Hot Rod Forum banner
1 - 20 of 24 Posts

·
Registered
Joined
·
230 Posts
Discussion Starter · #1 ·
I am building a 383 671 blower motor and i dont understand the difference in crank balancing. I am having a hard time trying to pick out a crank because i dont know the difference between them. does it have to do with the harmonic balancer, and does it make a difference on a blower motor

thanks alot
Marc
 

·
Stroked Z28
Joined
·
230 Posts
Externally balanced engines (ie. stock 400 sbc) use the harmonic balancer and the flywheel/flexplate to help balance the crankshaft. Thus, when the engine is balanced, all three pieces should be balanced together.

Internal, or neutral balance, mean that the harmonic balancer and flexplate/flywheel are evenly balanced (ie. stock 350 sbc). The balancing of the engine is accomplished by the crankshaft coutnerwieghts.

You can build a 383 several ways. You can turn down a 400 crank (external balance). You can buy an externally balanced stroker crank that has the longer stroke and samller main journals. Or, you can buy the same crank as in internally balanced unit. Select your balancer and flywheel/flexplate accordingly.

I'll defer to others on which is the best choice for a blower.
 

·
Administrator
Joined
·
18,631 Posts
If you want to make power, the whole assembly, pistons, rods, crank, etc should be balanced. This is besides your choice of internal or external balance. Its 2 different things

You order your crank, whatever you want, with the correct balancer and flywheel.

You then have the choice if you want the whole assembly balanced or not.
 

·
Registered
Joined
·
249 Posts
My Pontiac 400 was balanced when I built it in the early 80's, I took the clutch and flywheel off the other day and noticed it probably could use replacing. But if I do, will this throw the balance off? They drilled a lot of holes in the backside of it getting it lighter and balanced. Can I get just the flywheel balanced?
I don't really want to disassemble the engine that's for sure.
 

·
Administrator
Joined
·
18,631 Posts
Hard to say.....Did they use the flywheel weight to balance the engine originally? Looks like it.

Is the 400 Pontiac externally balanced.

I always figured that they balanced the recipricating assembly (engine parts) as a unit and that the flywheel etc was neutral.............unless, they are externally balanced where that would come into play.

We have probably totally confused the original poster now..............

Time for MachineShopTom to step in.............................over to you Tom.
 

·
Stroked Z28
Joined
·
230 Posts
I'm curious, too.

My builder insisted that I give him the flywheel & balancer I intended to use for my 383. He dynamically balanced the crank accordingly. I went with an SFI approved balancer in hopes of preventing future headaches (in case the balancer slipped---like my 400's did right before it proceeded to start eating my main bearings).

Back to the original post, I have been told not to blindly trust the "balanced" rotating assemblies that you can buy. I was lead to believe that the 383 rotating assembly should always be dynamically balanced. It can't hurt - especially if you are putting a blower on top.
 

·
Registered
Joined
·
249 Posts
I always thought a balanced engine was where all parts are static balanced and then the assembly was balanced as a whole.
When we balance rolls at work we static balance them then dynamic balance them to a certain RPM.

When I built my engine I know the pistons and rods where weighed and matched separately, the crank and flywheel was supposedly balanced separately and then the crank, balancer and flywheel were assembled and balanced as a unit.

What constitutes an internal vs external balance?
I have a hard time believing anytime you change a flywheel you'd have to remove the crank and have everything re-balanced. How is someone suppose to know what type balance they have?
It's very confusing :confused:
 

·
Stroked Z28
Joined
·
230 Posts
Hang in there... you just have two different ideas going on here. First, you can have an engine that is externally or internally balanced. Second, you can balance any engine (like you would balance a tire).

Internally balanced means that the crankshaft counterweights alone balance the engine. The harmonic balancer and flywheel are just along for the ride. They spin like perfectly balanced tires on either end of the crankshaft.

Externally balanced means that the crankshaft journals alone do not balance the engine - you must have the flywheel & harmonic balancer attached to act as counterweights for the crankshaft. It is just a design difference.

You can tell the difference between the int & ext balance flywheels & harmonic balancers by looking at their backsides. If I recall correctly, the external balanced ones have an area scalloped out on one side (they are not perfectly symetrical on the backside). There are probably exceptions to this.

You can BALANCE any rotating assembly. For example, my piston/rod combos were each individually weighed and machined to be within a couple grams of each other. This is static balancing. Then, my externally balanced crank had the harmonic balancer & flywheel bolted on each end. My machinist spun them at a pretty high RPM on a machine that worked much like a tire-balancing machine. It told him where to drill out the crank journals (which journal & how much). That is dynamic (moving) balancing.

