[MHenricks]

Can someone explain the difference between internal and external balance, this is not real clear to me, 454

[ericnova72]

External balance was used when during the engineering design phase it was determined that the required crank counterweight size would not fit inside the confines of the engine block dimensions. So, in order to get enough counterbalance weight to balance the rotating assembly some of the needed weight was put on the flywheel and/or the harmonic damper, hence the term External(outside the crankcase). Examples are the 454 Chevy big block, 400 Chevy small block, all of the small block Ford Windsors(221, 260, 289, 302, 351W) and many others.

Internal balance has all the necessary weight on the crank itself and inside the crankcase.

[Cape Cod Bob]

I/m interested too. What I know is some motors are balenced on the crank itself , and others are balanced on the flexplate. Why, I don;t know.

[MHenricks]

Ok, I'm building a 454 with a procharger and hopefully will put out around 700 hp, which would be better int. or ext? I don't have a harmonic balancer or flexplate with this motor. The motor is approximately a 1996 model.

[techinspector1]

Originally Posted by MHenricks

Ok, I'm building a 454 with a procharger and hopefully will put out around 700 hp, which would be better int. or ext? I don't have a harmonic balancer or flexplate with this motor. The motor is approximately a 1996 model.

I too would like to hear from some of the professionals on this board concerning this question. I have a friend who is building a 7000-7500 rpm 383 to run on E85. The motor should produce nearly 600 hp (AFR's, 12:1, lots of cam) and he has asked me whether to use internal or external balancing. I'm frankly at a loss to help him decide and would like some help from the board. I'm thinking that at this level of power, you would want the balance to be pretty close, so would you have to tweak on the crank any with heavy metal if you used external? This may be dependent on rod and piston weight as compared to production weights, I don't know for sure. If you were building such a motor, which way would you go and why?

[ericnova72]

As far as the 454, at 700 hp I wouldn't worry about changing to internal balance, plenty of them have been built external balance at that power level over the last 30 years without a hitch.

The 388 is a bit different, the crank is physically smaller than a BBC crank, and you are getting up there on power and rpm and weight out on the ends of the crank loads the bearings more than weight inside on the crank, so it is a judgement call based on the crank type mostly. If you are buying an aftermarket forged crank most (and some of SCAT's cast cranks)are configured for internal balance already and if you don't get crazy with heavy rod and piston weights it won't need any mallory($$$) to balance it internal. It usually only gets expensive mallory when you convert an external balance crank to internal balance. If this is a street/strip engine I personally wouldn't worry with either balance type, it isn't at high rpm for more than a few seconds in each gear. If it is a road race or circle track engine I would recommend internal balance.

[techinspector1]

Thanks Eric.

[SSedan64]

One benefit with internal balancing, if the neutral balanced Flexplate or Harmonic Balancer is damaged you can easily buy a replacement without having to worry about more costly balancing as would be need with an Externally Balanced assembly.

[timothale]

Internal balance is always better from a design standpoint but sometimes there is not enough room inside the engine or it might be very hard to forge the crank shaft to have the weight where it's needed. think of a teeter totter in the play ground. as an external balance with 3 kids on each end , the plank will be bending. like the stress in an external balance crankshaft. I had a billet crank out of an early 50's crosley hot shot 4 cyl engine I used as a demo in my engine class ..they started with about a 5 inch shaft and cut it . the counter weights were machined on each side of each rod . the hot shots were able to turn very high rpm, with a little work. they were small displacement high rpm engine.

[MHenricks]

Ok, so if I have an externally balanced enginne, 454, shouldn't I still have the rotating components balanced?

[timothale]

When doing a rebuild or , performance upgrade you should have it rebalanced. A performance balance should match weights of all components to closer tolerences than the assenbly line specification. All pistons should be matched, all piston pins, all ring sets, the rods are suspended on a fixture and the small ends matched then reversed then the large ends matched then the total weight of the rods matched. bearing sets are usually within tolerence. The damper and flywheel you will be using should be included when doing the rotational balancing..... A little bit of balance theory....
, think of a teeter totter If two little kid are on one end and two big kid have to be closer to the pivot point to balance. If you visualize a one cylinder engine at top dead center the piston assy. and rod starts moving down and as the crank rotates the counter weights weight are starting to rotate and move up and it can be offsetting forces . when the piston is near half way down the counter weight is rotating at one side and the large end of the rod and the crank at the opposite side but the piston is in the bore above the center line . Like one of the kids moved above the pivot point so the teeter totter is not now balanced. when computing the weights added to the crank for the rotational balancing an average is used so the engine is in balance, then out of balance one direction then in balance then out of balance the other direction ...then repeating the situation. that is why some engines have an "extra" rotating 'balance shaft" at a different speed to smooth out the vibrations. When doing the rotational balance you can hear the crank smooth out when you start getting the counter weights matched up.