To Balance or Not To Balance Crank?

[Green993]

I have my 993 engine totally apart. I am replacing the crankshaft with a new (used) one from a lower mileage car (original was damaged). The crank was inspected, polished and magnafluxed at Ollies. I have balanced the rods and pistons already. The pulley is new. I'll be re-using flywheel, pressure plate and clutch disc.

Is it worth the trouble, effort, money, etc to get the crank, pulley, flywheel & PP balanced?

My goal is a smooth running engine that will last another 100K or more. I do not race it. It is a daily driver. I just don't want to regret NOT doing something that I could have done pretty easily given it's all apart. Also, does the 993 crank follow the standard rule of 100% rotating weight plus 50% reciprocating weight?

Opinions?

[HawgRyder]

An opposed engine is in pretty good balance to start with.
As long as you don't mix types of barrels and cylinders (size, weight) you should be OK for the normal RPMs.
We tried using mis-matched barrel/piston combinations on a VW once (to cheat the rules in class racing...bad us!)...and we put 66mm barrels on the rear (pulley end) of the case...and 92mm barrels on the flywheel end.
As long as they were opposed the engine ran very well.
And yes...we got caught...LOL.
But it did prove that if you match opposing P/Cs you can get away with a lot....just make sure that the heavy ones are on the same end of the crank...etc.
Bob

[Green993]

The pistons, cylinders and rods are an original matched set. The pistons, however, were 3 grams apart from heaviest to lightest (without rings or pins) and my machinist said the rods were also out of balance (now in balance).

[HawgRyder]

So...pair up the heavy pistons and use them on opposing rods....and so on.
Bob

[Steve@Rennsport]

Originally Posted by Green993

Is it worth the trouble, effort, money, etc to get the crank, pulley, flywheel & PP balanced?

Yessir, it sure is.

[AlfonsoR]

Steve,

Is it customary to component balance the flywheel, crank, and pulley, then final balance as an assembly? Or do you just do a single balance job on the complete assembly? If necessary, where is material removed from on the crank?

[Steve@Rennsport]

Originally Posted by AlfonsoR

Steve,

Is it customary to component balance the flywheel, crank, and pulley, then final balance as an assembly? Or do you just do a single balance job on the complete assembly? If necessary, where is material removed from on the crank?

Crankshafts are balanced, factoring rod & piston weights. Flywheels, pressure plates, and pulleys get zero-balanced so they can be replaced without having to disassemble the complete engine and re-do everything.

Material is ground off the counterweights in various placed to achieve objectives.

[Green993]

Steve,

Does the 993 crank follow the standard rule of 100% rotating weight plus 50% reciprocating weight?

[chris_seven]

I don' think that the 50% rule is applicable to a Flat 6.

I believe that a Flat 6 Crank has good primary and secondary balance and can be balanced as a simple rigid rotor to a maximum imbalance of 10 g.cm (0.139 in.oz) without bolting on additional weights.

I would balance a flywheel separately to ISO G2.5 and the same with the clutch cover.

I would match weigh the pistons and rods (I would also match little end weights)

The crank pulley is such a small diameter it may not have much influence.

I am not sure what 'zero balanced' means - is this a Class 0 according to the German Standard VDI 2060?

[Aurel]

A crank itself should be balanced by design. I do not see how it could become imablanced unless material is removed from it, which is exactly what is done to balance it with pistons, rods, etc...But then, that particular crank becomes imbalanced by itself, and will not function correctly with another set of rod and pistons, right? It makes a lot more sense to me to balance the whole assembly via the piston pins weights, and removing material from rods whenever possible. Please enlighten me...

[HawgRyder]

You are correct as it pertains to flat or opposed engines.
With an inline or V8/V6 it is a different matter.
As long as you are not pulling high RPMs IMHO you can forgo the balance.
If you plan on racing in any fashion..by all means spend the cash and balance it.
Be advised...there is balancing and there is BALANCING!!
The amount of offset or allowance is where the big bucks come into it.
To balance to zero grams...you are in the stratosphere (and remember...the clutch assembly is part of the rotating mass...as well as the pulley, etc).
The engine will be glass smooth...and probably last a lot longer...so it just depends on your wallet...LOL
Bob

[AlfonsoR]

Originally Posted by HawgRyder

The amount of offset or allowance is where the big bucks come into it.
To balance to zero grams...you are in the stratosphere
Bob

I'm pretty sure Zero balance, means component balance. In other words, each component is dynamically balanced individually. This way, if one needs to replace the fly wheel, it can be done so "without affecting" the engine balance.

The truth is that you balance to a specification, such as ISO1940 G2.5. or whatever you prefer, there is always an inherent in-balance. The in-balance is a small amount of course, and it is within spec, but it is still there.

If you only component balance each peace and you don't do an assembly balance then you have to make sure you know the phase angle (location) of the in-balance so that when assembled you don't align the in-balance of each component into the same phase and potentially exceed the spec you are trying to achieve.

Otherwise, you component balance each piece, then you assemble and balance the assembly. This is the way large turbomachinery is balanced, except that in large machines, you may also consider an at-speed balance if the rotor operates above the first critical (natural frequency). In a car engine, the run speed is always below the first critical.

This is also why on any rotating equipment, you want to match-mark one piece in relation to the other...so that the balance of the assembly is not disturbed.

[afterburn 549]

Always shoot for "0" Its cheap, it right, there are no wrongs .1 gram is a LOT of weight at RPM

[911pcars]

I would consider balancing the reciprocating parts a basic part of rebuilding an engine. How do you know the conn rods are end balanced.... other than assuming they are or measuring them?

Have engine balancing costs increased significantly (~$250-$300 to elect not to perform? The more you know about the engine, the less you have to assume.

