jfreeman74

I am planning on buying a 400HP 383 from Recon engines. They say that the motor is externally balanced but I will still need to buy a flexplate and harmonic balancer. I thought that if an engine was externally balanced that the flexplate that was used for balancing the engine had to stay with the engine. Are all 400 block flexplates the same when it comes to balancing?

Mr_Metal

The 400 SBC is externally balanced by the flexplate.
If you examine the flexplate, you will see that there is a 'balancing weight' attached to it for just that reason.

The engine shop was correct in stating that the 400 engine is 'externally balanced', however it's externally balanced by the flexplate and 8" dampner (harmonic balancer)... which you will still need to pick up down the line.

Great engine choice by the way.

I have a 388 Stroker for the '70 Camaro w/ Weiand 142 Supercharger (roots-type), Bowtie Phase II heads, Holley 750cfm DP, and Hooker Super Comps.

"Some Heads Are Gonna Roll..."

jfreeman74

But wouldn't the balance change if I used a different flywheel and dampener than what they used when they balanced the motor. Are all 383 stroker and 400 motors externally balanced and why? Is there some reason why they can't balance them internally?

ede

can't be internally balanced because they use 5.56 rods, the rods are too short which would cause the piston to crash into the counterweights if mass wasn't removed from the counter weights. since mass was removed from the counter weight mass has to be added somewhere else to get it all to balance so you end up with the extra material on the flywheel and dampner.

jfreeman74

But wouldn't it be different depending on what flexplate and dampener you use? I thought if it was externally balanced that that flywheel had counterweights added to it to balance the engine. This would change if I added a different flexplate from what the engine was balanced with, correct?

ede

the flex plates should have the same weights and the dampners too, so changing wouldn't change your balance.

jfreeman74

Ahhh, so all flywheels and dampeners are created equal? Well, as far as overall weight goes.

RB83L69

Stock 400 flex plates and balancers and flywheels are all the same. There is a spec for them. It's not like every single one had to be custom matched to its motor at the factory. Think about it....

An externally balanced motor should be built to the factory spec. That way, you buy a 400 damper from wherever, and a flywheel or flex plate from wherever, bolt it up, and go.

Any machine shop or engine builder that wants to custom balance your engine by altering the flywheel, doesn't need to stay in business. That's backwards.

The reason that a rebuilt short-rod motor can't be balanced internally, is the same reason that made them have to be externally from the factory. Think about it.... the crank has counterweights, that are large round things directly opposite the rod throws. The longer the stroke, the larger the counterweights have to be, in order to be of equal moment of inertia to the rod throw and rod and all that. The problem is, if the rod is short, then when the piston is at BDC, the counterweight is a TDC so to speak; piston all the way down, counterweight right under it. There isn't enough room to fit enough counterweight to completely counterbalance the rod. The center 2 rod throws counterbalance each other mostly, so there's not so much of an issue there; but the 2 end rod throws don't, so there has to be extra weight added at each end of the crank to make up for the part of the counterweights that won't fit.

TheMysticWizard

iTrader: (1)

Ok is it just me, or am I loosing my mind.. I have seen 383's that are internally balanced, I believe they were using 6" rods tho...

maybe I am lost.

jfreeman74

I have seen some internally balanced 383's but they were not using a 400 crank.

RB83L69

If you use long rods, it's possible to internally balance a 383 (or 400 or 421 or 434 or whatever).

A stock 400 crank already has a large amount of metal shaved off of the weights to accomodate the stock short rods, and so cannot be made internally balanced without the use of heavy metal.

MrDude_1

you CAN internally balance a 400, but its not normally done, and it isnt cost effective..

what the machine shop does is drill a hole in the counter weight, and fill it with a even heavier metal called mallory metal.

this heavier metal adds the needed weight to counterbalance the crank without making the weights larger.