Engine balancing

[Pierre]

Looks like I'm getting a set of Mike Rich's pistons for my sport i. Question - they're 20 grams lighter than the stock pistons they'll be replacing. the engine was balanced with the stock pistons a few years ago.

 

So, what gets balanced in an engine balance? I'm assuming the new pistons are identical in weight - one to the other - but lighter. I don't know the stock pistons were identical in weight.

 

Big deal? Not enough weight change to worry about? Anyone know just what is involved when motor gets sent out for balancing? Falco or something like that in FL did mine.

 

TIA.

[Steve G.]

You may indeed feel it. I put 10 thou larger pistons in my Norton, and the increase in vibration, while minimal, was evident. I've got 30,000 miles on them, so obviously it has not caused internal destruction. I don't think this would be the case with your engine.

Ciao, Steve G.

[dlaing]

I think balancing is voodoo.

The counter-balance in the crankshaft is almost never in perfect balance with the piston, and the air pressure going on in the combustion chamber and even in the the crankcase throws a huge wrench into the balance equation.

On my bike it is silky smooth going down hill with light throttle, but give it some gas and it starts to vibe at certain RPMs...funny thing is it used to vibe at 4500 to 5000 and now it vibes 4000-4500

Ignoring cylinder air pressure and vibration from the valve train, if there was no counter balance, a lighter piston would produce less vibration.

If you have to go lighter or heavier with a piston, go lighter.

I have no experience at this, but suspect 20 grams lighter out of "balance" will mostly just change when the vibes occur, more so than how much they occur.

You might ask the question on Wild Guzzi. I am sure Will Creedon could give an interesting and accurate answer.

Weren't you the one that told me Will was a balancing guru????

Can't wait for you to get your bike back running

[dlaing]

From Motoman's articles he discusses how the most stressful points for the con rods are obviously just past TDC of power stroke and less obviously at TDC of exhaust stroke.

"The 2 times that the bearing's speed in relation to the journal surface is increased, also happen

to be the 2 most stressful points in the 4 stroke cycle. These 2 points are the first half of

the power down stroke, and the last half of the exhaust upstroke."

TDC of exhaust stroke is stressful (and increases vibration) because the piston is being pushed by the gasses in the crankcase into the vacuum in the combustion chamber.

You can only imagine that a lighter piston will put less stress on the connecting rod and produce less vibration

[mike wilson]

So, what gets balanced in an engine balance?  I'm assuming the new pistons are identical in weight - one to the other - but lighter.  I don't know the stock pistons were identical in weight.

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Not neccessarily. I usually weigh them and make them all the same weight. That would be the weight of the lowest......

 

Big deal?  Not enough weight change to worry about?  Anyone know just what is involved when motor gets sent out for balancing?  Falco or something like that in FL did mine.

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Everything you do helps. Balancing the crank and flywheel to match the mass of the pistons and con rods (weigh and equalise them, too) will make the bike seem smoother and more pleasant to ride. Some people say that a "rough" bike feels more powerful. YMMV.

 

What _is_ a black art is the decision over what balance factor to apply. That's where experience counts as much as theory. The good thing is that Guzzis have reasonably good primary balance, so your crank should only need adjusting for the changed weight of the reciprocating parts.

 

I would do it. It shouldn't be too expensive, it _will_ make a difference (although that difference may not be immediately obvious) and it will help with the life of components.

 

If you want to get an idea of the forces involved, get someone to throw small change at you as hard as they can. The change in forces on the reciprocating parts are more than that and are happening up to 16,000 times per minute.

 

mike

[g.forrest]

and i thought guzzisti liked vibes, i believed it was the soul they speak of. as long as pistons are of equal weight.

[mike wilson]

and i thought guzzisti liked vibes, i believed it was the soul they speak of. as long as pistons are of equal weight.

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Power pulses are not the same as vibration due to unbalanced mass. You'd soon get sick of the latter. Unless you like that sort of thing, of course......

[motoguzznix]

Pierre

 

When the crankshaft was balanced with the heavier stock pistons, a decrease in weight would change the balancing of the engine. Vibrations will increase, despite the fact the pistons are lighter.

A 90 ° V engine has theoretically a very ideal balancing when 50 % of the oscillating weight is calculated for the master weight. In this case only horizontal forces remain. As more the balancing gets away from ideal, the more horizontal and vertical forces occur.

I was balancing the crankshaft of my 2000 V11 a year ago due to the very severe engine vibrations.

I calculated the master weight for the 500g piston + 106,5 g pin to 1832 g (balancing 52,5 % of the piston weight). If I would reduce the piston weight by 20 g, the master weight on the crank pin would be reduced to 1811 g. This is a difference of 1,2 % in the weight, hard to say how much this would worsen the engine vibration behavior.

Normally reduced weight is always a goal for oscillating parts in an engine, thus the best solution would be to change the crankshaft balancing for the lihgter pistons. If the engine is already out of the frame, this would be my route to go.

[Mike Stewart]

Pierre,

 

I would talk to Mike Rich, he should know what the out come would be. I thought he could also do custom pistons to match the weight of yours

 

Mike

[dlaing]

What _is_ a black art is the decision over what balance factor to apply.  That's where experience counts as much as theory.  The good thing is that Guzzis have reasonably good primary balance, so your crank should only need adjusting for the changed weight of the reciprocating parts.

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Assuming that the pistons, rings and pin really are exactly 20 grams lighter, and his engine was balanced "perfectly" How much weight would you take out of the crank?

And since I am ignorant of the process, I have questions: how do you take the weight out of the crank? with a drill, a lathe, a mill, or what?

And what does the 51 or 50 or 52.5% represent? Motoguzznix threw down some hints, but I still don't get it. I guess I could google the answer, but it would be good to hear it from those who know Gooses

[badmotogoozer]

Twenty grams is very little. The easiest way to match the balancing you currently have is to take out the original pistons. keep track of which one goes with each cylinder. Then have a balancing shop match the weight of each new piston to the weight of the piston it is replacing. They will take the necessary amount off the inside of the piston skirt on a lathe. Now you have new pistons at the exact same weight as your old ones. Put them in their respective cylinders and your engine will be just as well balanced as when it left the factory - if it did leave the factory balanced...

 

Just making them the same weight doesn't take into account possible differences in rod and wrist weight. (bring your rings and wrist pins to the balancer too). You don't need to rebalance your crank this way (gotta strip the motor to the crank for a proper job).

 

 

Rj

[Pierre]

Hey guys, thanks for the responses. Left town yesterday and just back this morning.

 

I did as advised, and revisited the issue with Mike Rich.

 

The balancing procedure is as you guys have described. However, Mike tells me he varies the balance factor depending on what rev range the bike will be making power in.

 

His response to the impact of 20 grams per piston was "+- 1% to the balance factor" - so well within his parameters. Seems Falicon used 55 as balance factor. Mike goes as high as 61 and as low as 51 - again depending.

 

The primary reason for inserting his pistons is valve recess. I've got an RR3 cam in there and existing valve clearance was the width of a crushed head gasket. Lived fine that way for 7000 miles. Now, however - with a stage 3 (screw and glue) porting job, I'm going to be upping the rev limit to closer to 9000 RPM. I already had Carrillo rods, and stock pistons would have been fine even at that RPM, but Mike's pistons have more relief at the valve pockets so it delivers substantially more piston to valve clearance. That, the superior "squish" design of his pistons, and the fact he threw them in "no charge" made it a pretty easy decision.

 

FWIW, I'll have a 10.9 / 1 static CR (as measured by Mike). That's just about where I was with the stock pistons - his have a wider dome, but porting resulted in greater head volume. The bike is twin plugged, and of course the cam has lots of over lap which makes the 10.9 / 1 less a detonation issue than would at first appear.

 

I'm now trying to determine if the Daytona / Centauro throttle bodies will bolt up to my intake manifolds. Seems sport i is 45 and Centauro is 51 or thereabouts. Also have to check that my existing manifolds can be bored out that far. Anyone done this BTW?

 

If you guys are curious at all about the history of this project you can find it here: http://www.guzzitech.com/Sporti-EngMods_PierreP.html

 

Project started in the dark early days of FI. Since I began this adventure of course, Todd has managed to get Dynojet to build the PC III and now the PC III usb for the bike. I discovered Will Creedon's chips - and even prevailed on him to flash a new ignition map. Will be updating that with factory twin plug curve - but modified at full throttle. We'll see what comes of it. Now if I can just get Cliff to make his MY ECU work with the PC III so I can continue to use the tuning link ...

 

FWIW, I'm probably a poster boy for Ratchet's warnings. OTOH, I've enjoyed the process and the bike (+-90 RWHP to date) is a rock and roll riot to ride. Good drivability and no motor related issues. took her on a 7000 mile summer tour in current (RR3 cam) configuration summer of '04 (including Guzzi national) and the bike never missed a beat. Hope to retain that same drivability and reliability, but with significant additional power everywhere. We'll see. It'll go back on the same dyno for comparison. Heads are "in transit" and shop has bike scheduled for work next week.

