72 340 Rebuild

-
L

ltrripp11

Well-Known Member
Joined
Jan 23, 2006
Messages
107
Reaction score
1
Gentleman

New To The Site And Need Some Advise. Going To Be Rebuilding A 72 340 Motor In The Near Future. Need A Recipe For Success However. This Being A Low Compression Motor I Want To Bring It Back To The Original 10.5.1 On The Early Models. Heres What I Had I Mind After Reading Many Posts For A Good Strong Street Motor But Not A Drag Queen.

"30" Overbore
10.5.1 Pistons (not Sure On Brand)
Comp Cams Xe268h Cam Kit
Edelbrock Performer Rpm Air Gap (any Hood Clearance Issues?)
Stock Exhaust Manifolds
Holley 650 Vac Secondaries (currently On Stock Manifold) Have Thermoquad
2.5 Dual Exhaust With Dynomax Mufflers
Has Stock 3:54 Rear Gear Ratio
Stock 727 Tourque Flight W/ Shift Kit
Unknown Tourque Converter Size (what Size Suggested?)

Thats It. This Is A Complete Numbers Matcing H-code 340 That I Want To Look Stock And Show. See Any Problems Or Suggestions Here. This Will Be Proffesionally Rebuilt By Local Race Shop. Thanks.
 
KB are good pistons. I have forged KBs in my 360

Good cam

Air gap intake RPM range is 1600-6800 (i think). Hood clearance issues I guess would be dependant on what car you have. I know that for my 67 cuda with stock hood it would be tight. I thought about using that intake but didn't because of that very reason. Wasn't sure???

3:55 is a all around good gear. That's what I run but I don't drag race so it is street only.

727 is a beast ... it will take beating.

Converter Size. Talk to Dynamic converters. Give them every piece of info that you have and let them custom build you a converter. They are high but well worth it. Don't be shocked if the stall seems to be a bit high in RPM. Everyone that I've seen was shocked at the stall they suggested but after they ran the converter they was happy with the suggestion.
 
The early 340s were rated at 10.5:1 compression but were actually 9.5:1 compression in the real world as built by the factory. But 9.5:1 compression with the stock open chamber iron heads will make for a good strong street motor.

Pay attention to piston weight. The factory pistons were very heavy. Most after market pistons like KBs are much lighter. Don't skimp and get a cheap replacement piston that is heavy like the originals were. The weight really effects performance and is harder on the rods and crank too.

You will have to rebalance when you switch to the lighter pistons. If you have money to burn a nice set of Scat I-beams would bring the weight down a lot too. If not recondition the factory rods but price out both options before you decide.
 
KB 243 pistons...585 grams about 100 grams lighter then stock high compression pistons...they are hypereutictic which will work just fine...

Scat I beam rods....595 grams...over 140 grams lighter then stock....
 
ltrripp11 said:
... I Want To Look Stock And Show. See Any Problems Or Suggestions Here...
Click to expand...

The Air gap will not look stock, and the purists will nit pick your engine. You could try painting it engine color, but they will still notice. DAMHIK

Should be close on hood clearance. My hood has a high spot where the air cleaner stud hit. I turned it down a couple of twists and it cleared. Good luck with your build!
 
Remember the 72 340s were the cast crank 340s. They were externally balanced. The harmonic balancer is Asymetric and the torque converter is counter weighted

make sure you have the rotating assy. balanced either to factory balance specs OR internally balanced (if possible). Then you can make sure of the proper harmonic balancer and TC weighting.

Also, if going to with stock balancing spec.s, get a 340 ext. balanced flexplate. that way you can swap neutrally balanced converters.

good luck.

PS. I'm doing a stock 72 340 now....so I'm in the same boat!
 
Holley 650 Vac Secondaries (currently On Stock Manifold) Have Thermoquad
Click to expand...

Are you running an adapter ? Stock 72 intake was spread bore and a square bore holley wont bolt up.
 
