v-12 engine balance

[jake197000]

does the fact that the old Lincoln zephyr v-12 is a 75degree engine affect the way it is balanced.they all seem to have a shake at about 12-1500 rpm some worse than others.

[SchmidtMotorWorks]

The vibration is torsional, unfortunately, there is nothing that a crankshaft balancer can do to change that.

The options available to you are damping the vibration or making a stiffer crank or reducing mass (if there is anything to get rid of).

[jake197000]

I know you don't need bobweights on a v-12 but would it be worthwhile to try it to see what happens and if so what factor would I use to make them up.its a 75 degree engine.

[Circlotron]

For static balance you don’t need bob weights, the same as a 4 cyl doesn’t, but adding them takes a load off the mains in both cases.

[Baprace]

Hines balancing has a V12 H series 1941 lincoln engine using 50% reciprocating and 100% rotating.

[Dan Timberlake]

just under load, or revving in neutral too?

[panic]

IIRC there is still rocking couple in a V12, but there shouldn't need to be reciprocating compensation unless the bank angle (or combination of bank angle and pin offset) is not 60 degree (720 degrees / # of cylinders = ideal firing order interval): the 75 degree bank requires it since the mirror-image effect on an L6 where the front and rear three cylinder groups counter-act each other is not present.
Just a guess: the choice was made for packaging, 75 degrees reduces the height, lowers the CG, and increases the intake port length.

[Schurkey]

does the fact that the old Lincoln zephyr v-12 is a 75degree engine affect the way it is balanced.they all seem to have a shake at about 12-1500 rpm some worse than others.

Is the shake from poor balance, or from uneven cylinder firing?

(or just from poor tune?)

[jake197000]

thanks for the good info.the engines run very well it feels like there out of balance.when they are rightyou cant hardly tell there running butthey start vibrating as you bring the revs up some more than others.i had one that the welch plugfell out of the rod journal and thought oh that's it.nope didn't even change it oil preasure was still good toothey only have 4 mains.feels like a car with an out of balance clutch.not terrible but noticeable.seems theres got to be a way to make them better.im pretty new to balancing.used to send everthing out but now have my own equipment.had some real butcher jobs done and started doing it my self.

[jake197000]

its not from a poor tune up.i set up the distributors on my sun tester.

[SchmidtMotorWorks]

If your balance is different with the bob-weights on or off the crank then the problem is the accuracy of the bob-weights, the precision of the positioning or the stroke and index of the rod pins.

There is nothing that bob-weights can do to tell you anything about the balance of a V-12 crank.

[jake197000]

interesting.i guess ill spin the crank,match the piston weights.should I balance the rods big and little end ? ill balance flywheel and clutch and that's as good as it gets ?

[SchmidtMotorWorks]

interesting.i guess ill spin the crank,match the piston weights.should I balance the rods big and little end ? ill balance flywheel and clutch and that's as good as it gets ?

Yes, of course, balance the moving parts.

Key here to understanding the problem is that a V-12 crank being very long can twist along the length.
That twisting reverses due to changes in the load on the crank moving to different points along the length due to the firing order.
The crank assembly has a natural frequency that it will reverse at if the load is applied and released just one time, similar to striking a tuning fork.
When the engine is running at a speed that the load coincidently varies at a similar frequency the twisting vibration will increase.
A harmonic dampener (incorrectly called a balancer) can fight that problem, but they are not magic, it is difficult to solve such a large problem with a device attached to the end of the crank.

If the dampener you have is not effective, it might be worth a try to fit a more effective one.
Finding one that is right for the engine might be difficult.

[jake197000]

you've been very helpful with your explanation.they have a damper with the fan mounted to it and it has a spring loaded plate that allows it to slip if needed.i work on a fair number of these and therepretty well worn out by the time I get them been rebuilt onlyhow knows how many times.just noticed they all have the same shake.just trying to make it as good as it can be.

[Schurkey]

they all have the same shake.

Is the shake from poor balance, or from uneven cylinder firing?

A 75-degree V-12 is going to shake like an odd-fire Buick V-6 due to the uneven firing impulses--unless it's got an even-fire Buick-style split-pin crankshaft.

Wild Guess: You're dealing with both problems. The engine balance is way off, and the firing impulses make it even worse.