Perhaps taking a deep breath, concentrating and thinking might bring a result:
"What exactly do I want to know?"
"How do I phrase the question?"

Just a thought.

Benta.

Yup. I've worked on balancing for 3 cylinder engines, V8s, I4s, i6s, V6s, and I can't remember a single one that needed anything odd for primary balance.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

The referenced discussion link shows conflicting info. on both rotating percentage as well as reciprocating. The crank is a 120 deg. design with either 18 deg. or 22 deg. offset rod journals - again depending on who's data you believe. It appears that rotational should be 100% but the percentage of reciprocating mass varies significantly. Obviously VW must have specified percentages though no one seems to be able to obtain that information? That is why I asked here. :)

Here is a 7 year old discussion over on SpeedTalk.
http://www.speedtalk.com/forum/viewtopic.php?f=1&a...

Some input by folks who were working backwards from actual parts, some with leaked OEM info, some just guessing.

Thanks. Yes, that is the discussion that I linked to in the original question. I posted the question here hoping someone with more crank design experience or someone who happened to have access to VW's specifications, might be able to resolve the issue which would be useful for quite a few people.

I have contacted several engine balancer machine manufacturers in an attempt to obtain the "official" VW percentages for balancing this engine, without any current source being able to supply this information. So far the only consensus is that the rotating mass should be 100%. The reciprocating percentage is believed to be 30 percent via reverse engineering, but the sample rate is so low as to need more testing of original, unmodified, OEM components to validate if 30 percent is correct for recip mass.

The current recommended range on recip mass varies from 30-38 percent, which is a rather broad range, IME. One company recommended no lower than 32% with up to 38 percent depending on peak engine RPM and/ or modifications or with aftermarket components of differing weights to the OEM components. This particular engine will be limited to the 6700 rpm OEM rev limit so over-balancing is not likely to be needed or appropriate.

You appear to be talking about counterweighting, not balance. The size (mass, eccentricity phase angle) of each web is typically calculated to minimise the peak force in each main bearing. If you look in an engine design book such as Heywood you will find that the counterweighting guidelines are extremely broad.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

Thanks for the recommendation on Haywood's book which I am familiar with. Charles Fayette Taylor also has extensive discussion on engine balance. What I am trying to determine is exactly how to properly balance the VR6 crank assembly for both the rotational as well as the reciprocating forces. The counterweights are only part of the equation so to speak and in some instances no counterweights are used at all yet the assembly can be in full balance. The primary issue is firsthand reports of various percentages being used for recip mass delivering "perfect balance". Some have speculated the counterweights on the VR6 engines are unneeded.

The point being that the narrow angle (15 deg.) VR6 engine experiences some forces in a different manner than an inline six and similar to a traditional V-6, as the VR6 engines use one journal per cyl. but the recip forces are in two different planes that are non-canceling. This design offers some of the balance benefits of an inline six but also some of the challenges of a V engine design. It can be viewed somewhat as two inline 3-cyl. engines back to back on one crank with the exception that each engine would be offset at a narrow 7.5 deg. angle from vertical. Without knowing what VW chose to use as a recip mass bobweight percentage it's impossible to know if the engine will run as smooth as original/intended regardless of being in balanced. I'm well aware that the combustion loads can impact overall engine balance depending on the magnitude but in my particular application this is not likely to be a significant issue with a PCP ~1725 PSI.

I guess you'll have to go back to first principles and work it all out. Sometimes that is the only way.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

The firing order of the VR6 is the same as an inline 6.
The 15 deg. won't make a large difference in balancing (no need for the dual CBS drives).

VW added the 15deg. mainly because of how they could package the head and camshafts, vs. an inline 6.

Why don't you just aim for the factory counterweighting percentage?

je suis charlie

gruntguru - Because no one except VW knows what that counterweight percentage actually is and they ain't talking. :)

My continuing research into this matter has led me to a series of tech articles which exposes that the current balancing methodology used by anyone outside of auto makers or large engine manufacturers is basically a SWAG that appears to provide acceptable balance in many applications, even though there are numerous errors in determining recip mass percentage and even con rod rotating vs. recip percentages. To make matters even worse the balancers being used by most of the world other than auto makers/engine mfgs. are set up for the "average engine crank correction radius of 3" and as such they induce primary balance errors before you even start if the crank you are balancing doesn't use a 3" correction radius distance. To complete what amounts to a SWAG regarding primary balance, the location on the counterweights where mass is added or removed is often at the incorrect crank radius and thus even when neutral balanced to a "zero" imbalance, the crank is noticeably out of primary balance.

For those who want to see why the current non-auto/engine mfg. balancing methodology is simply a SWAG that may be acceptable to many but not actually proper first order balance and what some race teams in NASCAR / NHRA are doing to achieve better crank balance, the short tech articles at the links below might be very informative. I also noticed in my research that cranks that appear to not require counterweights for primary balance, often used counterweights to eliminate the rocking motion that occurs in a primary balanced crank from non-force canceling multiple geometric cylinder planes.

And last but not least is an interesting SAE paper on counterweight "phasing" where NC State found amazing improvements in main bearing load distribution by minute changes in the counterweight phasing on a small block Chevy engine without any change in the counterweight mass or radius.

http://www.onallcylinders.com/2016/03/17/the-basic...

http://www.onallcylinders.com/2016/04/15/finding-b...

http://www.onallcylinders.com/2016/05/04/finding-b...

http://victorylibrary.com/mopar/crank-bal-c.htm

http://www.onallcylinders.com/2015/10/16/guide-to-...

