Engine balancing

[xb-60]

Could I draw on the forum’s experience again for explanations on engine balancing terminology and processes?

For example, what does balanced to 28oz mean when describing an engine?
How does this affect clutch choice?
Also what is external balancing?
How does a harmonic balancer work?
What items are subjected to internal balancing? And is this balancing static or dynamic?
Is material typically added (weights) or removed?
Do flywheels need to be balanced?
Do clutches need to be balanced?
I assume gearboxes, as supplied, don’t need balancing unless it’s for all-out racing applications?
Do tailshafts normally need balancing?
At what rpm point does extensive/comprehensive balancing become essential?
Does extensive/comprehensive balancing affect engine life significantly?

I understand the common-sense logic of balancing rotating and reciprocating components, but apart from simple static balancing, I don’t know how it’s accomplished.

Advice or input on any or all is much appreciated. Thanks.

Cheers,
Glen

[patrickt]

Glen, I think you asked too many questions in one single post. That said, if none of the real engine builders give you a detailed response, then I'll tell you the story of my 428 build, that started out life as an externally balanced engine with a goofy flywheel from the Ford factory, many, many, years ago. Then, almost 50 years later, a stork would drop my new bundle of joy on the doorstep and it was now an internally balanced FE of 447 cubes. The harmonic balancer, unfortunately, stayed the same (maybe that's why it spun apart). Hopefully blykins, barry, or keith will give you an experts post and you won't have to rely on an enthusiastic hobbyist and his story (that has absolutely nothing to do with a small block anyway).

[bobcowan]

Most SBF's are externally balanced. Meaning that the extyra weight is added to the blancer AND the flywheel. Early engines add 28oz to balancer and flywheel; later engines use 50oz.

Some custom engines will be internally balanced. And then the harmonic balancer and flywheel do not have weights attached. Internally balanced engines add Mallory metal to the crank throws if needed, or metal is drileld out of the crank throws.

Balancer and flywheel weights need to match the crank.

The crank is balanced dynamically - spinning. Bob weights are usually attached to the crank throws to simulate the weight of the reciprocating assembly.

Clutches and transmissions are generally not balanced, except in extreme rpm applications.

Any time the engine is tressed - regardless of rpm - then balancing becomes more important. It's a time consuming and tedious process, but worth the effort for racing.

[blykins]

Q: For example, what does balanced to 28oz mean when describing an engine?
A: It means that the engine uses a 28oz imbalance (externally balanced). This typically means that the harmonic balancer and flywheel/flexplate are 28oz items.

Q:How does this affect clutch choice?
A: It doesn't. However, some guys want to have the pressure plate balanced with their rotating assembly as an option.

Q: Also what is external balancing?
A: An externally balanced engine uses a weighted harmonic balancer and/or flywheel to keep the engine in balance.

Q: How does a harmonic balancer work?
A: It uses different materials, such as rubber, fluid, polymers, etc. to help eliminate the vibration harmonics out of an engine.

Q: What items are subjected to internal balancing? And is this balancing static or dynamic?
A: Everytime you balance a rotating assembly, it's dynamic. Go to YouTube and look for some rotating assembly balancing videos. The crankshaft is placed in a balancer and spun up to rpm. There are bobweights bolted to each rod journal which simulate the weights of the pistons/rings/rod bearings/rods/parasitic oil, etc.

Q: Is material typically added (weights) or removed?
A: It depends on which setup you have. You can do it either way, although the crankshaft manufacturer will design the counterweights on the crankshaft so that a target bobweight will be used.

Q: Do flywheels need to be balanced?
A: Not typically. They come from the manufacturer intended for a specific application.

Q: Do clutches need to be balanced?
A: The pressure plates are balanced from the manufacturers, but I have seen a few that need to be installed in a certain position to be perfectly balanced. You always balance the pressure plate only as a clutch disc is a consumable item.

Q: Do tailshafts normally need balancing?
A: If you're talking about a driveshaft, then yes.....always.

Q: At what rpm point does extensive/comprehensive balancing become essential?
A: There's really not an "extensive" balancing. It's either balanced or it's not balanced.

[madmaxx]

I thought the harmonic balancer was to counteract the torsional elasticiy of the crankshaft under load.

[blykins]

Originally Posted by madmaxx

I thought the harmonic balancer was to counteract the torsional elasticiy of the crankshaft under load.

What you said = what I said. Except you just used bigger words.

[Jerry Clayton]

I balance a crankshaft about every other day---there's more to it than what is easily visable or talked about on the internet. it sounds much simpler than it really is. I have been working on a book type thingie and have recently taken lots of digital photos of portions of the balance procedure, however haven't gotten all aspects covered yet--I should have enough photos to post in a few days (middle of next week) so be patient with me please and I'll get on it as I have one tomorrow--

[Jerry Clayton]

this is a trial upload--if it works notice the 2 pieces of heavy metal to internal balance this crank


On this pic of a rear counter weight you can see two pieces of mallory heavy metal installed into the crank and then a drill point drilling to remove what was then excessive weight--
Engine shops have charts of the weight of drilled holes, weights of different sizes of mallory AND then the differances of the weight of the removed iron and installed mallory--besides the numbers of the hole sizes, its important to know the weight of the mass removed by the POINT of the drill also---and of course theres a differance in 118and 1358 points

[madmaxx]

No, you said vibrations, I meant when the cylinder opposite of the other end fires the crank "twist" and the harmonic balancer helps resist this twisting. I dont think it balances anything it is more of a gyroscope.

