Working with various engines over the years, manufacturers balanced motors either internally, or externally, or a combination of both. Reason I am asking is my motor has a vibration around 1200 rpm. Yeah I know bad motor mounts will enhance this-I just changed those. It is much smoother and most might not even notice it. But a well balanced motor should not vibrate in this manner. regardless of mounts. Its a five speed and vibrates under all conditions, so that would rule out the TT.

My thoughts are (if internally balanced) that when a po did the clutch job, perhaps the components were not balanced. Or, if externally counterwieghted installed improperly. But by reading many posts here on vibrations, perhaps porsche just did a lousy job on balancing the motors and used nice hydraulic mounts to hide it.

When I've balanced motors in the past, it included all immediate rotating external parts and the reciprocating internals. This always lead to more hp, faster rpms, and a very stable motor.

Any thoughts or input. Curious when you racers had balancing done what you found with stock components.

 

Well i would say its both internally and external, after all there is a fairly large vibration dampener on the crank. I havent really looked at the crank, although i have an engine with the oil pan off that i can, i would assume its got heavy metal in the crank as well

Ive heard one poster that also rebuilt an S4 engine i think it was that they took the components to be balanced and the shop foreman said, what for this is already in spec, or something to that effect.

 

Isn't the V-8 configuration inherently hard to balance? The combustion pulses make it so, I've heard, while the most stable and smooth engine is a V-12. Can anyone here confirm that?

 

These engines are internally balanced. Because of this the flywheel at the back and front are each uniformly balanced.

Some cars use uneven weighting of the flywheel(s) to reduce vibration. The 928 is not one.

I'd look for a weak spark plug wire or fuel injector.

 

http://www.bmw850.de/engines_e.html

 

Isn't there a rubber bushing in the harmonic balancer?

 

The "Balancer" at the nose of the crankshaft is really a torsional vibration absorber. It is 'balanced'. as we commonly know it, independently of the crankshaft.

The effect of applying periodic torque at different points along the length of the crankshaft results in some odd twisting in sections along its length. All of those funny twisting forces conspire to break the crank when they reach frequencies that generate harmonic oscillations (ringing) at the non-drive end of the crank. Hence the need for a damper only at the one end. The engine depends on having adequate load on the other end to dampen those resonant oscillations there. This is why there are reports of torsion bar damage inside the torque tube when an automatic car is driven in four-cylinder mode for any extended period; the sharper torsional flexing and oscillation at the rear of the crank are coupled directly to the torque tube. No flex coupling or clutch disk springs to soak up the 'impacts', and you have all the ingredients for a fracture.

 

Has this vibration always been there or is it a recent developement ? In my '84 5spd there was a vibration similiar to yours. My ac comp. was loose and caused a vibration without belt squeal. I hope it is a simple one.

 

6 and 12 can be "perfectly" balanced, all the up/down, rotating, and combustion events have an opposing force from one of the other engine parts. The smooth feeling a V12 has is kinda nice, but I also remember when the first Mazda rotary engines came out and you could balance a nickel on a running engine. The 911 also has the smooth feel, almost electric. I kinda also like an engine thats more like releasing a demon from hell too though.

 

I remember reading somewhere that the engine short block is internally balanced through tolerance control, as one should expect from a high performance precision machine. Within engine family, all pistons and rotating parts are interchangeable and will not affect balance. What I read stated that new factory pistons are ready-to-go and require no balancing because there is, purposely, very little variance including weight(s).

From the WSM, page 13-24a: Since 1980 models pistons and piston pins are paired according to weight. Pistons are weighed with components (piston pins, piston rings, circips).

Piston pins must always remain in the corresponding pistons and must not be mixed up within a set of pistons for one engine. This is important when disassembling and assembing an engine and pistons must be marked if necessary.

If pistons and pins are mixed up, they must be rearranged by checking the total weight.

Weight = 772 g
Permissible tolerance = +/- 4g

In other words, so long as the piston assembly total weight is within that spec range, you are good to go.

 

Stock stuff is heavy but very close to same weight, +/- few grams. Even oversize pistons are same weight as stocks.

 

Thanks for the link!

 

OK "How did porsche balance these engines ?" ......very well

 

All that being said, and thanks by the way, I am thinking something external is a little out of whack!

 

iirc, fact specs are within 4gr. had mine done to 1gr, but I doubt anyone would be able to tell the diff in bal just from driving it.