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Rotary Performance Underdrive Pulleys
Here is a decent view of the double alternator belt pulley, along with a pair of new belts, of course!

Idle seems a little smoother, perhaps attributed to the motor not having to work as hard to spin the water pump and alternator.  I have noticed that when I have a load on the electrical system, my fuel pump slows down slightly.  That could be attributed to a number of things besides the fact I am only spinning the alternator at 68% of factory speed (namely my old battery, or a 140k mile alternator).  I really question the overall power gain (or the freeing of already present power) because the stock crank pulley is not THAT much heavier than the underdrive pulley.  Weight being the key factor governing the power increase (read below for facts about the source of underdrive pulley power increases).

Installation went well overall, however you must do some research prior to installing these because Rotary Performance does not provide you with ANY directions, whatsoever.  For those who are thinking about doing their own underdrive pulley installation, scroll down to my installation section for instructions and tips!  Do yourself a favor and have the parts store that you purchase your new belts from perform the pulley swap for you.  It makes life easier on you and it is usually free.


Crank bolt size:

Belt size for Rotary Performance underdrive pulleys (crank and alternator used together):

Dayco Top Cog part number: 15310 x 2 (which is 31.0 inches long -- you will need two)
Factory: 153210  (You guessed it, 32.1 inches long)

You probably could get away with using the factory size if you are not using an underdrive pulley on the alternator, but I would highly recommend it, or atleast getting a factory size double groove pulley.

Note: It is VERY important for those considering doing your own pulleys, be prepared with a large 1/2" drive breaker bar and extension pipe for leverage.  I broke a 3/8" drive ratchet (before I bought the breaker bar), a 3/8" breaker bar and a 3/8" to 1/2" adapter.  Not good.

1.  Remove the negative battery cable so you don't short anything when removing your alternator (that 10 gauge wire bolted to the alternator is hooked directly the battery).

2.  Remove the alternator and set it aside for now.

3.  Remove the flywheel inspection place attached to rear housing, driver side.

4.  VERY IMPORTANT:  Prop the clutch pedal to the floor to prevent the bearing from sliding down on the front of the eccentric shaft (I used a 21mm box end wrench, with the box end against the back of the fuse box and the open end "gripping" the clutch pedal).

5.  Rotate the crank until you see the long (approx. 3") gap where the flywheel lip is gone.  Use a fat crowbar, or something metal (and strong) to prevent the flywheel from spinning when the crank bolt is loosened.

6.  Remove the 4 10mm bolts that holds the crank pulley retainer ring in place.  Take the ring and pulley off and set them a side.

7.  With a 1/2 breaker bar and 1/2 drive 19mm socket (may need the help of an additional pipe), loosen the flywheel bolt (it is on there TIGHT!).

8.  Once the crank bolt is removed, be sure you do not loose the copper washer if it came out with the bolt, otherwise, it is still in the pulley mount recess, so be sure to get it, you will need to reuse it.

9.  Carefully remove the crank pulley mount, while not disturbing the woodruf key on the eccentric shaft.

10.  Clean the crank bolt, copper sealing washer and apply a small amount of silicone sealer to the bolt flange and the back side of the copper washer so there is something to hold the copper washer in place (it WILL slip out of place if you are not careful, causing an oil leak!).

11.  It is best to insert these as an assembly, keeping the crank bolt and pulley pressed together at all times so the copper sealing washer does not slip down.  Hand tighten until it is time to get the breaker bar back out.

12.  Before tightening, be sure that the crowbar (or whatever) is secure when you torque the crank bolt back down to avoid the flywheel slipping past, breaking your knuckles!

13.  Tighten the crank bolt down to spec (I don't know what that is... it's very tight though!).

Now you are ready to either put the alternator back on, or take your alternator to a shop when you get your new belts and have them put the new pulley on for your.  Most parts stores have the tool for this, since they routinely swap pulleys themselves and will do it for free to boot!  Don't forget to remove your crowbar, and clutch pedal propping device.  Also it would probably be a good idea to put the inspection plate back on as well.

That's it!  Enjoy.

What they are supposed to do:

The RP Underdrive Power Pulley is designed to underdrive the water pump and alternator approximately 15% which gives you an increase in power and reduces cavitation of the water pump at high RPMs.  The alternator underdrive pulley reduces rpm of the alternator further by another 20%.  

How underdrive pulleys give you more power:

(taken from

Weight loss is the most critical factor in HP production, each lb. of mass taken off the crank/eccentric shaft is worth approximately 2.7 HP and the figure increases with forced induction, nitrous, VTEC and increased RPM's. