Originally Posted By: Scott_Tucker
Originally Posted By: mechtech2
The highest cylinder pressures are at wide open throttle and mid/mid high RPM.
This is your torque peak - the most cylinder filling.
At high RPM, you have less filling, but at a higher rate.
The biggest stress from high RPM is the unloaded TDC of the exhaust stroke. The rod changes direction with high loads on it's bearing and pin.
^ This. The largest mechanical load on the engine is on the rod bolts at TDC on the exhaust stroke (they are in tension). On the exhaust stroke, there is no gas pressure in the cylinder to cushion the inertia of the rod and piston.
If you take your fist and put it to your chest and then punch outward as hard and far as you can, it's probably not going to hurt your arm. If you put a 10 lb dumbbell in your hand and do the same thing, your arm will probably pull out of it's socket. This is the same thing as the piston. Think of it as, the higher you rev the engine, the heavier that dumbbell becomes.
I agree, but the concept also leads to the fact that the faster you push that dumbell away, the farther that your arm will come out of the socket.
My point above was that the more ft/sec that the piston is moving the more elongation on parts that may occur, thus moving them in a range that wouldnt otherwise be touched at the ends of travel. This may cause cleaning of those sections, wear of those sections, or something negative.
Not sure what hitting a higher max pressure at the change direction point does that is good. You have highr chance of overcoming any oil films.
Moving the full range of motion though Id think is a good thing, no?