Over the past couple of months with us pushing the envelope of what the 991 motor is capable of we took a step back and wondered, “How much torque can we make on the stock engine?” With cars running bigger turbo’s, nitrous and just bolt on setups we wanted to know how strong the stock 991 rods really were, without just speculation.



This car made 815ft/lbs to the wheels on the stock motor! Courtesy of RobTuned.


So we ordered a set of bone stock 991 Turbo rods from Germany. Once we got the rods we decided to send one out to our friends at AMS Performance in West Chicago. For those that are not familiar with AMS they are a company in Illinois that manufactures parts for a wide range of cars, most notable the GTR. What some may not know about AMS is the amount of hardcore engineering that goes into their products. Having worked at AMS for a number of years I figured they would be perfect to analyze the rod for us. So we sent one rod out to get some tests completed on one connecting rod.





Before we get to the results I want to talk about the connecting rod that comes in the 997.2 Turbo and the 991 Turbo as they have a very interesting method to create them.

For the 991 and 997.2 turbo Porsche uses a manufacturing technique referred to as fracture splitting to make the rods. Porsche is not the only one that does this, many other manufacturers do the same thing as it is cheaper to make rods for mass produced cars and motorcycles. There are a lot of positives to making the rods this way as opposed to a more “traditional” method of making the rod in one piece and cutting for mass produced connecting rods. (Less cost in machining and doweling the mating surfaces) The other big advantage is a better registration of the 2 surfaces. Because they are not flat but fit together like a puzzle piece it can increase bearing life as there is less of a chance of the rod cap being able to move under high axial loading. The rod starts life as a single forged connecting rod, and then goes through the fracture process where it is notched and then using hydraulic force using a special device that moves quickly or is dropped with applied force of approximately 160 tons of force.




This is Yamaha’s method of fracture but similar in how it works to give an idea.




Comparison of the two methods- "Traditional" vs Fracture




This is a pic of what the two mating surfaces look like after the separation.



So after the analysis this is what AMS came up with:





The test was conducted using a 997.1 rod as a comparison. The reason for the 997.1 rod being the comparison connecting rod is that the 997.1 rod is thought to be good to pretty decent torque numbers so we figured it would be a good control to compare it against. While the 997.1 rod is pretty good, we can see that overall the 991 turbo rod is better by a decent margin!

Here is what AMS President Martin Musial had to say about the rod construction:

“We feel the 991 has a better overall design. The 991 rod transitions from the beam portion to the big end (rod bearing journal side more gradually with a larger radius reducing points at which stress risers can be found. The 991 turbo rod also has more material in areas that would lend it strength in compression (forces exerted from higher cylinder pressure/torque). The material of the 991 rod has a higher tensile strength by 7% and less overall elongation by 3.2%. The rod bolt on the 991 has a higher tensile strength than the 997 stud by 8%. However, the 997 rod has more material around the wrist pin area, which from a power/torque perspective would have no advantage, possibly only in high RPM situations (but as you can see below this is negated because of the 991’s rod to stroke ratio advantage).


Rod to stroke ratio
The 991 Turbo connecting rod is longer (center to center) than the 997.1 This means less side loading of the pistons on the cylinder walls, and also a longer dwell time near top dead center, reducing the acceleration and thus forces on the piston/rod. A better rod to stroke ratio favors higher RPM horsepower generally.

In summary the 991 turbo connecting rod is a better design than the 997.1, it should hold up to more cylinder pressure (torque) and also work better in a high RPM environment.”

The results were measured using a Rockwell hardness test. There are different types of Rockwell tests that can be performed, this one was measured using the Rockwell C scale due to the high hardness range. It is tested by putting a load on a localized specific point. This is usually done over a few different points so that you can create more of a range of hardness across the tested material using a diamond cone. Once they make an indent it can be measured as well as the force required to make the indent in the test material. That is the number in the hardness column. As you can see the 991 turbo rod has a higher hardness than the 997.1 turbo rod at 33, 33, and 34. These numbers are arbitrary numbers in the Rockwell C scale.

The other big components you see in the test are the tensile strength and the yield strength. Tensile strength is a measure of how much force it takes to pull the part apart, yield strength is a point where a material starts to deform in this case using elongation. To give an example a Carrillo forged aftermarket rod has a tensile strength of approximately 195,000psi. So while the stock rod is weaker in terms of the tensile strength we can see that is still has a good UTS compared to a high end aftermarket rod. Yield strength is also higher than the 997.1 turbo at 117,700psi. Again for comparison an aftermarket Carrillo rod has a yield strength of around 182,00psi.