 

Also FWIW, although my hit and miss approach has been pretty pricey (including some disasters along the way - set of heads lost by UPS in AU is one) I think knowing what I now know, and given current resources available - I could do it again for about 1/3 to a half of what I spent. Now , if I'd known the MGS-01 was going to be released I'd not have done any of it and I'd own one of those beauties now. I'm pretty sure they can be made street legal (in fact it's been done, I understand) even in California. However, I'd have missed several years of riding a pretty potent two valver - so I'm not complaining. I'll post updates when available.

 

Thanks again for the input.

[g.forrest]

i know power pulse. its vibes. nothing wrong with that..unless your used to super smooth power.

Power pulses are not the same as vibration due to unbalanced mass.  You'd soon get sick of the latter.  Unless you like that sort of thing, of course......

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[mike wilson]

Twenty grams is very little.  The easiest way to match the balancing you currently have is to take out the original pistons.  keep track of which one goes with each cylinder.  Then have a balancing shop match the weight of each new piston to the weight of the piston it is replacing.  They will take the necessary amount off the inside of the piston skirt on a lathe.  Now you have new pistons at the exact same weight as your old ones.  Put them in their respective cylinders and your engine will be just as well balanced as when it left the factory - if it did leave the factory balanced...

 

Just making them the same weight doesn't take into account possible differences in rod and wrist weight. (bring your rings and wrist pins to the balancer too).  You don't need to rebalance your crank this way (gotta strip the motor to the crank for a proper job).

Rj

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It's very little weight but (at a density of about 2.7g/cubic centimetre) it's a hell of a lot of aluminium to cut off a component. Even allowing for piston alloy to be more dense than pure aluminium, it's about seven cubic centimetres per piston, or just under 1/4 of a cubic inch. I know these pistons are big rascals but manufacturers don't put material where it's not needed - unless they buy it in as stock and that's the nearest size. On a heavily machined component like a piston, it makes sense to do it right. If that means leaving more material on, for example to prevent skirt breakage some thousands of miles down the road, then that is generally what happens. IIRC, the pistons are not full skirt jobbies, so you will be taking mass off the parts that carry all the thrust loads. It's a good idea for small discrepancies (which I would classify as being in the order of fractions of a gramme) but not in this instance. I wouldn't do it.

 

mike

[motoguzznix]

QUOTE(badmotogoozer @ Mar 31 2006, 08:13 PM)

Twenty grams is very little.  The easiest way to match the balancing you currently have is to take out the original pistons.  keep track of which one goes with each cylinder.  Then have a balancing shop match the weight of each new piston to the weight of the piston it is replacing.

Milling 20 grams out of a piston seems to me a lethal dose.

A max of 5 g to bring the pistons on the same weight is practical, but 20 g will kill most pistons (ecxcept cast iron ones from pre WW1).

And as I understood in this case, the new pistons are lighter, so you should add 20 g to every piston to get the same balancing factor as before.

And what does the 51 or 50 or 52.5% represent? Motoguzznix threw down some hints, but I still don't get it. I guess I could google the answer, but it would be good to hear it from those who know Gooses

To balance the crank, you have to calculate which master weight you have to apply to the crank pin to get it right.

For a 90° V-engine you take the complete bottom weight of the rods (you have to weigh bottom and top of the rod) which represent the rotating mass of the engine parts. The reciproating mass ist represented by the Piston with rings and pin and the top of the rod. The theoretical ideal is to take 50% of these masses and add it to the rotating mass. This is the amount of weight to be applied on the crank pin.

Balancing then can be done on an expansive machine or statically like a tire is balanced. I removed the material with a small angle grinder with grinding paper. You have to be bloody careful not to distroy the main bearings.

Why in practice the balancing factor is above the 50 % depends on the oil which is on the crank while the engine is running, and there is oil inside the crank pin too. This condition is not when the crank is outside the engine to balance it. When you take care of this, the balancing factor gets higher. And the amount of oil on the crank depends on the rpm too, this explains for me the different balancing factors like Pierre explained for engines with different rpm targets.

On my V11 the crankshaft lost approx 50 g to get the 52,5 % balancing factor. This does not mean the the imbalance of the crank was 50 g, as you often cannot take away material where it is needed. Mostly you have to take away from the crank pin side and not the opposite side where the counterweight is located. And the balancing was pretty successsfol as the engine now runs a lot smoother than before.