conrod weights

[Damomma]

I read somewhere (maybe in Wayne's book?) that weighing the conrods to group like weights for opposing cylinders was a trick to fine tune the balance on a rebuild. I understand that Porsche spec is a 9gr max spred across all rods but no matching for opposing cylinders is done.

Is this worth doing? or does this just throw off the balance on something else crank related?

[ChrisBennet]

Yes weigh them and balance them with the heaviest one nearest to the flywheel. It's free.
The spec is 9 grams but the ones I've weighed were a lot closer than that.
-Chris

[snowman]

It ain't quit that simple either. You must be concerned with TWO weights, the rotating, and the reciprocating. In the case of matching weights of opposing cylinders, you would match just the reciprocating. The reciprocating is composed of the small end of the rod, plus the piston and all its components.

YOu need to learn about all these things BEFORE you go about doing anything that changes your engine. If you do not you may do more harm than good. They are not difficullt to learn and a couple of days with a good book is all that is usually needed. I would suggest a book like "engine blueprinting". All the same things apply for a Porsche.

[BK911]

For a race engine or a high reving street engine, I would end balance the rods. For a street engine, why bother?

This is the way I balance a street engine...

Weigh the pistons and piston pins.
Group the heaviest piston with the lightest pin.
Weigh the rods.
Group the heaviest rod with the lightest piston/pin combo.

Assemble on the crank in the following order starting from the flywheel end:
Heaviest combo
2nd heaviest
4th heaviest
3rd heaviest
5th heaviest
6th heaviest (lightest)

Is this the best way? No
Will it be better than nothing? Yes
Does it cost anything? Just a little time!

JMHO!
BK

[911pcars]

As applied to Porsches, there may be little difference between a "street engine" and a "high reving street engine". Having the exact same piston/rod weight in each cylinder is a better alternative and doesn't add much to the cost of a rebuild (but also not absolutely necessary). In experienced hands, one could also remove a few grams of excess weight from the rods as well. And in case of a single unit replacement down the line (dread the thought), it's also simpler to have them all be the same.

JMHO,
Sherwood

[snowman]

I guess its all relative to the magnitude of the weights involved. Porsche rods are matched within a reasonable number to begin with and you should keep the same parts in the exact same positions when you rebuild an engine.

I can't agree that you would be makeing things better by matching with the method proposed. If you can measure the total weight of a piston or rod you can measure the correct parts and do it properly. If you think you are going to do better than the factory, know what you are doing.

[Damomma]

BK911, It seems that I would want to weigh my piston pins together with their respective rod so as to not mix up which rod went with which piston pin. My thinking being that the piston pins wear in with the rod more so than the pins wear in with the piston. Then separately weigh the pistons and install them as you say.

But then should I move the cylinders and their respective heads to match the pistons? Or since I'm re-ringing that doesn't matter?

[snowman]

Another note: Only mix up wrist pins, pistons, rods, when you are using new pistons, pins, and even then be careful, ie make sure fit is same. With most new Porsche pistons, they come with matching cylinders, they should not be mixed up. On the other hand if you have the capability to properly measure, EVERYTHING and are VERY confident on what you are doing, y ou probably will and should ignore this info. But then you will be measuring the pistons and cylinders to the nearest 10th of a thousanth, you will know to weigh the big end and small end seperately and how to match weights ect.