I assume this is just a packaging issue.


You're missing the point of the balance shafts, it's not to counteract the *rotating* mass, it's to counteract the *oscillating* mass - meaning the pistons and top halves of the connecting rods.

Knife edging reduces some rotational inertia, and it also might reduce some frothing of the oil in the sump, but that's about it.

 

I stand corrected then. After re-reading everything, I can see how having only one shaft is actually a terrible idea... and machining the rotating assembly in the pursuit of balance would largely be a waste of money. Cool, I'm just gonna run no balance shafts. This motor is going to be all business.

 

This is not the case as balancers don't counteract weight of the crank but the secondary vibration which is caused by pistons that accelerate faster on the upper portion of the cylinder vs the lower portion. This is the reason pistons don't counter balance each other and balancers are used.

EDIT: I see van already answered.


One thing that I don't get with the balancers is why the early balancers are much lighter yet early engine is as far as moving parts go basically identical to later engines. Maybe the weight of balancers is not that important after all.

 

Just a guess, but maybe the weight change affected a different RPM range of vibrations...that could have been possible because of the updated engine mount design.

 

I studied the article more in-depth that StoogeMoe posted and can see how machining the crank cannot balance a 4 cyl engine, because the definition of secondary forces are those which are 2x the frequency of the primary forces (tied to crank rotation).

The balance shafts in my '82 motor are definitely different than an '86 I serviced the balance shafts on - they are crescent shaped lobes of steel, whereas the late shafts have a metal lobe and the rest of the "cylinder" is plastic, held on with cap screws. I'm sure that's common knowledge to those who are familiar with these engines, though.

 

Could be but since the x2 speed is maintained trough complete rev range, I think it work its magic all the time.

Could be the early design is Mitsu's idea of mass relations and later design is porsche's own..

 

I always assumed that the plastic caps were to improve "windage" and reduce frothing up the oil before it drained back in the sump.

I didn't know there was a weight difference between early and late shafts... Has anyone done the math to know if the rotational inertia is different (mass times radius squared)?

 

The balance shafts counteract the second order forces (vibrations) which are inherent in inline 4-cyl engines. This is an effect due to the reciprocating imbalance forces. Normally on engine displacement above 2.0l the occupants will notice more or less unpleasant vibrations and a (buzzing)noise if an engine without balance shafts is revved >4000 rpm (ask the 924 owners). So it’s mainly a question of traveling comfort and engine acoutics.
The height offset of the balance shafts has the by-effect to counteract the torsional oscillation of the crankshaft (due to gas pressure loads) and the breakdown torque of the longitudinal axis of the engine.

The late 944 was an evolution which improved the early one. Improved cylinderhead, camshaft, balance shafts, windshield etc. As for the late balance shafts, I agree with Van, it's mainly a question of windage. But also improved bearing to increase driver's comfort.