engine balance question
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From: Clifton, NJ
engine balance question
Hey guys, was reading the aftersales-training pdf available at pcarworkshop, and in the connecting rod section, it indicates that rods with weight deviations of less than 6g can be fitted to one engine.
I'm wondering if these standards were used when the new engines were assembled, because that seems like an awful lot especially without a harmonic dampner.
From my work with performance engines, we always shot for a reciprocating mass weight difference of half a gram or less so I'm kind of surprised to see this (i'm assuming/hoping that they are using heavy or light pistons to get the balance closer).
Anybody know?
Thanks
I'm wondering if these standards were used when the new engines were assembled, because that seems like an awful lot especially without a harmonic dampner.
From my work with performance engines, we always shot for a reciprocating mass weight difference of half a gram or less so I'm kind of surprised to see this (i'm assuming/hoping that they are using heavy or light pistons to get the balance closer).
Anybody know?
Thanks
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From: Costa Mesa, CA
Yes, 6g seems like quite alot. One thought is that a boxer-6 is an inherently balanced engine. It does not have the higher order harmonics that v-8 engines or 4-bangers that require harmonic balancers. As such maybe there is more latitude allowed.
Thread Starter
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From: Clifton, NJ
I'm kinda surprised porsche didn't feel a harmonic dampner was necessary, especially if they went with such a big weight differential on the rods.
Maybe the document I looked at just didn't specify that the heavy rod should be used with the light piston and vice versa.
Racer
Joined: Nov 2009
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From: SF Bay Area - Marin
"The straight-6, flat-6 and V12 designs have none of these forces or moments of vibration and hence are the naturally smoothest engine designs. (See the Bosch Automotive Handbook, Sixth Edition, pages 459-463 for details.)"
Maybe the V's are also naturally balance, but not as much so as the flat & straight 6 engines...therefore handling looser tolerances?
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From: Costa Mesa, CA
While the flat 6 is inherently balanced, so are 90 degree v8s and 60 degree v6s.
I'm kinda surprised porsche didn't feel a harmonic dampner was necessary, especially if they went with such a big weight differential on the rods.
Maybe the document I looked at just didn't specify that the heavy rod should be used with the light piston and vice versa.
I'm kinda surprised porsche didn't feel a harmonic dampner was necessary, especially if they went with such a big weight differential on the rods.
Maybe the document I looked at just didn't specify that the heavy rod should be used with the light piston and vice versa.
But it is interesting that they decided not to use the harmonic balancer as it is apparently used for the purpose of damping the torsional vibration due to an individual cylinder firing that causes the crank to twist. Maybe Porsche decided that since the other vibrational loads are not present in the boxer layout, the torsional vibration is acceptable. The elimination of the HD does reduce rotating mass.
BTW, which document were you reading?
Drifting
Joined: Apr 2008
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From: Calgary, Alberta
I've always thought of balance as the "cancellation" of primary and secondary forces in a reciprocating engine. But just because they're cancelled, the forces don't necessarily go away, and BMW introduced a counterbalancer to its 1200 boxer twins in 2005. The non counterbalanced 1200 in the HP2 is buzzy.
Hoag (in vincer's linked Car & Driver article) mentions the balance of the 90 degree V twin, and the smooth Ducati engines are a good example.
Hoag (in vincer's linked Car & Driver article) mentions the balance of the 90 degree V twin, and the smooth Ducati engines are a good example.
Last edited by Rinty; May 23, 2011 at 03:22 PM.
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When I did my rebuild I went sub gram on the balancing. Had the crank checked as well, it was almost perfect. Also included weighing the wrist pins and pistons.
I would have to check my lnotes to see how far out the stock weight were.
I had to rebalance anyways since I switched to APR rod bolts - very important if doing a bottom end.
Cheers.
Mike
I would have to check my lnotes to see how far out the stock weight were.
I had to rebalance anyways since I switched to APR rod bolts - very important if doing a bottom end.
Cheers.
Mike
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Joined: Aug 2001
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From: yorba linda, ca
The DMF will act somewhat as a harmonic balancer. For the 993 3.6 engine Porsche focussed on removing as much weight as possible, perhaps this was the driving force behind removing the HB?
Drifting
Joined: May 2006
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From: Paris
I have a brand new set of "Matched" Mahle pistons in original sealed box they were, when I bought them. Wrist pins, rings, all mounted from factory. Lightest is 601 grams - heaviest is 606 grams
where can you
apart from the wrist pin, nowhere
and from the conrod bushing
Never tried, but will be trying soon to balance to the max.
What about the crank pulley ??
What correlation has it got with the flywheel ?
In other words: I have three pulleys and each has total different weight (difference 50 grams), and one has PLENTY of counter weights riveted on the back, other two have few.
where can you
apart from the wrist pin, nowhereand
from the conrod bushing Never tried, but will be trying soon to balance to the max.
What about the crank pulley ??
What correlation has it got with the flywheel ?
In other words: I have three pulleys and each has total different weight (difference 50 grams), and one has PLENTY of counter weights riveted on the back, other two have few.