Originally Posted by blykins

What you said = what I said. Except you just used bigger words.

[Jerry Clayton]

Here are some pics of part of the process of balancing a 3.250 Callies crank for a twin turbo charged small block ford

[Jerry Clayton]

The next group of 5 pics

[blykins]

Originally Posted by madmaxx

No, you said vibrations, I meant when the cylinder opposite of the other end fires the crank "twist" and the harmonic balancer helps resist this twisting. I dont think it balances anything it is more of a gyroscope.

No, I said harmonics. Actually I said vibration harmonics, which is really a redundant phrase....but I digress.

It's a damper. Harmonic damper. Whatever you want to call it, it's a shock absorber for harmonics in the engine.

It's easy to do a Google or Wikipedia search and pull off a big long string of engineering associated words, but I was trying to get the jist of it across, as this is more of a fundamentals type post.

[Luce]

Let me take a stab at this....

A crank is sort of like the crystal wine glass the fat lady sings to and makes it break. If you tap the rim, it rings. If you sing the exact same frequency to it, in the right location, the pressure waves will reinforce the vibrations causing the ring, and the vibration will increase until the forces are stronger than the glass, and it breaks. If you keep a finger lightly touching the rim, the vibrations are dampened and absorbed, and can never reach a dangerous level.

The soft mounted weigh of the harmonic dampener is that lightly touching finger. It dampens high frequency vibration that the pistons and rods can induce. A good crank will ring too if smacked, but not anywhere near as long if the harmonic balancer is installed.

And another reason the clutch disc doesn't need to be balanced WITH the rest of the rotating mass is because it's not indexed. Each time the clutch is engaged, it stops at a different place.

[madmaxx]

As I understand, you dont balance a crankshaft with a harmonic balancer, if the crank is out of balance changing balancers will not solve the problem.

[AL427SBF]

Originally Posted by Luce

Let me take a stab at this....

A crank is sort of like the crystal wine glass the fat lady sings to and makes it break. If you tap the rim, it rings. If you sing the exact same frequency to it, in the right location, the pressure waves will reinforce the vibrations causing the ring, and the vibration will increase until the forces are stronger than the glass, and it breaks. If you keep a finger lightly touching the rim, the vibrations are dampened and absorbed, and can never reach a dangerous level.

The soft mounted weigh of the harmonic dampener is that lightly touching finger. It dampens high frequency vibration that the pistons and rods can induce. A good crank will ring too if smacked, but not anywhere near as long if the harmonic balancer is installed.

And another reason the clutch disc doesn't need to be balanced WITH the rest of the rotating mass is because it's not indexed. Each time the clutch is engaged, it stops at a different place.

... or just say you need to dampen the engine's resonant frequency so you don't end up like that bridge caught in the wind oscillating at its' resonant frequency until it disintegrated.
----

[blykins]

Originally Posted by madmaxx

As I understand, you dont balance a crankshaft with a harmonic balancer, if the crank is out of balance changing balancers will not solve the problem.

It just depends on how you look at that...

If the rotating assembly has been balanced in a 28oz imbalance and you stick a 50oz balancer on there, then yes, changing balancers will solve the problem.

[Bill Hughes]

I believe that I comprehend the concept of an externially balanced system. What I don't understand is, what radius is this 28 or 50 OZ external weight at each end of the crank supposed to be at?

[Keithc8]