It would appear that the more we learn the less we know about engine balance. Apparently the major car/engine makers do all the calculations, produce appropriate hardware, instrument everything they can and run the engine on the dyno to see what results they actually obtain. One crank balancer company told me that in their work any time a customer requests a change from neutral primary balance, i.e. over/under balance to eliminate an issue or improve balance at a specific RPM range, they make alterations of the mass in 1% increments. Then the engines are dyno and often track tested to see what the real world results are. Then it's back to the balance machine and they change another 1% of mass and re-run the dyno/track evaluations. They can spend weeks and a lot of $$$ and gain little or in some cases a noticeable improvement for the specific engine in a specific form of motorsport or other application.

The incompetence of aftermarket engine balancer operators is nothing to do with the counterweighting percentage of the crankshaft.

I can two plane balance any engine with an accelerometer (preferably two) and a PC, in fact that is exactly how the balancing strategy for the LT5 V8 for manufacture was done, because I did it.



Cheers

Greg Locock


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TrackRat. I don't believe it would be that hard to reverse-engineer the factory crank-rod-piston.

Short of that - what is the starting point for your engine? Not a VW factory crankshaft? If it is, it is not hard to alter the counterweighting on the factory crank in line with your changes in piston and rod mass.

Plenty of unscientific claims/errors is those links BTW.

je suis charlie

As noted by the balancing personnel discussion in the original link above in my first post, it's not possible via reverse engineering to obtain the OE recip mass percentage as the crank will balance with any number of recip percentages.

I'm using the OEM crank, unmodified with slightly heavier pitons than OE and a significantly heavier billet steel Crower rod. In the Speedtalk discussion sources stated that the recip mass percentage provided by reputable sources was: 30%, 38% and 50%. The crank balances out fine with all of those percentages yet they can't all be the correct percentage for the OE application. That is why I posted the question here hoping that someone had direct info. from VW. It's also why the folks in the Speedtalk discussion were trying to determine the correct recip percentage. One person in that discussion had used 50% recip until he was advised that it is suppose to be 30%. Other sources state that it's 38%. So he, I and everyone involved at this point is unsure what the true correct recip percentage really is.

This is the dilemma.

Not possible using a balancing machine. Easy using a scanner to produce a 3D model.

Or you could take a stab. Measure your increase in recip and rotating mass. Lets say it comes to 30g and 25g.

If the correct counterweight percentage is 30% you will need to add 25 + 30 x 30% = 34g
If the correct counterweight percentage is 50% you will need to add 25 + 30 x 50% = 40g

If you split the difference and add 37g you will be about:
- 0.4% heavy if the correct percentage is 30.(actual counterweight % will be 30.48% instead of 30%) (assumes correct counterweight = 700g factory and 734g modified)
- 0.35% light if the correct percentage is 50 (actual counterweight % will be 49.52% instead of 50%) (assume correct counterweight = 820g factory and 860g modified3)

If it was my engine, I would take an even bigger stab and throw the factory crank back in.
If it started at 30% it will now be 24.6%.
If it started at 50% it will now be 43.7%.

je suis charlie

Let's try again once more. You are worried about counterweighting. Fine. Counterweighting does not affect whether you can balance a crankshaft on a balancing machine. Anyone who confuses the two will be confused.

So what you could do is put all the rumors and discussion in your links into one big database, and delete everything by anyone who confuses the two. This will leave you with a very small percentage of your original reading matter, and may contain a kernel of truth. At worst it will contain less confusion.

Incidentally, optimal counterweighting is somewhat affected by cylinder pressure diagrams, and of course, redline rpm, so if you are changing either of those then you would ideally come up with a new set of weights, eccentricities, and phases, even with the same pistons and conrods.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

If I had the tools I'd figure this out for you. I would start with a known good factory engine and preferably in new condition.

As discussed you can not reverse engineer the reciprocating mass accurately because any recip mass from 30%-50% will appear to neutral balance.

The point being VW decided on a specific mass to cancel recip forces and that percentage is unknown resulting in shops using anywhere from 20%-50% recip mass and then assuming the crank is properly balanced, when it's highly unlikely to be properly balanced. Obviously with this wide range of recip mass the crank is going to actually be under or over balanced unless someone by accident got it right with: 20%, 30%, 32%, 36%, 38% or 50%. Without being able to run the engine and measure the shaking forces after the balance work no one knows if they got it right or wrong. Chances are they got it wrong without knowing exactly what VW used and / or why because determining the true dynamic forces is very difficult.

The primary balance on a VR6 engine is not straight forward as it can be on many other (not all) engine designs. I am also aware and have mentioned that there are secondary forces including high gas pressure, camshaft forces, etc. that can require attention. I have reviewed several engineering texts on the subject of engine balance/design and specifically the difficulty in determining primary forces with the VR6 engine because it has some typical I-6 and some V-6 forces.

I was hoping when I posted the original question that maybe someone who worked for VW might provide this information as it should not be some national secret. It's not like a VW owner can take their crank assembly to their local VW dealer and have them dynamically balance the assembly per (unknown) factory specs.

"The primary balance on a VR6 engine is not straight forward as it can be on many other (not all) engine designs." Either you don't know what those words mean to engineers, or you are typing rubbish.

I'm out of here.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

"As discussed you can not reverse engineer the reciprocating mass accurately because any recip mass from 30%-50% will appear to neutral balance."
Nonsense. Scanning is cheap. Once you have a 3D model it is not too hard to calculate the counterweighting.

Did you read my post above? It suggests a simple procedure to put you within 1% of the factory counterweighting percentage.

je suis charlie

I wonder who scared/P'ed him off?

je suis charlie

His wife.