Non-counterweighted crank?

[tharbert]

911T-E 1972, Bosch mechanical fuel Injection, type 911/51 engine, 140 hp.

The top end needs a rebuild. I figured as I had the thing apart, bumping the cams up to E spec and installing 2.2E pistons might be a fairly inexpensive way to get a few more HP. I was also planning to have Mark Jung test/recalibrate the T pump to make the most of the changes. This is certainly a road many have traveled.

While trying to figure out exactly what I need for this mod, I was reading a Peter Zimmerman post on Rennlist about my engine.

Peter mentions my 911/51 motor has a non-counterweighted crankshaft. The E and S had counterweighted cranks. This is the first I'd heard of two different cranks in this MY. I was wondering if that crank would be fine with the increase in top end performance associated with 2.2E pistons (CR 9.1-1) and an E cam.

Why have two cranks? Would the lighter crank be OK with the E upgrade?

[gamin]

I have seen a 2.2 T engine modified with E P/C's, E cams, and E distributor. Retained the non counterweighted T crank. Used carbs. Seemed to run great. Not sure what the down side would be, if any.

[javadog]

I thought all of the 2.4 cranks were counterweighted. In fact, I'm almost positive they are... Maybe an error in the book?

JR

[Matt Monson]

Originally Posted by javadog

I thought all of the 2.4 cranks were counterweighted. In fact, I'm almost positive they are... Maybe an error in the book?

JR

That is correct, all models got the same crank starting with the 2.4l.

Just as an aside, a lot of guys like to build high revving race engines around the non-counter weight crank. They balance them, but it weighs less, so revs faster and robs less horsepower.

[TimT]

Just as an aside, a lot of guys like to build high revving race engines around the non-counter weight crank

SSShhhhh!

You will be fine with a non c/w crank in your application...

[tharbert]

Good call Javadog.

Found this tidbit:

[Plavan]

Originally Posted by Matt Monson

That is correct, all models got the same crank starting with the 2.4l.

Just as an aside, a lot of guys like to build high revving race engines around the non-counter weight crank. They balance them, but it weighs less, so revs faster and robs less horsepower.

Yeah- SHHHHHHHHHHHHHH!!!! Haha. No need to balance them- I just shove them in- No issues in a proper Aluminum Case.

[chris_seven]

Non-counterweighted cranks in 66mm stroke engines will have a shorter fatigue life in race engines due to gas torques causing torsional vibration at the higher rpm. These vibrations would be absorbed by the counterweights used in the S and E cranks.

Torsional vibrations don't cause the same rough running as lateral vibrations so all seems well until they fail, the real question is how long do they last, which could be quite difficult to predict. The 906E cranks also retained counterweights even though they were generally lighter.

The 2.4 cranks are generally more prone to this type of vibration as they are not as stiff as the 66mm crank and I don't believe that making a non-counterweighted version of this crank would have been a good idea.

[Matt Monson]

Originally Posted by chris_seven

Non-counterweighted cranks in 66mm stroke engines will have a shorter fatigue life in race engines due to gas torques causing torsional vibration at the higher rpm. These vibrations would be absorbed by the counterweights used in the S and E cranks.

Torsional vibrations don't cause the same rough running as lateral vibrations so all seems well until they fail, the real question is how long do they last, which could be quite difficult to predict. The 906E cranks also retained counterweights even though they were generally lighter.

The 2.4 cranks are generally more prone to this type of vibration as they are not as stiff as the 66mm crank and I don't believe that making a non-counterweighted version of this crank would have been a good idea.

In theory that holds true.

In practice a high strung race engine is something you time and tear down at planned intervals to refresh and inspect. As I said, I know of a bunch of 2.0l race engines that have used that old 66m T crank. Some of them rev to 9000 rpms and failures are few and far between.

If I was building a 7000 rpm street rod engine I wouldn't hesitate in the least to base it on one of these non-counterweighted cranks. Porsche has historically been a very conservative company and over builds a lot of their components.