14 replies to this topic

[fredeuce]

There is an old thread in the archive but can't be replied to. So a new thread is in order.

I am interested in the art/science of engine balancing.

I have read that when calculating the mass for the bob weights when undergoing the dynamic balancing process the technicians perform a number of steps in this process.

Firstly , they determine the rotating mass which includes the big end of the conrod the rod bolts and an allowance for oil.

Secondly , they then determine the reciprocating mass which includes the piston , rings , gudgeon (and any retainers) and also the mass of the small end of the rod.

The next step is that bob weight is determined by adding the rotating mass to 50% of the reciprocating mass.

My question is how is it determined that 50% of the reciprocating mass is the appropriate proportion to add to the rotating mass? What is the maths /physics behind that ?

Next question. I have also read about overbalancing whereby the proportion of the reciprocating mass that is bought into the equation is not 50% but perhaps slightly higher say 52%. This is purportedly to deal with high rpm vibration problems. This of course depends upon the proposed use and the applicable working rev range needed. The article in question was discussing a big block FE Ford V8.

Next question how does this play out with the early Holden cranks and their habit of failing in that zone between 6000-7000 rpm but okay either side of this range. That is to say is overbalancing a solution here ? This begs the question as to what the cause of those failures is.

Over to the gurus!

[Greg Locock]

My question is how is it determined that 50% of the reciprocating mass is the appropriate proportion to add to the rotating mass? What is the maths /physics behind that ?

Next question. I have also read about overbalancing whereby the proportion of the reciprocating mass that is bought into the equation is not 50% but perhaps slightly higher say 52%. This is purportedly to deal with high rpm vibration problems. This of course depends upon the proposed use and the applicable working rev range needed. The article in question was discussing a big block FE Ford V8.

That's not the way it is done on a modern engine. The counterweight mass for each web is set by considering the forces in the main bearings. However, the 50% number is a good place to start from.

[Grumbles]

The failure mode of the Holden six cranks is related to torsional vibration of the crankshaft and has little to do with balancing - though of course changing the bobweight percentage will raise or lower the natural resonant frequency of the crank slightly. The balance factor used will always be a compromise and while "overbalancing" is currently fashionable I'm not convinced of any real advantage to factors much above or below 50%.
If you want a Holden 6 crank to live you need to control the torsional vibration, and that's a different subject altogether..

[fredeuce]

Greg and Grumbles , thanks for the responses.

Can either of you expand on the 50% reciprocating mass allowance and why that was/is used ?

[pugfan]

Greg and Grumbles , thanks for the responses.

Can either of you expand on the 50% reciprocating mass allowance and why that was/is used ?

Imagine the case where you have the crank alone and add some counterweights so it is balanced. Add the piston but now you have no reciprocating balance which your main bearings won't like. So you add 100% of the reciprocating weight to your crank. Your crankshaft is now out of balance and again your main bearings are not happy. By using 50% rather than 100% your main bearings are the least unhappy they can be, i.e. peak forces will be lowest. This is my understanding and may not necessarily reflect reality.

[pugfan]

The counterweight mass for each web is set by considering the forces in the main bearings.

Does this imply including forces acting on the piston?

[Grumbles]

Greg and Grumbles , thanks for the responses.

Can either of you expand on the 50% reciprocating mass allowance and why that was/is used ?

You might be interested in this, this, and this. More targeted at the seven bearing red motors than the old greys, but the principles are the same.

[gruntguru]

Imagine the case where you have the crank alone and add some counterweights so it is balanced. Add the piston but now you have no reciprocating balance which your main bearings won't like. So you add 100% of the reciprocating weight to your crank. Your crankshaft is now out of balance and again your main bearings are not happy. By using 50% rather than 100% your main bearings are the least unhappy they can be, i.e. peak forces will be lowest. This is my understanding and may not necessarily reflect reality.

To enlarge a little - consider the 100% balance scenario. Along the vertical axis (parallel to cylinder) the reciprocating masses will be completely balanced by the counterweights. Unfortunately at 90* to that (horizontal) the side-to-side component of the counterweight motion is not balanced by anything. Consequently by adding 100% counterweights, all you have achieved is to transform the reciprocating imbalance from a vertical one to a horizontal one. 50% counterweights are an each-way bet. The vertical imbalance is halved and the horizontal imbalance is a similar magnitude (and 90* out of phase so the resultant is a rotating imbalance. Can one of you experts tell me if it rotates the same way or opposite the crankshaft?)

Edited by gruntguru, 22 January 2011 - 05:27.

[Lee Nicolle]

You might be interested in this, this, and this. More targeted at the seven bearing red motors than the old greys, but the principles are the same.

