Crankshaft balance question

[Stude Light]

On a modern V8 engine, I'm told that the piston/rod weights have to match what the crankshaft was originally balanced to, otherwise you have to mess with the counterweight balance. On a straight 6, I'm told that it doesn't matter if you change piston weights (assuming you keep all six piston/rod weights the same) as the crankshaft is zero balanced. So, if you change from cast iron pistons and you can take a bit of mass out with aluminum pistons, it's not an issue. Assuming this is all true information, what about a flat-plane V8 crankshaft? The 1921 Northway V8 I'm working on sure looks to have a zero balanced crankshaft.  The reason I'm asking is I'm thinking about getting some new aluminum pistons made and I'll need to make a decision on mass.

 

A little background reading....this engine is in an Oldsmobile Model 46 7 Passenger Touring Car and had been knowingly run with water in the oil (no glycol). I have been disassembling the engine in the vehicle to determine the source of the water and found that. I'm told the engine ran fine and made no noise.  I pulled all the fork and blade rods/pistons out and the crank and babbit actually look really good and clearances are at the high end of spec.  The bores are all standard and have some wear but look good otherwise. I have one cracked piston and the rest have excessive play at the ring to ring grooves (like 0.012" vs 0.0015"). Normally I would just have the engine rebuilt with new bores, pistons and bearings but since the bearings aren't too bad and the car body is a number 4 for condition, it's really not worth putting much into this nickel era car. So, I am leaning to just have Egge make me up some new aluminum pistons with the standard bore and a bit narrower rings vs the original 3/16". I would like to go as light as possible to reduce the bearing loading but wanted to make sure that wouldn't mess up the balance.

 

Thanks for any insight and experience.

Scott

[hidden_hunter]

1921 won’t have been factory balanced, Cadillac introduced it with the v63 in 1923. At best it would be that the pistons were approximately the same weight

 

Personally if it was me I would match the weight of the originals 

[Stude Light]

5 hours ago, hidden_hunter said:

1921 won’t have been factory balanced, Cadillac introduced it with the v63 in 1923. At best it would be that the pistons were approximately the same weight

 

Personally if it was me I would match the weight of the originals 

Hmm. That is surprising to me. Northway was making V8's for Cole, Oakland, Cadillac, Oldsmobile. Studebaker was already advertising balanced engines with fully machined rods and balanced pistons before 1920, so it would seem an engine manufacturer, like Northway, would be leading the industry in design and balance. The 1923 Cadillac V63 was the first to introduce a cross-plane V8 engine but definitely wasn't the first to be balancing their engine components. I know the Northway engines used by Cadillac were different from what Olds was using.

 

[JV Puleo]

At this point the value of balancing was fully appreciated but, what we call "balancing" and what they were referring to are slightly different. They could do static balancing. To go further, they could make the weight of all the reciprocating components equal. The best way to accomplish this was to machine them all over to exacting tolerances. The dynamic balancing machines we are familiar with were not invented until a bit later - I think in the early 30s. In any case, if you dynamically balance the parts you should not have a problem provide4d all the individual parts, pistons, rods etc... are the same weight.

[Stude Light]

We’ll, I’m not planning on pulling the crank as I would need to pull the engine and separate the halves. So it’ll just be making all the piston and rod assemblies weigh the same. Just wondering if I can go lighter.

[Larry Schramm]

If you are asking if you can replace cast iron pistons with aluminum, that is a very common exchange on old engines.  It will give you less rotating mass which means less wear on the rod bearings.  Should improve oil consumption also.  I have done it on any engine I have to take apart.

