste of a good short block ?
- Thread starter rebeldart
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Post the 4 digit number on the front left of the block and I or someone else can give you the build date. Blocks can be cast dated way before the build date.
I'll take it if you don't want it. LOL Like 70AARCuda said, up the compression and larger intake valves. tmm
Check the balancer to see if it's a cast crank, the rest of the stuff is interchangeable inside anyway!! That should give you a clue. A 71 cast date is a 71 or later motor, and could be a 72 assembly. If it's late enough in the 72 model year, they changed to a cast crank in Feb. I believe!! Geof
X2 on the balancer 72 ish had a big cast crank only on the balancer.
My cousin bought a 73 RR this summer factory 340 4-speed car.
He got extra lucky after we got it home, it drove with no issues.
It had a 70 340 forged crank motor in it.
67Dart273,did this in the day.Swapped a 340,in a Road Runner.PM him,or hope he chimes in.
Aside from proper balancing, there is not much difference between a cast and a forged crank anyway. It takes a heck of a build up to compromise a cast crank!! A buddy of mine has been running the same cast crank motor since I sold it to him over 5 years ago. He's into the 11's now and HP's up in the 500's somewhere, no problems!! And he runs it!! Geof
67Dart273
Well-Known Member
Mine was in a 70 RR. I don't know how much help I'll be here. Mine was a 71 engine, high compression 340, completely stock except for (eventually) headers, and playing with various spread--bore carbs.
I am to this day amazed at how well that little engine hauled that thing around. It would stay with a typical stock 383RR, and would get over 17 mpg on the highway. That was a heavy car, for a RR, as it had the Dana axle, air grabber, and "hang on" A/C. I always had about 150--200 lbs of junk in the trunk.
You sayin' you got junk in your trunk, Del???LOL!!
C- I suspect this is a G which denotes 1971? C would be 1967
W- Windsor engine plant
340- CID
P - Premium fuel
3374- 114 day of 1971 (April 24 1971 build date)
0473- was the 473rd engine built that date
This is the info I came up with.
That's what I guessed - just hoping for the higher compression motor. Like I said, a little disappointing not being numbers matching, but if I got a better motor (performance wise) I won't cry about it, just know the heads are suffocating the poor thing if it's a 10.5:1 motor .............. only 95,000 original smiles on the car, all original otherwise, don't know why they ever swapped the block, must've been catastrophic (or took an easy road), one of the exhaust manifolds has yellow marker (junk yard/swap meet) reads "71 Dart", but part number is correct for a 73 as well, dunno. Also - what was last year for the 90 degree oil filter mount ?
Just checked - cast crank motor. How do I tell if it's a 10.5:1 vs the 8.5:1 ? Don't mind pulling heads - but all runs well as she sits (little smoke at start ups after sitting - will be replacing seals). I'm on a really tight budget for this resto due economic conditions resulting in job loss/change (oooh, did I say "change", now who asked for that ?) ..... 
ukel:
ukel:Whoa ! Just found this great read on another site - I know I've got the weenie heads , just need to I.D. the pistons (?) or were ALL cast crank 340's the 8.5:1 or was there a run of cast 10.5:1 motors ?
Hey,
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.
Hey,
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.
Pretty sure all the cast crank motors had the low comp. pistons.
Yep - pretty sure it's a 72 spec motor. Odd that the block is stamped 70 and serials reflect April of 71 build. Also motor is blue to the bone , no hints of orange anywhere. Guess I'll live with it for now - need to get back on my Dart and will probably sell the Roadrunner to finance the Dart resto (original 70 340 4-spd). Thanks - always learning !
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