930 Engine Balancing Results
 

For the benefit of those who are curious about how well porsche motors are balanced I have decided to share my balacing project results with you all. :D

Over the past few weeks I have spent many hours balancing my pistons, pins and rods in an attempt to weight match everything as well as can be done. In this case my limit was the accuracy of my scale and balancing equipment.

The good news is that the worse weight difference was only a few grams, but everything is now within 0.05 grams of where they should be. For regular jobs getting everything to within 0.3 grams is concidered good enough.

Here is how far out everything was to start with.

The rods' total weights were all within 0.5 grams of each other but their distributed weights were way off. The largest big end weight difference was about 3 grams and they all were different.

The wrist pins were within about 0.5 grams of each other so I made 3 matched sets to minimise having to remove weight from most of them.

The beauty of the flat six is that the reciprocating weights only need to be balanced between each opposed pair of cylinders.

The pistons were obviously in weight matched sets. Here are the details for those of you who are curious how close porsche decided to balance these weights.

444.2g - 443.4g
442.4g - 442.7g
441.2g - 441.6g

I took a lot of care to remove piston material from a location that would not effect the structural integrity of the piston in any way. On 930 pistons there is not a lot of excess material to choose from.

Luckly I didn't have to do anything to the crank. The crank was extremely well balanced from the factory.

To put it into perspective we usually balance a couple reciprocating assemblies a week and nearly everything we see starts out with a measurable imbalance of several inch-ounces at each end of the crank.

A balance job will usually involve in getting that down to less than 0.5 inch-ounces.

After spinning it up my 930 crank we measured less than 0.05 of imbalance at each end with no flywheel or pulley. Thats amazgingly good. Next week I will spin the assembly again with the pully and flywheel+PP attached so I can balance those items seperatly.

If I have a chance I will try to post some pics up of my recently lightened items.

Thanks for the info. I have a question though. First let me say that I have never balanced an engine so I am not questioning your methods. I am just curious. Could you have matched lighter pistons with heavier pins and visa versa to save having to remove as much from the pistons?


Jeff

What you want is each opposed cylinder piston and pin (ie 1&4, 2&5, 3&6) to have matched weights. For that reason you would want to balance each set that is already close in weight to each other so that the weight removal is minimised.

If you took the heaviest and lightest piston and tried to match them up you would have to remove more weight than if you took each closest pair.

If you look at my piston weights that I posted above they are listed as best matched sets. Do the math yourself and you can see which method is more effective.

I think CATCA meant that you might be able to match the total weight of the assembly - rod, pin, and piston to equal the opposing assembly. For instance, if you had a piston that was 1 oz below the average and a wrist pin and rod that were 1 oz fat you could match them all together like a chemistry equation and perhaps have to remove even less material.

I see where you are coming from on the wrist pin and piston combo, but the pin differences were not enough to balance out the pistons so I balanced both for the sake of perfection. Technically I could have saved removing about 0.1 grams from each piston but then I would have to keep each pin matched with each piston instead of just having 3 balanced sets for the 3 cylinder pairs. When I get down to deciding which clearance specs I use to match items I can be more flexible this way.

When it comes to rods you have to treat them differently since they act as both rotating and reciprocating mass so there is no practical way to relate their imbalance to that of the piston/pin combo.

are you using a beam scale or electric one??

Electronic.

I think if you could post pictures and directions of where and how you removed material that would be helpful. For example, did you mill, file, dremel, sand, etc to shave the weight? Same for the rod, pin and piston locations.

Just more data for everyone on the board. Did you make a jig to measure rod end weights? I assume that you balanced each rod end and then removed a bit from the middle (near COG) to balance overall weight

This is where the term "0" grams comes from if you shoot for everthing to be the exact same in weight you can claim "0" grams but in reality you can be out a 10th here and there.....I use a triple beam Cause i am told by other shops the electric will not give you the same results every time. What is your experiance?

As long as there are no large temp changes between measurements the scale always returns the same result for each item. If you can't get repeatable measurements from an instrument its junk IMHO.

