70" 340 vs 72" 340 specs please

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mopar410

mopar410

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I cant seem to find the changes made from the early 430s to the 72,73 340 motors.I have read that the compression went from 10.4:1 to 8.5:1 but I couldn't find why.Anyone know what they changed on the motors or was it just gross and net HP and they were closer then they made them out to be.PRE 72 275hp 72,73 240 hp.
 
pistons were changed...compression height changed from 70 340 with a 1.84 to about 72 340 with a 1.74.....

cylinder heads changed from 2.02 intake valve to 1.88 intake valve

carb/intake change....70 340 had carter avs carb with square bore intake manifold...72 340 used carter Thermoquad with spreed bore intake manifold...
 
Allpar states the "J" heads went to a 1.92 exhaust valve with bigger tunnels but everywhere else I looked show the 1.88 valves.
 
mopar410 said:
Allpar states the "J" heads went to a 1.92 exhaust valve with bigger tunnels but everywhere else I looked show the 1.88 valves.
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incorrect...340 went to 1.88 intake and always had a 1.60 exhaust.
 
70aarcuda said:
incorrect...340 went to 1.88 intake and always had a 1.60 exhaust.
Click to expand...
This is from allpar where I got that info but I always thought it was 2.02 or 1.88 also.

1972 - The 340 four-barrel dropped to 240 horsepower:
  • Compression dropped to 8.5:1; the compression height of the piston via wrist pin location changed by 0.10”
  • The crankshaft was switched from forged to cast somewhere in the production cycle, believed to be in early April 1972, with engine 39118000 (thanks, Karl Thomas); a milder camshaft was used
  • 340/360 were moved to “J” casting heads with 1.92” intake valves; exhaust valves remained the same
  • Paint went from orange to corporate blue
http://www.allpar.com/mopar/mopar340.html
 
as i note..compression height changed from 1.84 to 1.74...which is .10 change...

J heads started in 1970...but still had 2.02 valves..changed to 1.88 valves in 72..

oh,,,yeah ..the paint color changed....lol
 
70aarcuda said:
as i note..compression height changed from 1.84 to 1.74...which is .10 change...

J heads started in 1970...but still had 2.02 valves..changed to 1.88 valves in 72..

oh,,,yeah ..the paint color changed....lol
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I knew that orange paint was faster.Good for about 10 hp.
 
So basicly you could put all the 70 parts in a 72 and get a reliable 275 hp
 
not sure but i believe the camshaft changed also

the way they rated the hp changed also so the 240 is not a fare comparison
it should be noted that the crank although went to cast iron is still a very strong piece
 
The only difference were pistons, intake valves to 1.88, Carb and intake, and exhaust manifolds. Somewhere in 72 the crank was changed to cast, but my two 72 340s were forged. If the crank is cast, then you need cast 340 damper and flywheel/torque convertor. Early pistons with the stock cam will ping and blow head gaskets.
 
Dual point distributers to electronic ignition in 72. Don't think the electronic would hurt H.P however.
 
It would be interesting to see what the 72's "gross" HP, would have been versus the 71's 275 HP rating. Guessing there's a formula out there some where that would calculate gross versus net HP calculations.

Pat
 
hi the 72 340 is 8.5 cr, versus 10.5 for 70/71 the cams are the same period!!!
net hp was measured with accesories attached. gross hp was with none. as far as power, the 72 340 is as good as the earlier 340's. 240 hp is way underrated. the 72 used 1.88 valves, 70/71 had 2.02 valves.the 1.88 valve head does flow very well, has higher port velocity. in stock eliminator the 72 is as fast as a 70 340. just food for thought.
 
Correct me if I'm wrong but, doesn't one point in compression generally mean about 20 hp gain on a street engine?
 
ive read before that a good rule of thumb is 3-4 % of torque per point of compression honestly dont now how accurate that is
 
If the 275 hp is at the crank(goss) and the 240 hp is at the wheel (net)thats eqauls out to be about the same or a little more for the 340.Seems like you can lose up to 50 hp depending on the drivetrain.A 35hp drop would be considered good,right?
 
mopar410 said:
If the 275 hp is at the crank(goss) and the 240 hp is at the wheel (net)thats eqauls out to be about the same or a little more for the 340.Seems like you can lose up to 50 hp depending on the drivetrain.A 35hp drop would be considered good,right?
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Both are rated at the crank
 
I don't think the 1.88 intake valve hurt those engines at all. Lot of people said they saw no increase in power going to the bigger valve on a stock port head.

The '71-'72 340s had the larger thermoquad carb and better intake then the earlier 340s.

The earlier 340s really only had about 9.5:1 compression from the factory. The lower compression of the '72-'73 340 does hurt performance and requires a piston change to correct.

Stock 340 pistons are heavy on all the years. Pay close attention to piston weight when shopping for replacement pistons. Lighter pistons allow engine to rev quicker and put less stress on rods, crank, and bearings.
 
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