Carl Fausett

Does anybody know if, when they factory was assembling our engines, did they balance them internally (without the harmonic balancer and flywheel mounted) or externally?

They reason I need to know is I am thinking
of taking my harmonic balancer to a machinist and have them lighten it.

Light-weight flywheels are a well known performance improvement, lowering the moment of polar inertia so you can blip and get into the RPM range you want fast...
but the harmonic balancer on the front is heavier, and it does not have the complications of having to be structurally strong like the flywheel does.

If our 928 is balanced without the harmonic balancer mounted, then the harmonic balancer is balanced by itself and I should be able to lighten it without a problem.

What do you guys think of this?

dr bob

I'd think twice about removing mass from the balancer. First off, you can't remove very much, since you have to maintain enough metal outside the rubber layer to make sure you don't have a centrifugal explosion. Next, consider that you have to have enough mass there outside the rubber to actually dampen the ringing in the crank itself. Don't be misled by the name "harmonic balancer". It should really be called a "harmonic dampener".

I would not consider lightening the harmonic balancer, considering that the crank will destroy itself if it is allowed to 'ring' for any extended period of time. Metal fatigue will cause cracking at the high-amplitude nodes, and total crank failure follows quickly. Some bearing tests (on gas turbines...) show that ringing in the rotor also disturbs the lubrication film on the bearings, sometimes inducing oil whorling which can exacerbate the ringing. I suspect that our 928 engines would suffer the same way.


Typically, each rotating piece is balanced individually, and this includes the harmonic balancer. Many of these are balanced by drilling a little metal out of the heavy side, and that drilling is almost always on the section outside the rubber layer. Flywheels are balanced separately, and are drilled at the heavy points. Flex plates for the ring gears on automatic cars are typically balanced with weld-on metal tabs at the light points. Crankshaft counterweights are drilled to adjust weight, and a good crank balance is done while accounting for weights end to end as well. Kind of like a dynamic wheel balance, as compared to a bubble balance.


For street driving, a lightened flywheel makes the engine more responsive, since it takes some torque just accelerating the various masses in the driveline. A carefully lightened flywheel has the same effect as adding horsepower at the crank. Downside is driveability, as the car is much more throttle sensitive when launching and shifting. You can get used to it, though. I drove a street-plus 911 for many years with a very lightweight flywheel. It was so sensitive that a new set of plugs would "cure" what seemed like a worn clutch.

<img src="graemlins/yltype.gif" border="0" alt="[typing]" />

John Struthers

Dr Bob,
You might know...
Would there be a difference if you machined the outer circumference -reducing the diameter over turning the width, "FACING" re: the fragging problem?
Another question:
When lifting my foot off the gas of my 82' Weissach, 3 spd, auto I get comments from passengers that it feels like I'm already braking.
Would that engine brake effect be more or less pronounced with a lighter balancer/dampner?
Last, would it be safer to remove weight from the flywheel or the dampner?
Since time lag is reduced -throttle response- could you reduce the jerkiness -******?- by adding a little slack to the throttle cable, or, are you stuck with an overresponsive pedal?
TIA
John S. <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />