Let's talk short-stroke rods

[Tyson Schmidt]

Well, I'm at a crossroads with my short-stroke 2.5L wizz-bang, high revving race motor. It's a 2.7 7R case with all the mods, including case-savers, shuffle-pins, and boattailed mains. I plan on revving this thing to around 7,500 for now, and after some top-end mods down the road, I'd like to see 8,300.

Right now, I'll just run S cams and 36I/35E ports for POC rules issues. But down the road it will be a 2.5 VARA motor, and I'd like to maximize it's potential with as much cam/port/rpm as it will stand.

So. I need a bulletproof bottom end. I've got a 66mm counterweightd crank that's been magnafluxed, micropolished and balanced. I've got 2.0 rods, and 2.2 rods.

Therein lies the rub. Which do I use? The 2.0 rods are lighter, and ARP rod bolts are available for them. But, I assume they are weaker than the beefier 2.2 rods.

The 2.2 rods are heavier, but beefier, so they should be stronger. But ARP and Raceware do not make rod bolts for theses rods.

My twisted way of thinking would be to use the lighter but weaker 2.0 rods, and the ARP rod bolts, since I would expect the rod bolts to be the weaker link in the chain.

Discuss amongst yourselves. (in Linda Richmond voice)

[304065]

You are going to the trouble of building a 2.5 with all the mods, why not 390g Ti rods?



It is, after all, only money!

[garibaldi]

The titanium rods definetely wont last. You can use them, but you will have to be pulling the motor apart often to check them, and most likely be throwing them out after a short period of time.

[Tyson Schmidt]

Ti rods are out of the question for the reasons Christian noted, and of course, my budget.

[304065]

Hi Christian,

Why don't they last, in your experience?

[garibaldi]

Titanium does nto exhibit the same physical properties as steel obviously, While lighter, it requires different machining practices, and forging. Titanium will gall, and it will creep. We had a set of ti rods from a 996 GT3R here that had been run for a race or two and the big end was no longer round, there was excessive bernelling of the bearings, and the center to center was off. Basically they were junk after hard use.

The same goes for titanium spring retainers and such for other engine internals. Run a set of ti retainers for a while, then pull them out and look at how the keepers dig their wya into the material.

[Grady Clay]

I find nothing wrong with the proper Titanium alloy rods for racing
except they are very expensive.
I don’t know who makes the best Ti race rods for a 66 mm crank.

I used the 2.2 (non-S) rods that we reworked by lightening, polishing,
and shot peen.
I used OE Porsche bolts and nuts.
Everything passed Magnaflux, Zyglow, and X-ray.
For three engines I had over 100 rods.
Out of that number we probably didn’t use two as “suspicious.”
Never found an actual defective part.

These engines ran 8300 max all the time, 8800 intentionally on occasion,
and probably saw 9000 unintentionally.
Rebuilt every 25 track hours.
Never had a failure or found a potential problem on rebuild.


Here is a 2.0 steel rod (66 mm) and a 935 Ti rod (70.4 mm) for comparison. I
can’t find a 2.2 rod here at home.
It would be useful for someone to post a picture of
a 2.0 rod and 2.2 rod side-by-side on some graph paper.


There are some good comparisons of the various Porsche
rods on p. 65 of Bruce Anderson’s 2nd Edition.
Everyone should have this book in addition to Wayne’s.

Best,
Grady

[KobaltBlau]

Great info, Grady!

Originally posted by Grady Clay
It would be useful for someone to post a picture of
a 2.0 rod and 2.2 rod side-by-side on some graph paper.

Yes, this would help those (like myself) who are not intimately familiar with these parts (although perhaps our advice isn't worth so much ) A flatbed scanner would be better for this than a camera.

[Tyson Schmidt]

Grady, thanks a million! That's just the kind of info I was looking for. I'm still bummed that they don't make ARP rod bolts for 2.2 rods though.

What do you think of using 2.0 rods with ARP bolts? I have another 66mm crank and the 2.0 rods, so I'm thinking of building a spare short-stroke motor.

[Grady Clay]

It is important to look at the different modes of rod failure.

The four most important are:
From a bearing failure
From a structural failure.
From acceleration failure
From detonation shock.
And of course there are others.

Preventing a bearing failure is pretty obvious. New bearings, oil that is always sufficient flow, clean oil, correct lubricating properties, everything within temperature limits, proper bearing clearance, eccentricity within limits, proper journal surface condition, and not exceeding design limits. This is true for both bearings in the rod.

A structural failure usually is from a flaw during manufacture. We are fortunate Porsche specs such good parts and the suppliers are some of the best in the world. Through and careful inspection goes a long way to prevent a flaw from finding its way into an engine.
Drop a rod on a cement floor - well....

