GM balancing on crate engines
01-12-2009, 08:43 PM
#1
Team Owner
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GM balancing on crate engines
In case you haven't seen this over in the Engine Mods section:
I promised to post the results of the balancing of my brand new ZZ4 GM short block. (Also posted in C4 Tech) This came assembled, so I had to pull the pistons and press out the pins. I have to assume that since this was assembled by GM, that these weights were within their tolerance for a balanced engine. All of these weights are in grams.
Pistons Rods Pins Pistons & Pins
1 533.9 598.5 159.8 693.7
2 531.4 600.8 160.0 691.4
3 533.8 601.5 159.4 693.2
4 533.6 600.8 159.2 692.8
5 530.8 599.2 159.7 690.5
6 533.3 596.1 160.0 693.3
7 533.5 602.4 159.4 692.9
8 534.8 596.9 159.6 694.4
As you can see, the greatest difference between two pistons is 4.0 grams and 6.3 grams for the rods. There is not too much you can do with the pins to balance them. And the piston and pin combination is the end product that counts. Just think of large rocks at the end of sticks twirling around, you sure would want them to all weigh the same. After balancing the piston/pin combination, they all weigh 690.5 +/- .1 grams. We threw the pistons on the scale and they all weight 690.5 or 690.6 grams. The rods were balanced to 596.1 grams +/- .1 grams.
The balancing was done by Ed's Crankshafts in Hayward California. The shop is run by two brothers, Ed and Carlos. I have know both for years and Ed's wife used to work for me years ago. There are not too many crank grinding shops left in the area. Ed's does most of the cranks for the East and South Bay machine shops.
The GM balance may be okay for those that drive down the freeway or side roads at no more than 2500-3000 rpm. But if you want an engine that will run smoothly and last....BALANCE IT.
Pistons Rods Pins Pistons & Pins
1 533.9 598.5 159.8 693.7
2 531.4 600.8 160.0 691.4
3 533.8 601.5 159.4 693.2
4 533.6 600.8 159.2 692.8
5 530.8 599.2 159.7 690.5
6 533.3 596.1 160.0 693.3
7 533.5 602.4 159.4 692.9
8 534.8 596.9 159.6 694.4
As you can see, the greatest difference between two pistons is 4.0 grams and 6.3 grams for the rods. There is not too much you can do with the pins to balance them. And the piston and pin combination is the end product that counts. Just think of large rocks at the end of sticks twirling around, you sure would want them to all weigh the same. After balancing the piston/pin combination, they all weigh 690.5 +/- .1 grams. We threw the pistons on the scale and they all weight 690.5 or 690.6 grams. The rods were balanced to 596.1 grams +/- .1 grams.
The balancing was done by Ed's Crankshafts in Hayward California. The shop is run by two brothers, Ed and Carlos. I have know both for years and Ed's wife used to work for me years ago. There are not too many crank grinding shops left in the area. Ed's does most of the cranks for the East and South Bay machine shops.
The GM balance may be okay for those that drive down the freeway or side roads at no more than 2500-3000 rpm. But if you want an engine that will run smoothly and last....BALANCE IT.
01-12-2009, 09:48 PM
#2
Race Director
what would you expect from a mass produced engine......? If I was going to tear a brand new engine apart for a rebuild, I'd start with a seasoned engine and save a few grand!
01-12-2009, 11:25 PM
#4
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It's exactly what I expected, which is why I'm doing what you guys suggest.
Well, that and you can't get LTX shortblocks from GM anymore.
I'm just having trouble finding a block
Well, that and you can't get LTX shortblocks from GM anymore.
I'm just having trouble finding a block
01-12-2009, 11:37 PM
#5
Le Mans Master
Let's look at this from another perspective.
First, consider that the unit weights stack up for the assembly
the Bob Weights used to balance 90º V8 cranks.
Bob Weight is calculated by adding 100% of Rotating Weight
(big end and bearing weights for TWO rods, plus an oil allowance)
and 50% of Reciprocating Weight (small end, piston, ring set,
pin and locks all for ONE cylinder.)
From this perspective, my vote is that the variance between the
unit weights of John A. Marker's parts becomes less significant.
Using some assumptions, a theoretical Bob Weight can be
calculated to consider the merits of my position.
His rod weights lie in the vicinity of 600 g. For a 600 g rod, it is
reasonable to assume that that the big end would weigh 420 g,
while the small end would weigh 180 g. It is also reasonable to
assume that a bearing weighs 50 g, a ring set weighs 47 g and
to use an allowance of 5 g for oil.
Using those numbers
heaviest and lightest complete assemblies is small. Considering
that for calculation of Bob Weight, this difference is split in some
proportion between Rotating and Reciprocating Weight, my vote
is that the resulting imbalance is negligible for production purposes
and probably for a variety of motorsports applications, too.
The effect of windage on balance in a wet sump at 6,500 RPM is
more of a concern, IMO.
