Is it advisable to "upgrade" to a lightened crank pulley?

[projectRDM]

Originally Posted by Def

Sorry Russ, but you're wrong on this one. That bonded piece of rubber and the AC ring is there because they are targeting a specific frequency and that piece provided the right spring constant and inertia to meet that frequency. IIRC, the equation is frequency = sqrt(Inertia)/k, where k is the spring constant of the rubber portion.


Now as far as engines being fine, I have no idea where the natural frequency of the stock crank is, but it might be above the stock redline, which can frequently happen with short/stiff 4 cylinder cranks. I'm still of the opinion that there are easier ways to make power with these engines that aren't as "risky" as this, because you know SOMEWHERE you are getting a HUGE amount more vibrations. Otherwise I'd think the factory wouldn't bother.


Here's an illustration of a 2nd order mechanical system, the middle one is what happens when you have a properly designed dynamic vibration absorber(i.e. a stock crank pulley):

I won't disagree because there may be truth to that in some cases, but I've seen a lot of import cars not equipped with factory A/C (not Nissans though, but still) that do not have any kind of damper in the pulley. The KA is particularly prone to NVH anyway, I've worked on cars where accessory bolts were loose or missing, but I've never seen/heard any issues from running a lightened pulley other than oil leaks from the seal on lesser designed copies. I'd like to think after over a dozen years I would have come across a broken crank or bearing damage somewhere if it was a definite possibility. My car also had an aluminum flywheel for a portion of it's life, again no issues, no weird vibrations, no loose bolts or torn mounts. It actually idled and revved smoother with the lighter components.

On your BMW with a long ass crank I'd suspect there'd be way more harmonics in place. Same goes for any V8 engine. On a 4cyl like a KA that's only half counterweighted you're not introducing any more resonance than you would anywhere else. I still stand behind that, but as there are numerous things that people question (banjo vs. flare brake lines for example) I'm not going to put a life on it. Modifying a car in any way will always increase the risk of failure, especially when pushed harder than normal. You run the risk of damage with even an air intake if you really think about it, but the pros outweigh the cons all day long.