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The 915 heads with the original 400 pistons won't get you anywhere near 10-1 compression.

You probably wouldn't need to rebalance the assembly with a forged 383 crank because the piston weight isn't that much different. While the 400 piston is larger in diameter, the pin height is lower.

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The 915 heads with the original 400 pistons won't get you anywhere near 10-1 compression.

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Balance factors: 383 steel crank vs. 400 cast crank.
They both use the same rod.
They both use a piston that's the same or very close weight.
383 cast cranks used exactly the same factory imbalance as the 383 or 400 cast cranks. And they were somewhat interchangeable on the assembly line, between cast and forged 383s.

My opinion is if you remove the 400 cast crank and swap in a 383 forged crank with the 383 harmonic damper and flywheel/torque converter, you will have a typical Mopar factory balance job on the result.
The Mopar balance jobs aren't known for their precision, but we didn't hear about any problems back in the day. Now that everyone is balancing to a gram or less, ANY Mopar factory balance job can be improved. But don't expect a noticeable difference.
So swap away!

And, if you're hoping for 10:1 compression without changing pistons, dream on...you'll never get there.

R.

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Please excuse my lack of knowledge but what does the OP need to get 10-1 in his engine? Closed chambered heads, mill them, deck the block a bit, thin head gaskets, yes? How much more would he need?

Would turning down the rod journals to 010 undersized but doing it like 440 rod journals are offset ground to BBC size do anything to help or is that wasted effort?

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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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Some actual numbers would bolster your argument. As you know, machine shop balance and factory assembly line balance are way different.

The forged crank for the 1972 400 has the same part number (2268114) as the 383 from 1962-1971 which tells us there is no difference in the cranks.

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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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Some actual numbers would bolster your argument. As you know, machine shop balance and factory assembly line balance are way different.

The forged crank for the 1972 400 has the same part number (2268114) as the 383 from 1962-1971 which tells us there is no difference in the cranks.

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently. The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.
At some point you'd have to decide what is close enough, right? So in your opinion, what would be close enough? 30, 40, 60, 100 grams? typically anything more than 1 Oz.-inch will cause problems. (28 grams)

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The factory tried to maintain 14 grams or less.. but
I know that varied somewhat

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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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Some actual numbers would bolster your argument. As you know, machine shop balance and factory assembly line balance are way different.

The forged crank for the 1972 400 has the same part number (2268114) as the 383 from 1962-1971 which tells us there is no difference in the cranks.

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently. The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.
At some point you'd have to decide what is close enough, right? So in your opinion, what would be close enough? 30, 40, 60, 100 grams? typically anything more than 1 Oz.-inch will cause problems. (28 grams)

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The big issue is whether or not the crank balance is internal (no added weight to flywheel/converter) due to being forged instead of cast. Weight balance is way differemt due to offset balancers and weight pads (flywheel/converter) on the cast crank assemblies. CANNOT mix parts between the two, otherwise you will eventually have a bad vibration and a very short engine life! ( BOOM, BOOM, POW!! ) Either type of rotating assembly, I would DEFINITELY spend the extra cash re-balancing it!! Free horsepower, way smoother operation and extended engine life.

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Thanks everyone for all the good info.i wasnt sure what the compression was on a stock 400 but was hopeing that useing the 915s and some mill work that i could get real closr to 10-1 comp.i knew that the comp was low but did not realize that it was that low......yes its just going to b a half ass street motor for my 17 year old son.i had some parts laying around so i thought instead of having the 400 crank polished,i would polish a 383 crank and b somewhat better. Lol

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If you use the 915's, zero deck/flat top piston combo, choose a cam that will pump up cylinder pressure to "create" a ratio closer to 10 to 1 cr (effective). Definitely need to play with the ignition timing for MORE SNAP!!

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently.

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The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently.

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I understand that but the crankshaft was individually balanced to a set bobweight, if the 400 bobweight was that much different than the 383 they would have given the crank a different part number and a different balance but they didn't...so a parts room 2268114 crank would have been supplied as a direct replacement for either the 383 or the 400.

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The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.

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Again, depends on how much imbalance is considered "too much"....evidently the factory thought it wasn't enough to warrant a different part number.

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