Yeah no kidding. Unfortunately we have another victim. This time a PE guy. Check gurgens post. His also appears to be fatigue.

 

Just a word about fatigue. Aluminum has no fatigue limit. What this means is that every stress applied to it adds to metal fatigue. After a sufficient number of stress cycles, it will fail. The main way to combat fatigue of aluminum parts is to make them much beefier than required, so the point of failure is moved many many cycles out on the life curve.

Steel is different. Steel has a fatigue limit. What this means is that any stresses below the fatigue limit *will not* accumulate and lead to metal fatigue. Stresses above the fatigue limit will accumulate, and like any stress to aluminum, will lead to metal fatigue. *Good longevity design* mandates that steel parts be operated below the fatigue limit. So metal fatigue *never* becomes an issue.

If the Nissan engineers did their homework, there should never be a metal fatigue issue with the stock steel rods at stock levels of stress. But running high boost can push the rods above the fatigue limit, which will then let fatigue stresses accumulate in the rods and lead to their eventual fatigue failure.

But I don't think any of our engines have even approached the number of stress cycles needed for steel fatigue failure. The failures experienced so far are most likely simple overloading to yield point failures. That's not dependent on age or the number of stress cycles. It is simply a function of the alloy, cross section, and design of the stressed steel parts.

 

F.Y.I. I believe I saw a recent post where someone had a failure at under 6lbs of boost. Just a word of caution.

 

shopdog, you lost me in part of the logic. So you are saying that if a rod is operated below its fatigue limit, then fatigue doesnt accumulate. But what if the rod is operated beyond its fatigue limit, as is likely the case with stock rods, since the Nissan engineers certainly didnt design our rods for F/I applications.

Than I assume that fatigue will be a factor...right?

 

Dear Christ! There isn't even a comparison from the stock rods to the Pauter Rods. Those things are huge!

How much heavier are they? And the Pistons, Has anyone weighed them?

What effects will the increased rotating mass have on the engine?

Jeff

 

The pistons are around 50grams lighter than stock....and the Pauter rods are probably about 100-150grams heavier than stock. I feel like a broken record..LOL...but weight isn't the issue with forged rods. You really need strength about all else, when using them for F/I applications, so the rods will definatlely be heavier than stock. Pauter makes lighter rods, but they are not suited for F/I applications.

Based on the dynos that Kudos, SGP and others have posted, there is no power loss from these rods....since the boost is being turned up dramatically, to compensate for both the extra weight of the rods, and the lower compression ratio of the pistons.

 

Thanks for the explanation.

Looks like I'll be investing in some Pauter Rods and forged pistons when I decide to go FI. I would rather pay the price of reliability up front, than risk possibly blowing my block apart later on.

 

Would Ti Rods be too much for a daily driver?

 

If you have $4800 to spare..go for it! vs. about $1200 for forged steel rods.

Yes..I would say it is overkill. Unless you are building a race motor, and absolutely need the lighter weight, or intend to rev your motor to 10,000rpm. The Ti rods are just as strong as steel...just 30% lighter.

 

The fatigue limit for steel is roughly (very roughly) between 10 and 25 percent of the yield limit of the steel (depends on the particular steel alloy). If yield is 100,000 PSI (a reasonable number for a non-exotic alloy), then the fatigue limit falls somewhere in the 10,000 to 25,000 PSI range. As long as stresses stay below that level, fatigue is not a factor.

To determine if our stock rods fall above or below the fatigue limit, you need to determine the smallest cross section of the rod, and the maximum force it will see in operation. Divide the force in pounds by the cross section in square inches to get PSI.

A very rough back of the envelope calculation says that the stock rods will see roughly 8,000 PSI at the thinest part of the section in a stock engine at WOT and redline. So they're below even the more conservative value for fatigue limit.

OTOH, at about 10 PSI of boost, you exceed the optimistic fatigue limit figure at WOT and redline by about 2,000 PSI. So fatigue will build up under those conditions. Note that it may take *many* cycles before fatigue failure. The further over the fatigue limit you go, the fewer cycles are needed to reach fatigue failure, but even at 10 PSI boost, you'd need at least 1,000,000 cycles before fatigue will become a factor. That's roughly 150 minutes running at redline.

 

so guys, stay under 10psi

nice explanation SHOPDOG! Thanks!