I was reading through the GMPP catalog and Chevrolet Power and it says to use "unbalanced" flywheels for all 86+ engines with the one-piece rear seal. My question is this: If the engine requires a non-neutrally balanced flywheel to counteract the unbalance of the crankshaft and the rest of the rotating assembly, why is is necessary to balance the rotating assembly? It would seem to me that you'd need specific flywheels for each crank/piston/rod combo.

For example if I'm building a 350 from scratch and I have the rotating assembly balanced, wouldn't I want a neutrally balanced flywheel since the rotating assembly is already balanced???

Secondly, are our engines (86+ SB chevy w/ 1-piece rear seals) considered internally or externally balanced engines?

Any help on engine balancing is greatly appreciated,

Jason

[This message has been edited by 89BlwnRs (edited January 16, 2001).]

 

Pretty sure the 86-up is considered internally balanced, but IMO it isnt. The flywheel or flexplate has the imbalance you mentioned. When you get one of these lower ends balanced, they need the flywheel/flexplate with it. If you happened to purchase one already balanced, and then bought a replacement flywheel or whatever that is for the 86-up, its probably close enough in most instances. If you are building some sort of high-RPM 8000HP monster, I would definitely have the flywheel you are using sent in with the rest of the bottom end to be balanced.

 

Ok Madmax, so if I buy the ZZ4 short-block from GM that comes already balanced and I wanna throw a flywheel on for a MT, do I use a standard 86+ flywheel with the counterweights? The ZZ4 comes with a flexplate, but I was unable to determine if the flexplate is neutrally balanced or has the counterweights on it.

Could I just use a stock 86+ flywheel on this balanced rotating assembly? I guess this would depend on if they balanced the assembly with or w/o the flexplate. If they did it without the flexplate, it would seem the flexplate would have to have a neutral balance. If they did it with the flexplate, it wouldn't matter b/c they were gonna balance it anyway.

Just doing some thinking on engine building,

Jason

 

You would just need to use an 86-up flywheel. The only crate engine I know of that still uses the old 2-piece rear main seal is the cheapie Goodwrench motor. The imbalance on the 86-up either flywheel or flexplate is supposed to be the same, not sure what exactly the number is. Small block Fords have been like this since the beginning of time, with an imbalance in the flywheel, and its considered ok to run what you want as long as you have the correct part, which in your case is an 86-up flywheel.
That help?

 

Yeah, you're helping a lot. One more question I've been pondering. If I don't have my assembly balanced, and I'm using a regular forged crank for 86+ w/ 1-piece rear seal, and then I use that 86+ flywheel we were talking about. Is the engine balanced now? Technically the engine is not ever really balanced (without taking it to a shop) because even if the flywheel counteracts an unbalance in the crank, it won't overcome all the possibly rod/piston combos. What if I use aluminum rods and forged pistons instead of cast. There's obviously a big weight difference, so this engine isn't really balanced if the same flywheel would be used on a different engine with stock PM rods and cast pistons.

I guess what I wanna know is where the imbalance initially occurs. Is the crank "balanced" by itself? If I spin the crank by itself at high RPM, will it be stable? What exactly is a counterweighted flywheel balancing out?

Do you see what I mean about different pistons/rods changing the mass of the rotating assembly and thus the balancing? Any ideas?

Jason

 

Yea...

If you use lightweight rods+pistons, then you have to lose material from the crank counterweights. I think you are getting beyond what I know but the older cranks used to have a flange on the back that was anything but round. The one piece rear seal cranks are round, so the extra material had to be added to the flywheel/flexplate. The idea is that once its all assembled, then its balanced. Some places even go to the extent of sending the clutch plate and cover in with the flywheel. Cant have too good of a balance I guess.

"If I don't have my assembly balanced, and I'm using a regular forged crank for 86+ w/ 1-piece rear seal, and then I use that 86+ flywheel we were talking about. Is the engine balanced now?"

Yes, it would be.
Basically the only difference in having your flywheel balanced with the rest of the parts takes into account any manufacturing or machining errors when the parts were made. They are supposed to be certain weights, and the crank is balanced using that information.

"Technically the engine is not ever really balanced (without taking it to a shop) because even if the flywheel counteracts an unbalance in the crank, it won't overcome all the possibly rod/piston combos."

Exactly. Its going to be close, but the more the actual weight of your components is off, the more off the balance is.

