Actually... that is not the reason. All of this great info stimulated me to study up a lot on this in the last 24 hours and have found this is not the case.
The most 'turning torque' on the crank is exerted by the piston+rod around 70 to 90 degrees. This is what Brian is thinking of above. People misinterpret this as the same as total rod force, but it is not. To see why:
The rod force can be divided into 2 parts: tangential and radial. The tangential part is going through the rod journal in a direction that is 90* from a line between the crank center and the rod journal center. Your can think of that like the force you exert on a bicycle pedal and it is what turns the crank. Tangential force peak generally around the 70 to 90 degrees range and THAT is the 'turning torque', like what Brian identifies. But it is not the only force on the rod.
The other part of rod force is the radial part, which is what is going straight into the center of the crank. If you put a piston at TDC, and pushed down on it, all the force would go straight down into the crank, and the crank will not turn; that is radial force. The piston+rod's radial force has NO leverage to turn the crank,
BUT it is still force that the rod and rod bearing and oil have to deal with.
This radial force peak soon after TDC, when the combustion pressure peaks (ideally around 14 degree ATDC) and is considerably higher than the peak tangential rod force in the 70 to 90 degree range...
for most engine uses. The 'primary' force being 2-3 times higher, or more, than the peak tangential force at 70-90 degrees, would be a good guess.
Sooooo.... while you are at lower to mid RPM's, the rod force graph actually peaks soon after TDC due to the radial force component. If you only think of the turning force (tangential force) from the rod, then you will miss this higher peak of radial force acting on the rod and bearing and oil.
HOWEVER..... when you get higher and higher RPM's, then another effect starts coming into play, and a 2nd peak emerges in the rod force versus crank angle graph.
- Imagine #1 piston and rod moving down the bore at high speed; at 8000 RPM for a SBM, this moves as fast as 1500 inches per second (over 100 mph), so the piston/rod has a LOT of energy in it at that speed. And know that if you increase the RPMs from 6000 to 8000 the RPM's, this rod+piston energy goes by by 1.8 times more...it increases as the square of the RPM's.
- As the piston and rod moves down the bore, something else is happening: the next cylinder in the firing order is on the compression stroke, and that work put a 'backwards' force on the crank; that wants to slow the crank down. Yes, the flywheel/TC try to keep the crank turning at a constant speed, but regardless, the crank DOES slow down a bit as the next cylinder's charge is compressed.
- Now piston and rod #1 also HAS to slow down, and that deceleration has to come from the crank 'fighting back' and exerting a force 'backwards' on the #1 piston and rod. At higher and higher RPM's, that reverse force has to rapidly get larger and larger because the piston and rod inertial energies are growing as the square of the RPM's, and it takes a more and more of reverse force on the rod to take out the energy needed to slow the piston and rod.
- That reverse force adds to the normal combustion force (tangential + radial) on the rod and produces a 2nd peak in the 70 to 90 degree crank angle range, that grows rapidly with RPM's and eventually becomes THE BIG peak in the rod force, bigger even than the one just after TDC.
So that 2nd rod force peak nearing 90 degrees of crank angle, that shows up with any significance only at high RPM's, and which gets bigger and bigger when you to higher and higher RPM's, looks to be the reason for rod oil timing.
I never would have dug into this if this thread did not turn this way. So I'll express my thanks to everyone and to FABO.
As a side thought.... If you change from a stock 340 piston+rod weight down to something like a KB+SCAT rod weight, the weight drops around 18% and that drops that 2nd rod force component by the same %. I'd imagine that would be good for a few hundred more RPM before the oiling fails....
