Interesting insights into Crank and Cam Harmonics in I6's

[TheDarkSideofWill]

I went with the 4wt crank for ease of installation, availability at the time, quick "spool up", and the snout being right for the pulleys. So far, so good.

Does it *actually* spool up faster?

[Cheromaniac]

I think you'd notice the difference more between the 4cwt crank and the 12cwt crank when you watch the engine revs drop after you've blipped the throttle. The revs will drop faster if you have the lighter crank.

[lafrad]

you also notice a difference on shifting... the lighter crank will match the new gear's RPMs MUCH faster than the heavy one... no matter what load is on the engine. It has the potential to extend the life of an Auto Trans... and it would, in theory, make shifts quicker on stick shift...

[TheDarkSideofWill]

Has anyone who has actually compared the two cranks in identical or nearly identical engines have anything to say about it?

The crank with the lower *Moment of Inertia* spins up or down faster. The trick is that this is *NOT* necessarily the *lighter* crank.

In fact, for a given bobweight, the heavier crank *should* actually spin up *FASTER* than the lighter crank. In order to be balanced, at the given bobweight, the heavier crank will have to have smaller radius counterweights. The small radius reduces MoI MUCH more than the larger weight increases it.

So I'm interested in actual observations from people who've run both cranks in the same engine, identical engines or nearly identical engines.

[lafrad]

Everything I have seen puts the counterweights at the same or similar diamater on all cranks, for both the 4.2 and 4.0L cranks.

Since the I6 is a "natural balance" engine, there is very little need for ANY counterweight: the pistons and rods all balance EACHOTHER from one end of the crank, to the other. The counterweights that are present are mainly present to do "cleanup" for the fact that all 6 pistons aren't in the same PLANE of rotation motion.

Since the diamater of the counterweights are the same, that does mean the moment of inertia is higher for the heavier crank.


Now, something interesting that this inspired: since the PISTONS are required to work through the CRANK to balance their weights out... that would mean any balancing effects would effectively be stressing the crank... (even though, the engine as a whole is in balance, the weight will put force along the crank).... the 12CW crank would effective put that entire force/counterforce at each crank throw... reducing the twisting and torquing stress ALONG the crank. That would suggest that the 12CW setup would be more durable for High, High rpm operation. (remember, inertial forces are not linear with RPM, they are at least the "Square", so, the difference in forces at 3000 rpm would be 4 times LESS than 6000 rpm... )

I bet those main cap failures with the 4CW crank are due to extended amounts of time at High RPM with a crank that is twisting back and forth and rocking the caps in ways they weren't expecting. That would suggest that either the 4CW crank isn't strong enough to support that type of work...

I would venture to guess the amount of stress difference is WELL within tolerances of the block/crank when kept below 5000 rpm.

[TheDarkSideofWill]

Ahh... ok. If the counterweights are the same diameter, then the heavier one will spin up more slowly. I'll just have to concentrate on getting weight out of my rotating assembly, then.

The firing order is 1-5-3-6-2-4.
The pairs that balance each other around the rotating axis of the crank are 1 & 6, 2 & 5 and 3 & 4.
1 & 6 in particular are going to produce a strong fore-aft rocking motion around the engine's lateral axis if the crank had no counterweights.

I'll use the 3&4 pair as an example of the benefits of a fully counterweighted crankshaft. At extreme RPM, the centrifugal loads between the pair of cylinders actually get large enough to try to "straighten out" the crankshaft between them. This rocks the main journal in the bearing and causes it to touch the bearing surface. Once that starts to happen, bearing failure follows quickly. A fully counterweighted crank "contains" the centrifugal forces within that one throw and does not allow those forces to rock the main journal in the bearing. I don't think that any AMC/Jeep based stroker turns fast enough for that to be a problem. The primary selling point for the fully counterweighted crank in the Jeep world seems to be that the extra mass affects the torsional vibration modes.

Even BMW had problems with crankshaft torsional vibrations when they started spinning their inline 6's really fast (stock redline on the E46 M3 engine is 8000 RPM).