Sherwood

[mattC2993]

Crankshaft should be balanced for factory rod & piston weight. The biggest question is the pressure plate balance but then again no one check balances these except oddly enough the 356 guys whose engines turn 2000 rpm slower . Considering the cost of a 993 build checking the balance seems like a good investment IMHO.

[Green993]

Originally Posted by 911pcars

I would consider balancing the reciprocating parts a basic part of rebuilding an engine. How do you know the conn rods are end balanced.... other than assuming they are or measuring them?

Have engine balancing costs increased significantly (~$250-$300 to elect not to perform? The more you know about the engine, the less you have to assume.

Sherwood

The reciprocating parts are already balanced. Supertec balanced the rods and I did the pistons. It's the rotating parts that need balancing. The only thing keeping me from balancing the crank is knowing which formula to use. My local machinist has a list of various makes/models provided by Hines but it does not include Porsche. According to Hines website if it is not on their list then the standard formula of 100% rotating and 50% reciprocating. All I need to do is confirm this and I can go ahead.

I have searched all over the interwebs and can't find this info. The machinist is not a Porsche specialist but otherwise is an excellent shop and well regarded. I am also amazed that there are not many shops that do crank balancing (at least locally here in the Bay Area).

I had an interesting conversation with Ted at German Precision. He said people are just not rebuilding as much as they used do so demand for this service has declined. This, coupled with California's stringent chemical laws, may be why automotive machine shops are on the decline in CA.

[Green993]

Originally Posted by mattC2993

Crankshaft should be balanced for factory rod & piston weight. The biggest question is the pressure plate balance but then again no one check balances these except oddly enough the 356 guys whose engines turn 2000 rpm slower . Considering the cost of a 993 build checking the balance seems like a good investment IMHO.

I agree. See my last post.

[911pcars]

Originally Posted by Green993

The reciprocating parts are already balanced. Supertec balanced the rods and I did the pistons. It's the rotating parts that need balancing. The only thing keeping me from balancing the crank is knowing which formula to use. My local machinist has a list of various makes/models provided by Hines but it does not include Porsche. According to Hines website if it is not on their list then the standard formula of 100% rotating and 50% reciprocating. All I need to do is confirm this and I can go ahead.

I have searched all over the interwebs and can't find this info. The machinist is not a Porsche specialist but otherwise is an excellent shop and well regarded. I am also amazed that there are not many shops that do crank balancing (at least locally here in the Bay Area).

I had an interesting conversation with Ted at German Precision. He said people are just not rebuilding as much as they used do so demand for this service has declined. This, coupled with California's stringent chemical laws, may be why automotive machine shops are on the decline in CA.

If the rods and pistons are balanced, then they should be a null factor when balancing the crankshaft. Not sure why there's such a mystery in balancing a flat six crank despite the reduction in automotive balancing shops. Contact a Porsche engine rebuilder/machine shop like Ottos, Competition Eng., Patrick Motorsports (or equivalent) and ask them the pertinent question(s). Don't assume a Porsche 6-cyl. crank is inherently balanced w/o confirming. It's a relatively minor expense compared to the overall budget.

Sherwood

[Walt Fricke]

My understanding is that balancing a Porsche 911 crank does not involve bobweights. I'm pretty sure that's what the balancer guy I used to use (RIP, alas) said. He took only a very little off here and there for race use.

And he was a stickler. He wanted to balance the pulley, flywheel, and PP together, and had to be persuaded that I wanted those components zero balanced so, as has been mentioned, they could be separately replaced.

Pistons can be an easy DIY balance job with an inexpensive digital gram scale. I don't think it is worth getting pistons within one gram, because carbon willl deposit unevenly and it doesn't take much of that to add a couple of grams. And by matching weights, you get as good a balance as you could anyway. Maybe at 20,000 rpm it matters, but I don't think having pistons which are close enough to the same weight get a bit lighter as they move out from the flywheel is going to put a twist on the crank.

Not so rods, which require a jig for end for end balancing, and have only a few small places where you can safey grind off material. The web on the bottom is not like a lot of Detroit iron - a big blob there to be ground down for balance. A shop once ruined a set of nice stock 911 rods by shaving away on the web. They weren't used to 911s.

[AlfonsoR]

Originally Posted by Green993

The only thing keeping me from balancing the crank is knowing which formula to use. My local machinist has a list of various makes/models provided by Hines but it does not include Porsche. According to Hines website if it is not on their list then the standard formula of 100% rotating and 50% reciprocating. All I need to do is confirm this and I can go ahead.

Perhaps too late, but the 50% vs 100% sounds like a general design rule. I searched too, but could not find any info. I would recommend you contact a couple of race shops and ask them what spec they use for balancing their cranks. It might be beyond what you need, but it more than likely will be a spec that allows x amount of imbalance. Come to think of it, Costa Mesa R&D has a video on balancing cranks. They would be able to help you with a spec for your crank.

In machines such as compressors and steam turbines, it is a fairly common practice to balance rotors to 4 x W / N, where W is the weight and N is the max run speed. This calculation will be in oz-in. This is essentially an Navy spec. It may be of some guidance. You can read more here: http://www.irdbalancing.com/downloads/shoptolerances.pdf

What did you end up doing?

P.S.: found a site that explains it better than I did and it is focused on cranks....http://www.cwtindustries.com/whybalance.htm

Notice that their computer screen shows two balance planes, which means they are assuming for the purposes of balancing and correcting weight that the rotor is supported by two bearings, one at each end. So to apply the above formula, W is the weight at each end or essentially half of the total weight. The oz-in value is the max allowable in-balance. The balance software will calculate that for you and tell you at what angle the imbalance is so that if there is a correction needed you know where to add or remove weight. Anyway, probably clear as mud now.