[automaschinewerks]

this will be interesting to follow.  in-line engines are easier to balance because of the cylinder pairing. 1-6, 2-5, 3-4 in a six cylinder for example.  it is fine to change weights like lighter pistons as long as all six are kept the same weight or in other words not putting in two cast iron pistons and 4 aluminum pistons.  this is because the cylinder pairs are up, down, sideways in relation to the length of the block at all times, if anybody can follow that picture inside a rotating engine.  the problem with v - type engines is the cylinder pairs cant be equally spaced from side to side and end to end.  even on flat plane engine, piston on one side is coming down and on other side is on its way up, on whichever pair along the length of the block.  the problem is the crank and/or flywheel is balanced for the weight of cast iron pistons.   any v-type engine i have balanced needs to have bobweights on the rod journals with a weight formula calculated from the rod/piston/rings weight (reciprocating mass).  on an in-line engine the crank and flywheel is spun by itself in the balancer with no need for these bobweights (due to the even cylinder pairing) and only the pistons and rods weighed separately and weight removed to make each the same as the other five.

that said ive never had to balance a flat place v-type crank so like i said, this will be interesting to follow.

terry

Edited November 29, 2021 by automaschinewerks
weight removed to make each the same as the other five, not six (see edit history)

[Stude Light]

I know that the flat-plane V8s are likened to two inline four cylinder engines joined at the crankshaft and have an inherent second order imbalance issue. I’m just wondering if I can go with lighter pistons without messing up first order balance. It seems any reducing in rotating mass will be kinder to the bearings and help reduce the second order vibration.

 

So, if we consider each bank as a 4 cylinder engine and, examining the design of the crankshaft counter balance, which appears it is designed just to offset the mass of the throws on the crank, it seems logical that piston mass doesn’t matter but I wanted to get some feedback from the vast level of experience our forum members have. 

[Brian_Heil]

Engine balance.

 

There is reciprocating mass which is the piston, rings, wrist pin, any pin retention devices and the small end of the rod.

 

And there is rotating mass which is the the large end of the rod, bearing shells (if so equipped) and the fasteners along with the counter weights on the crankshaft and the whole crankshaft.

 

The way the two rod masses are determined is by placing a rod on two precision scales with the bores of each end supported on each scale.

 

Reciprocation mass is second order (twice per rev), rotating mass is first order (once per rev).

 

The layout of an engine is done for a number if reasons.  One of those is to cancel secondary vibration or in other words the piston masses going up and down.  This is why V8s are 90 degrees for the V and 8 cylinders not 7 or 6.

 

If you change all the reciprocating masses with say lighter aluminum pistons, and make them all still weigh equal, they will all still cancel.

 

Now back to primary balance.  The position and shape and mass of the crankshaft counter weights (is a complex design issue and not just there to look nice), they are there to offset or balance the large end mass of the rod assembly and the offset journal and throw of the journal of the crank and since this is rotating at engine speed it is primary balance.  To balance the crankshaft, a ring weight is fabricated to match the large end rod mass and these are affixed to each rod journal.  In the case of a V8, eight ring weights would need to be fabricated and attached and then the crankshaft is spun on a crankshaft balance machine and weight is added or removed from the counterweight(s) to bring it in to balance.  In production, the end counterweights are made just a little bit too big so a hole is drill in them as required to bring the assembly into balance.  Adding mass gets a bit difficult to keep it attached a 5000 ERPM.  But I have seen it done.

Edited December 1, 2021 by Brian_Heil (see edit history)

[Stude Light]

6 hours ago, Brian_Heil said:

Engine balance.

 

There is reciprocating mass which is the piston, rings, wrist pin, any pin retention devices and the small end of the rod.

 

And there is rotating mass is the the large end of the rod, bearing shells (if so equipped) and the fasteners along with the counter weights on the crankshaft and the whole crankshaft.

 

The way the two rod masses are determined is by placing a rod on two precision scales with the bores of each end supported on each scale.

 

Reciprocation mass is second order (twice per rev), rotating mass is first order (once per rev).

 

The layout of an engine is done for a reason.  One of those is to cancel secondary vibration or in other words the piston masses going up and down.  This is why V8s are 90 degrees for the V and 8 cylinders not 7 or 6.

 

If you change all the reciprocating masses with say lighter aluminum pistons, and make them all still weigh equal, they will all still cancel.

