iNHERENT WEAKNESSES/KNOWN FAILURE OF 2.7 CRANK/RODS?

[eimkeith]

hey guys - still deciding which way to go with my 77 C3 motor, now that I have 10 to 1 JEs for it.

I was considering going 2.8SS, but that will lower the CR with the pistons I have, so now I am thinking through rebuilding it as 3.oL with the stock bottom end stuff.

I know the case is strong, but I don't have any useful knowledge of the crank and rods in this motor.

Any wisdom on this?

[jluetjen]

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.

[Steve@Rennsport]

Hi John:

Having raced 3.0 RSR's for many years, I have some first-hand experiences with these things,....

Indeed, the vibratory signature of the 70.4mm cranks did shake flywheels loose at 8000 RPM and that was a major issue. We resorted to many "creative" solutions to try keeping the flywheel attached as the RPM hit 8000 and above. Below 7900, no problem at all if it was assembled correctly.

This was/is a problem for all 70.4mm 6-bolt cranks and this was not fixed until the introduction of the 9-bolt cranks.

Further, RSR's had a special crank, rods and bearings with larger fillets at the rod journals to prevent high-RPM cracking. We also did other modifications to the crank, case, and bearings to get more oil to the #2 & #5 rod journals at the center of the crank; the last place to get oil.

One may use the 2.4-2.7 70.4mm 6-bolt crank as long as its properly prepared and with the understanding that its safe to 7800 RPM with all the mods. We also use aluminum flywheels to help keep them attached at those RPM levels.