How critical is balancing?
 

I have just finished installing my very first rod and then it dawned that perhaps I should attempt to balance my units.

Here are my units:

1. Rods have been reconditioned by Competition Engineering.
2. I have new ARP bolts.
3. Piston's are new JE pistons.

I thought I was slick :D by going to the post-office on a Sunday and use their scale to measure the rods and ARP bolts. Unfortunately, I do not think their scale is delicate enough as all the rods came out to be 1 lb 9.90 oz. The bolts each weigh 1.60 oz. I have not weigh the pins and pistons yet as I think I am using the wrong scale.

Are these #'s close enough? Should I bother with balancing? Man, I can't wait to assemble my engine.


Does anyone have a chemistry scale I could use? If you are close enough, I am even willing to drive so you do not have to part with your unit. I have loan tools before and I understand how it feels to part with your tools.

Thanks.:D

BTW, make sure Walt of Competition Engineering does not see this thread. I believe he spoke at German AutoFest and mentioned about how crazy/an?l we are about balancing. :D :D

There are approximately 454 grams in 1 ounce. Engine parts are balanced using the gram scale. An ounce tolerance is not .... tolerable.

And there are some jig requirements to measure rotating and reciprocating parts, not just "place on scale and weigh". CE is a good source as are some strictly engine balancing shops.

Some guys here have built their own jigs for measuring rod balance. You could check the archives. Crank balancing is another matter.

Sherwood

Ummm, there is 454g in a pound and a little over 28g in an ounce. But either way, an ounce variance is alot.

Cheers

I agree. The Post Office scale was showing 2 decimal places and I was hoping it would at least give me 100th-of-an-oz accuracy. But it does not.

Ummm, wrong scale. My apologies for the obvious mistake.

Sherwood

Bruce Anderson's book says this is not as important as a V-6 engine because it is horizontally opposed, so balance is not critical.

FWIW I did notice a difference in my engine after balancing at higher RPM's. Previously all were within about 6 grams, they were balanced to within 1/2 gram. Spin balancing on a computer rig is the best way to go.

Balancing on a street motor is unnecessary on Porsches, they are already really good. You can match weights left/right front/rear to help without a lot of work.

For weighing pistons, just buy a scale at an Office Max or the like. The one with the smallest maximum weight. Not expensive, plenty accurate to use to get all your pistons weighing the same. You can't weigh really heavy stuff, but so what. And since what we care about is comparative weights, it doesn't matter much if there are inaccuracies, as long as they are constant.

You could weigh rods as well, but since they need end to end balancing, you'd want to build a jig (and know what you are doing), so that is more of a specialist thing.

And nobody does DIY crank balancing, do they? I haven't even figured out how a tire balancer magically gives its readout, much less how the crank balancers do it. But do it they do.

How much is enough is a fine topic for endless discussion. A half gram isn't much. Way less than the differential buildup of carbon on a piston, no? At some point there is no measureable gain for closer balancing.

Since I have the scale, I've been thinking of weighing rocker arms. Maybe I'd want to figure out how to weigh each end separately. Hmm.

Walt Fricke

As Walt points out, there's no easy way to determine the balance of a crank, especially at speed which it requires. B. Anderson's statement assumes the crank is already balanced, but we know what happens when one assumes too much.

Sherwood

When I did my pistons/pins/clips, I use a scale like Walt recomends. When I was done, they read the same number of grams. I think my scale went to .5 grams...

Cheers

I've twice balanced a single replacement piston (we all know how that kind of thing happens) to match the balanced set of 5 I had. Even though the new ones were of the same weight group, each time I think I chickened out on grinding off more aluminum on the edges of the pin boss before I got within a couple of grams.

But now, having snooped on enough Pelican discussions, I realize I could have snagged the odd gram or three or maybe five on the pin. (can you even get more?) So next time I can waste time getting things to within the tolerance of the scale. But I won't have to worry about what the practical limit really is.

Walt

Postal scales are not good enough, but if you want to drop $125-200 you can get a gem/jewelry scale that does .1g

http://www.oldwillknottscales.com/index.asp?PageAction=VIEWPROD&ProdID=39

I was looking around and found a rod weighing fixture that actually uses this scale... forgot the link though :( I was thinking about it, but I could use the money elsewhere for sure...

best regards,

Michael

A hundred bucks here and there and things add up. Maybe I can use this to replace my powder-scale. :p

In order to balance any of those components properly, you need an accurate and respeatable scale that has very good resolution. So bathroom scales, and post office scales that the public throws there overweight boxes onto on a daily basis prrrrrrobably wont give you the result your looking for. The rods shoudl be balanced end to end, weighing the whole rod and taking that as it wont do. The big end and wrist pin end could be off on each. Thats will determine where you would take meat off if any. There are adjustable jigs used to weigh each end, you cant just go grinding off material whereever you want....well I mean you can...but its not the optimum way. The pistons usually have material under the dome you can remove, and weighing them is pretty straight. The crankshaft, well thats a pain. I balanced all my cranks on an old Stewart Warner balancer, that gave beautiful results, it was pretty tricky to learn the nuances of how it worked, but once you got it, it did a great job. The cranks are balanced in two planes, dynamic and kinetic if I remember, one is your basic up and down balance, or centerline type plane, and the other is the end to end, which basically makes the crank want to sort of crabwalk at speed when the imbalance comes in. Forgive me, its been a while. I would balance the crank, get it dead on, then do the flywheel, pp, and pulleys, so by the end the whole mass was in balance.

I used to spend hours lightening and balancing my FV engines. It was a real pain in the arse, but since I was a home builder I used one of the readily available digital food scales that you can get at your local Williams/Sonoma or kitchen supply store. It was good enough and went to .5 grams.

Then I rebuilt my 2.0 six last year and was blown away by the accuracy and matched weights of the rods (total and end to end), pistons and rockers. I didn't put a grind mark or drill bit dimple in anything. Then again we are talking about the difference between VW and Porsche.... :)

Dion

I learn something new everyday. I was exposed to the 'crank' balancing from Wayne's book --- which I could never do.

Then, I though I read somewhere on this board that in lieu of actually taking grams off the units, you simply:

1. Identify the weight of each unit.
2. Having identified the weight, you install the 2 heaviest units on cylinders X/Y, the 2nd middle weights on cylinders A/B, and finally the lightest units on cylinders C/D --- or something like that!

BTW, I am building an early SC (big exhaust port), S cams, carbs, and mainly for weekend-canyon carving.

So, since my project would probably not see the track, balancing would probably be somewhat a waste of time and $$? If so, I am all for the assembly go-ahead. :D :D

Just curious....I have new Mahle P/C's, Carillo rods and a brand new crankshaft, fresh from Porsche that'll fit in a brand new '84 case....

Do I need to bother balancing anything at all? I was sorta hoping that those fine suppliers at least kept their parts equal in weight....

Opinions?

Dennis

I would put this in the category of assuming all is well because the parts are new. Proceeding without checking depends on your level of confidence. Some folks are more comfortable than others about this.

Sherwood

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

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

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

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.

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