It seems the nitrous bent the rod. Other than RUF, has anyone else bent a rod without nitrous?

 

ebaker,

Nitrous isn't voodoo to those of us who understand. Nitrous supplies additional oxygen. Oxygen plus fuel in the proper proportions makes power. My nitrous system is not instantaneous, it is progressive ...... 10 hp added every tenth of a second.

In addition, examine the pistons. They are perfect, no detonation. The plugs color was perfect. Power is power, no matter how you generate it if there is no detonation present. The 996tt rods are a weak link somewhere between 620 and 750 rwhp when this power is being delivered to the road. With the monies involved in a 996tt motor the rods need to be addressed if you want to insure your investment. I was very lucky that all I experienced was a bent rod. If it wasn't for S Car Go's experience, the car could have continued to be driven a little more and the motor would have gernaded. The ticking noise was not very noticable. We all originally believed something was amiss in the upper end.

Oh well, free advise is worth exactly what it costs you.

 

Chad,
I looked at your pic of the rod and want to offer something here. After seeing your pic, it is clear the failure is out of plane with respect to its plane of motion.



Technically speaking, the rod is simply a two-force member. That means, it is not subjected to flexure, only tension and compression. When I look at this, I clearly see a compression buckling failure. What I mean by this is that the force on the rod is applied at the wrist pin and crank bearing. Regardless of where the piston is in the stroke, and hence the angle of the rod, it is still a two force member only subjected to tension or compression. My assessment is that the force of the explosion in the combustion chamber applied to the piston top induced an excessive compression force in the rod that exceeded its buckling yield strength. Forces less than the yield strength of the rod will allow the rod to operate in its elastic range. That means the rod can elongate and shorten under tension and compression loads respectively. The rod can also bow out of plane much like the analogy of pressing on a thin metal yardstick standing upright. It will flex outward within its elastic range but will eventually permanently bend after reaching its yield strength.
This is the wonderful property of steel and its related alloys. The stress strain curve allows for a range of operation in the elastic region with a gradual deformation leading towards ultimate failure. The yield point is where the metal permanently deforms but does not yet break. This is why safety factors are used in Allowable Stress Design and Load Factor Design in steel construction. They allow the element in question to operate in the elastic range without permanent deformation and/or failure.
Now, if you look at the cross section of the rod, you will see it is an I shape. If you orient the rod such that its cross section is like a capital I (as opposed to an H), the strong axis is horizontal. The weak axis is vertical. Clearly, in textbook fashion, the rod yielded about its weak axis. Although impractical, a much better and stronger rod would incorporate a square cross section giving both axes equal buckling resistance.
Just my $0.02.

 

Ken,

Rob at S Car Go also told me it was a compression stroke with excessive force that bent this rod. My question is why did only one rod bend? Do you have any ideas?

 

The compression capacity of a member is based on several things:

• Member cross section
• Member length
• Material
• End fixity

We know that, for all practical purposes, all of the above are essentially the same for a given set of rods in a given engine. I would have to guess the End Fixity has the greatest possibility of variation though. As an example, the factors associated with various end fixities can vary between .7 and 2.0. As you can see, that is a large envelope. So, what does all this mean??

Well, if the connection to the crank or to the piston is even slightly hung up due to oil starvation, misfit, etc., the capacity will be affected. I can expound on this further if you wish, but those are the basics.

Keep in mind, I am talking about the out-of-motion-plane of buckling. Here is an example, albeit a bad one.................
Let's say you have 1/8 inch wide journal bearings on #4 and 1/2 inch wide journal bearings on #5. The wider #5 journal bearings will provide a "stabilizing" effect for out of motion plane buckling. Therefore, with all else being equal, the #5 rod will have a higher compression capacity. Or......back to the metal yard stick example........
Take the yard stick and put its bottom in a vise. Now push down on the top of it and it will take more force to buckle it as compared to the yar stick simply bearing on the floor!! Get it??

 

Can you elaborate on this?

 

Tyson,

In 2003 or 2004 Porsche redesigned the 996tt case. All the new motors since then have this stronger case. There is visible webbing reinforcements on the new cases. I believe PorschePhD posted some pics on this subject awhile back.

 

Thanks Ken, I get your point. One thing that I was aware of is the 996tt's have a weak oil system when placed in a heavy track enviroment. The excessive G forces could have easily contributed to this problem. I have never heard of it on acceleration but I have in high G handling.

 

I thought the oil system under high G loads was only a problem on the regular 996 block. The GT1 based block uses a dry sump lubrication system with scavenging pumps, doesn't it?

 

Tyson,

You are partly correct. The 996tt uses a a dry sump with a single index (pickup) pump. The GT3 uses a two index pump. The GT1 uses a three index pump

Cost of 996tt pump about $500.00, GT3 pump about $1,400.00, GT1 pump about $6,000.00.

I'm sorry to say but if you have a good upgraded suspension, upgraded sway bars and competition tires and you seriously tract your 996tt, you are at risk of oil starvation.

 

So does that mean upgrading to a GT3 pump solves the problem?

Does the GT2 use a single pickup pump as well?

 

My sources say the GT3 oil pump solves the problem. I am not positive as I have never seen a GT2 motor apart, however my understanding is the GT2 uses the two index pump like the GT3.

Maybe Stephen, Todd or Kevin could clarify which oil pump the GT2 utilizes.

 

I still can't belive that noise was a rod. It sounded like something else.

 

Carefully look at the pics and you can see where the bent rod was being rubbed by the crank.

 

Glad the rod did not fail. It would have been real ugly.
Can't wait for Rob to finish your car. It will be sick indeed.