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Discussion Starter #1
no wonder we need balance shafts, rebuilding my 900 n/a 2l 16v and decided to do the lot, ive lighterned the flywheel, replaced the crank, and no 3 con rod, replaced the crank because it has 030" run out on the centre bearing, only found this when i put it on my crank grinder to regrind it, balanced the crank flywheel and clutch cover as an assembly, just been on tonight balancing the con rods and pistons, the pistons all with 1/2 gram of each other, the con rods well different story, the 3 original ones of the car were not with in 20grams of each other, the replacement was 10 grams lighter than my lightest one, even checked the damaged one 10grams heavier than my heaviest one, so the best i can balance them is within 10 grams of my replacement, maybe if gm spent a little less time cutting corners this may not be as bad, in contrast did a set of corsa ones today all weighed within 1 gram of each other, ground off the casting marks all the same, wish me saab ones were so easy
any one else measured them??
 

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You've confirmed a suspision of mine it seems, the engine dynamic balancing [Lanchester Shafts] is to reduce time lost maintaining tolerance in the first place; I'm sure the rubbish Vauxhall V6 I have is better balanced than the 4 pot engine in most other Saabs.
 

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ragtopcav said:
You've confirmed a suspision of mine it seems, the engine dynamic balancing [Lanchester Shafts] is to reduce time lost maintaining tolerance in the first place; I'm sure the rubbish Vauxhall V6 I have is better balanced than the 4 pot engine in most other Saabs.
Actually, I believe this is why we have ballance shafts:




A mis-ballanced con-rod would create a first order force, which wouldn't even be ballanced much by the ballance shafts; they deal with second order forces. ;)

This diagram shows the second order vibrations in an inline engine (that big nasty spike is what the ballance shafts are there for):




Adrian~
 

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cheers for the head up Adrian, I like to learn.

I'll go and read some more now, in the Lanchester Library no less :cheesy:
 

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Discussion Starter #5
well i learnt somthing there thanks, any one know if lancia had owt to do with the design? looks very mich like a lancia idea the balance shafts
 

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champy2k said:
well i learnt somthing there thanks, any one know if lancia had owt to do with the design? looks very mich like a lancia idea the balance shafts
F. Lanchester - first dynamically balanced engine, about 100 years ago, hence they're called Lanchester shafts [he also did the first disc brake, first go at fuel injection and he's accredited with the first description of the turbocharger. He was more prolific than W.O. Bentley!] and my uni' Library is named after him.

I think my rather flippant post needs refining, coming at this from as an auto engineer [brakes are my bit, not engines] looking at how Saab optimise the power [especially from a turbo engine] to be in the 2000 - 5500 rpm range, and they do run raggy much over 5000rpm I wonder if this is what the balance tolerance is optimised too? my shedtastic Cavalier 1.8 [factory engine, 19years old, un modified] revs nicely up to 7000rpm with more finesse than any 4 cyl Saab I've driven, this implies the GM unit with its red line over 6600rpm will be to a higher balance tolerance; now before anyone gets uppity, I'm not implying the old engine is in any way better, but if you have a working limit that requires a given tolerance of component that is what you work to and little or no higher - all engine designers/manufacturers do this. My thoughts are that the apparent 'poor' Saab parts are probably well within the normal balance tolerance for the duty cycle limits; my V6 has a monster flywheel [15kgs - 33lbs] that gives a smoothness, I wonder how it'll run with a 8kg steel flywheel?
 

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Discussion Starter #7
i see were your coming from on this, i have fitted lighter flywheels to the gm v6 a couple of times the seem to give them mor punch and increased engine braking, i am at present collecting rods and getting a set to match, long job but worth it
 

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champy2k said:
i see were your coming from on this, i have fitted lighter flywheels to the gm v6 a couple of times the seem to give them mor punch and increased engine braking, i am at present collecting rods and getting a set to match, long job but worth it
Why not just shave the rods to weight?

Ragtop, have you had a chance to drive a Saab with the B235R like a Viggen or 9-5 Aero? They're very free-revving compared to the older Saab B-series engines. The rods, pistons, and crankshaft are all lighter, and the ballance shafts are aluminum instead of iron.

If the ballance shafts are ballanced for one RPM they should be ballanced for any. The only question is whether Saab designed the ballance shaft's bearings to withstand higher than 6K RPM. Hard to say.

In any case, the rods being mismatched has little, if anything, to do with the ballance shafts. As I pointed out before, ballance shafts ballance second-order vibrations; a mis-matched con rod will make a first order vibration. The reduction in second order vibration might make the mis-matched con rod less noticable though. :)
 

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Discussion Starter #9
was going to take approx 15-20 grams off the heavys ,but i didnt want to go to far so matching on to match the heavys
 

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Adrian W said:
Ragtop, have you had a chance to drive a Saab with the B235R like a Viggen or 9-5 Aero? They're very free-revving compared to the older Saab B-series engines. The rods, pistons, and crankshaft are all lighter, and the ballance shafts are aluminum instead of iron.

If the ballance shafts are ballanced for one RPM they should be ballanced for any. The only question is whether Saab designed the ballance shaft's bearings to withstand higher than 6K RPM. Hard to say.

In any case, the rods being mismatched has little, if anything, to do with the ballance shafts. As I pointed out before, ballance shafts ballance second-order vibrations; a mis-matched con rod will make a first order vibration. The reduction in second order vibration might make the mis-matched con rod less noticable though. :)
My point exactly when I made a far more 'intelligent' post, Champy's observation was that the parts tolerance is not that great, my theory [and defence of Saab engineering] is that because of generally lower optimum operating rev' range, the tolerance of moving components will be proportionally changed to suit - sound manufacturing engineering practice.

To add to your comment, no I haven't driven the newest Saab engines, my findings are based on 7 to 25 year old technology, but, to add to how cars are now engineered, Ford Puma diesel engines [compact euro spec' units] have no grade range for pistons, they're all made to a fixed spec' no grading and matching - such is the quality of mass production now; Saab product must achieve or exceed this which a Ford product can
 
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