[Eric_Oz_S2]

So, how do you balance a 968 engine - you have the piston+rods, crankshaft, dual mass flywheel and balance shafts to contend with? It would seem that there is so much rotational mass, that as long as the piston+rods were all matched, the 100g difference between stock and aftermarket rods would do little to upset the balance.

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[flash]

100g is HUGE! i can tell you exactly what the difference is. my engine is now glass smooth. all of the lumpiness of a stock engine is gone. it's actually a little disturbing. it feels quite weird to me after 9 years of feeling how it was stock, and then how much lumpier it got with the light flywheels. having gone to the extent of balancing and matching everything perfectly, the engine feels like it would rev 9k.



because this is a 180 degree crank, it isn't as much about the piston/rod to counterweight difference as it is the cylinder to cylinder difference. in a V8 you can have a little difference between cylinders and not worry about it. in an inline 4 the offsetting masses need to be the same, especially if you want to run high rpms.



how the heck are you going to match the pistons without balancing? they should be currently matched to the rods that are on them. they have to come out the top, which means new rings and honing, and at that point you have pretty much torn the motor down. i don't understand why you wouldn't balance everything. it's not expensive and really makes the motor sing.

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[Eric_Oz_S2]

I'm not saying the pistons and rods won't be balanced as matched sets. That is easy. I am querying how you would balance the WHOLE engine? ie do you account for the lower piston+rod weight by grinding the crankshaft counterweights.

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[Cloud9...68]

My machine shop showed me the machine they used to balance my engine. It's actually a surprisingly simple device, but very hard to describe in a post, plus it's been awhile since I saw it. First, they weighed the pistons and rods, and balanced them to match. Then, they mounted the crank in it the machine, with the rod bearings, rods, pistons, rings, flywheel, clutch disk, and pressure plate all attached. Then the machine spins the entire assembly in free space, and it somehow measures the degree of imbalance, which they then correct, check, and repeat as needed. They found a measurable amount of imbalance in my assembly, but it wasn't really anything too dramatic. Would I notice a difference if I hadn't gone through this process? Who knows? I'm sure it's possible to get much more elaborate with the balancing process, but this is what I did; hopefully it's sufficient for such a relatively low-revving engine.

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[MB968]

As Cloud says, balancing of rotating masses (including the reciprocating masses) is fairly simple process. His shop probably used a balance stand that is very lightly suspended. The imbalance of the assembly rotated at a relatively slow speed causes motion to be sensed in the two support planes. Corrections can be made either by removing mass (have you ever noticed the counterweights on old engines that had a series of holes in the counterweights), or by adding (usually not preferred but can be accomplished by adding lead into holes, or in more extreme cases depleted uranium). Automated equipment acts like a tire balancer and calls out the oz. inches of imbalance to be corrected, their location along the axis of rotation, as well as their angular location. This process is repeated until the level of imbalance is achieved. For machinery there is an international set of "G codes" to which equipment is sold to. I've personally done all the above to machinery in my early days as a vibrations engineer. Looks tricky, but is quite simple once you learn the process.



What I don't know it to what G level a typical engine rotating mass balance is done. They are definitely not all created equal, although I assume there must be a typical level of residual imbalance that they try to get below. Does anyone know what this is.

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[Eric_Oz_S2]

Thanks for the replies. How are the balance shafts taken into account in the balancing?

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[flash]

that's a whole lot of very complicated math to really answer that one, and i can't remember any of my harmonics math.



on a purely anecdotal note, i expected problems when i did not alter them to match the lower masses of the flywheel and rods and such, i thought it would have gone to hell. it feels smooth as glass. i would guess that they are only dealing with higher order harmonics, and i can't feel that, so i have no idea how much damage if any will occur. it's either that, or they were dealing with a specific harmonic, which no longer is an issue, or has been pushed out of the range of the engine.

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[Cloud9...68]

Yeah, like flash says, I don't know how you would do that on any sort of balancing tool, which, as I said, spins all the components directly connected to the crank in free space. It would take some crazy complicated machine to enable attachment of the balance shafts.

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[MB968]

[quote name='Cloud9...68' timestamp='1346866033' post='132176']

Yeah, like flash says, I don't know how you would do that on any sort of balancing tool, which, as I said, spins all the components directly connected to the crank in free space. It would take some crazy complicated machine to enable attachment of the balance shafts.

[/quote]



And, the function of the balance shafts does not relate directly to the balance of the engine in the static sense that most people think of balance. So, you wouldn't want to mock it up on a balance stand even if you could. If memory serves me, an in-line 4 stroke 4 cylinder engine when balanced in the traditional sense still ends up with rotating couple that is at twice engine speed. The two counter rotating balance shafts on our engine that spin at twice engine speed are designed to deal with that rotating couple. Again if memory serves me correctly; had this in school in the late 60's. If I ever come across my engineering book that goes through this I'll try to post a lay man's version.

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[Eric_Oz_S2]

Thanks again.



I'm now debating whether it is worth changing piston rings. I have no intention of removing the engine from the car as it is not actually broken in any way. Granted the pistons need to come out and ring gap can be checked at the time. If they are to be replaced there may not be a need to hone/etch the cylinders. There is always a debate about honing/etching when putting new rings in, but my opinion is if the bores are in good condition it is simply not necessary. I know there will be arguments to the contrary, but if the bores are uniform there is no point as long as suitable mineral oil is used for run in.



As the engine is not going to be removed, really I am stuck with changing the rods, rod bearings and piston rings (if required) only. So it seems the options are heavier forged turbo rods (if I can find suitable ones) or just try an aftermarket one (not a lightweight version - but will still be lighter than stock).



Probably going to be difficult to find the '86 turbo rods in good condition.

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[flash]

i would not mess with the rods (other than just changing bearings) until you are ready to pull the motor and do it right.



changing rods is really not going to buy you anything all by itself.

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[DaveN]

In response to taporite6, don't bother with the sleeves just do what I eventually came to, get a new block and start again <img src="/forum/images/smilies/968/sad.png" class="smilie" alt="" />

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[Eric_Oz_S2]

[quote name='flash' timestamp='1346551019' post='132066']



the last i heard, even michael has never never gotten that out of an NA. the only one in know of that he got even close with was an ITB motor, and that one was $40k and still fell short by a long way (325 i think). pete has one that does 300, but that one cost over $20k.

[/quote]

nope, michael got 404hp at 8000rpm on an engine dyno. yes it was certainly custom.

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[flash]

i've not heard of that one. i'd love to see what he did to get that, but i sure wouldn't want to have to pay for it.

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