340 questions

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QwikDart73

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I recently accuired a 70 model 340. i'm fairly new to the world of 340's being as they have many differences from the standard, more common LA engines. I am wonder if what i've been hearing is true in that the came stock with a forged steel crank and forged rods. if so, what material are the rods? what other components are "weaker links"? i look to build the engine fairly stout and would like to see some high-reving, pump-gas-friendly results. any tips? please, overload me with recommendations. extremely curious.
 
forged steel crank, and forged steel rods came stock on the early 340's. Strong stuff but it has it's limits. I would say keep it under 450hp and 6500rpm and the stock stuff if fine. If your stepping it up, the rods I think are going to be the first to go. Just my two cents.
 
Thanks for the input, Buddy. valuable info. sounds like i'll need to upgrade, so far!
 
By chance do you know what car your 340 came in orig?

If your lucky it could be a Six Pack engine although rare! I believe they came with some factory upgrades, someone correct me if Im wrong!
 
That 340 was only found in the Six pack Challenger T/A and cuda AAR it had thicker webs for the ability to install 4 bolt mains on 2 3 and 4 but same rods I think.
 
The biggest issue with the 340s is the weight of the stock pistons. They are over 700 grams. This excessive weight will put a lot of stress on the rods. Pay very careful attention to the weight of your replacement pistons. This is an area where you want to spend some money. I have heard you can actually feel the difference lighter pistons can make in a 340. Its that significant.

The other thing to be aware of is the stock compression ratio is not 10.5 to 1 like the factory advertises for the early 340s. Its more like 9 to 1 which is good for pump gas just not what you think it might be.
 
Great core... You got the best heads, the good internals. Only thing better wuld be the '71 intake and carb...lol. On the hp, the safest thign is the lighter pistons. Put good pistons in it and dont sweat the rods until you really need to. Mopars dont routinely break rods.
 
how long the crank and rods last depends on RPM Vs weight. I had a 340 with a 525g piston and pin combo, Reworked stock rods and stock forged crank. After 120 passes when it was freshened up it needed a new crank because the old one was developing cracks. This motor was making 450hp at the time.
 
It made peak power at 6900rpms. We took it to 7300 on the dyno but saw no gain.
 
340dynosheet.jpg
 
thanks for all the input, guys! i was away form the computer this weekend cause i had to move. if anyone is still interested, the motor came to me as a short block with pretty much everything else in pieces. i do know that the heads that came with it were not the original. they are 360 heads that had been reworked with bigger valves (x-head spec.). i was told that the valves were stainless but found out upon taring them down that that was completely false. still don't know if i'll try and salvage the cast heads or just spend a little money and buy some edelbrocks or something (any suggestions?). "dodge freak" asked what rpm i would like to see and i was thinking pretty high, tword the 8,000 range. like i said, i plan on building a stout combo for drag racing/ street duty. (i do come up with some crazy ideas, then i have no choice but to make 'em happen!). i want the compression on the high end of pump-gas-friendly, thinkin 10:1-10.5:1 on 92 or 93 octane. and i am nearly 100% positive that it is not a six-pack engine, although i will run the casting # and post later. i do not know what the engine was out of. look forward to hearing from ya. thanks again
 
8000rpms and pump gas will be a real pig on the street. My 340 was pretty wild for a street motor and was hardly touching 7000rpms. You'll need huge ports and a huge roller cam to get that high and dont expect any torque below 6000roms then.

Look at the peak torque on my dyno sheet. 5300rpms and just over 400ftlbs. The cam was 260/265 @.050"
 
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