Here are the pics as requested.

http://forums.pelicanparts.com/uploa...1105036748.jpg

http://forums.pelicanparts.com/uploa...1105036828.jpg

http://forums.pelicanparts.com/uploa...1105036855.jpg

http://forums.pelicanparts.com/uploa...1105036875.jpg

http://forums.pelicanparts.com/uploa...1105036894.jpg

http://forums.pelicanparts.com/uploa...1105036917.jpg

http://forums.pelicanparts.com/uploa...1105036934.jpg

Enjoy . . . :)

I was just meaning the pin and piston together. But if there is not enough of a difference in them to help with the balance I can see why you wouldn't do that.

Thanks for the info, like I said earlier, I really know nothing about engine balancing!!

Cheers, Jeff

that would be my thought also(should get the same reading every time)

Did you balance the big ends(rotating), the small ends(reciprocating) as well as overall weight or the rods?

I usually balance the big ends first. I do this by hanging the rod on a very good ball bearing in the small end and placing the big end on the scale, with the rod perfactly horizontal. Balancing all big ends to match the lightest rod.

Next I balance the small end by just weighing the total rod and only removing material from the small end. Again matching the lightest rod. This balances both the small end and total rod weight at the same time.

Summary:
Material is removed from the entire bottom of the big end, or balance pads if they have them, and finally from the entire top side of the small end. Trying to make a smooth finished look as work progresses.

All rotating weights can be combined to acheive balance and all recipricating weights can be combined to acheive balance, eg piston, plus wrist pin, plus retainers, plus small end of rod.

i can see the smoothing on 350's small ends. Cool.

Is this a "why not" for everyone who is rebuilding an engine or does it make no difference unless you are seriously racing? It seems that you would have a smoother running engine if it was all balanced well.

My view is that the rods should always look like the pictures above, any engine. Biggest reason, stress risers, ie scratches, or gouges or sharp or rough edges will cause cracks to start and the part to fail even street cars. As for balance it makes any engine run longer, smoother. Better than production Porsche balance? Maybe overkill.

For any race car or really HP street car you should also have everything shot peened. This puts the entire surface of the material in compression, which prevents cracks from propagating or even starting. If you do balance your rods you should send them out to be shot peened after you balance them.

Good to know.

Yup, I had meant to snap a pic of our balance apparatus but forgot.

In addition to the crank balancing machine the rod balancer is nothing more than a scale with a special adapter with small bearings for weighing the small or big end of the rod, not to mention pistons or anything else that weighs less than a kilo.

The process is just as you describe, though if you have have to remove enough weight from the small end you may have to readjust the big end a little because it can create enough of a moment change about the wrist pin bore to increase the big end weight from its set value.

Since this part of the procedure requires nothing more than some basic skills, a scale and a grinder its possible to make this a DIY project.

It appears porsche owners don't need to worry about balancing their cranks.

Did anyone happen to notice the change in the lower profile of the one piston in the shot with them base to base?

http://forums.pelicanparts.com/uploa...1105065547.jpg

Do you mean the little casting "bar" in the center bottom vs the absence in the top photo?

While that material is missing, I was referring to the skirt material that is missing.

You can see how the unmolested piston has a circular area at the bottom to clear the crank while the lightened piston has a straight edge from the pin area to the skirt.

Update time . . . :D

Last weekend I had a chance to rebalance my crank with the flywheel and pulley attached so I could see how well those items were balanced.

My perfectly balanced crank bumped up to about 1.5 oz-in off on the flywheel end and 0.4 off on the pulley end. All that was required to get everything back in line was a half inch deep hole in the flywheel and a small notch in the pulley.

Here are some pics for everyone to enjoy . . .

Here is my modifiled flywheel.
http://forums.pelicanparts.com/uploa...1106020672.jpg

Here is a pic that shows the notch in the pulley.
http://forums.pelicanparts.com/uploa...1106020779.jpg

Here is a pic of the whole balancing apparatus.
http://forums.pelicanparts.com/uploa...1106021013.jpg

Here is a screen capture from the balancing PC.
http://forums.pelicanparts.com/uploa...1106021321.jpg

So ya only got it to 0.64gm instead of 0.00000000gms. Now you will have to limit the RPMs to 30,000 or less.

Very nice equipment.

How much would your shop charge to have this done?

A 'race balance' that gets all rods and piston sets within 0.1 grams of each other and the crank-flywheel-pulley combo within a 0.3 oz-inches costs $462.

