atb

Just received the rotating assembly back from the machinest.

Due to the lighter weight piston/pin/rod combo, over 400 grams of weight was removed.

Should rev a little quicker......

mark kibort

400grams? thats less than 2lbs, on a narrow diameter. 2lbs out of 50, wont be noticable, but im sure its balanced better so that may make it even smoother!

looks nice!

MK

atb

The machinest made up a bob-weight with the stock piston/rod/combo just to see what factory spec was. He said it was right on. Pretty impressive. (Although this is a Porsche we're talking about, so maybe not)

slate blue

Hi Adam, I wish I had one of those in my garage! Can you tell me how they removed the weight? The reason I ask is that when I get my crank done I will be using very light pistons and rods, my concern is that if my rotating assembly is too lite it may lead to a very expensive balance. My pistons complete weigh 500 grams and the rods 580 grams. As you know the standard S4 weigh is half kilo heavier per rod and piston.

Also do you mind weighing that crank for me?

Thanks and good luck

Greg

atb

Greg, If I get a chance I can weigh the crank, but it would be on a scale only accurate to a tenth of a pound. I don't know if that would be be accurate enough for you.

I was hoping to see the crank on the balancing jig, but didn't get a chance. I don't know what they use to drill the counterweights. This is the same shop that did the bore/hone on my stroker block, so they have a Sunnen machine there, but I doubt they would need that kind of precision for balancing. When the stroker crank goes in, I'll definitely take some pics of it on the jig.

slate blue

Thanks Adam and thanks Sterling, good info. Adam any scales will do. Thanks Greg

Imo000

Adam, I have an easy solution for you. Take you crank to your local butcher/grocery store and ask them to please weigh it on their deli scale. If that doesn’t work, you could ask your local paediatrician to put the crank of the baby scale!

Garth S

Most vets have a very accurate digital 'walk-on' scale in the waiting room: just put a leash on it & go ...

atb

imo wrote:


Doctor, I don't what's wrong with my baby, he's been a little "cranky", (nyuk, nyuk, nyuk,


Sterling, your reciprocating components are so light its criminal dude, do you know how much weight they took off your crank? Maybe there's an uninstalled DEVEK stroker crank floating out there that we could weigh for comparison.


I'll take my stock S4 crank, the lightened RearTurbo S4 Crank, and my 928 Intl Stroker crank and get them all weighed. It will be interesting to see how much weight they add to the stroker to balance when the time comes.

Carl Fausett

I like it.

BUT - and not to make you worry - but you have moved the nodal point of the crankshaft slightly as a result, and I wonder what effect that might have.

What is a nodal point? Machinery and rotating parts set up a certain harmonic resonance at certain rotating velocities. They hum, in a sort of a microscopic, not-always-audible sort of way. But - along the rotating assembly, there will be certain nodal points where there is no vibration or hum at all. Basically, this is where the sine-wave of the resonance crosses the shaft to the other side.

ANYWAY - they like to (engineers) locate the mounts of things at nodal points to be most effective. Nice write-up in EXCELLENCE magazine today of how the tranny mounts for our 928's were moved to nodal points behind the transaxle because of the cantilevered battery box hanging off the back.

On your crank - this MAY have the effect of changing the resonance, which MAY mean that the harmonic balancer is now wrong, or that the crank bearings are now in the wrong place. They were at nodal points before, and now they are not.

I don't think that you will have any trouble, 400 grams is not all that much. But if the crank does fail, it will snap in half, and this will be the reason why.

atb

Crap Carl, did you just come off decaf today?

Carl Fausett

what - cause I am all cranked up?

atb

Now your talkin'!

Your explanation of nodal points on a crankshaft I think is similar to the phenomena of harmonics on a guitar string. Can I get an "amen brother" from Widom on that, or am I in left field on this one?

slate blue

Hi Adam, or shall you now be known as nodal point? Anyway can you say what your other bits weigh too?

Thanks Greg.

Carl Fausett

...except for the fact the guitar string is not rotating or performing work, I like the analogy. If you have ever stared at a base string after it was plucked, you noticed the sine-wave shape of the string... the whole string does not vibrate the same direction at the same time, but the vibration sets up harmonics back-and-forth across the string.

Anyway, I'm sure you are going to be fine - especiually because this was done in response to matching, lighter rods and pistons.

When I balanced my engine, I found most things within 2 grams - which is about twice as accurate as your factory Ford or Chevy motor. We took them down to 1/10th of a gram anyway. Here are some fo the pictures:
https://rennlist.com/forums/928-forum/198474-balancing-the-928-engine-some-surprises.html