.
First, consider that the unit weights stack up for the assembly
1 598.5 + 693.7 = 1292.2
2 600.8 + 691.4 = 1292.2
3 601.5 + 693.2 = 1294.7
4 600.8 + 692.8 = 1293.6
5 599.2 + 690.5 = 1289.7
6 596.1 + 693.3 = 1289.4
7 602.4 + 692.9 = 1295.3
8 596.9 + 694.4 = 1291.3
Across the total weights of the rod/piston/pin assemblies,
the delta is 1295.3 - 1289.4 = 5.9 g.
Notice, too, that when grouped in crankthrow pairs, three
of the four throw sets are very close.
But simple total assembled weight is not what is used to calculate2 600.8 + 691.4 = 1292.2
3 601.5 + 693.2 = 1294.7
4 600.8 + 692.8 = 1293.6
5 599.2 + 690.5 = 1289.7
6 596.1 + 693.3 = 1289.4
7 602.4 + 692.9 = 1295.3
8 596.9 + 694.4 = 1291.3
Across the total weights of the rod/piston/pin assemblies,
the delta is 1295.3 - 1289.4 = 5.9 g.
Notice, too, that when grouped in crankthrow pairs, three
of the four throw sets are very close.
the Bob Weights used to balance 90º V8 cranks.
Bob Weight is calculated by adding 100% of Rotating Weight
(big end and bearing weights for TWO rods, plus an oil allowance)
and 50% of Reciprocating Weight (small end, piston, ring set,
pin and locks all for ONE cylinder.)
From this perspective, my vote is that the variance between the
unit weights of John A. Marker's parts becomes less significant.
Using some assumptions, a theoretical Bob Weight can be
calculated to consider the merits of my position.
His rod weights lie in the vicinity of 600 g. For a 600 g rod, it is
reasonable to assume that that the big end would weigh 420 g,
while the small end would weigh 180 g. It is also reasonable to
assume that a bearing weighs 50 g, a ring set weighs 47 g and
to use an allowance of 5 g for oil.
Using those numbers
Rotating Weight
945 + 919 = 1,864 g
Relative to the Bob Weight value, the 5.9 g variance between the420 - Big end (x 2)
50 - Bearing (x 2)
5 - Oil
470 + 470 + 5 = 945 g
Reciprocating Weight50 - Bearing (x 2)
5 - Oil
470 + 470 + 5 = 945 g
692 - Piston & Pin
180 - Small End
47 - Ring set
692 + 180 + 47 = 919 g
Bob Weight180 - Small End
47 - Ring set
692 + 180 + 47 = 919 g
945 + 919 = 1,864 g
heaviest and lightest complete assemblies is small. Considering
that for calculation of Bob Weight, this difference is split in some
proportion between Rotating and Reciprocating Weight, my vote
is that the resulting imbalance is negligible for production purposes
and probably for a variety of motorsports applications, too.
The effect of windage on balance in a wet sump at 6,500 RPM is
more of a concern, IMO.
.
01-12-2009, 11:44 PM
#6
Le Mans Master
new short block. But my reasoning has to do with the block, rather
than with the balance.
.
01-13-2009, 01:15 PM
#8
Le Mans Master
Natural aging, together with thermal cycling that occurs
in operation, acts to increase hardness and relieve stresses
arising from the casting and machining of engine blocks.
Without any special treatment, a block that is machined
shortly after being cast may be prone over time to dimensional
accuracy and wear issues due to changes in the metal's properties
that occur until the block stabilizes.
Enough.
The ideal situation is that there has been enough use for the block
to have stabilized, but not so much that it has too little material left
to be made serviceable.
Perhaps 1k is insufficient. On the other hand, an engine with 80k on
the clock might be found such that:
a good prospect for further preparation.
There are processes that have the aim of replicating the seasoning
effects which are applied to virgin castings with the aim of eliminating
the need to sort through and clean up used cores.
.
in operation, acts to increase hardness and relieve stresses
arising from the casting and machining of engine blocks.
Without any special treatment, a block that is machined
shortly after being cast may be prone over time to dimensional
accuracy and wear issues due to changes in the metal's properties
that occur until the block stabilizes.
Originally Posted by eogel
... but used how much?
The ideal situation is that there has been enough use for the block
to have stabilized, but not so much that it has too little material left
to be made serviceable.
Perhaps 1k is insufficient. On the other hand, an engine with 80k on
the clock might be found such that:
- the cylinders would clean up with a 0.010 - 0.020 cut.
Leaving sufficient wall thickness for good sealing & cooling,
despite core shift. - the crank would look good with a light polish
- the rod bores would look good with a light resizing after
installing better fasteners.
a good prospect for further preparation.
There are processes that have the aim of replicating the seasoning
effects which are applied to virgin castings with the aim of eliminating
the need to sort through and clean up used cores.
.