I'd skip the Comp cam and go with a Lunati. With the Comp you've got picked I would say go with a Lunati 60402. I say this because Lunati cams take advantage of the mope's larger lifter (.904). Comp cams are set up for a chevy lifter, .8-something. The larger lifter allows a more aggressive ramp profile for better power.

I also know of a few people on C-C that run the 60403 with a fairly stock setup with lots of success.

Other than that, I just rebuilt a '68 340 for my challenger. Not finished yet, but I went with the KB 243's and Edelbrock RPM air gap intake as well. I'm planning on running hooker 5115's, edelbrock heads and a street avenger 770 though. The KB pistons look good, although I'm no expert. The air-gap manifold definitely doesn't look stock if thats a concern, but it also looks like a much better design for making some power.
 
72bluNblu said:
I'd skip the Comp cam and go with a Lunati. With the Comp you've got picked I would say go with a Lunati 60402. I say this because Lunati cams take advantage of the mope's larger lifter (.904). Comp cams are set up for a chevy lifter, .8-something. The larger lifter allows a more aggressive ramp profile for better power.

I also know of a few people on C-C that run the 60403 with a fairly stock setup with lots of success.

Other than that, I just rebuilt a '68 340 for my challenger. Not finished yet, but I went with the KB 243's and Edelbrock RPM air gap intake as well. I'm planning on running hooker 5115's, edelbrock heads and a street avenger 770 though. The KB pistons look good, although I'm no expert. The air-gap manifold definitely doesn't look stock if thats a concern, but it also looks like a much better design for making some power.
Click to expand...



comp makes many camshafts based on mopar 904 lifters..you just have to pick out the right series ....

also...all of hughes camshafts are based on mopar 904 lifters...
 
The 71-72 Thermoquad intakes are the single best Mopar made. I would have the Thermoquad rebuilt properly and use that combo. I ran it for 2 years daily driving and it outran plenty of aftermarket packages.
 
thanks for all the replys and info fellas. it would appear i am at least on the right track with a few issues to look at. one thing not really mentioned or cleared up is the tourque converter sizing. any comments or experiences with similar application? also forgot to mention this motor has the x heads. how are these different then the j heads?
 
ltrripp11 said:
thanks for all the replys and info fellas. it would appear i am at least on the right track with a few issues to look at. one thing not really mentioned or cleared up is the tourque converter sizing. any comments or experiences with similar application? also forgot to mention this motor has the x heads. how are these different then the j heads?
Click to expand...

Read this http://www.forabodiesonly.com/mopar/showthread.php?t=46931&highlight=dynamic

Also click on "search" above and then type "Dynamic" and it will show numerous threads about converters.
 
X and J heads are basically the same heads except the X heads have the bigger intake valves. Some J heads have the bigger intake valves and others don't. The X heads will work well for what you are trying to do.
 
That nobody mention. That was the year I think that they started hardening the valve seats to help with the unleaded gas. I have a 72/340 and went with ROSS 99625 pistons. Mine are .030 over and Ross calls the compression of 9.7 and the pistons only weigh 514 grams each. I think the price is just over $500 per set now. You get what you pay for and I like their track record. I ues to go to the dirt tracks around here and over half the cars I ask about said they ran ROSS because they would hold up and were cheaper than most of the top line pistons.
 
Combo looks pretty good,i'd take a look at the Lunati cams too i've had great results with the ones i've used,as for the "therm-o-bog" throw it in the trash,the 650 holley should be fine for what your running...
 
he is a member here too. he rebuilt mine and i love it. no problems at all..
 
1971/1972 TQ and manifold is an EXCELLENT set up. I feel them to be THE best 340 carb/man. combo that Mopar used on the 340. Set up correctly, will deliver Great torque , throttle response and power. I feel the 1971 340 delivered the highest output of any year. NHRA IMMEDIATELY refactored that engine to 330 HP. Even more than the six-pac engines, they were factored to 310 HP. The 1971 340 had the same combo as the '68-'70 engines, only difference was the TQ and TQ intake, Heads were the same performance, different casting numbers. As always opinions will vary..Just my experience and JMHO. Good luck with the rebuild...this IS the fun part.