When you put the statically balanced pistons/rods on the dynamically balanced crank, you have a balanced rotating assembly.
 

·
Save a horse, Ride a Cowboy.
Joined
·
5,122 Posts
428ho said:
What constitutes an internal vs external balance?
I have a hard time believing anytime you change a flywheel you'd have to remove the crank and have everything re-balanced. How is someone suppose to know what type balance they have?
It's very confusing :confused:

An external balance uses a harmonic damper and flywheel that have a heavy spot on them to assist in balancing the rest of the assembly. A heavy weight at a larger distance from the center of rotation than the crankshaft counterweights themselves allows for adding or subtracting less weight on the crankshaft itself.

Advantages = less expensive/ easier balancing procedure.
Disadvantages = The weight hung on either end of the crankshaft to balance the center of the crankshaft can/ does cause crankshaft twisting forces that might be detremental in high performance applications, even if a proper damper is used to reduce harmonics. = possible broken crankshaft. Other theories are bearing damage or loss of power due to crankshaft twist.

For our intents and purposes, it makes little difference.

A competent machine shop when balancing any engine of either type will NOT alter the damper or flywheel specification weight, only the crankshaft counterweights. i.e. So that any spec external flywheel will interchange exactly.
 

·
Stroked Z28
Joined
·
230 Posts
When my externally balanced 400 started to act up, it began shaking. The harmonic balancer had slipped. Thus, it was like an out-of balance tire going down the road. You know, whomp, whomp, whomp... technical terms... :)

The end result was the the front main bearings were worn all the way around. The middle bearings were worn on the bottom. The rear bearing was worn on the top. The rotating assembly was shaking itself to death... The cylinder walls were wearing unusually as well.

Had I had an internally balanced engine, the slipped harmonic balancer would have just meant that my timing marks were off. It would have run about the same.

That is why I personally plan on having my rotating assembly rebalanced if I ever swap flywheels. But, that is just my experience...
 

·
Save a horse, Ride a Cowboy.
Joined
·
5,122 Posts
Big Mouse said:
When my externally balanced 400 started to act up, it began shaking. The harmonic balancer had slipped. Thus, it was like an out-of balance tire going down the road. You know, whomp, whomp, whomp... technical terms... :)

The end result was the the front main bearings were worn all the way around. The middle bearings were worn on the bottom. The rear bearing was worn on the top. The rotating assembly was shaking itself to death... The cylinder walls were wearing unusually as well.

Had I had an internally balanced engine, the slipped harmonic balancer would have just meant that my timing marks were off. It would have run about the same.

That is why I personally plan on having my rotating assembly rebalanced if I ever swap flywheels. But, that is just my experience...

Interesting. :confused: I thought that the balance portion of the damper was locked solid to the crankshaft, that is, independent of the outer rubber mounted damper oscillation ring. :confused: Maybe we should go take a look at a 400 damper to be sure.

I think that what you experienced is not a "balance" failure but rather an elastnomer failure that cause the damper ring to slip and thus causing the damper to fail in its job of preventing crankshaft oscillations which in turn destroyed your bearings.

Harmonic dampers are incorrectly commonly termed harmonic balancers. On any engine, even an internally balanced engine they do "balance" but their main purpose is to dampen oscillations of crankshaft twist and whip.
 

·
Stroked Z28
Joined
·
230 Posts

·
Stroked Z28
Joined
·
230 Posts
xntrik said:
I think that what you experienced is not a "balance" failure but rather an elastnomer failure that cause the damper ring to slip and thus causing the damper to fail in its job of preventing crankshaft oscillations which in turn destroyed your bearings.
I can't argue with that. Either way, I'm playing it safe ;)
 

·
Save a horse, Ride a Cowboy.
Joined
·
5,122 Posts
Big Mouse said:
Xntrik - have a close look at the bottom of the pic for this balancer:
http://store.summitracing.com/partd...908395+4294840135+4294889107+115&autoview=sku

See the scalloped edge?
You are correct on that one. We always have used aftermarket dampers that do not use elastnomer rings for any performance engine. ATI etc. It has been a long time since I messed with a stock 400. Thinkig back I think it was 98 since I saw a 400 using a stock type damper...... :thumbup:
 

·
Save a horse, Ride a Cowboy.
Joined
·
5,122 Posts
Additional note: Obviously on that type of damper if the elastnomer fails the static and dynamic balance at the nose of the crankshaft, as well as the harmonics, are disrupted, with the results you experienced.

