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jluetjen jluetjen is offline
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Join Date: Oct 2001
Location: Westford, MA USA
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I've never heard of specific problems with the rods, but Porsche did have lots of problems with the cranks in race engines. When they first started using the 70.4 mm cranks in the 2.5 liter 911ST's, they had a lot of vibration related issues that resulted in the flywheel bolts backing out. You can read more about it in this thread. People came up with clever solutions to keep the bolts from backing out. Mark Donohue's book mentions a case when he was racing against Peter Greg that Greg's mechanic neglected to pass on their solution and as a result Donohue's car broke and Greg's car won the race. It's not explicitly stated, but this thread seems to refer to the problem that I'm thinking of . The problem was so bad that Porsche went back to the 66 mm cranks for their 2.5 liter motors and used bigger bore pistons.

To make a long story short, Porsche eventually solved the fundimental problem with the crank and went on to racing greatness with the 911RSR where they combined the big bore pistons with the debugged 70.4 mm crank. But prior to that time, they had a lot of grief getting the 70.4 mm stroke engines to stay together above 7500 RPM. My understanding is that the cranks in the 2.4 and 2.7 street engines did not get the machining required for them to be successful above 7500 RPM.
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'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman
Old 07-01-2005, 06:34 AM
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