cranks neutral balance, internal/external

[fords2fast4u]

Question here is differences in external/internal balanced cranks.

External balanced cranks require that the balancer and flywheel be on when you balance the shortblock... right.

I THOUGHT that internal balanced cranks meant that the rotating assembly is balanced without the balancer and flywheel, and then you can add whatever balancer and flywheel you want after that, so long as they match... leaving you the option to change them after the motor is together.

Now, does neutral balance, simply mean internal balanced? I'm guessing not, but that's what i need to know. I have a handle on a neutral balanced crank, and when i asked the seller if my 28 oz flywheel and balancer would work with that neutral balanced crank, that he later called internally balanced, he said that no it would not.

So internal and neutral balance are the same thing? and you need a nuetral balanced balancer and flywheel to match them? thanks.

 

[FastDriver]

Internal balance = neutral balance.

The counterweights essentially equal the bob-weight of the opposing side of the crankshaft. Internally balanced cranks don't need to be balanced with the balancer or the flywheel because the counterweights do all the balancing. The balancer and flywheel can be balanced on their own separately from the rest of the rotating assembly.

External balanced crankshafts require additional weight outside of the crankshaft to help to balance it. The crank must be balanced with the balancer and flywheel because they provide the balancing force that the crank needs. Obviously, we've reached standard external balance weights now, so one 50oz balancer or flywheel can replace another without having to rebalance the entire assembly.

Just to make sure there's no confusion: with a 50oz imbalanced crank, you'll need a 50oz balancer and a 50 oz flywheel. With a 28 oz imbalanced crank, you'll need a 28oz balancer and a 28 oz flywheel. And, with a neutral balanced crank, you'll need a neutral balanced balancer and flywheel.

The problem I have with what you wrote is that you can't just by a "neutral balanced" crank and expect it to be neutral balanced to the rest of your rotating assembly. For one to be neutral balanced, it has to be balanced with a specific set of rods and pistons. Swapping these parts with others will require a rebalance.

Also, the more imbalanced a crank is, the less it will rev without causing problems. The centripetal forces exerted on the crank and on the main bearings increase exponentially as RPM increases. So, the more neutral balanced a rotating assembly is, the more RPM it can handle. The more imbalanced the crank is, the more the crank will want to flex/oscillate, and the more it will pound the main bearings. This can lead to part failure, and the effects are cumulative over time. Which means that the 50oz imbalanced engine may be alright going to 7000 rpm for several years, and yet it may break the following season even though it isn't exposed to a higher force than it has regularly been exposed to.

Generally speaking a 50oz balanced rotating assembly isn't meant for high RPM. However, I've seen 28oz balanced rotating assemblies live at 8000 RPM. With a well-done neutral balanced rotating assembly, the sky is the limit. However, I'm not recommending that you go out, get a neutral balanced rotating assembly, and then assume you can rev it to the moon without giving any regard to the rest of the moving parts, especially the valvetrain.

Hope this helps,

Chris

 

[Brian@PAR]

Very good post

One other thing to consider when having a balance done. Some shops will work your flywheel and/or balancer. If this is done then you can't just swap them out. You can however have the new part matched to the old part and not have to have the whole balance redone. Just something to keep in mind.

Also ask the shop what you need to bring for the balance. If they don't require you to bring them bearings and rings find another shop. These weights are minimal but all add up and need to be included in the bob weight. We have a shop here that offers a "street" balance. All they do is match up the parts and even out the bob weights. That is a waste of time and money.

 

[fords2fast4u]

thanks for the info gents! That's what i needed to know.

Looks like I'm looking for an external balanced crank then, b/c i don't want to buy a new balancer and flywheel, and 28 oz balance will support the 6k rpm max that I'll run.

Curtis

 

[FastDriver]

I remember you now, Curtis. I'll drop you a line on email where we can catch up.

Chris