[oldman]

Originally Posted by DorkMissile

Well unfortunately the weight of everything goes completely out the window if you change rods. If the rod weighs the same weight in Grams as the OE rod, but it heavier on the little end, and lighter on the big end, it is pretty much a crap shoot without rebalancing IMO.

My GPI rod had a scary amount of metal removed from the small end. As you can see from my other post drop-in at least IMO are good to 850 engine HP, above that I say get some thick I beams and so real forged pistons (aka heavy) with much lower ring lands.

[DorkMissile]

Update - received the call from Tesar Engineering today - balance is now at .27gr, which I would think suffices.

We were "calculating" about 24-26gr of unbalance on paper. When he got it on the balancer, it came in at 18gr out of balance (with the crank being heavy). The scary thing is, looking back on this, is that the concept of "drop ins" really doesn't work. Well I guess it works given a large enough tolerance.

Talking about the concept of drop ins with my machine shop (he is a "old timer"), he says the only way it would work would be by accepting up to +/- 15 or 20 grams of balance in each direction. Like he said - we were "back figuring" what the factory bobweight should be based on what the factory rods / pistons / pins weighed, and it turned out that the crank was still 10gr or so lighter on the rotating weight than what it "should" have been based on the reciprocating parts

A couple of 3/4" holes about .5" deep is all it took. $260 for them to balance the assembly, the flywheel and polish the crank. Money well spent IMO.

[Eldi Z]

Originally Posted by DorkMissile

Update - received the call from Tesar Engineering today - balance is now at .27gr, which I would think suffices.

We were "calculating" about 24-26gr of unbalance on paper. When he got it on the balancer, it came in at 18gr out of balance (with the crank being heavy). The scary thing is, looking back on this, is that the concept of "drop ins" really doesn't work. Well I guess it works given a large enough tolerance.

Talking about the concept of drop ins with my machine shop (he is a "old timer"), he says the only way it would work would be by accepting up to +/- 15 or 20 grams of balance in each direction. Like he said - we were "back figuring" what the factory bobweight should be based on what the factory rods / pistons / pins weighed, and it turned out that the crank was still 10gr or so lighter on the rotating weight than what it "should" have been based on the reciprocating parts

A couple of 3/4" holes about .5" deep is all it took. $260 for them to balance the assembly, the flywheel and polish the crank. Money well spent IMO.

DorkMissle - This is another piece of extremely valuable data.
It clearly proves, that as suspected, the term "Drop-In" does not really reflect reality - especially when rods are involved as well.
I wonder if opting for Forged "Pistons Only" option and actually being able to source a set in which all the pistons' components, Incl. Pins, Rings, Clips (+ Piston itself of course), actually weigh exactly like the OEM piston assembly. Then we might get away with really dropping them in without having to balance the complete RA.
What is your take / your engine builders take on this? Would be interesting to know.

[oldman]

The true problem is the 50% reciprocating weight is just a rule of thumb. It just happens to work, there are some over-balancing that also just works. The big end is rotational, the small end and piston is reciprocating. Anything in between is hybrid. My drop in rods are longer than stock, my piston has a different pin height, these small changes affect the hybrid portion of the weight. so 50% to 51% seems to work. Heck these cranks are entirely missing the two center counterweights.. as "good enough". Even things like the center of mass, are you talking geometric or ballance (mass center)? There is a 3% flex right there. Nutshell is it probably is very important that all rods weight the same, and that their weight distribution is the same, factory cast rods are very good vs old fashion forgings. Modern cnc rods even better. Pistons weight the same and 50% of the total is recipricating as a rule of thumb. Is that cener of gravity or is the geometric center is a whole other can of worms. Long stroke short rods engines function differently vs short storke long rod engines but we still use the 50% factor which "just works" but it is NOT based on what is actaully going on inside the engine. I'm more of a inline engine and as long as the stuff is the same weight like pistons... no balancing is required.

Given all this, I just trust, because it lets me sleep better at night. IMO a engine above 850 HP should really go with much thicker I beam rods and stronger thicker pins and to a lesser extent the piston, the whole thing rebalanced. The factory rod is pretty thin IMO and trying to mimic the weight (not the distrubtion) makes for a non-optimal rod. Good enough but at some point you should get "real rods and piston and pins" and rebalance.

Thanks for the good work DorkM. I should have weight mine, I just went off the mfg documentation.