Balancing a 400

[Donniejr81]

Ok here's what Im working with. I have a 71 Charger which was originally a 383 car, but at some point throughout its life a 1974 2bbl 400 was swapped into it. It has the factory 71 727 auto. Im pretty sure the 400's are externally balanced and the 383's are internally balanced.

Im rebuilding the 400 and am going with the KB240 pistons. The machine shop says they have to balance the 400's internals since Im going with new pistons. That has me a little confused since its externally balanced. I am pretty sure though that I will need a weighted torque converter for a 400...right?? This is my first 400 build so where is a good place to start? I want to do this thing right...input?? Thanks.

 

[PoppaSmurf]

because you have changed pistons, you have changed the rotating mass weight and I am thinking to the lighter side.....therefore you would want to have the entire assembly Re-balanced to the new rotating mass weight....BE sure to have them balance the pistons as well.....they should all balance within 1 gram if not less of each other make sure they include the wrist pin as well and locks(if using) and the rings your going to use ....chances are they will need to pull a few grams of weight off the counter weights and maybe touch the harmonic balancer to get it where it needs to be

 

[racedodge]

just a recommendation, if they are going to balance it anyway, you might as well set it up to be balanced internally. you can get an aftermarket harmonic balancer to match and then you no longer need a weighted converter.

 

[YY1]

Hijack alert!!

What's your compression ratio going to be with those KB pistons?

 

[racedodge]

Hijack alert!!

What's your compression ratio going to be with those KB pistons?

KB240's are 9.5 to 1 pistons with an open chambered head.. closer to 10 to 1 with closed chambered.

 

[Donniejr81]

Thanks for the input thus far. I was actually wondering if I could change it to an internal balance since Im getting it re-balanced anyway. That opens up a few more choices as far as torque converters, balancers, and flywheels go. Maybe just supply the machine shop with a 383 flywheel, balancer, and torque converter when I drop my 400 stuff off for balancing.

...After me and Dad got the head off he was like "man those are some big pistons"

With the right compression, cam, and exhaust I see the potential for a lot of torque that'll wind up RPM with the best of them.

 

[383man]

just a recommendation, if they are going to balance it anyway, you might as well set it up to be balanced internally. you can get an aftermarket harmonic balancer to match and then you no longer need a weighted converter.

Thats what I would do. In fact we did with my sons Dart as we used a 383 crank in his 400 and had it internally balanced. The shop will need everything that rotates on the bottom eng including bearings and piston rings with the rods , pistons and crank. They may want the balancer also. But this way you dont have to run a weighted converter or balancer. On my sons 400 we used a neutral balanced harmanic balancer and of course no weight on the converter. His 400 is .030 over so its 406 cubes. He uses Eddy RPM heads and the MP .557 cam. And the intake is the Performer RPM with a 750 DP. In his 3550 lb Dart with 3.91's it has run 11.40's @ 117. Ron

 

[Donniejr81]

Thanks for all the input I think Im headed in the right direction now.