 

Now back to primary balance.  The position and shape and mass of the crankshaft counter weights (is a complex design issue and not just there to look nice), they are there to offset or balance the large end mass of the rod assembly and the offset journal and throw of the journal and since this is rotating at engine speed it is primary balance.  To balance the crankshaft, a ring weight is fabricated to match the large end rod mass and these are affixed to each rod journals.  In the case of a V8, eight ring weights would need to be fabricated and attached and then the crankshaft is spun on a crankshaft balance machine and weight is added or removed from the counterweight(s) to bring it in to balance.  In production, the end counterweights are made just a little bit too big so a hole is drill in them as required to bring the assembly into balance.  Adding mass gets a bit difficult to keep it attached a 5000 ERPM.  But I have seen it done.

I knew a Powertrain guy would speak up and set things straight. Thanks Brian.

[Brian_Heil]

17 hours ago, Stude Light said:

I knew a Powertrain guy would speak up and set things straight. Thanks Brian.

I’ve still got all my files from when I designed the crankshaft, flywheel, dampolator and balance shaft for the 1988 3800. 

[automaschinewerks]

sooo...... the part i cant get my head wrapped around is when i make the bobweights necessary to balance a v-type engine in my engine balancer, suppose i balance the rotating assembly to a reading on the unbalance gauge,  suppose i take and add a 20 gram washer to each bobweight, then spin the crank again, it shows a more unbalanced reading on the unbalance meter.  as long as all piston weights changed are equal to eachother, the crank should be in static balance.  but dynamic balance, which is when the crank is spinning is a different story though.  remember ive only had to balance modern v-types engines with uneven clocked rod journals, not an older single plane crank like yours, which is like a regular in line 4 cyl. crankshaft,  which would not need

bobweights if it were balance for an in line 4 cyl. engine,  but im not sure what it would need for bobweights to balance for a v-8 engine like yours.

[Brian_Heil]

1). What is the engine layout. V8?

 

2). Does the crank have counterweights as part of the casting?

 

3). Pics?

Edited December 3, 2021 by Brian_Heil (see edit history)

[automaschinewerks]

hi brian, if you were asking me about the crankshaft balancing,  then yes a regular 50s,60s,70s,80s,90s v-8 crankshaft ford,chev,dodge or whatever make, in a stewart warner electronic engine balancing machine.  i dont have any pics of my balance jobs setup in the balancer, but im sure google will show lots of pics of same setup.

[Stude Light]

Regarding the flat-plane Northway V8. It is probably balanced like 4 cylinder which is why it has the inherent 2nd order imbalance. Thus it seems lighter pistons means lower 2nd order….but that’s me guessing.

Edited December 4, 2021 by Stude Light (see edit history)

[Brian_Heil]

When balancing the crank forget about the reciprocating mass, you are only balancing the rotating mass.  And if the person who designed the crankshaft did it properly, the bob weights when affixed should bring the crank in close.  
 

Now, there are a number of engines where you can’t get enough counterweight size/mass on the crank to balance with the correct bob weight.  There are packaging issues that limit how big a radius counterweight can be swung within the crankcase or to the piston skirt when it comes to the bottom of the bore and hangs out.  Or the width of the counterweight is limited because of narrow cylinder bore spacing that dictates narrower counterweights etc.  When this happens a counterweight is added external to the engine on the front balancer and on the rear flywheel. When this happens you have to adjust the bob weights to compensate for this external counterweighting or include the balancer and flywheel with their counterweights on the balance machine fixture. 
 

Remember, the reciprocating masses on a V8 cancel themselves out. If they are equal, you don’t worry about them That’s why a V8 is 90 degrees in the V and there are 8 cylinders and one piston from each bank share a common crank pin. In that configuration they (the second order balance forces) cancel out. 

[Brian_Heil]

3 hours ago, Stude Light said:

Regarding the flat-plane Northway V8. It is probably balanced like 4 cylinder which is why it has the inherent 2nd order imbalance. Thus it seems lighter pistons means lower 2nd order….but that’s me guessing.