Grandpa always said. "It only costs a little more to go first class." :D

He also said...... "If you can't afford to do it right the first time, how much will it cost to do it over?"
 

·
Registered
Joined
·
173 Posts
OK....here is some more input on this issue. Some of what I am going to say has already been said.

Some engines, such as all SB Fords, 383 and 400 SB Chevys, and 454 and 502 BB Chevys, are externally balanced. What this means is that due to design limitations in the engine, they are unable to add enough weight to the crankshaft to bring it into balance. This is taken care of by adding weight on the damper and flexplate/flywheel. Because this weight is outside the engine, it is referred to as "external" balance.

Engine builders can convert external engines to internal by drilling holes in the crank and inserting an extra heavy metal called mallory metal. This is kind of expensive. Some crank manufacturers, like Eagle, have been able to redesign external style engine cranks so that they are internal.

In my opinion, if you can convert an externally balanced engine to internal, do it.

Here is where some of the confusion comes in. As several people have stated, on an externally balanced engine there is additional weight on the balancer and the flexplate. In the case of a stock balancer, this is achieved by cutting away, or scalloping as some have said, some of the weight off of the inertia ring which is the outer part of the damper. On the flexplate or flywheel, there will usually be a heavy weight tack welded to one area.

If you have a stock factory engine and want to switch balancers or flexplates you can do that without rebalancing the engine, as long as you use externally balanced dampers and flexplates on an externally balanced engine and neutral balanced dampers and flexplates on a neutral balanced engine.

Where problems start to arise is if you have a built engine and your engine balancer attempts to bring the entire assembly into balance by drilling holes or adding weight to either the damper or the flexplate or flywheel. If this has been done, then if you try to replace those components with new ones, your entire engine will be out of balance. Actually this is a no-no and no competent engine balance shop would do this.

As mentioned above, stock externally balanced dampers have a section of the inertia ring removed. Unfortunately this is a very bad deal if the rubber fails and the ring on the damper begins to rotate because it seriously throws the engine out of balance big time. If this happens while you are revving the engine up, it could throw the engine out of balance enough to break the crank. My company makes aftermarket performance dampers and we accomplish the external balance by using a bolt-in counterweight that bolts to the hub, not the ring. In this instance, if the ring should move, it does not affect the engine balance significantly.

In damper and flexplate/flywheel manufacturing, the units still have to be balanced irrespective of the added weight. By this I mean that the parts have to be balanced properly before the counterweights figure into the mix. With a bolt-in counterweight this is a simple task. The damper is spun on a balance machine and balanced by drilling holes in it. Then the counterweight is bolted in to provide the external balance.

With a factory style damper, they have to be bolted to a plate that has a counterweight on it to offset the counterweight on the damper when the damper is balanced.

So looking at your damper and flexplate/flywheel to see if they have balance holes drilled in them so you can determine if your balance shop has drilled them won't work. Because the manufacturer of the balancer and flexplate/flywheel will have drilled balance hole already. You might be able to tell if your balance shop has drilled additional holes because if the original part was painted, the new holes will show up.

I hope this information helps clear up some of these issues.
 

·
World Class ASE tech
Joined
·
704 Posts
Internal means the crank can be balanced by itself
External means the crank has to have the balancer and flywheel to be balanced.

The internal engine will have no weights or very smal one on the flywheel and balancer. The external will have big weights on the flywheel and balancer.

On a blower motor you don't need a balancer. So BDS tells me. But you should use a crank with 2 keys 180 deg from each other and a billet drive hub.

http://blowerdriveservice.com/
 

·
Registered
Joined
·
117 Posts
Since we're yacking about balancing...heres my prefernce..

I like to run nuetral balanced engines...i/e..non SCB 400 stuff, why?

Well, this way I take my crank in and have it balanced...have the rods and pistons matched..be done with it.

Then I grab near ANY flywheel/pressure pate/flexplate/balancer I want, and run the darn thing.

This way when my flywheel/pressure plate/etc goes bad or needs to be replaced, i don't have to take the engine apart and have it all balanced as a unit again.

Seems simple to me..lives very well to 7K rpms (probaly more) and is simple to fix when you want it to.

Just some of my thoughts, hope it helps.
 
1 - 20 of 24 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top