2.8 RSR on 1R mag case! SWEET!!!

kenikh · 06-13-2006, 07:14 PM

OK, given the folly of the title, I thought I'd get to the point. I can't find a single thread that states:

a. 2.8 P/Cs stretch the limits of reliability on ANY mag case
b. Installing anything bigger than an 'S' cammed 2.4L set-up into a 1R case is the equivalent of building an expensive grenade.

I know these details by heart, but I wanted to share this mantra definitively with another Pelican:

"Thou shalt not bore the spigots of a 1R case."

All replies welcome!

tadd · 06-14-2006, 02:06 PM

What are the failure mode(s) of the non-7R cases? All I have read about are pulled studs and out of round main bearing supports. The pulled studs, if I understand correctly, are due to the difference in expansion between the steel studs and the aluminum cylinders while the out of round bearing saddles are due to the dynamic vibrations of the crank pounding away with each rotation.

So... assuming the stud problem is sorted with the proper choice of studn and case savers, should not the crank pounding problem be greatly lessened by reducing the rotating mass (e.g. Ti rods, good ballancing, crank lightening, etc)?

In other words: How much does actual power production contribute to case failure and how?

tadd

Steve@Rennsport · 06-15-2006, 10:40 AM

Tadd:

The failure modes of ALL the mag cases subjected to continual high RPM operation are cracks found between spigots and in the rear main bearing webs behind the flywheel.

Proper preparation lessens these stresses but the two cranks; 66mm & 70.4mm, have different vibratory signatures and place dissimilar stresses on the case.

For me, the final choice of a suitable case rests with the engine displacement, compression ratio, engine RPM range, and usage (street or competition).

Lots of RPM and high compression is what finally cracks these cases and its exacerbated in the larger displacement motors. This is why Porsche finally went to aluminum in late '73 with the 2.8 and 3.0's. Don't forget, those used 70.4mm cranks and ran up to 8200 RPM. If you keep your peak RPM below 7400, you'll be just fine with a properly prepared mag-cased engine.

kenikh · 06-15-2006, 10:49 AM

I can read Tadd's next question: what prep can be done to get a higher reliable redline on an early mag case? For context, his spec is a Long Stroke 2.1 (you can take credit for validating his choice) on 2.0S P/Cs with lighter racing rods (I won't steal of of his thunder by saying which). I think this meets the critreia you mentioned above: street usage, high compression, high desired redline, but fortunately small displacement.

Steve@Rennsport · 06-15-2006, 10:54 AM

One gets maximum durability by very careful balancing of the crank, rods, piston assemblies, good main bearing saddle preparation, shuffle-pinning, and bearing fitment.

tadd · 06-15-2006, 11:45 AM

Steve:
Thanks for the input. Observation is the best (and only) science.

Have you ever seen anyone put a second pair of shuffle pins in the four 'large' center main bearing saddles? I was reading about the 'reemed fit' thru the case bolts and then got curious about extra shuffle pins.

Just to clarify (and so that I am not reading too much between the lines), I noticed that less rotating mass was not on your list. Is that evidence that the 'case wearing forces' are torsenal from the 'wind up' of the crank from the power pulses or will a loss of 200-300g from the rods (as well as less inerta from a lighter flywheel) be useful?

tadd

Steve@Rennsport · 06-15-2006, 01:10 PM

Tadd:

We simply install one set per main bearing web to reduce (never eliminate) the shuffle and fretting on the parting lines. The harder one turns the engine, the more movement we see.

Reducing rotating mass and MOI improves engine response and reduces rod & bearing loads. It doesn't seem to reduce the movement on the case parting lines nor change the crank vibration issues.

tadd · 06-15-2006, 01:25 PM

Super info to chew on... Thanks!

I guess short of a new crank design, you are stuck with the band-aid measures you listed out before.

tadd

Steve@Rennsport · 06-15-2006, 08:58 PM

LOL,....The 911 crank is pretty good for what it is: an 8 main bearing unit made for a small package.

My only complaint about Porsche cranks is the small & convoluted oil galleys and less-than-optimal rod oiling. We go to great measures to improve oiling to the #2 & #5 rod journals to ensure proper flow at high RPM.

Wayne 962 · Wayne 962's Garage

I like bulletproof solutions - taking an early aluminum case to build anything up to 2.9. Or, taking a Euro 3.0 case and building a short-stroke out of that one. The aluminum cases are truly the best bets out there for durabiliy...

-Wayne