Time to balance the crankshaft, got a Q before it.

[Jocke]

The mecanic has told me i have to balance the crankshaft with the flywheel mounted, here´s a problem i dont have the flywheel i gonna use, so do you guys think it's unnecessary to balance the crankshaft only?

Thanks
Jocke

[ChrisBennet]

I can't answer your question but I'm curious, did your mechanic tell you why you needed to have the flywheel?
-Chris

[porschetech]

When balancing your crankshaft, it is a good idea to balance not only the flywheel but the front pulley too. You can also balance with the clutch disc and pressure plate but most don't as you do not want to have to balance the crank while replacing the clutch. Be sure that if balancing with flywheel that the locating pin is installed (if applicable).

[HawgRyder]

The balance factor of a horizontally opposed engine is usually good to excellent as manufactured.
Balancing will help with high rpm's on our engines, but not the same as a V8 or inline (they are inherintly out of balance).
All the same...when balancing....everything that turns should be balanced together....this means with weights attached to the connecting rod journals equal to the appropriate weight of the rod, piston, rings, and clips too!
A balanced engine can be a thing of beauty....but the extent of the balance program will determine the outcome (good, better, best) and the outcome is directly tied to the money spent.
Personally, my engine will go together as original....meaning...no balance (other than checking weights of objects as built by Porsche).
Bob

[350HP930]

Porsche did a pretty good job balancing the crank before it left the factory.

Unless the crank has been machined I would not be too concerned about having it rebalanced.

[Jocke]

So you meaning i probably don't need to balance it??

I'm scared as hell, something will go wrong when I starting to raise the HP...

/J

[350HP930]

Balancing just makes for a less vibrating engine at high revs. All cranks are balanced at the factory but some manufacturers are more exacting about the process than the others. Since porsche was in the business of making engines that could spin at 8K you have less to worry about their balancing accuracy than you would a GM product for example.

That being said, balancing won't do anything to make your engine stonger since the vibration from a mild imbalance is not going to break anything (though it can help improperly secured fastners come loose). The normal stresses an engine experiences from intertia and power strokes far exceeds what a few extra grams in the wrong place can do.

The drilling and grinding material from a seasoned crank to rebalance can make it weaker though from the removal of material and the addition of stress risers. Do you really need that kind of compromise?

I assume you will not be taking your engine over 6,500 RPMs much so spend the money on something that will give you more benefits for the dollar.

[911pcars]

If all the engine parts are as they came from the factory, you could probably skip the balancing. If you replaced or rebuilt any parts, I'd suggest spending the extra $$ to balance the parts. For example, connecting rods have two ends (big end/small end) that must match the other rods. One end is reciprocating (back and forth); the other end is rotating. If the rod is rebuilt, the weights won't necessarily match the other rods. Are you going to assume the soon-to-have flywheel (a large inertial weight) is going to be balanced?

It all depends on how thorough you want to be.

Sherwood

[RoninLB]

I balanced the pistons and rods by shaving the inside of the piston pins, and having the higher weights twords the flywheel end.

[Jocke]

Originally posted by 350HP930
I assume you will not be taking your engine over 6,500 RPMs much so spend the money on something that will give you more benefits for the dollar.

Well i building it with parts to handle 8,000 RPM so i guess i have to balance then...

Thanks everyone
Jocke

[jluetjen]

If you are building an 8000 RPM engine, YES -- you should have the whole thing balanced. The forces involved go up significantly (exponentially) as the engine speeds go up.