"I guess what I wanna know is where the imbalance initially occurs. Is the crank "balanced" by itself? If I spin the crank by itself at high RPM, will it be stable?"

Not sure where the imbalance occurs to tell you the truth. The crank needs the flywheel attached to be balanced, so no I would not run it at high RPM's without it (like you were going to do that anyway).

I guess you could look at it as a tire with no weights to balance it, it will wobble and shake and generally tear things up. The weight added to the rim is just like the imbalance in the flywheel or flexplate, and together it balances the system.

 

Thanks a bunch MadMax, I appreciated your help. I think I've got a good idea of what I wanted to know.

Thanks again,

Jason

 

No, a crank will not be balanced by itself. The counterweights on it are there to counteract both the imbalance of the crank journals themselves, and also the added weight of the big ends of the rods. Each incremental section of the rod as you move from the big end to the small contributes less and less, until you reach the small end which contributes basically zero to the balance situation.

The weight of the pistons is generally negligible when balancing a motor; they don't rotate (you hope). As long as they are all the same mass, their reciprocating effects will balance each other, since at any given moment there are exactly the same number traveling upwards as downwards, at exactly the same speed.

The reason for the external balance system in a 400 is alot easier to see if you have one in front of you. Imagine: you have x number of inches from the crank centerline to the top of the deck. You have crank journals a certain offset from the center of the crank, specifically ½ the stroke. You have an offset from the top of the piston to the top of the pin, specifically to accomodate a ring package. Subtract the diameter of the piston pin from that, now you have the place where the center of the pin must be.

OK, now if you're the factory, one of the most important things to you will be the reliability of the rings; the wider they are and the farther apart, the less warranty claims you will have.

You can see that the distance you have left when you subtract the piston height thing and ½ the stroke from the block height, will be the rod length. In the case of the 400, the factory kept the same ring package and deck height (unlike Brand F), so the only thing they could do from that point was to shorten the rod.

Now, look at the crank counterweights. They are fully "up" toward the deck of the block when the piston is down. That means that the counterweight's maximum dimension is limited by the rod length! So, the shorter your rods, the less counterweight will fit under the piston without it hitting the bottom of the pistons at BDC.

That's why the 400 is unbalanced: there simply isn't enough room in there to cram enough counterweight. That's also why you can build an internally balanced long-rod 400, but not an internally balanced short-rod one.

The 86-up small block has a small "unbalance" in the flywheel, to make up for the difference in the shape of the crank flange. 85-back cranks have an asymmetrical crank flange and an exactly neutral flywheel.

Again in the case of the 400, there is a "counterweight" at the front of the crank where it can counter the effect of one counterweight's shortfall, and a like one at the rear. There must be an asymmetry on the damper and on the flywheel to make up for the unbalanced unbalance at the other 2 counterweights.

Kabish?

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"So many Mustangs, so little time..."

[This message has been edited by RB83L69 (edited January 16, 2001).]

 

Wow! THat was a lot to cram in there RB, but it does make some sense! I think the best thing to do would be have the entire assembly balanced, but that's not even what I'm gonna do. I think I'll be fine with it as stock.

I'll have to look into it though, and see if anyone around here does it, and how much. If it's not all that expensive, I may have mine fully balanced.

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Working on:
'84 Z28 LG4 305 with 200,000 original miles!
Added dual elec fans.
145 MPH IROC Speedo
Building 430 HP 350
using primarily GMPP parts.
Block is in shop!

ASE Certified Master Tech

Starting to look like the Kicker poster child!

 

Kabish RB. Thanks a lot for that info, I see why the flywheel needs that unbalance since I looked at the two different cranks (1 and 2 piece rear seals) next to each other.

Thanks for the info on the 400 too,

Jason

 

all sbc's after 87^ are externally balanced from the factory. when you balance a engine you balance the piston, rods, crank, flywheel, and har-balancer. this allows higher revving and little more power, and greater durability. and it just naturally runs better.

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1989 firebird formula

Mods: converted from T.B.I. to a carburator 305 to a 350. Flowmater exhaust,hedman shortie hedders,202 heads,350 horse cam,bored.40 over, Edlebrock torker2 intake.

Future mods performer rpm air gap intake (polished) and 600 edlebrock carb, comp roller cam, and way better headsa 400 defintely in the works!