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Old 01-26-2017, 07:40 AM   #1271
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Member#: 395882
Join Date: Jul 2014
Chapter/Region: MWSOC
Location: GR, Michigan
2016 WRX


Originally Posted by DaveC113 View Post
Maybe a fraction of a second...

I assumed the rod failures were all from fatigue, not sure if there was any plastic deformation.
They are most likely fatigue failures, given that people aren't just turning up the boost and having it blow up instantly. People are getting 5-10k+ miles out of the motor before the rods pop.

Originally Posted by wadeh View Post

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.
I think you missed half of it. Not only does it see the high pressure for longer, but it also sees higher pressures. Combustion is not instantaneous. It happens during most of the power stroke. The burn gets further along before the cylinder really starts to move down. Here's a diagram showing the general shape of the curve. That peak is the deathknell for rods. Slow down the crank, and that shape shrinks along the X and grows along the Y, as the rate of combustion becomes more similar to the dwell time at TDC.

Compression isn't fatiguing the rods, but them doing this is:

Do that a bunch, then rev the thing out to redline a bunch (which will slowly fatigue the rods as well, albeit no more than the stock setup) and eventually the rod is going to go. That's why you will see some bent rods and some that look like they just flew apart. The failure can occur in either mode, but the buckling mode is a new one introduced by the higher than stock boost at low RPM possible with bolt-ons.
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