Internally vs. externally balanced

[btsai]

Engine gurus -- what are the practical application differences between an externally balanced engine and an internally balanced one? For example, besides the bore and stroke, how much difference is there between a 390 and a 428? (Yes, I know -- 38 cu in....). The 428 is known for lots of torque, right? But I don't think of the 390 as a high horsepower (again, relative to the displacement), higher revving engine. BTW, I'm intentionally leaving the 427 out of the equation for this scenario...

What's the scoop?

Ben

[SFfiredog]

There are theoretical advantages to internally balancing a rotating assembly. Idealy, one would like to have as much of the balancing weight in the center counterweights of the crankshaft as possible.
Sustained high RPM engines sometimes will have one or two extra center counterweights to help spread out the load on the crankshaft. This, of course, adds weight to the rotating assembly that would not be desirable on a road race or drag race engine.
The main advantage, and the only real reason to internally balance a normally externally balanced engine is the rotating assembly is not dependant on the flywheel or dampener being matched to the engine at the time the balancing is done.
Since these parts are dynamically balanced to themselves one can change dampeners or flywheels on a internally balanced engine without throwing off the balance weight.
In my opinion, unless you have an engine you plan to spin over 7500 to 8000 rpm (small block) or 6500 to 7000 (big block)regularly or want to swap lighter or heavier flywheels at any time the added expense of internally balancing would be better spent towards upgrading some other internal component (lighter custom pistons, aftermarket rods, etc.)
HTH,
--Mike

[btsai]

So if I read you right, having an externally balanced 428 doesn't really put you at a theoretical disadvantage compared to an internally balanced FE -- as long as you're not trying to build a high RPM motor.

I'm guessing they took the reverse approach with the motor they put into the "turd" -- a de-stroked 390 block that was designed for high RPM (although it didn't last -- nice idea, but tough to execute, I guess...).

Thanks for the great info, Mike.

Ben

[Mr.Fixit]

Very few people bother with internal balancing. It really isn't necessary, even for drag motors that see high rpm's.

[btsai]

OK -- now I'm a little confused. I thought 428's were externally balanced and all other FE's internally balanced. There's some piece to this puzzle that I'm missing...

[CJ428CJ]

BTSAI,

I believe you're correct. 428's are externally balanced and all other FE's are internally balanced. But, 428's can be internally balanced when they're being rebuilt if you prefer (and if you have a machine shop that knows what they're doing). Balancing a 428 internally involves the machine shop drilling holes in one of the crank counterweights and adding "slugs" to get the balance right. If you have it internally balanced, then there's no need to use a 428 flywheel that incorporates the counter weight.

CJ

[btsai]

Thanks, CJ. I see you run a 428. Funny how much attention is placed on the 427. 428 has similar look. I gather you can make a 428 go pretty good, too. eh?

[SFfiredog]

Originally posted by Mr.Fixit


Very few people bother with internal balancing. It really isn't necessary, even for drag motors that see high rpm's.

Fixit is right. I used to rev my Boss 302 drag motor to 8800 rpm (it saw 10,000 RPM during one really big wheelie).
It was externally balanced and I never had a bottom end problem. Now valve springs were a different story...
--Mike