Don Terrill’s Speed-Talk

What's the real pros & cons of converting an externally balanced engine to totaly internal balance?

SKeown

Think about how much stress is on a crank snout with 50 oz on one side of the balancer at 7000 rpm.

It costs money.

External balance = a bandaid. Oops forgot to internally balance it.

panic wrote:It costs money.

Maybe it's priceless.

It all depends on what you're going to do with the engine. Under 7000rpm external balancing will be fine and it's used because it can allow the engine to be balanced less expensively and also the counter weights inside of the case can be smaller/crank can be lighter which could be an advantage when it comes to power.

Internally balanced engines are nice because it allows the parts which are on the outside of the engine to be more universal. If the flywheel needs to be changed it only has to be balanced to itself, if it is then the rest of the engine is unaffected.

One thing that I always do when balancing an externally balanced assembly is use a standard amount of balance weight on the flywheel and damper. For instance, if it's a 454 keep everything 454, not almost a 454 weight wise in order just in case there is a problem farther down the road and the customer has to change something, balance won't be thrown out the window.

DaveMcLain wrote:if it's a 454 keep everything 454, not almost a 454 weight wise

"Almost a 454". I had to laugh at that. Not because you are wrong but because you are right. Just never saw it in print.
Ever run into one from any mfg that actually met the oz/in target and position? Kind of rare.

Just did a Ford 460 flywheel match for a previously balanced assembly. The oem flywheel was a little light on the target and 3 degrees one way. The aftermarket replacement was way heavy on the target and 6 degrees the other way. Much fun balancing and moving the vector point by 9 degrees at the same time.

An externally balanced crank can have less stress on the 1st and last rod throw because the 2nd CW is too small due to block space limits so the end CWs are made too big to compensate. The external balance takes part of the excess and straddles it over the end mains thus reducing the stress on the 1st rod throw.

I'm currently doing a cheap light weight rotating assembly .060 over 302 to eventually replace my 289. I really want to go internal balance since the plan is for a shortblock that will handle 7,500 shift points and an occasional accidental bump off a 8,100 rpm chip in the rev limiter. Just depends on how much Mallory metal it'll take. For an appication like this I see no down side other than the cost of Mallory metal to get it to zero.

If you do chose external make sure you have 2 wheels balanced the same when you do your balance job, kill the starter ring gear and you are sunk.

Since are on balance and a couple guys on here know what they are talking about...

I have heard numbers from engine builders 1-2% overbalance and some underbalance. Seen some stuff in my OLD Hines book, but not the new ones...

If you have a 1600g BWT 2% is 32g and I know guys pulling thier hair out to get down to a couple thenths of a gram.

What do you guys think ???

289nate wrote:I'm currently doing a cheap light weight rotating assembly .060 over 302 to eventually replace my 289. I really want to go internal balance since the plan is for a shortblock that will handle 7,500 shift points and an occasional accidental bump off a 8,100 rpm chip in the rev limiter. Just depends on how much Mallory metal it'll take. For an appication like this I see no down side other than the cost of Mallory metal to get it to zero.

Cheap and internal balance with a 302 Ford don't really go together, and REALLY don't go together with the later 50 in oz cranks. With the relatively compact crankshaft/rod/block assembly so long as you stick with the earlier heavy casting (2MAE) crank with the early 28 in oz harmonic/flywheel I wouldn't sweat the internal/external debate, they don't seem to mind 7500. If you're heart's set on an internal setup, you might be better off starting with a standard weight Scat 3" stroke forged crank since it should need less mallory and shouldn't have any factory balance holes to intersect with said mallory.


SMALL BLOCK FORD 302 FORGED
Part No Short No Engine Description Stroke Rod Length Rod Pin Weight
4-302-3000-5090-2123 43021 302 302 STD WEIGHT 3.000" 5.090" 2.123" 48

raceman14 wrote:If you do chose external make sure you have 2 wheels balanced the same when you do your balance job, kill the starter ring gear and you are sunk.

Since are on balance and a couple guys on here know what they are talking about...

I have heard numbers from engine builders 1-2% overbalance and some underbalance. Seen some stuff in my OLD Hines book, but not the new ones...

If you have a 1600g BWT 2% is 32g and I know guys pulling thier hair out to get down to a couple thenths of a gram.

What do you guys think ???

Not sure what other builders refer to as percentage of under/over balance, I refer only to the reciprocating weight, not total weight. So if the reciprocating part of that ficticious 1600g bobweight is 700 grams, 2% of it (for me) would be 14 grams.

I have heard numbers from engine builders 1-2% overbalance and some underbalance. Seen some stuff in my OLD Hines book, but not the new ones...

Ever hear a physics based reason for using overbalance?

I checked Einstein's and Hawking's work. You are correct, they make no mention of it...but then they make no mention of 50% either.
It's race tech or as one scribe put it years ago "a California snow job"
Some swear by it. Some swear at it. Some use it to cut corners (averaging style balance job).
I don't care. I do it when requested.

I like external balancing in some cases. Thee further out from center the less weight is needed.