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Old 08-06-2006, 08:47 PM   #12
hotrod
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Member#: 14141
Join Date: Jan 2002
Location: 13.239@102.85 @ 5800 ft on 13T
Vehicle:
2002 Impreza WRX
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Beat me too it, that thread shows some of the differences, I know the 2.0L STi crank is drilled differently but so far have not held one in my hands so cannot comment on them from first person knowledge. I assume they used one of the drilling styles where the oil feed to the connecting rod journal intesects the main drilling near the surface to eliminate the issue with centrifugal force as much as you can.


On aluminum blocks thier higher expansion rates actually open up the bearing clearances on the mains as they get hotter, so the mains should not be much of a problem.

On the rods the issue is one of the detail of the construction of the rod itself and how strong it is.

The highest load the connecting rod big end sees is not on the power stroke it is at TDC as the intake stroke starts. Especially on throttle lift engine braking, where intake manifold vacuum is trying to pull up on the piston as the crank is trying to pull it down.

At TDC the load is upward (toward the piston) as the recipocating assembly changes direction and this pull tends to try to stretch the big end of the rod into an oval shape. Depending on how rigid the rod is (and how high an rpm your turning) this can pinch in the rod on the rod journal at the parting line. If it pinches in enough the journal grabs the bearing and instant engine salad follows immediately after as the big end of the rod gets ripped off as it tries to twist with the crank pin.

This is why engine bearings have extra clearence on the bearings at the parting line, but sometimes in high rpm usage this extra clearence is not enough. When that happens you have to open up the clearence, or get a rod that is stiffer on the big end so it can better resist the tendency to pinch in on the crank pin.

Larry
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