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Old 08-06-2006, 09:23 PM   #13
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Join Date: Aug 2004
Location: SoCal

Originally Posted by bugeyes
If it was a balancing issue (and I'm not saying its not), why do the engine builder which balance their engines still modify the cranks?

I recently started a thread based on balancing of subaru crankshafts. The fact is the subaru crank cannot accomodate counterweights large enough to cancel out the weight of the rod/piston. Therefore the crankshaft is considered to be a "coupled" type crank which relies on the two adjacent journal loads to cancel each other out. What I am saying is the 2 front cylinders cancel each other out due to them being 180 degrees apart on the crank. This puts extra load on the main journal between them.

What I am getting to here is if you fit heavy rods and heavy pistons you are increasing the load through the crank which would increase any amount of twisting and hence increase the chance of oil failure (if this theory is correct??).
Yeah, I posted in that thread. The engine is "considered" a coupled type, but not such crank truly exists. I wouldn't be suprised if heavier rods/pistons did increase the chance of failure, but they may not necessarily do so.

Originally Posted by hotrod
Balancing is helpful but not the primary issue.
Maybe, maybe not. But the inadequacy of the Subaru balancing system is going to send some very strong first-order vibrations all up and down the rotating and reciprocating mass in the engine. That could easily wreak havvoc on oil-film stability and cause significant fretting wear. Just the second-order vibrations inherant to inline-4 engines can cause problems with the oil film on the cyllinder wall and create issues with fretting wear.

I still think there's a strong possibility that, were the Subaru crank capable of being fully balanced, you wouldn't need to modify the oiling system until a significantly higher RPM. But, it is just a hunch and, since it isn't possible to fully balance a Subaru crank due to space restrictions, we'll probably never know.

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