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Originally Posted by why?
This is a great way of saying it. Low rpm's and high load are an awful combination for boosted vehicles. If you go over to nasioc you'll see tons of people say number one about an STI is to never floor it at low rpms in 5th or 6th because that is where most issues occur.
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You have issues on turbo charged cars because you get boost spiking when the load is very high.
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Its the inertia on the rotating parts. If you cut fuel the engine keeps spinning due to inertia right? so when you are WOT the intertia pulling the piston down, helps relieve some of the stress put through the rods. The effect is stronger as you go up in revs due to more inertia in the rotation parts.
Think of a merry go round at a play park. When its spinning slowly you can push really hard, but as it builds speed the inertia makes it harder for you to push it as its already moving.
The effect is nothing like as strong as people like to make out though, and we still see plenty of cars with centrifugal supercharges blowing rods once they go much past 280whp.
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If the inertia of the crankshaft pulling the piston down relieves some of the compression force in the rod you are effectively saying that the net force on the rod is lower. If the net force on the rod is lower the net force on the crankshaft is lower. If the net force on the crankshaft is lower you would measure less torque on a dyno. This does happen and it is why most NA motors do not generate max torque near redline. When you stuff more and more air in as rpms rise you can create this (ie centrifugal supercharger).
Second paragraph, no. If a machine can generate 100 lbs of force it does so regardless of how fast it is applied. You have a tougher time pushing a faster moving wheel because of timing issues and the fact that your are not a machine; if you were mechanically constrained to the rotating system and your 100 pounds of force did not require a ramp-up time like muscles do your force would always be applied at the correct time and you would generate the same amount of rotational acceleration.
If the above argument is correct it really comes down to how long the maximum compression force is applied to the rod. If that is the case and someone can point me to a reference I would be happy. I would think from a materials standpoint it would be the opposite; if maximum torque occurs at higher rpm the transition from compression to tension would lead to work fatigue faster.