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Old 01-25-2017, 10:20 PM   #1264
Scooby Newbie
Member#: 426075
Join Date: Jul 2015
Location: Snohomish, WA
2015 WRX


Originally Posted by playslikepage71 View Post
YOUR ENGINE IS MOVING. If you take POWER and DIVIDE it by TIME (RPM) you get WORK not FREAKING TORQUE. Torque is the lazy way to describe what's happening. Each piston is doing work and that's being translated through the crank mechanism into torque, but if your engine stops moving, no torque for you.

The reason low RPMs are bad are not because of higher BMEP (that's cylinder pressure averaged over the stroke, and gives you this "torque" figure that has nothing to do with rods blowing up), but because of higher PEAK pressures. When the volume stays small for longer, as in the piston moving slower at low RPM, the pressure goes up enormously for a time, until the piston starts moving down to allow for expansion. At higher RPM, the piston moves much sooner, in terms of time, and peak pressures don't get as high. The ~same amount of energy is added to the system over the stroke, but the high compression force in the rod isn't there because the burn doesn't complete until the piston has retreated.

If you are having trouble understanding how torque and power are truly related, this is a the best video I've found to explain it:
Nothing in that video contradicts anything that I have said. In particular:

- Power is the relevant number regarding moving the car, not torque. It says that specifically.
- Power = Constant * Torque * RPM (which contradicts your second sentence above, btw...but that said, after some more reading, I agree with you that we are using the term torque, when we should be using work. You have convinced me on that.)

One thing that you said above is enlightening and I had not considered. Specifically, an engine at lower RPM experiences high cylinder pressures for a longer period of time than an engine at lower RPM. This undoubtedly changes the stress experienced by different parts of the engine and is detrimental to the life of the engine.

The one thing that I still cannot reconcile is how all of this relates to the rod failures that we've been hearing about. Pretty much everything I'm reading says that (assuming proper lubrication and no defects) rods specifically are damaged by tensile stress associated with high RPM, not high cylinder pressures. EvenWikipedia cites over-revving as a cause, but does not mention low RPM operation.

Some people have rightfully pointed out that small turbo engines experience higher cylinder pressures than naturally aspirated engines (and this likely applies to all forced induction engines). But diesel engines experience even higher cylinder pressures, and compression stress on the rods in those applications is not generally cited as a particular worry.

Ultimately, I am just trying to understand why we're seeing rod failures. Telling people not to use lots of throttle at low RPM is good advice and will likely prevent certain failures, but I don't see the connection specifically to throwing rods.
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wadeh is offline   Reply With Quote