While there is a lot of interest in those items i will disagree with a lot too.
Balancing by experts is really nesecary on all engines that are rebuilt. Most modern pistons are lighter and that needs to be compensated for. Lighter is better ofcourse.

Harmonic balancers will always be an item of discussion. The Romac ones seem ok, as are the fluid ones. The Holden ones need to be bonded or they spin all the time, but when bonded makes them harder and they damp a little less. The XU1 plate is a requirement on these engines. As is a locking bolt and washer to hold the balancer on. 3/8 Grade 8 is enough.
Short stroke reds dont usually ever have a major harmonic problem, balanced properly, good rod bolts, Loctite the flywheel bolts in and turn them to 7500.

But 202s are a drama, Really a better damper is required, the flywheel has to be shrunk on or it will come off. Plus use new bolts everytime and use loctite and torque them 5lb more than recomended. Though these are VERY hard to get now. Redrilling and using Chev bolts never hurts, BUT, be very careful doing it or they will no longer fit 100% properly and they will cock and break.Dowels help too. Balancing is imperative as is a bigger oil pickup, preferably external.
The counterweight crank helps some of this but it is 15lb heavier, and that is a problem in itself. Most Nc engines are using them now but I still think they are too heavy.

While long and aftermarket rods are and always will be very nice the slowfire/blue motor ones are pretty good. But pick them carefully and crack test them [and bolts] before you start. Standard bolts are ok but ofcourse ARP are better. But then the rods will need to be resized. End of story, always. And cracktest new ARP too there has been crook ones.

Pistons, use the best you can afford, For mild engines standard Duralites are fine. Steel support original style are junk. Never use them as they fail at about 3000 rpm! Some HQs still use them. The Race series Duralites were very good, but nigh on impossibe to get now, as are the APEP ones too. Pity they were cheap and good. And will take 7000rpm ok. Or more on 186s

I have built a lot of red 202s with nothing trick and assembled properly will not fail. I have never cracked a block, never broken a crank and since shrinking flywheels on [of all weights] have never had one come loose.
The magic RPMs seem to be 6800 max[6500 safer] for a 202 and about 7500max [7200 safer] for a short stroke. And compression above 11.2-1 will cause a lot of trouble. That is what cracks blocks etc. not hamonics. Though I guess detonation couples with harmonics kills them!!. Now the HQs are back to lower compression they are not cracking blocks, or hammering bearings etc. The 11.2-1 originally came from John Lewis [Maxwills] and I will agree with him 100%. And ofcourse that is 100 octane territory. 98 ULP is 10-1, and 91 about 8-1 !!

Though the rest of the engine can be a problem without regular checking. Starters come loose, alternators come loose and break the adjusting bracket, engine mount bolts come loose. And do not solidly mount them in preference, and if you do use an engine plate. Not just the side mounts. As an aside with alternators. When revving them hard remove the fan or it will centrifuge and jam against the bracket, which in turn will spit the belts off. If you do not have to use an alternator, DONT. Big pain in the bum!!
Really if you have to use one put the biggest pulley on it too slow it down or it will fail, fall off and short out either way. Been there done that!!

Slow the waterpump down too to crank speed, saves the pump and aids cooling considerably

While I do not know the ins and outs of balancing I have had Maxwill Motors do mine for 30+ years and as I have said no real problems.Holdens,6 and V8, Chevs, Clevelands [expensive], Windsors, A Chryslers and 4.1 Fords.
While on that my Chev Sports Sedan engine cracked cranks, no real problems but when time for a rebuild it was scrap the crank.[It would not do another run] It turned out my balancer was too big and heavy. New lightweight trick one [about $250] and no more troubles.

I feel grey motors are possible better off with lighter pistons, some of those old race pistons are bloody heavy, I know there is some better balancers around now. Though I have seen injected midget engines without them at all. {and no flywheel ofcourse]
Once again balancing is a major factor, use the experts. Most engine reconditioners really do not know what they are doing. Standard is usually ok, after that they are lost.
I feel that too much compression may be a problem with those too, even on methanol.Like the red motor the chamber will never be very good.
Longer rods may help too, as they are less agressive at TDC and will smooth out the stroke.But no I have no idea what fits, I have heard Mitsubishi of some sort? X6 ofcourse

[jimmy8v]

Vibration free have some interesting articles on this, they tend to be at a more practical level than say, a dynamics textbook but interesting all the same.

http://www.vibration...blications.html

Edited by jimmy8v, 21 January 2011 - 09:22.

[NoNoiseLimits]

When i was at Prodrive we did some extra balancing to the Subaru engines, to improve on the factory balance(WRC engines start life as an STI from the factory, and are stripped upon arrival), and measured no difference in power or durability. This may be something related to the nature of the flat four, but the old aircooled VW definately benefits from balancing.

