CrankShaft - Equilibrate

[UrS4boy]

In my aby the original pistons was all 350 gram each without one who was 349. Pretty good to be original, after a little adjustment everyone was 349 gram, my new h-beam rods was all 551 grams each after a little bit of adjustment and every rod+piston+rings+studs is 1009 gram each. I did not balance it more than checking the weight of it and adjust it, i know people who drives 530+ without balanced engines more than this weight-adjustment.

Not that this is 100% correct but to put Nuno's 95 gram difference from stock, 95 gm is less than 10% of 1009 gm. Probably within factory tolerance.

[Losgard]

In my aby the original pistons was all 350 gram each without one who was 349.
Pretty good to be original, after a little adjustment everyone was 349 gram, my new h-beam rods was all 551 grams each after a little bit of adjustment and every rod+piston+rings+studs is 1009 gram each.
I did not balance it more than checking the weight of it and adjust it, i know people who drives 530+ without balanced engines more than this weight-adjustment.

[UrS4boy]

Ffs how infuriating it is to read such a load of waffle.

I hope what I was saying wasn't part of the "waffle".

[JP]

Ffs how infuriating it is to read such a load of waffle. Couple of basic points.

-I5 cranks are external balanced cranks.

-This means the crank is balanced by adding/removing weight from the pulley and flywheel.

-The counterweights on the crank are designed to be an offset based on a percentage of the piston and rod assembly. Where a 10-20gramm difference is permissible as a percentage 750-800 grams that each assembly weighs.

-Some of the counterweights in the crank are further offset to counteract the extra weight in the front pulley for the locking tab.

-You can get away with changing items around within a certain percentage range exceed it and the engine won't idle and rev as smooth and is more susceptible to bearing and bore wear.

-Changing your Flywheel will put an externally balanced engine out of balance. If you're lucky the replacement will be within the percentage tolerance. If unlucky you will suffer horrible clutch judder.

Those are the main points, rebalance is the proper route to take. However if you are within a given percentage change in your weights it is permissible on a road going vehicle to get away with it so to speak.

[Error404]

Really its the difference between doing a boggo backstreet rebuild or a professional job. you pays your money you takes your choice

[UrS4boy]

Factory balance is done factoring in the weight of stock components within tolerances, if you change the weight of one piston and rod assembly by 10% or more (which is very likely using h-beam rods and lightweight pistons) then here is a significant imbalance. At the end of the day Nuno can do what he wants, but his mechanic has quite rightly pointed out that best practice dictates have the assembly rebalanced together.

Disagree (mostly). The crankshaft is balanced. Period. As long as the five pairs of pistons and rods are within a certain weight tolerance of each other (I don't know about 10% but, say, sure), then things are fine. If Nuno was to just install one new rod and piston combo into one cylinder, then I would agree that dynamic balancing of the rotating assemby is needed. However, he isn't doing just one, he is doing all 5 so no "significant imbalance" unless the pistons and rods are not matched within weight tolerances.

I agree that best practice dictates have the assembly rebalanced together. As for being necessary for a street driven, non-race, engine, I say waste of money.

[UrS4boy]

Question here: Does the workshop manual dictates that when changing the damper or flywheel one should balance all three as a unit?

No.

We have massed produced engines with mass produced components that are manufactured within in specified tolerances for size and weight. Assuming that the factories producing these components reject parts that are outside these tolerances, assembling an engine does not require rotating assemby balancing. I am sure the Veyron and the like have their assemblies dynamically balanced but that is a whole other league.

[Billman]

Question here: Does the workshop manual dictates that when changing the damper or flywheel one should balance all three as a unit?

[Thuppu]

Sorry. I have to say something. This thread implies that if Nuno does not balance all his rotating parts (crank, pistons, rods, flywheel and clutch), the end of the world will be upon us or, at the very least, his engine will blow up in only a few revolutions.

Let's get real. The factory did NOT do any such balancing. Yes, the crankshaft is balanced (i.e. counter weights drilled out in strategic places) but it was NOT balanced with rods, pistons, etc. attached. The factory allowed for some tolerance in the weight difference between the combinations of rods and pistons that would be pulled from the parts bins, such that the total weight of a rod and piston combination could not be more and "n" grams different than its other 4 neighbours.

