Balance L6 Rotating Assembly

[EvilC]

I asked the question below in another thread and got some really good info so I thought I would give it it's own thread question and answer:

 

 

EvilC: Is our L6 motor an internally or externally balanced motor? The reason I ask is: say I am building a motor, do I get the crank, piston/rods and flywheel all balanced together or just the internal components together only. My train of thought is, I would not get the flywheel balanced with the “motor” because say 5 yrs down the road I change flywheel and clutch setup, I would have to pull the motor apart. If I statically get the flywheel balanced alone each time, no mater how many changes I make…the motor itself will always be in balance. The flywheel is just the wild variable that will always change and only play a factor in the rotational mass that the motor has to over come.

 

Is my train of thought correct or does everything has to be balanced together?

 

Thanks guys!

 

Answers –--->

 

Careless: As stated in one of my building books (not word for word).

 

balancing the rotating assembly, and then the rotating assembly with the addition of the damper, and then with the addition of the flywheel will get you into a closely balanced range so that any flywheel or balancer you put on from thereafter will be closer to a completely balanced package than a balanced rotating assembly with a separately balanced damper and flywheel.

 

For the same reason, people balance with the disc and pressure plate bolted on too using the same procedure to install on an engine prior to installing the trans.

 

Balancing all these components at once will ensure that further changes of the same parts (if they are zero balanced from factory, like most performance parts... or if you have them re-balanced alone if your engine is already together) will not cause severe imbalance.

 

This is as close as you can get to having a rebalance of the entire rotating assembly (which is ridiculous for a hobbyist if you're changing just a flywheel or balancer) because you actually have something hanging off the ends of the crank that are closer to the finished weight/mass/balance than having nothing there at all.

 

Doug71zt: You're on the right track. It is harder to balance the damper and flywheel without the crank to mount them on. So, you zero out the crank to have a starting point, then add and balance the flywheel, then the damper. Everything can then be replaced down the road without having to worry that you are throwing the balance. If you mounted up all the components at once, would you drill the crank, flywheel or damper to zero the assy out?

The process I use to balance a new build is:

Match the piston/pin weight

Match the rod total weight/small end weight/big end weight.

Now the recip. components are done, on to the rotating components:

Zero balance the crank

Zero balance the flywheel

Zero balance the clutch housing

Zero balance the damper.

 

If you are rebuilding a factory-built Nissan motor with all the original parts, I have found that it is not really critical to balance the assy as they are very close from the factory. Worst I have seen is around 4 grams. They were correctly balanced from the factory, not thrown together like a GM V6..... If you are building something from scratch or a Frankenstein motor, then it is a good idea to balance it. I didn't do the clutch housing on this last engine because it was custom-built and balanced by the shop. They are internally balanced - to answer your first question. No external weights are required and the assy is at zero balance without damper or flywheel installed.

[z-ya]

Clive,

 

IMO if you are building a moderate turbo motor (300-350WHP), the stock internal balance is fine. Spend that extra $300 or so on engine management, the turbo, or IC. The only thing you should check the balance on is the clutch pressure plate as a number of people have received ones that were out recently.

 

If you are building a 400+WHP turbo motor that will make peak power over 6000RPM, then I would spend the $ for balancing. In that case balance everything that is connected to the crank (damper, flywheel, pressure plate, rods, pistons).

 

Pete

[rags]

I can't see how NOT balancing a rotating assembly with a balancer and flywheel could be a smart move ever. What is the worst thing that can happen? Putting everything on the balancer and finding out the whole assembly is spot on? My thinking is it's a gamble to not balance and since I don't like to gamble I have balanced every motor I have built. I can't tell you if I'm just lucky or not, but all of them are still working fine. Just my 2 cents.

 

Joe

[woldson]

Ok, I may need some edumaction here.

 

If my fly wheel is balenced, its balenced, right?

 

On a side note my pressure plate did get balenced, five holes through the chassie.

 

Alos me needed a unberstnable reason, not for me but for my kin folk (son). He is young, 4 , I can still use this....teehee.