Terry
 
D-mailman said:
1971/1972 TQ and manifold is an EXCELLENT set up. I feel them to be THE best 340 carb/man. combo that Mopar used on the 340. Set up correctly, will deliver Great torque , throttle response and power. I feel the 1971 340 delivered the highest output of any year. NHRA IMMEDIATELY refactored that engine to 330 HP. Even more than the six-pac engines, they were factored to 310 HP. The 1971 340 had the same combo as the '68-'70 engines, only difference was the TQ and TQ intake, Heads were the same performance, different casting numbers. As always opinions will vary..Just my experience and JMHO. Good luck with the rebuild...this IS the fun part.

Terry
Click to expand...

I agree with the above.
 
i just finished my 72 340,9.5 to 1, an air gap, 670 holley street avenger,comp 270h, headers, tci sizzler tc, 727w/shift kit and 3.23 sure grip it all sits in a 72 dart. the onlyclearence issues i had were the cheap hedman headers. and what can isay the car gets upand moves if i can get traction and i still have ignition troubles. by the way i run 93 octane
 
D-mailman said:
1971/1972 TQ and manifold is an EXCELLENT set up. I feel them to be THE best 340 carb/man. combo that Mopar used on the 340. Set up correctly, will deliver Great torque , throttle response and power. I feel the 1971 340 delivered the highest output of any year. NHRA IMMEDIATELY refactored that engine to 330 HP. Even more than the six-pac engines, they were factored to 310 HP. The 1971 340 had the same combo as the '68-'70 engines, only difference was the TQ and TQ intake, Heads were the same performance, different casting numbers. As always opinions will vary..Just my experience and JMHO. Good luck with the rebuild...this IS the fun part.

Terry
Click to expand...
I think this must have been the "availible" ticket, back in the day.. Maybe thats why my 69 340 had these on it. when I aquired it. but now-,, airgap and holley, ain`t goin back. But if anybody`s tossen TQ`s throw em in my trash can. heck I got to where I enjoyed rebuilding them, once a year or even less. Far as hood clearance. my breather nut is just- rubbing a hole in hood insulation. not hitting metal. and I`m using 1/2" spacer. 71 mani + quad set was @ same height. you would`nt think that, by comparing them. btw my car 67 cuda:-D

parts 047.jpg


parts 056.jpg
 
moparpat88 said:
i just finished my 72 340,9.5 to 1, an air gap, 670 holley street avenger,comp 270h, headers, tci sizzler tc, 727w/shift kit and 3.23 sure grip it all sits in a 72 dart. the onlyclearence issues i had were the cheap hedman headers. and what can isay the car gets upand moves if i can get traction and i still have ignition troubles. by the way i run 93 octane
Click to expand...

what heads did u use on this motor? where they the open chamber x or j?
 
swing69 said:
Remember the 72 340s were the cast crank 340s. They were externally balanced. The harmonic balancer is Asymetric and the torque converter is counter weighted

make sure you have the rotating assy. balanced either to factory balance specs OR internally balanced (if possible). Then you can make sure of the proper harmonic balancer and TC weighting.

Also, if going to with stock balancing spec.s, get a 340 ext. balanced flexplate. that way you can swap neutrally balanced converters.

good luck.

PS. I'm doing a stock 72 340 now....so I'm in the same boat!
Click to expand...


The cast crank was introduced April 11, 1972....D


http://www.hamtramck-historical.com/images/TSBs/1972/09-07-72C page1.jpg
 
340sFastback said:
I agree with the above.
Click to expand...

The 72 Thermoquad is noted for being the best example manufactured due to the metering arrangement. Mine is all original save for the notorious main well o-rings (TSB ISSUED ON THIS)& body gaskets. I can tell you it is spot on and does not bog whatsoever! Errrrr, the few times I have opened up the secondaries8) D
 
You must log in or register to reply here.
-

Back
Top