Complete rotating unit balancing might be something that high revving F1 engines get but for our 3B, AAN, ABY and ADU lumps, it is NOT necessary. Sure things *might* be better, i.e. smoother, higher RPM (but why do you need more than 7200 anyway?) with a complete balance but for non-race applications it is overkill.

That said, for Nuno's install, he should weigh all five pistons and weigh all five rods separately and then match the rods and the pistons together such that the combined weight of the pairs minimizes the total difference in weight between the pairs. Easy with a computer spread sheet (or a piece of paper and a pencil).

Hope that makes sense.

This is just how I think of it. Why do we need the damper and flywheel in the balacing operation?

[Error404]

Factory balance is done factoring in the weight of stock components within tolerances, if you change the weight of one piston and rod assembly by 10% or more (which is very likely using h-beam rods and lightweight pistons) then here is a significant imbalance. At the end of the day Nuno can do what he wants, but his mechanic has quite rightly pointed out that best practice dictates have the assembly rebalanced together.

[mcandmar]

You are right in saying it will work, but its not right. As you point out Crank, flywheel, and front pulley are balanced from factory assuming a rod/piston assembly of X weight. And sure there will be some deviation between each piston and rod as per manufacturing tolerances. Problem is aftermarket rods and pistons have very different weights from stock components, and personally i would want it balanced. I made sure my own engine build was balanced because of this, and also because i replaced the original ABY flywheel and front pulley with 3B items. If you spending a few grand to rebuild an engine, spend the extra few hundred quid and do it right IMO.

[UrS4boy]

Example Rod and Piston Pair Matching Exercise

One the left are the hypothetical weights of the five rods and five pistons, with their average weights and their average total weights. On the right are the weights of the rods arranged heaviest to lightest and the corresponding piston weights arranged lightest to heaviest, i.e. the heaviest rod is paired with the lightest piston and vice versa.

In the end, even though the pistons weights have a 4 gram spread and the rods have a 5 gram spread, by carefully matching the pairs as shown, the difference in weights across the pairs is only 1 gram. (If purely random pairing had been done, it would have been possible to have one pair 254 gms (152+102) and another pair 245 gms (147+98), an overall difference of 9 grams. By selectively pairing, that difference drops to only 1 gram.)

............ Pistons....Rods...............Rod.......Piston.... .Total

1........... 100........150................152.........98...... .....250
2........... 102........147................151.........99...... .....250
3........... 101........151................150.........100..... ....250
4........... 99......... 148................148.........101.........249
5........... 98......... 152................147.........102.........249

Average= 100........149.6...249.6.......................... .....249.6

[UrS4boy]

Sorry. I have to say something. This thread implies that if Nuno does not balance all his rotating parts (crank, pistons, rods, flywheel and clutch), the end of the world will be upon us or, at the very least, his engine will blow up in only a few revolutions.

Let's get real. The factory did NOT do any such balancing. Yes, the crankshaft is balanced (i.e. counter weights drilled out in strategic places) but it was NOT balanced with rods, pistons, etc. attached. The factory allowed for some tolerance in the weight difference between the combinations of rods and pistons that would be pulled from the parts bins, such that the total weight of a rod and piston combination could not be more and "n" grams different than its other 4 neighbours.

Complete rotating unit balancing might be something that high revving F1 engines get but for our 3B, AAN, ABY and ADU lumps, it is NOT necessary. Sure things *might* be better, i.e. smoother, higher RPM (but why do you need more than 7200 anyway?) with a complete balance but for non-race applications it is overkill.

That said, for Nuno's install, he should weigh all five pistons and weigh all five rods separately and then match the rods and the pistons together such that the combined weight of the pairs minimizes the total difference in weight between the pairs. Easy with a computer spread sheet (or a piece of paper and a pencil).

Hope that makes sense.

[Billman]

IMHO you'd have to very lazy and/or very stupid no to
If you change the mass of any of the reciprocating components you should have it balanced.

experience shows otherwise....how many people in this forum changed rods and leave the rest as is without to balance the assembly?

I once met a guy from Austria with a Lehmann tuned RS2 with 500+ and said that his crankshaft is untouched.
Dahlback said the same in an interview i was reading a couple of years ago about the Golf.

The flywheel has an imbalance from the factory. Also stock pistons and rods have massive deviations that shows that there is always imbalance.

however there also also many opinions about it. As JP states above there is much said in his thread also.

[JP]

Balancing is covered towards the beginning of my bugswatter build thread.