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RB25 pulleys

justified2.4

By justified2.4,
in Engines & Forced Induction

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the phantom Collaborator

the phantom

Posted
Posted

Hey Sydneykid,

I was wondering how long it would take for you to bite... :P

Yes harmonics are indeed a fascinating subject. Hmmmm...overbalancing...an intriguing concept...and one that I've only rarely seen made a frustrating mention in my theoretical travels. This will no doubt occupy considerable brain time on my part. My inital hypothesis of the notion is that an artifical harmonic is induced through the overbalance, such that it destructively interferes with the troublesome primary natural harmonic at 7500RPM....

I assume you have built engines to this specification. How do they perform and survive the onslaught?

The Phantom seeks harmony from the Oracle....

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Steve Grand Master

Steve

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It would be interesting if you could create a mirrored (180 degrees out of phase) frequency to the harmonic, with the same amplitude, and then it would be completely cancelled out.

they do this with sound, in high noise environments, so it shoud work with an engine.

Ah theory.....

The answer just hit me! a horizontally opposed 12 cyl 6 litre engine!:P

Sorry got a bit carried away.

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the phantom Collaborator

the phantom

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Having been left intrigued by Sydneykids mention of overbalancing vs crankshaft harmonics I have attempted to dig deeper into theory to try to explain it or at least understand what is going on. Very little is out there on this specific subject however, with only a few good texts found at the local Uni and state libraries, explaining engine balancing and vibration theory for the various engine configurations.

Anyway, based on closer scrutiny, I have changed my mind in regard to the previous hypothesis I made regarding the mechanism wherby this is achieved...it is much more complex.

Since the crank is essentially a complex torsional tuning fork, two mechanisms exist to alter this frequency. One is material stiffness, and the other is weight. Since there is not much you can do with the stiffness, other than perhaps nitriding or some other treatment, this leaves weight. By overbalancing I can only assume added weight on the counterweights over and beyond the factory level, possibly with a heavier-than-steel tungsten alloy inserts. This would effectively lower the frequency of vibration. Theoretically at least if you double the weight you would also double the inertia. Relations show then that the resonant frequency of each crank throw would also lower by a half. The resonances are still there, but have shifted in frequency leaving the more power productive 7500 RPM plus region clear for a hammering. Ofcoarse this leaves the 3750 RPM band at a peak, but since we are probably talking full on race engines here that isn't a problem, particularly if the critical RPM is quickly passed through and that relatively little power will be made there.

I am left curious as to how the standard RB30 crank handles the lateral as opposed to torsional crank deflections at these ludicrously high RPM's. Perhaps this is the reason for the heavy crank girdle made by Nizpro on their powerhouse RB30/26 hybrid with a standard based crank. Although heavier counterweights would help counteract this, the RB30 standard crank has NO counterweights directly over number 2 and 5 crank throws. Compare this to the OS Giken 3.0 billet crank which has EVERY crank throw counterweighted an as such would greatly assist in reducing the lateral deflection. No girdle is used on this engine either, which one can only assume is because of the superior stiffness of the billet crank. These counterweights present opposite all of the crank throws are also significantly bulkier, which is in line with what Sydneykid is suggesting....and the OS engines supposed ability to reach over 10000 RPM.

This thread however began with a discussion on light weight accessory pulleys and harmonic dampers, with their lower rotational inertias and theoretical power gains. The MUCH heavier OS crank would have significantly higher inertia, but since the engine can handle such absurdly high power outputs anyway, this is obviously of secondary relevance, even in the super time critical world of drag racing....

I await Sydneykids further input into this discussion with great anticipation...

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the phantom Collaborator

the phantom

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Apologies for appearing to hijack your thread and moving off topic. That wasn't the intention, just thought you and others might want to learn more about a related subject in which detailed information is very rare. Even rarer is finding someone with Sydneykid's experience willing to share some of this knowledge...

:D

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