Thanks for your respectful reading and replies.

The choice of what to do with rods is entirely yours. Many people simply choose to take the risk and wait until failure occurs. That is not an unreasonable choice to make, especially if when failure occurs it simply results in requiring the replacement of rods. The reason some tuners choose to upgrade as a pre-emptive measure is that they just don't want the higher risk of failure in the first place.

 

Torque=pressure. The only way it can be mitigated is by either not making it in the first place, or by using it less.

Do you think it is an accident that when Porsche built the GT2 RS based on the 997 GT2 engine, that they increased the power by 90hp, but the torque by only 16lb ft?

Porsche like every other engine builder knows that torque kills rods that's why they limit it in all their turbo-based road-cars.

 

This has become a circular discussion. Excess torque being a probable cause for the bent rods was mentioned in the first couple of pages of the thread.

Good points, Guy.

 

Excellent point and great posts Guy ,
I have a fumy feeling Porsche still left a little room for improvment in turbo engine.

 

Simon,

The post was made along with a follow up suggestion. No need to argue any points.

 

I have tracked mine many,many times.On my 6th or 7th set of rotors since I have owned it in 6 years and 16K miles of use. This engine build has 3-4K miles on it, 20 Mile runs are on this build and 500 track miles. Plenty of runs to ball games and Starbucks as well. I recently upgraded to E85 but the engine wasnt opened, just an inj swap...

 

That's great, respect to both you and your car. I think you would admit that you're in a minority though.

 

This is a great thread, and a lot of good food for thought.

IMHO, increases in low rpm torque can be an issue because the rod is submitted to those greater pressures for a longer period of time at lower rpm.
This would explain why tuners tend to limit lower rpm torque increases, and then bring it back in at higher rpm.

I know overall torque levels can cause rod failure, but can it also be a function of time spent at peak torque, and not just maximum torque levels?

Is this an incorrect thought process?
Maybe some of you guys who have greater technical knowledge can comment on this.
Perhaps I am reiterating what has already been said, but in a slightly different manner.

Certainly knock could cause these rod failures, but I think it would show itself with a very close inspection of pistons and especially spark plugs.

Were I to modify a factory TT engine, I would be very inclined to limit maximum torque output based on these and others findings in regard to rod failures.

Tom, I am curious how much cost is involved to replace the factory rods prior to experiencing an issue like this?

Thanks in advance.

 

Honestly...the biggest cost in a job like this is not the rods themselves. You'll spend somewhere between $250-$300 (EACH) for a good set of rods. The expensive part is the labor to have your engine removed, torn down, rebuilt, re-assembled, and re-installed. It's a very labor intensive process unfortunately. Then the issue becomes "well if I'm in the engine anyway, why not do some head work, etc?" Again parts for some of these upgrades are not that costly...it's really the labor you're paying the most for.

 

I understand.
That is what I was asking, but I should have been more specific about the hours involved to replace them.

 

I should correct myself....that $250-$300 is EACH....for the rods...lol.

Labor time...you're looking at roughly 50-70 hours for a tear down and rebuild...that's just time spent on the engine itself, and barring any other upgrades you do while you're in there.

 

Good information.
Thank you.
A fairly substantial cost, but I think money well spent if one is looking to really push the engine for the bigger power.

 

I think your right on. Basically, most of what is being discussed has to do with fatique phenomenon, a well known issue with materials under going repeated stress. Google S-N curves and you will get a ton of information. Each metal and geometry will generally have a fatique limit that corresponds to the number of cycles. The greater the stress the less cycles it will take to fatique if you're above the fatique limit. Certain metals like aluminum and magnesium do not have a fatique limit, meaning that the material will eventually fail no matter what after so many cycles (it could be a very very long time). That's why they grounded all those SW Boeing 737s because of metal fatique due to take-offs and landings (cycles). With steel rods, there likely exists a fatique factor that if you stay below, it will NEVER fail no matter how many stress cycles it encounters. For the internals parts of a motor, one would really have to know the fatique limits for each component, piston, rods, crank, etc...I'm sure Porsche knows these numbers. So if I ever plan to jack up the power on my car and keep stock motor, I know that I will probably be on borrowed time. Hopefully, within that time, I will upgrade the various components so I can enjoy a longer cycle time before failure.

 

You're spot on about fatigue...and this is why I will NEVER put Titanium rods in my engine lol..

But, if I can recall, a component that fails due to fatigue doesn't bend or deform, it just breaks, and the morfology of the breaking surfaces is almost "always the same".

This, to me, means that bent rods are due to the rod being put under a greater stress than the max tensile strength, even for a small period of time.

Correct me if I'm wrong.

 

This is a key point here TT. Not every tuner is involved in tearing down engines after failures to know the effects of ones work, and what to change or strengthen next time round. The best scenario is to have a tuner that not only writes his own code, but is also closely involved in the strip down, reassembly and modification process.

Good point Guy. However not all tuners have the luxury of being involved with all facets at the coal face. Its easy to make the decision to upgrade certain components as a pre-emptive measure however there is nothing better than having a practical understanding of the component limitations in an engine. Not to say that strengthening an engine isn't a good thing.....but there is no substitute for knowing the exact details behind the cause of any component failure. That will give you the true reliability.

Agree that Porsche would and, should know all here! However i suspect as the GT2 is a rear wheel drive only car that too much torque or a large spike can make the car slower if traction is lost at the wrong time. Therefore as 997tt has AWD it can make better use of higher low to midrange torque by maintaining traction. Maybe im wrong but i suspect that if the GT2 had AWD a higher increase in torque levels may not have been such an issue, and its where that big torque is delivered and how evenly spread it is delivered to the drive train that really makes a difference in how the car accelerates. No sense having massive torque that just causes uncontrollable wheel spin in low to mid range.

Aint that a true statement. Its the labour thats always the killer . Btw do you mind me asking whats your charge out rate per hour your way?