We also have a 'street balance' for $381 but its likely that most porsche motors would already be within that balance spec from the factory.

0.1 grams vs 1 gram is a TOTAL WASTE of money on an all out RACE CAR, even 1 gram is questionable. For example just how much difference in force at 9000RPM is there with 1 gram unbalance in pistons? Or how about 1 gram cm for crank balance?
The number can be calculated, but the answerr is still fly sh itt.

One could argue that balancing a porsche motor is a waste of money since its already 'good enough'.

A one by one inch piece of paper weighs about 0.1 grams so that should allow one to put into perspective how much weight we are talking about.

I guess the next time I am doing some machine work perhaps I shouldn't bother being accurate to 0.0001 +-0.00005" because getting to within 0.0005" of the target is 'good enough' and anything else is a waste of time and money. ;)

Thanks for posting the pics.

Please lets not get into pissing contest about this.....if you want to shoot for "0' grams thats ok with me and we all can learn from your exp.-If you want to shoot for something less,Fine. I think this is a good place to learn and help ea other.
I have noticed that in the last few months that the members here have aspired to be dignified and respectifull while helping,correcting and apoligizing for stuff.....NOT like other places.....where rude is the rule.
My 2chttp://www.pelicanparts.com/support/smileys/hiding.gif

Well, that was a good question and I suspected that it would add up to more than fly **** so I decided to make some quicky calculations using some of that old school physics stuff that is still bouncing around inside my head.

The following caculations use F=ma for linearly accelerating mass such as imbalances between the reciprocating sets and F=mv^2/r for the rotating mass like the crank and big rod ends.

A porsche motor spinning over @ 7K RPMs is going to see max piston and rotating mass speeds of 27.5 m/s (61.5 MPH).

Since the pistons accelerate and decelerate twice per revolution this means that there will be 466 accel/decel events per second where the approximate acceleration is about 12,815 m/s^2

Using these numbers for each 0.1g imbalance between reciprocating weight there will be an applied force of 1.28 N (0.288 lbs) to the motor.

For each 0.1g of imbalance in the rotating mass there will be a resulting 2.02 N (0.454 lbs) force.

If you arrange several of these of these 0.1g imbalances in the right configuration we could be talking several lbs of buzz inducing force applied to a motor at its max RPMs.

I think it would take a lot of fly **** to amount to a large fraction of a pound, let alone several.

:cool:

For imbalance, anything less than hundereds of lbs is fly s....t. Thats why Porsche uses numbers like 15gm cm to be good nuff.

Actually there is another reason as well. Oil.

Oil weighs a lot. How much oil is there on each piston, each part of the crank at any one time? It varies, a lot and we are talking about numbers ranging from 5 to 25 grams. every time an oil squirter hoses down the bottom of the piston is adds a lot of weight, not exactly uniformly, the crank is being splashed with oil and retaines varying ammounts both internally and externally.

I suspect thats the main reason even the ANAL types, and most really good Porsche machinests fit that discription, who like perfection, usually stop at one gram.

Even so its really nice to have equipment that can do 0.1gm and if you don't have to spend to much time doing it , its always nice to have perfection as in the fully finished undercarriage of a concours car, not practical at all but nice.

This is a great discussion, guys. I think everyone can learn something from it, and it's just one of the great things about these boards.

350HP930,
I will be in Tampa, for business, the third week of February. Any chance I can stop by and drool on your motor????

Well, about half of it is at home and the other half work, but we do give tours of the machine shop if you would like to stop by.

Tampa aint too far from Atlanta. :)

Balanceing Results
 

I'll share mine too...will pick up the rods tomorrow, but I was able to get my pistons, rings, locks and pins to within .8 grams of each other and a reciprocating mass zero imbalance across the banks. Here are the total mass of each piston + rings + pins + locks. I didn't remove any material, just mixed-and-matched. I'll scan and post my balancing worksheet later.

Piston Mass (g)
1 576.8
2 576.8
3 576.8
4 576.0
5 576.8
6 576.0

I'll install piston # 4 and #6 the #1 and #4 cylinders, so that there will (at least until the rods come in) be a perfect balance out to 1 dec place grams across the banks.

john

Good info thread, that deserves a "renewal" :)