Engine balancing

[Pat S]

What's the skinny on engine balancing? I mean I know everything that moves (crank, rods, wrist-pins/pistons, flywheel, clutch plate, pulley) really should be balanced for a perfect rebuild. But since the engine is naturally balanced from being horizontally opposed, at what point does the law of diminishing returns kick in? Meaning, is there a point in balancing where it become more academic (and expensive) than usefull.

If it helps... I'm using a std/std crank, reconditioned rods from the same weight group, stock flywheel/pistons/pully, Sachs clutch. I have no problem with having it balanced, I just want to know how anal I need to be.

Pat

[350HP930]

Lets just say that unless you are going to be running at red line all day long there are much better things you could spend your money on.

If you get an accurate scale you can always balance the pistons and rods yourself if you have the engine apart though.

[Pat S]

Thanks, that is more or less what I thought. I'm not quite the perfectionist that Wayne is but just the same I'd like to do it right the first time. Since I am gettting the rods reconditioned anyway I'll just have them balanced and I'll check the pins/pistons to make sure they are still close myself.

Pat

[Bill Verburg]

You can make sure that all recipricating components belong to the same weight group(see you spec book for this info)sometimes things are so close it doesn't matter. On my build all the rods were between 496.4 and 497.6 grams. Put the heavier ones closer to the flywheel.

[Pat S]

Hmm, heavier near the Flywheel. Makes sense and I never would have thought of that.

According to the rebuild book, there doesn't seem to be a piston weight group for the 2.7 (74-77). Spec just shows +- 6 grams. My rods are all the same group though.

Thanks,
Pat

[Bill Verburg]

Almost, the spec for 2.7 is max difference in weight of the set is 6g, that's &#177 3g

There are 9 weight groups for the con rods all are &#177 4.5g

Starting w/ the '78 SC for Mahle they speced 4 weight groups for each market, all are &3177 2g for production pistons but &#177 4g for service(repair) pistons. Similar arrangements for the KS pistons.

It' really interesting to note the weight difference between the ROW Mahles which start at 668-672g and the US/Japan Mahles which start at 636-640g

[Wayne 962]

Engines that are balanced will perform better, have less vibration, and theoretically last longer...

-Wayne

[BK911]

I am in the process of a budget rebuild on a 2.4T. I weighed the wrist pins, pistons and rods. I combined the heaviest wrist pin with the lightest piston. Then the lighest wrist pin/piston combo with the heaviest rod. With no machining, I have less than 2 grams between the heaviest and lightest assembly.

in order of heaviest to lightest combo: 3, 6, 5, 2, 1, 4. May not be perfect, but should produce a pretty smooth running engine. That's if I ever have time to finish it!

Good luck!
BK

[911pcars]

Remember, when balancing or equalizing the weight of connecting rods, there are two parts of the total; the reciprocating end (small end) and the rotating end (big end). Each is taken into account when balancing an engine.

Sherwood Lee
http://members.rennlist.org/911pcars