Internal vs external balance

[Steve.k]

At what point do you guys recommend or do internal balancing? Alot of small ford were external 28oz and seem to withstand alot of rpm. Where would you draw the line. Have a few cast cranks out spinning around 7300 with a h-beam rod attached. Thoughts?

[PackardV8]

How high is up? Maybe more specifics? The answer will come if those who have it get the question.

The SBF with 2.87"/3.0" stroke, short rods, are quite different forces at work than 3.75" - 4.25" strokes.

Then, there's the torque/time factor. What can survive a few 1/4-mile passes isn't always going to live with lotsalap roundtrack/hours-long road racing/offshore boat racing.

One advantage we have over the bad old days is light weight pistons and pins. Inertial forces increase with the square of the RPM, so beginning with a lighter piston helps with the balance.

jack vines

[Steve.k]

Jack 3.5 stroke looking for 7500+. Min,5.78 h-beam rod 4" bore and forged piston with pin weighs in at 669grams. Steel rods with bearings weigh680.

[Ken_Parkman]

External balance is a pain when you break a flexplate or need to change a damper or flywheel. I've run across this too many times, so now internal balance everything but a stock rebuild (AMCs).

[Dave Koehler]

If it is a warmed up street machine external is fine.
IF, however the above engine is really a toned down race engine in disguise with lots of expensive aftermarket parts then internal.
Race? yeah, internal every time.

[Steve.k]

Thanks guys. Got a 70 era cleveland with some serious head work. Every time we up the rpm 100 it picks up a fair bit. Currently running to 7300. Stock crank good rods,racetecs, titanium valves etc. However the motor is external balance and haven't changed. Not sure weather we should push lil motor much more.

[tuffxf]

Gday Steve,
Definently probably not ideal, i used to shift at 7550 - 7600 and across the line the same
H beams and cusom cps.
I didn't have any problems, you can see it by having a good look at the mains on 2 and 4, the crank will flex like the proverbial!
Run them loose and as you go ilghter on the rotating mass make sure whoever balances it keeps the external weight on each end as light as possible, take weight out there, it doesnt have to be 28 oz.
Other wise you end up with the counterweights looking like swiss cheese and full ext weight still there.
Is not good as it flexes more.
Cheers

[Steve.k]

Thanks tuff I'll spin her up a little more this season then give her the internal balance.

[DSRE388]

If your engine is physically capable of being internally balanced i can not see a down side to it, And ill second the PITA of breaking an externally balanced flexplate! Particularly one balanced exactly to that one engine!

[tuffxf]

Thats the go Steve!
Mines internal now as well.
Cheers

[Steve.k]

The heads would likely like to be at 8000 rpm + but I know it wont like that. Holding out for Trackboss iron block then will ratchet up a few notches.

[pdq67]

I always thought that Detroit externally balanced V-8 engines because it was cheaper and that the engines were mainly not going to be rpmmed above like 5 to 5,500 rpm so why bother?

I had my 496 externally balanced because I made it like a great big, "tow-truck", engine and it is cammed for max. Hp at 5,500 rpm and max. T at 4,500 rpm.

Cam is a good old CC 282S solid cam spec'd at 282/236, 110/106, .540 or so lift hot lashed so all I have to do to work it is to install my Erson HI FLOW AH hy-cam spec'd at 284/218, 111/111, .542" lift to turn it into a true work truck engine.

pdq67

[BirdMan]

When my main caps started walking around on 289 and 302 stock Ford blocks and cranks above 8000 I went to internal balanced and no problems. I have Mexican 302 blocks and Eagle forged stuff and no problems.

[Steve.k]

Thats pretty good rpm for external no doubt about it. We wil wick it up a touch.

[PackardV8]

I always thought that Detroit externally balanced V-8 engines because it was cheaper pdq67

External balancing is most often used because they ran out of room in the crankcase. The SBF began life as a 221"; as it grew, there was no more space in the crankcase for the needed counterweights. Same with the SBC which began as a 265" and no one believed it could grow to 400", but it did, albiet without space for the counterweights. The BBC was first a 396" and by 454", 496", 502" et al, there wasn't space.

The second reason external balancing is used is something changed with reciprocating weight in a subsequent version and was compensated by external balancing rather than a change to the crankshaft counterweights.

The third reason is some engines were given a final balance after assembly. The complete engine, with flywheel or torque convertor, is spun up by a 50hp electric motor. Any imbalance is corrected by drilling the front damper or flywheel, or by welding weights on the torque converter.

jack vines