Balancing an enigne is not exact sience but there are formulas that work with most engine under 7000 rpms just fine. You get into race engines over that and you may want to over balance or under balance the engine. That is a topic for another time.
The 302 Ford came originaly as a 28 oz balance engine. It had a weight on the damper and on the flywheel. The weight is bigger looking on the damper because it is closer to the center. The weight on the flwheel is not as big as the flywheel because it is farther from the center of the crank.
When you are balancing the farther out from the crank you get the less weight you need to balance. Most Fords are external balanced with a weight on the damper and flywheel. this is problem because most balance shops will drill on the damper and or the flywheel when balancing the crankshaft and all. If you have a problem down the road and need another flywheel or damper your engine will be out of balance. We use a factory 28 oz damper and flywheel to balance these engines to then check the damper and flywheel that we will use to make sure they are to spec. This is more time consuming but the right way to do it. you want to drill on the crank and not the other parts. If our customer have a problem with the damper or flywheel or want to change them we can then get them a new 28 oz damper or flywheel and they will be ok. We can even check the new stuff.
This all causes a problem and later Ford went to a lighter crank in the 302 with a 50 oz damper and flywheel, 1980 I think to help with the mass of the crank and to help power and economy. The external balance is very bad for race engines and anthing over about 6500 rpms. The longer the stroke the worse the case is on this. Well when Ford came out with this 50 oz stuff it caused even more problems because people would install a 50 oz set up on a 28 oz engine and vise versa. Got to love this Ford stuff.
The 352, 390 and 427 Ford are zero balanced but the 428 is external on the back only and the 428SCJ is external on the front and back. this is has also screwed a few FE guys over the years.
To make matter worse a lot of the pressure plates are out of blanced and need to be balanced with the flywheel and then marked so that they bolt back up the same as balanced. If the flywheel pressure plate bolt pattern is even shifted just a little it will affect the balance. If you surface the flywheel it will more than likely knock the engine out ot balance because most of the time a little more comes of one side than the other, I have seen that several times. And everybody thought this stuff was easy.
Then you get into making cranks that were orignally external to zero or internal balanced which takes heavy metal in the crank counter weights. You can also drill the rod pin side if it has not been drilled.
I have balanced engines now for about 30 years and seen about everything. On the first race steel cranks we were doing we have put as many as 16 pieces of mallory in a crankshaft and ended up with a 800.00 to 900.00 balance job. We now have much better cranks that are made for zero balancing. I remember working with Scat on the first Ford steel cranks back in the late 80s, counter weight placement and size to try and get them to zero balance much easier.
Well this could go on for a while and I would have even more mis-spelled words. Hope some of this helps. Street engine external ok up to about 6500 rpms, race engine just go internal/ zero balanced. The external weights start to flex the crankshaft at high rpms and can even break it but normally takes the #2 and #4 main bearings out first. Road Race and Circle track enigne are harder on them than Drag racing. We need to write a book on this stuff.

Good luck, Keith Craft

[xb-60]

Originally Posted by patrickt

Glen, I think you asked too many questions in one single post. That said, if none of the real engine builders give you a detailed response, then I'll tell you the story of my 428 build, that started out life as an externally balanced engine with a goofy flywheel from the Ford factory, many, many, years ago. Then, almost 50 years later, a stork would drop my new bundle of joy on the doorstep and it was now an internally balanced FE of 447 cubes. The harmonic balancer, unfortunately, stayed the same (maybe that's why it spun apart). Hopefully blykins, barry, or keith will give you an experts post and you won't have to rely on an enthusiastic hobbyist and his story (that has absolutely nothing to do with a small block anyway).

Too many questions? Maybe Pat, but there's too much I don't know....
Looks likely further down the list there's detailed expert responses a comin', but this forum is about the stories as well as the detailed, reasoned, analytical responses, so don't hang back, buddy!
Cheers,
Glen

[xb-60]

Reworded questions...

For example, what does balanced to 28oz mean when describing an engine?
A: It means that the engine uses a 28oz imbalance (externally balanced). This typically means that the harmonic balancer and flywheel/flexplate are 28oz items.
Re-Q: But what do you mean, that the h-b and flywheel/flexplate are 28oz items? And the engine uses a 28oz imbalance? You know me Brent, I'm slow on the uptake.

Q:How does this affect clutch choice?
A: It doesn't. However, some guys want to have the pressure plate balanced with their rotating assembly as an option.
Re-Q: I can't find the email, but I thought you asked me awhile ago what engine imbalance for a particular flywheel or clutch selection?

Q: Also what is external balancing?
A: An externally balanced engine uses a weighted harmonic balancer and/or flywheel to keep the engine in balance.
Re-Q: I take it this is the quick,cheap version of engine balancing. Is this static or dynamic?

Q: How does a harmonic balancer work?
A: It uses different materials, such as rubber, fluid, polymers, etc. to help eliminate the vibration harmonics out of an engine.
Re-Q: Does an engine that is internally balanced have/require a h-b as well?

Q: What items are subjected to internal balancing? And is this balancing static or dynamic?
A: Everytime you balance a rotating assembly, it's dynamic. Go to YouTube and look for some rotating assembly balancing videos. The crankshaft is placed in a balancer and spun up to rpm. There are bobweights bolted to each rod journal which simulate the weights of the pistons/rings/rod bearings/rods/parasitic oil, etc.
Re-Q: Excuse my ignorance, but is the rotating assembly just the crank, or is it the crank/rods/pistons assembly?

Q: Is material typically added (weights) or removed?
A: It depends on which setup you have. You can do it either way, although the crankshaft manufacturer will design the counterweights on the crankshaft so that a target bobweight will be used.
Re-Q: Does the crank manufacturer (say, Scat?) balance his cranks, or does he leave that for the engine man?


Q: Do tailshafts normally need balancing?
A: If you're talking about a driveshaft, then yes.....always.
RE-Q: Halfshafts too?

Q: At what rpm point does extensive/comprehensive balancing become essential?
A: There's really not an "extensive" balancing. It's either balanced or it's not balanced.
Re-Q: OK...is there a a rpm level at which it is advisable to have the crank and rods and pistons all dynamically balanced?

Apologies if this is tedious going, but there'a a lot here that I understand a little, and don't understand a lot. Thanks for the time spent in replying. It's appreciated.
Cheers,
Glen