Tackett

So. I'm not a newb, I obviously know how to use the search function. With that said, between google searches and site searches I am still left with some questions regarding pulleys in a NA application.

1: when you buy a underdrive pulley set, it's my understanding that only the crank is a shorter circumference, while the rest are simply lightweight aluminum. This makes sense to me.

2: I've heard people say that the underdriving ruins batteries and alternators. I really don't understand how this is possible. It seems like the alternator simply wouldn't charge the battery as much as a longer circumference pulley would. Therefore leaving your battery open to full drain easier. It seems like a trickle charger plugged up when parked would solve the problem. Couldn't this whole thing be offset by changing the circumference of the accessory pulleys to compensate for the loss? Or would that defeat the purpose?

3: are lightened, OEM size pulleys a better option? What's the disadvantage vs under driving? I would think that under driving would free up more HP, but I can't imagine it being much.

4: IF, one were to purchase a underdrive crank pulley only and utilize stock accessory pulleys, then deleted the extra idler pulley that exists on the '03 engine......WTF would the belt size need to be?


I'm going to be purchasing a new crank pulley and deleting the extra idler when I change the belts. I already have the correct belt for the OEM size crank pulley needed to accomplish the delete. I'm just on the fence about whether or not to get a oem size lightweight crank and use the new belt I have, or if its worth the hassle to get the underdrive crank and spend a lot of time and effort poking around for the right belt size to run the underdrive crank and the idler delete.

2004Black350z

iTrader: (37)

To be honest I'm lookin forward to a educated response out of someone bc my first 2 Zs I never went pulley route bc of previous experiences on different platforms.

terrasmak

iTrader: (8)

If the car is a DD, leave the stock pulley on it. Simple.

Also the belts are easy to find with a quick search

Classy

iTrader: (7)

I can't give you difinative reasoning that you ask for, but I can tell you from my experience that you should not run a UR crank pulley, mine notched it's way out where the key holds it to the crank, I was blowing oil out of this notch and it made quite a mess of my engine bay.

mattman87

Just an educated guess, but for question 4....

The idle pulley delete shortens belt length by 1" (remember reading in threads, think to 4.5"). Simply take the circumference of the OEM crank pulley, and subtract that from the circumference (pi*d) of your new underdrive crank pulley. Add that value to the (I think) 4.5" belt length.

More succinct: OEM Belt + (Circumference Difference) - 1" = new belt size.

Suwaidi

2: Some believe that by underdriving the alternator (and water pump in some cases), you could potentially damage them, also, larger sound systems would require more voltage and they wouldn't recommend running an underdriven pulley if you have plans for that.

3: Depends on your definition of better. Some companies (Stillen & NST) have mentioned that the underdriving would yield an additional 1-3hp. NST had a dyno on an HR which gained roughly 7HP (370z guys praise the NST pulleys). I, however, decided not to go with NST because of their shitty customer service and lack of response. They do however have a bunch of dynos on the 370z forums

Notes:

a) NST, Stillen and AMS make pulleys for our cars. UR does, but their website isn't working, but some dealers (ConceptZ) stock them
b) Make sure there is a steel sleeve - All listed previously have them
c) You will find a plethora of debates on whether you should run one or not - pick a side
d) Make sure that whoever installs it (you or shop) torques the bolt to specifications

bealljk

iTrader: (13)

This is solely my opinion - but underdrive and/or lightweight pulleys are a waste of money…

Underdrive pulleys work because a lighter & smaller rotating mass has a smaller moment of inertia as compared to heavier & larger rotating mass.

If you really really think of all the rotating mass and how that mass is proportioned away from it's axis the grams of weight savings is next to nothing in the big pictures…

here's what's rotating on your system…

tires, wheels, brake rotors, axles, differentials, drive shaft, transmission gears/parts, clutch, flywheel, engine internals, PULLEYS, peripheral elements(AC, Alt, WP), etc…

so when you factor ALL these rotating parts, dropping a few kilograms off one aspect of all these rotating masses is next to nothing…

I would love to see what NST did to get a 7whp gain - if you ask me, money is spent in better places...

terrasmak

iTrader: (8)

Btw , great mod, but search for cracked crank

Tackett

Thanks to all that responded. So lets review here:

1: belt size for underdrive+idler delete is:
OEM Belt + (Circumference Difference) - 1" = new belt size.
That makes sense to me, I guess all one can do is try. If I go this route ill post the belt size and the pulley I used.

2: correct me if I'm wrong, but sound systems and engine electronics get their voltage from the battery. Not the alternator. Which is why you can still run the sound system and everything else without the car running. I guess if that's the case, then the only problem would occur if the electrical system drains the battery faster than the alternator can charge it. But then, it seems like you could simply..........shhhhhrink(?) the size of the alternator pulley to compensate. Meaning that, while the crank turns slower than stock, the alternator pulley would be able to turn stock speed. But again, I think this defeats the purpose of under drive to begin with. Am I thinking about this all wrong?

3: pulley (whatever type) must be steel sleeved. I don't know why you would use one that isn't in the first place. For that matter, I can't understand why any would be manufactured that way to begin with.

4: the decision to underdrive or not should come down to the amount of electrical draw existent in the application. The more draw, the more likely the discharge rate of the battery could exceed the charge rate of the alternator.

Is that pretty much it?

terrasmak

iTrader: (8)

You forgot to mention that you will destroy your crack do to the extra vibration without the harmonic balancer you just removed.

Belt size is listed in the second to last post in the DIY section, I kinda hinted at that earlier.

Tackett

L.......O..........L


Wooooooooow.

I must really be getting senile in my old age. I completely forgot about even posting in that thread. Geeeez.

So you are saying that without a dampener, it's just a matter of time before the crank goes bye bye? If that's the case then ill never change pulleys. I usually take what you say as gospel.

terrasmak

iTrader: (8)

Yup, long term effects are not good. With that said yes I am running one, not really concerned because I expect to put less than 5k more miles on this motor. I will also install it on my next motor, the pulley will be balanced with the crank, flywheel etc etc at the shop. Cross my fingers and hope I didn't F myself.

kacz07

iTrader: (15)

I wouldn't mess with them. I threw in a Fluidampr crank pulley for the extra revs (8k) on my '06 Rev Up NA build and called it a day.

jerryd87

iTrader: (4)

even with a perfectly balanced rotating assembly its only going to help so much. its impossible to balance a assembly for both rotational forces as well as the explosions it will see at the various engine speeds. hence balancers, fluidamper will absord harmonics across all rpms but not as well as the ati, the ati will absorb better then the fluidamper but only to a certain rpm range(which is tunable on the pulley). stock isnt even that great because it has very little elastometer on it however even it is light years better then the underdrive pulleys.

not worth it in reality in the real world expect 1-3 hp as was stated, so you pay a couple hundred bucks, ruin your engine faster, and gain the performance advantage of running a gallon less fuel in the gas tank