How critical is balancing?

[euro911sc]

Dennis,

How much was your new case? crank? pistons?

Do you want to spend that again?

Instead, you could spend a lot less and get it all checked. I have heard/read enough sorries about pistons with the wrong compression ratio new in box etc etc that I just don't trust anyone anymore. I know, its sad, but its the way things are...

Best regards,

Michael

[Dennis Kalma]

Case was $1000 including studs and new Carrera Oil pump
Crankshaft (the expensive one) was $2800
Rods were about $1300
P/C's were about $3500 (Mahle, Nikasil 98mm 10.3 compression)

I would spend it again in a heartbeat....this is my retirement toy and I want it to last as long as I do....

And yes, the case was a heck of a deal. It appeared on eBay and no one bid on it....the price I offered was below reserve, but the guy just wanted to get rid of it and I was the fortunate recipient.

So far everything checks out as being right, I guess when I finally get it all together I will find out for sure.

Dennis

[Walt Fricke]

Dennis

My 2 cents: buy an inexpensive digital scale (the 2Kg kind?) and weigh your pistons. Do it because you can, and because Mahle makes a lot of stuff.

Without access to a balancing shop, not much else you can do. But I'd be inclined to trust Porsche on the crank, and Carillo on the rods. I suppose you could weigh the rods - if you found one that was off you'd then maybe send them all to a balancer for a proper end to end balance.

I should weigh my new Pauters and J&Es just to see what my scale says.

Walt Fricke

[chris_seven]

Guys,

Surely for the cost involved we should balance these motors to a relatively high standard as good balance must improve basic performance.

The force (F) generated by rotational unbalance is given by
F=mr(omega)^2, where m is the unbalance mass, r is the radius from the rotational axis, and ù is the angular rotor speed in radians per second and is given by:

omega = 2Pi.N/60 where N is rotor speed in rpm.

This means (approximately) F = 0.01(N/1000)^2.mr

F is expressed in kilograms, m is in grams and r is in centimetres.

For example if a conrod has a big end weight variation of 1g at 5 cm then at 6000 rpm it will produce an unbalance force of 1.8kg

The inertia force created by unmatched parts can easily be calculated using F = ma

Again F is in kilograms, m is the unbalanced mass in kilograms and a is the maximum acceleration on m/s/s

An engine with a stroke of 100mm, a con rod ratio of 1.5 and a piston weight difference of 0.5gram will produce an inertia force of about 1kg.

Flywheels that are not perfectly concentric produce very high forces and the concentricity
achieved in their mounting needs to be very well controlled. A run out of 40 microns in a 15kg flywheel can cause 15kg of force to be produced at 6000rpm.

It has always been considered that crankshafts are stiff enough not to be deflected by normal out of balance forces particularly when supported in a crankcase but if the case is light alloy and has a low modulus of elasticity I would be less confident that some bending does not occur and this will result in reduced life cycles.

The other issue is that unless the crank is balanced in more than two planes and out of balance forces could result in a reduction in the lateral critical speed of the assembly which could result in significant vibrations in the running range of a performance motor.

Bad balancing causes power loss, fatigue damage, shakes ancillaries and can cause sensor failure.

[Dennis Kalma]

Hey Walt, your wise council continues!

I guess you are perfectly right....it just makes sense to check 'cause I can....and the penalty for having a screwup is so high. My wife has a great digital kitchen scale (I think it is 0.5gm) that I will use, or I can bug one of my scientific types to loan me or give me access to a better scale....

Thanks, hope all is well on your end....I am off for a month of vacation in Holland in a few days, will consider whether I want to retire or not....

Dennis