[Magoo]

My question is how is it determined that 50% of the reciprocating mass is the appropriate proportion to add to the rotating mass? What is the maths /physics behind that ?

Next question. I have also read about overbalancing whereby the proportion of the reciprocating mass that is bought into the equation is not 50% but perhaps slightly higher say 52%. This is purportedly to deal with high rpm vibration problems. This of course depends upon the proposed use and the applicable working rev range needed. The article in question was discussing a big block FE Ford V8.

Next question how does this play out with the early Holden cranks and their habit of failing in that zone between 6000-7000 rpm but okay either side of this range. That is to say is overbalancing a solution here ? This begs the question as to what the cause of those failures is.

Over to the gurus!

On an inline four or six like the Holden, bobweights are not commonly employed in dynamic balancing because each cylinder has another cylinder in reciprocating opposition. Bobweights attached to these rod journals would mainly cancel each other out. On a 90 degree V8 the banks act at right angles to each other, so the 50 percent bobweight approximates the counterpoised reciprocating mass at each paired journal.

Over/underbalancing is a controversial practice in the performance aftermarket. It might best be described as an attempt to approximate, in a stab-in-the-dark way, the actual forces at work at some critical rpm as opposed to the masses. (as Mr. Locock alludes above.) While some say it works, they are unable to quantify the methodology or results to any rigorous degree. It's generally cut and try. Others find it doesn't work at all. One thing overbalancing is known to do is introduce a noticeable vibration period at some lower rpm, generally 2500-4000 rpm. This is justified by the adherents of overbalancing by noting they don't intend to run the engine in that range. OEs may publish over/underbalancing factors (usually under) for service procedures on specific engines (V6s being a good example) in which case we can safely presume there was some method behind it.

Commercial balancing machines typically divide the crank in two halves for imbalance measurement and correction, center back and center forward. Meanwhile, production engines may be internally or externally balanced. Externally balanced engines use counterweights on the crank damper and/or flywheel to achieve complete balance. Engines may be externally balanced in the front half of the crankshaft and internally at the rear, vice versa, or externally or internally balanced at both ends.

I know absolutely nothing about the Holden six, I can assure you, but from the description of the problem I am going to go out on a limb and guess that this engine employs a four main-bearing crank. If that is the case, balancing techniques may help but they are not likely to actually correct it.

Edited by Magoo, 22 January 2011 - 22:22.

[Lee Nicolle]

The grey motor that Fredduece is referring too is a 4 main engine, with standard a 7 port head. I believe similar in design to a GMC 6 from the 40s.
As they were very common here in the 50s and 60s they were raced a lot and alot of power and RPN were extracted from them. But they had and still have a harmonic problem between 6-7000rpm where they break cranks, often.
The trick was to either run them under that or over that. In midgets they often ran them to about 8000rpm but the power range is very small. In road racing ofcourse this was nigh impossible, and they made good power in that critical range.

The red 6 was introduced in 63 with 7 mains and a 9 port head and was a far stronger engine, though chase big power and they break too. As do most engines.

[Magoo]

The grey motor that Fredduece is referring too is a 4 main engine, with standard a 7 port head. I believe similar in design to a GMC 6 from the 40s.
As they were very common here in the 50s and 60s they were raced a lot and alot of power and RPN were extracted from them. But they had and still have a harmonic problem between 6-7000rpm where they break cranks, often.
The trick was to either run them under that or over that. In midgets they often ran them to about 8000rpm but the power range is very small. In road racing ofcourse this was nigh impossible, and they made good power in that critical range.

The red 6 was introduced in 63 with 7 mains and a 9 port head and was a far stronger engine, though chase big power and they break too. As do most engines.

Thanks for schooling me on Holden sixes. Sounds fairly similar to American sixes of the period. The older Chevrolet and GMC sixes were further hobbled by their lubrication systems, which were not fully pressurized. It's rather impressive what people managed to do with these engines -- the better ones were competitive with the Ford V8.

[Lee Nicolle]

Thanks for schooling me on Holden sixes. Sounds fairly similar to American sixes of the period. The older Chevrolet and GMC sixes were further hobbled by their lubrication systems, which were not fully pressurized. It's rather impressive what people managed to do with these engines -- the better ones were competitive with the Ford V8.

Grey motors did have unfiltered pressurised oiling, about 25lb standard. A bypass filter was an option later on.
Red motors were full filtered and standard were 45lb.
My father had a 54 Chev truck, splash and dip oiling! And from memory poured bearings too. Very antiquated for that era.Though 55 had pressurised oiling. Though compared with a Ford was at least OHV.

Edited by Lee Nicolle, 26 January 2011 - 00:25.