Engine Gurus: Balancing

[Pablo]

I'm trying to understand this whole balancing thing. So far I get that the V6 has an inherent imbalance which is why the 3800s introduced balance shafts.

What I don't get are these % figures thrown around for balance. Apparently our engines are balanced to 36.6% which from what I've read eliminates the vertical imbalance in favor of a horizontal imbalance.

I've also read that some have balanced our engines to 50% which reduces both the vertical and horizontal imbalance (but from what was implied in posts, does not eliminate either)

First off, what does the % figure represent? (some kind of measurement of the crank counter weight?)

Secondly, when one balances a crank to any of these percentages, does that simply represent which direction the crank will be imbalanced to?

Does the total amount of imbalance change depending on where the engine is balanced? From posts it seemed like the total imbalance was less at 50% at the expense of harshness transmitted to the motor mounts vs the motor mounts controlling the harshness when balanced to 36.6% at the expense of a greater total imbalance. Am I way confused here?

If I'm not confused, wouldnt it make more sense to balance an assembly so there was less total imbalance (for horsepower reasons) to eliminate any extra crank bending forces? Also, can changing the plane of imbalance theoretically change how our cranks and maincaps (highly stressed in the LC2) are loaded and thus possibly extend their lifespan?
It would seem to me that if the plane of imbalance was closer to where torsional stresses and lateral stresses (just the force of the pushrod wanting to move the crank in the same direction on the same plane) were focused, the crank, block, caps, etc would be stressed more than if you moved the plane of imbalance somewhere else.

Any thoughts?

I tried my best to research this but there seems to be almost 0 relevant information online anywhere. So if I've gotten this all wrong please forgive me :tongue:

 

[Pablo]

ok after posting this I thought of some new google search terms and came up with some interesting articles that explain the % balance is in regards to how much of a factor you assign to the reciprocating weight (small end of the rod with piston, rings, etc)

The circle track article I read discussed balance as though 50% were the absolute minimum and that depending on what % balance you use over 50% you could adjust your imbalances so that they occurred outside your peak operating range.

It would then make a lot of sense to me to find how this relates to an LC2 rpm wise and taylor your balance so that your sweetspot is in a range that covers your maximum torque and horsepower.

Still not sure how this changes the plane of imbalance that i read about in other posts.

 

[turbows6]

36.6% bobweight if you are putting it in a street car
50% for a track car.

 

[Pablo]

If you could explain the reason behind your recommendation that would be much more helpful.

 

[A2000RICH]

If you could explain the reason behind your recommendation that would be much more helpful.

the reason is because its impossible to efficiently balance any engine perfectly that has a combination of rotating and reciprocating masses-----while some engines can come really close ie V-16's it is always a compromise-------generally V-8's fare better than V-6's-------inline 4's can do well and 4 cyl boxers can do even better------a V-6 is the definition of unbalanced and attempting to balance one means to accept a series of compromises-----even the location in the car (inline or traverse) affects the end result------when it comes to our cars it even comes down to the choice of engine mounts-------for track it is usually accepted that stiffer mounts will be used and for street/strip its usually gonna have more flexible mounts-------that is one of several choices that is reflected in the 50 vs 36% decision for track vs street..........RC

 

[charlief1]

The only truly balanced engine is a radial. A flat or inline engine is the next. For a v6 it depends on the angle of the banks. 120 degrees is optumum but since we have a 90 degree bank design you have to compensate with either 36.6% or 50%. It's a comprmise either way.

 

[Pablo]

I'm aware of some of the science and theory behind balancing.

That still does not answer why you suggest balancing a race engine at 50% vs 36.6% for a stocker. What is your reasoning for this?

I have done a fair amount of research since I posted this and I think I know why it is a good idea though it is a very dynamic situation with many variables that cannot be completely accounted for.

I would still like to hear anyone elses rationale for balancing at 50% vs stock.

 

[charlief1]

50% gives the best mechanical balance but induces vibrations in both the horzontal and verticle plane. 36.6% reduces the veticle and increases the horizontal vibration whick is absorbed in the motor mounts better producing a smother driveline for the driver. For performance 50% is better for the driveline but at 36.6% the vehicle is more comfortabe for the passengers.