Acceleration failure is the main issue here.
There are two distinctly different aspects to this.
First is the mass of the rod, wrist pin, piston, etc. that must be accelerated “up & down” in the bore.
Second, the mass of the rod must be accelerated “side-to-side” as the crank rotates.
The rod, especially the big end, experiences the vector sum of these forces, sometimes to failure. Obviously it is high RPM that limits. So is the mass of the piston assembly, mass of the rod, where that mass is located, etc. The structural strength of the rod in various directions is the limiting factor.
Things that improve the situation are light weight piston assemblies, light weight rod assemblies, shorter stroke, smaller journal diameter, stronger rod assemblies, and lower RPM. Note that some are mutually exclusive or not possible in our situation.

If the engine has “high speed detonation”, the driver usually can’t tell. There are lots of bad things that can happen. In the situation with rods, the most common failure is the large-end bearing. These shock waves can also cause failure in the bolt and in the rod itself.


Some on the Forum may disagree or have more to add, please chime in.

Best,
Grady

[911pcars]

My $.02.

The engine components one chooses, especially for racing applications, depends on how often the engine will be inspected. That is, disassembled with parts replaced and renewed as required. If the budget allows, you can run the ragged edge, use maximum allowable engine speeds and carve away reciprocating and rotating weight to ensure max. revs.

On one extreme are F1 cars, whose engine life is projected in hours and designed to produce max. power for two qualifying sessions, a race distance and (hopefully) a short run to victory circle. The other extreme are commercial vehicles that are expected to last well over 500K miles.

Thus, with a more frequent rebulding schedule, one could run the lighter, but less stout 2 liter S rods (assuming their rated life is indeed shorter than the heavier 2.2 liter versons). Weigh that with the potential of quicker lap times afforded by a higher rev limit.

I can't recommend a rebuild/inspection schedule. You may have to extrapolate a rebuild interval based on the input from the forum racers who use similar equipment and with experience in matters of longevity (Minkoff, Scott, Plavan, Streit, and many others too numerous to list).

Sherwood

[Tyson Schmidt]

I appreciate all the input. I'm getting some great feedback here.

I plan on teardowns every 40 track hours, once I switch the top-end to full-race specs, with the 8,300 rev limit. With the initial S cams and ports, and 7,500 limit, it should be pretty bulletproof.

I'm tempted to run the lighter 2.0 rods with ARP bolts just for the reductiuon in mass, and plan on more frequent teardowns. I guess I'll sleep on it.

[Vintage911Racer]

Tyson,

I am getting a spare case and parts to build a 2.4 short stroke.
Do you want to help me put it together?

[beepbeep]

Do you have to run Porsche rods? Otherwise, you can have them made in any dimension you want.

When we want to go fast here in Sweden, we order custom set of H-profile Verdi rods. They'll make dem in any way you please. Carillo should have something similar in US of A.

Thay are always lighter that the ones they replace and much much stronger (most people use them on turbocharged cars).

So instead of chasing unobtanium Ti rods, you could go check custom made steel ones?

[Tyson Schmidt]

Originally posted by Vintage911Racer
Tyson,

I am getting a spare case and parts to build a 2.4 short stroke.
Do you want to help me put it together?

Sounds like fun! When I'm done with mine, I'm sure I'll have some spare time to kill.

Why a 2.4 short-stroke? What P's and C's? Are you going to overbore a set of cast iron 84's? I think the max is 86.7mm, and that is not quite 2.4 liters, if I remember correctly.

[BURN-BROS]

What Christian is trying to point out with ti rods is that Titanium has the ability to work harden. It does not require heat for this to be acomplished and over a period of time will become brittle.Once this happens you can send them out to be normalized, heat-treated, straightened and resized. They do have a lifespan to them but are renewable. Grady does bring up a good point that this life span may be quite longer than what has been recommended.

Aaron

[Vintage911Racer]

Tyson,

I meant to say 2.5 Short stroke. My lack of typing skills caught up with me again.

[Henry Schmidt]

We use 2.2 liter rods. Shot peen, nitrated with 935 titanium rod bolts with 928 rod nuts. With this set up as with any other rod, if the engine is over revved tear down is necessary but failure is rare. The bolts are reusable so cost can be distributed over more than one rebuild.
Porsche Motorsport might have the bolts and it is my opinion that there is no better set up. Carrillo rods break at the small end causing catistrofic failure (magic windows in the case, broken everything.) I have never seen a Porsche 2.2, 2.4, or 3.0 rod break at the small end except once. A big hp Turbo. It appears to have had a flaw when manufactured. Now we x-ray used rods.
After being involved in over 40 rebuilds of this type (2.5 IMSA) engine, I can say without fear of contradiction that I like the 2.2 Porsche rod and it is a great choice.

[Tyson Schmidt]

Thanks a bunch Henry! Great info.

[KobaltBlau]

Originally posted by Henry Schmidt
935 titanium rod bolts with 928 rod nuts.

please forgive my ignorance, but are the "935 titanium rod bolts" simply the steel bolts that were used with the 935 titanium rods, or are they actually titanium bolts (seems unikely)?