Correct. 

[automaschinewerks]

brian, yes thats right about the external balance engines, i balance with the front pulley and flywheel installed.  i dont see any instruction on how to calculate adjustments to the bobweights to compensate if i didnt have or couldnt install the pulley and flywheel.

with this old crankshaft i would imagine there is no couterweights that have been balanced to the weights of cast iron pistons, the only time balance would go out is on a crankshaft with counterweights.  

is this what is meant by zero balanced crankshaft.  ive heard people call cranks with counterweights that were internal balanced, as in no extra weight lumps on flywheel or front pulley.  ive heard people give engines and parts more stupid nicknames than i can keep up with.

[Brian_Heil]

51 minutes ago, automaschinewerks said:

brian, yes thats right about the external balance engines, i balance with the front pulley and flywheel installed.  i dont see any instruction on how to calculate adjustments to the bobweights to compensate if i didnt have or couldnt install the pulley and flywheel.

with this old crankshaft i would imagine there is no couterweights that have been balanced to the weights of cast iron pistons, the only time balance would go out is on a crankshaft with counterweights.  

is this what is meant by zero balanced crankshaft.  ive heard people call cranks with counterweights that were internal balanced, as in no extra weight lumps on flywheel or front pulley.  ive heard people give engines and parts more stupid nicknames than i can keep up with.

Think of the external unbalance of the balancer and flywheel as just an extension of the crank counter weights, an additional counterweight on each end of the crank if you will.  They just couldn't fit that mass on the crank, not enough room so they hung it outside.

 

The counterweights on the crank can only balance so much bobweight.  When the required bobweight (due to the lower mass of the rod) exceeds what the counterweights of the crank can balance, then the balance has to be made up by the external weight on each end.

 

People get hung up on wanting to put all the counterweighting internal to the engine.  This is actually less efficient.  That distance end to end along the centerline axis of the crank is key.  Without getting into physics, you are balancing a dynamic couple so the longer this distance, end to end you hang the weight, the less weight required.

 

In the pic in the link below, the distance d is what I am referring to.  As d gets longer, F, the forces generated by the counterweights, can get smaller since they are multiplied by each other in the equation.

 

Simplified lets say F x d needs to equal 48 to balance the engine.  An F of 8 times a d of 6 would work.  But if I can make d longer, say 12, then F only needs to be 4.

 

And, you can go too far with too much external balance when someone says great, I will hang all the mass externally, then you get issues of crank bending and stress and fatigue at high erpm's when you do that and knock out front main bearings or snap the nose off the crank.

 

 

https://www.google.com/search?q=rotating+couple+physics&tbm=isch&ved=2ahUKEwjBh_6cm8r0AhURT6wKHfcECuYQ2-cCegQIABAA&oq=rotating+couple+physics&gs_lcp=CgNpbWcQAzoICAAQCBAHEB46BAgAEEM6BQgAEIAEOgYIABAIEB5Q9wdY8UFgo0loAHAAeACAAWSIAYkNkgEEMTcuMZgBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=_2erYYGME5GesQX3iaiwDg&bih=697&biw=1366&rlz=1CAXGER_enUS915#imgrc=SnuJK2r0Xrr2HM

 

 

Class dismissed  

 

 

[Brian_Heil]

And those crank counterweights get involved in bending stress and design of the strength of the crank too.  Sometimes you can balance the crank fine only to next determine those 'fine' counterweights at those positions are not optimal for reducing bending and stress in the crank.  So you get the stress where you need it by moving or changing counterweight shape and see how bad that messed up your balance.  But did that last change you just made remove casting material where you need to drill an oil passage?  And the list goes on. 

 

It's all done by computer now but someone who knows what they are doing has to write the program and load the crank model.

[trini]

The crankshaft on the Dodge Senior 6 is touted as having no counterweight . The engine runs smooth and purrs as a kitten. Were the dodge brothers that good ? Compare the Chevrolet of the same year. It is like chalk and cheese.