The other curious thing is the rod bolt that Porsche uses stock is pretty good. Aftermarket rod bolts in standard form (using H-11 tool steel) have a tensile strength of around 220,000psi from Carrillo. ARP’s standard chrome moly bolt has a tensile strength of 200,000psi. The stock 991 rod bolt is just barely below this which is pretty great for a stock rod bolt!

Now there are a lot of other factors that go into the strength of the rod, by no means is the tensile strength the end all way of testing how strong a rod is. Material composition and manufacturing process plays a huge roll in how strong a rod can be overall. What we are simply trying to show is how well the stock rods are constructed.

So what does all this mean and where are you guys going with this? Well what we are trying to show is that the stock rods from a first analysis is that the stock rods are pretty good. Better than previous generation water cooled Porsche turbo motors. How much torque can the stock rods take? Well we have not found the limit really. What we do now is that our increased torque output over some of the other tuning solutions appears to be very much on the safe side. When we make calibrations we go to great lengths to measure everything we can to be sure that we are offering the safest possible calibrations while still making good power and torque. Now what I would also say is that you want to make 800ft/lbs all day long, go with a good aftermarket rod as the stock rod was not designed to do that day in and day out.

Next we will work on an FEA analysis to see where the weak points are in the rod and what is going to be first to break, based on estimated cylinder pressures. So be on the look out for part 2!

Thanks for reading!

Mitch Mckee
Cobb Tuning

 

very good info !

 

Hats off again Mitch. All of our discussions over the time we have been working on the 991 platform are what give me the confidence to proceed with pushing these cars. Truly providing a top-notch product with confidence! Wonderful work and an especially wonderful contribution to the community for sharing it!

 

Rob tuned the nitrous car and it never reached that kind of torque. Weird TQ Dyno reading...possible?
Even the HP vs TQ curves crossed too late. Dyno on 5th gear or something?

 

The dyno plot above was simply an example. This was done in 4th, however regardless of gear Tq and Hp will always cross at 5250rpm as they are directly related.

Our post was simply to supply the good folks of this platform with our empirical data showing the strength of the stock rods in an effort to drive further innovation within the Porsche aftermarket. It was also to show that the torque gains over our competitors are 100% safe. Internally Cobb has had this data for some time which gave us the confidence to push torque like we have. We obviously had no issues finding torque limiters from day one 😘

Do we recommend running 800ftlbs in a daily driver... Not at this time, but 700ftlbs shouldn't be an issue and this has been shown over time exclusively on 991 turbos custom tuned on the Cobb Accessport!

Thanks again for reading guys! We truly appreciate your enthusiasm for our product and we look forward to continuing our cutting edge innovation for this platform backed by facts, data, and, most importantly, happy customers!

If anyone has any questions or would like further clarification we are always available!

-Jon

 

Thank you Mitch, Jon, Cobb and AMS for taking on the project and costs associated with determining the make up and relative strength of the OEM rods. Valuable information for any tuner, builder and end user in determining a safe operating range for added power.

 

Great write up with a lot of good data! Shows some serious dedication to the tuning in these cars! I'm a big Cobb fan!

 

These are excellent news. I did have a feeling that Porsche wouldn't go backwards on the strength for their motor internals on newer model Turbos

Dzenno@PTF

 

very cool - info like this or should I say the sharing of info like this is what makes this forum great - thank you!

 

Great write up! That was very informative and gives me more confidence in my plans. Is 700 ft lbs achievable without a turbo upgrade and are you talking at the wheels or flywheel?

 

Yes, you can even get pretty close on the factory turbochargers.
To the wheels! Crazy right?!

 

That's great! Is that only on 100 octane or meth? I'm stuck with 91 in California. I have my zr1 running a flex fuel setup and it's great! Does Cobb have an e85 tune yet? Is there the ability to do a flex fuel setup or do you have to switch tunes each time?

 

Flex fuel as we know it, no. But Cobb has been working up the ethanol ladder and testing. There are fuel system limitations. I feel you on the 91! There are options and meth is only one if them. We have gotten to 700 wheel on 93+. Have a nice 91 octane stage 3 also that's about as tough as you can get here in Cali on pump. Feel free to inquire with me anytime!

 

Yes more or less.

Mitch

 

E85 and fueling upgrades will be done very soon