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Piston weight vs rod strength (993 content) + bearing recommendations?

My math skills aren't strong enough to end up with the correct answer so I have come looking for help!

The project is a 993 C2 rebuild but the principle and math should apply to any engine.

Much has been said in regards to the 993 rods being a potential weak point once the rev-limit is raised past the stock 6800rpm but in my mind this weakness must stem from the rod's inability to withstand the forces of the piston going up and down. If this is the case what happens when the weight of the piston changes? The original 993 100mm piston is said to weigh ~625g per and I plan to use forged pistons that weigh ~500g per.

If my theory regarding piston weight vs rod failure is correct, is it possible to do the math and determine that my build with the 500g pistons will allow XXXXrpm at the same stress or load on the rods as the stock pistons do at 6800rpm? In an ideal world I would like to have a 7200rpm rev limit on my engine, but would rarely go past 7000rpm, especially not for extended periods of time.

While on the topic of 993 rods what are the best rod and main bearings to use for a street/DE motor rebuild? Original Glyco bearings from the P dealer or something different like Porsche Motorsport bearings or Patrick Motorsports CT-1 Low Friction Dry Film Lubricant Coating OEM GLYCO / CALICO COATINGS ?
Old 11-19-2017, 11:53 AM
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I haven't done the exact math, but the force on the pistons are proportional to the weight and to the second power of the rpm (piston acceleration).
With a 20% reduction in piston weight, you should be able to allow a 4.5% increase of rpm, and still have the same force on pistons and rods. That corresponds to around 7,100rpm.
Above that rpm, you should change to ARP high strength rod bolts. Typically, the rod bolts are the weak points on high rpm's. I would estimate that safe rpm's would be around 7,500 with the ARP bolts.
Another weak point might be the pistons themselves. With re-enforced rods/bolts, really high rpm (missed downshift...) can rip the wrist pin out of the piston.
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Old 11-20-2017, 09:34 AM
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I would not use OEM 993 rods in any engine that will be operated over 7K. There are FAR better choices than those things.

Any 993 engine using the original 993 crank that will be operated to 7500 RPM should get all of the oiling mods to the case, crank and #4 main bearing for reliability.
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Old 11-20-2017, 08:18 PM
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Quote:
Originally Posted by Steve@Rennsport View Post
I would not use OEM 993 rods in any engine that will be operated over 7K. There are FAR better choices than those things.

Any 993 engine using the original 993 crank that will be operated to 7500 RPM should get all of the oiling mods to the case, crank and #4 main bearing for reliability.
out of curiosity does this also apply to 964 motors?
Old 11-21-2017, 01:36 AM
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Quote:
Originally Posted by clutch-monkey View Post
out of curiosity does this also apply to 964 motors?
Yes
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Old 11-21-2017, 09:37 AM
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Quote:
Originally Posted by Fly911 View Post
With a 20% reduction in piston weight, you should be able to allow a 4.5% increase of rpm, and still have the same force on pistons and rods. That corresponds to around 7,100rpm.
Above that rpm, you should change to ARP high strength rod bolts. Typically, the rod bolts are the weak points on high rpm's. I would estimate that safe rpm's would be around 7,500 with the ARP bolts.
If your math this correct it is more or less the increase I was hoping for, i.e. +300rpm over the factory limit.

Who makes the best rod and main bearings to use for a street/DE 3.6 motor rebuild? The original Glyco bearings from the P dealer or something different like Porsche Motorsport bearings or Patrick Motorsports CT-1 Low Friction Dry Film Lubricant Coating OEM GLYCO / CALICO COATINGS ?
Old 12-31-2017, 03:58 PM
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