Does anyone know if the crank pulley is what supplies all the gains... or if the two accessory pulleys provide ANY sort of gains at all... Anyone with dyno sheets, or at least some sort of theoretical information to prove one way or the other?

 

I believe the crank pulley alone produces the vast majority of gain.

nissanperformancemag.com actually got slightly more dyno gain adding the crank pulley alone to an Altima VQ35DE, than they did by adding the whole pulley set on a 350Z. Now maybe that is somewhat apples to oranges, I don't know if both of those VQ35DE variants have the same pulley setup. But it's an interesting observation.

Also, physics says weight loss off the pulleys will not produce a significant gain on the dyno. A dyno is done in a higher gear where a little extra rotational weight isn't as much a factor (because we're spinning the pulleys up slowly anyway), and so we're talking a very small horsepower difference (1 or less). Thus the underdriving must be producing most of the gain on the dyno. And the crank pulley alone provides ALL the underdriving... the only advantage the whole set has is maybe in say 1st gear (where we have to spin the pulleys up quickly and rotational weight is more a factor) you might see a little more benefit from weight off the additional accessory pulleys.

 

Actually, if you simply read the UR info instead of speculating, they claim just the opposite. The vast majority of the power gain comes from the decreased rotational inertia and not from the underdriving. They also state that the accessory pulleys are the same diameter as the factory ones and their function is simply to provide the look of a pulley kit.

But don't take it from me...read their site.

 

Actually I have already read the site, and instead of taking their word for it I did my own research. If you do the math, what they are saying does not add up, and this pulley topic has come up on multiple forums with the same explanation. For one it is completely dependent on what gear you are in, which I don't even see mentioned. But if we are talking about dyno numbers (higher gear), you won't see anything significant from shaving a few lbs off a pulley. Taking 10-15 lbs off a larger diameter flywheel will have several TIMES the rotational inertia benefits of merely shaving a few lbs off a small pulley, so ask yourself why a lightened flywheel only shows a few HP gain on a dyno. Now you can see why the gain from the lightened pulley is more like 1 hp or less on the dyno, so the underdriving is giving the majority of gain. Now as I said in lower gears it is different, decreased rotational weight offers more benefit here. By the way the accessory pulleys are in fact lighter and are not only just there to "provide the look" of a pulley set.

 

umm I'd say out of all the single mods I have done, EXCLUDING NOS, the 3 peice UR Pulley set made the biggest single difference as far as how your car feels when driving after the install. Sure at the qrt mile mabe it took a half tenth off, but the engine does rev a bit more free. GET NOS OR A F/I if you want to really see a difference.