Piston and Connecting Rod Weight

[porschedude996]

Hi P-Nuts,

I'm looking at piston and cylinder sets and also connecting rods. My target is a smooth running stock engine with a 964 grind on my cams. My guess is to weigh the existing and try to purchase the same weight. My problem is that how can I assure that a piston within the cylinder-piston set is the same weight range of acceptability? Would matching of the total mass of the piston and rod would be sufficient?

[buster73]

If I recall correctly, the factory spec for piston-rod assembly variation is 9 grams. During my 3.0 engine project it was quite easy to achieve a tolerance of +/- 2 grams. Good enough for a street engine for my taste and my impression is you can feel a difference (but might also be wishful thinking). Of course you can achieve even smaller variations and also look into the weight of individual components, rod end weight balance, etc.... but the required efforts will soon grow exponentially. When looking at pistons of course all pistons should be in the same weight class to start with (which I would expect from any P&C set).

3.000 € Budget / Rookie / DIY 3.0 rebuild - successful

[PeteKz]

Because it's an opposed piston engine design, absolute weights of pistons and connecting rods do not matter much. What matters is that they are all close to each other. You should try to get the pistons within a couple grams of each other, but I generally shoot for less than one gram because I've built other engines where the balance is more important. The rods also should be within a couple grams of each other, but in addition, the big end weights should match. That's because the big end is what goes around in circles at high speed on the crank journal (ad you had the crank balanced, right?), so if they are not the same, you will get a secondary imbalance and vibration. However, this is more important on V and inline engines than these flat 6's.

I'd suggest that you turn this all over to a good shop that has the right equipment for balancing, rather than do it yourself. Unless you just really want to get "the whole experience."

Some builders recommend that you get the crank and flywheel, and even clutch all balanced together for optimum smoothness. I don't think that necessary either. I go for these components to be balanced separately as close as possible. I figure at some point I will end up replacing the clutch, and I'm sure as hell not going to pull apart the engine to rebalance everything together! So I just go for best balance on the components. NOTE: Some engines are designed with a counterweight in the flywheel and harmonic balancer (not 911's). For those it IS important to balance those components together.

[dannobee]

If you have one of the cheap digital scales, you can do it yourself. Pistons should be close. You would never feel a few grams imbalance, no matter how sensitive you are. But weighing the pins separately can help even out any variance by mixing the lightest pin with the heaviest piston (although nowadays, there is likely very little or no variance in pin weights).

On opposed engines, the two heaviest pairs of pistons go closest to the flywheel, with each pair (1-4, 2-5, 3-6) being as close to equal weight as possible.

"V" engines are balanced differently and require "bob weights" when the crank is spun on the balancer. Opposed engines are inherently balanced and the cranks are spun sans any bob weight, making balancing easier.

[PeteKz]

Dannobee: Thanks for adding that the heaviest weights should go closest to the flywheel. That's also in Bruce Anderson's book.

[930cabman]

Originally Posted by PeteKz

Because it's an opposed piston engine design, absolute weights of pistons and connecting rods do not matter much. What matters is that they are all close to each other. You should try to get the pistons within a couple grams of each other, but I generally shoot for less than one gram because I've built other engines where the balance is more important. The rods also should be within a couple grams of each other, but in addition, the big end weights should match. That's because the big end is what goes around in circles at high speed on the crank journal (ad you had the crank balanced, right?), so if they are not the same, you will get a secondary imbalance and vibration. However, this is more important on V and inline engines than these flat 6's.

I'd suggest that you turn this all over to a good shop that has the right equipment for balancing, rather than do it yourself. Unless you just really want to get "the whole experience."

Some builders recommend that you get the crank and flywheel, and even clutch all balanced together for optimum smoothness. I don't think that necessary either. I go for these components to be balanced separately as close as possible. I figure at some point I will end up replacing the clutch, and I'm sure as hell not going to pull apart the engine to rebalance everything together! So I just go for best balance on the components. NOTE: Some engines are designed with a counterweight in the flywheel and harmonic balancer (not 911's). For those it IS important to balance those components together.

