Connecting rods
 

Folks,

To what RPM are standard stock connecting rods considered applicable? The 2.5 mid-size engines that are on this thread http://forums.pelicanparts.com/911-engine-rebuilding-forum/575530-2-5-mid-size-six-what-needed-build.html what rods are being used? Just thinking ahead.

Thanks for your time and patience,

W

You get 2.5 on a short stroke 2.2 crank with 90mm pistons or on a 2.7 crank you need 86 or 87mm pistons to get 2.5.
Bruce

JMHO,....

I consider 7400 RPM continuous to be the maximum practical limit for OEM rods, with ARP bolts. These are much heavier than good aftermarket ones such as Pauter, Carrillo, or Arrow and this reduces the loads on the bearings.

Steve,

How many RPM with light piston and pin on a 70.4 stroke?

7400 RPM continuous without all the oiling mods to the crank, bearings, and case.

The RPM limits on the 2.4/2.7 crank are based more in the crank design than in the rods.
Highers RPMs with the 2.7 crank are tenuous no matter what rods you install.
The crank counterweights are too narrow and that allows the crank to flex and crack through the counterweight. That flexing also causes a harmonic imbalance that causes the flywheel to vibrate off.
This design flaw led Porsche to widen the web dimension (narrowing the rod journal) in all engines after the 2.7 (70.4) crank. Even the 2.8 RSR had a custom crank to deal with this issue.

I have never read this in Fere or anywhere else. What was done to the crankshaft? All I have read is that they used thread locking compound and tightened the flywheel bolts more, but still could occasionally get failures.- Although maybe that was the ST 2.5 engines before the SS and Nickasil

Henry speakth the truth,.....:)

The RSR cranks (and matching bearings) had a wider filet where the journal met the counterweights to improve fatigue resistance at high RPM: remember, these were turned to 8200 and beyond.

I have an RSR crank in my own engine and I cannot get bearings for it anymore. :)

Interesting. Thanks for that bit of info. :)

Steve, if memory serves the RSR bearing can be made by simply machining a set of production bearings and dry film coat the machined area.

BTW: I have a NOS set of 2.8 RSR single tang bearing that have oil pockets for oil stabilizing characteristics. Some RSRs had double tang rod bearings.
The box says "Bill Smith" on it. He was responsible for servicing some of the IROC cars back in the day. At Bill Van maybe?


Steve: How badly do you need them?


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Well, my sand cast case 2.7 race motor, with stock rods and Raceware rod bolts, ran happily up to the 8,000 rpm shift point I was using.

Stock crank, the usual oil pump mod, and a turbo oil pump, Mahle nominal 10.5/1 90mm pistons. I have had various issues over the years (stripped IS shaft aluminum gear, valves bent from missed shifts, broken aftermarket racing valve springs, ingestion of little machine screws through loose intake stacks, etc.).

And after I finally put the car on a chassis dyno I started shifting at 7,600, because that was the optimum for my gears with the torque curve I recorded.

But the motor has at least twice gone over 100 hours between bearing changes, and the bearings when changed still looked OK. More to the point here, most of the Raceware rod bolts relaxed back to within the specified tolerance of their original length.

Of course lighter is better for the same strength. Pauters aren't all that expensive, especially if you add in the cost of having stock rods balanced for you.

The 2.7 has been a very good motor for me, and only the felt need to keep up with my racing buddies with 2.8s (and now larger) has prompted me to build a bigger motor with more squeeze. And since I have a bit more $ available than when I built the 2.7 in the mid '90s, of course it has some lighter and more race prepared parts.

Of course, maybe I have just been lucky. Friends with stock CIS 2.7s which don't get wound up so tight have broken cranks. the mag case may have contributed some to that.

Frere mentions problems Porsche had with the 70.4 2.5 race motors, which caused it to switch to a 66mm version. And that these issues had not been resolved for the 2.8s, although apparently a harmonic balancer reduced breakage of the cranks. As did the larger filets. But I don't recall him discussing use of thread locking compounds, nor increased torque on the bolts.

One of my shop buddies asserted that Porsche dealt with longevity issues on its 2.8s by rebuilding the motors after every race. And at least replacing and retorquing the flywheel bolts. But I don't recall what his bona fides for making such statements were. Makes sense, though.

Based on an Excellence article I read back when, I believe the use of thread lockers for the flywheel bolts was an innovation of one of the Florida race shops. It came to light when the factory had trouble unbolting a flywheel from one of his motors. Maybe the trick of 150 lbs/ft torque on the bolts started that way too. Bruce Anderson knew of it when crewing for professional race teams.