Disagree, if you are balancing, why not do it right and spin balance all of the rotating components as an assembly? The correct way is the crank first, add the flywheel and balance again, add the clutch and do it again, ..... making sure everything is match marked

[PeteKz]

Sure, you can do that. Add $$ and time. I doubt you'll notice any difference in smoothness.

No need to match mark the flywheel or pulley--they go on the crank only one way.

[Neil Harvey]

Either do it right or not at all. That's my motto. Close is not good enough. Who is to say how close?

You cannot feel some detonation but that can destroy your engine.

Do the math. At 6000 RPM, 1 gram is a huge imbalance. So far, all you are talking about is a rotating imbalance. Now add in the compressive forces and see how much your crank twists and moves. The crank moves the most at the nose. That is why that front bearing is a sleeve. Except, nothing holds the sleeve to the case other than the interference crush.

All that noise travels up the chains into the cams and all your time setting the cam timing goes out the window. You already suffer from a horrible secondary imbalance every time the Piston leaves TDC to BDC and back again.

Some say you will never feel it. Feel what? If you lost 10 -20 HP or maybe never had it, would you care? I do. That's the easiest performance you can "buy". All it costs is to build it properly. How much does that cost?

[PeteKz]

I'll defer.

[Neil Harvey]

No need to explain. It is me that needs to explain.

As a professional engine company, we have to do it properly.

I have been thinking about writing up more technical papers about how we do certain processes in rebuilding these engines.

Recently we reconditioned some early Connecting rods. Someone in the past had 'tried" and failed to do a decent job. Every rod had its parting line in a different place. CC lengths were all over the place as was the BE bore. Weights were all over the place as well.

The more knowledge out there the more others will understand and hopefully have a better engine experience.

[dannobee]

To add a little more to the mix. Many of you know that I was a nascar crew chief for quite a long time. For long tracks we would routinely overbalance the crank by sometimes up to 4%. With a 2000 gm bob weight, that's 80 grams. On an engine turning 9000+ RPM. For 500 miles. And my drivers still had all of their teeth. Well, most of them.

And I remember many-a-time throwing together an engine late at night before a race with hobbled together engine parts and parking said car in the winner's circle the next day. Balancing wasn't high on the priority list when you barely have enough parts to put it together, much less the time to go down to the engine shop and disassemble pistons and rods, then fire up the balancing machine and spin the crank.

Engine balancing is one of those things that we try to get within a gnat's eyelash close. In theory it should make a difference. In practical terms, I've never seen it make any difference on the dyno, with one small caveat. If you "underbalance" the crankshaft by grinding down the counterweights, this can reduce the windage losses as the crank doesn't have to slice its way through so much oil when going round and round. Many engine builders who I've discussed this with (with collectively tens or hundreds of thousands of dyno pulls) say the same thing, no difference in horsepower.

With respect to Porsche engines, the workshop manual has a spec for what Porsche calls acceptable balance. 9 grams, 6 grams. I've seen both specs. On an engine that doesn't even have a torsional damper (harmonic balancer). If the factory really cared, it would have a proper torsional damper on it.

So, if the engine is apart, by all means, get it as close as possible. It's the right thing to do. But don't think it's going to magically make umpteen more horsepower or last another million miles.

With respect to modern components, the advent of better machining techniques, tighter tolerances, and better quality control, chances are really good that your (new) components are well within tolerance anyway. But check anyway, just to be sure. Here's a set of brand new Mahle's for one of my own cars. I won't lose any sleep over a half a gram difference.

[PeteKz]

Neil and dannobee: I really appreciate your information and experience.

[Discseven]

Originally Posted by PeteKz

..."the whole experience."

Eloquent Pete.

[930cabman]

Originally Posted by 930cabman

Disagree, if you are balancing, why not do it right and spin balance all of the rotating components as an assembly? The correct way is the crank first, add the flywheel and balance again, add the clutch and do it again, ..... making sure everything is match marked

100%, there is no other right way to get the job done, I was a balance tech in the '70's

[930cabman]

Probably a good idea to take a lead from the Nascar crew chief, might know a thing or two. Most of us on these boards don't know ....... you know what I mean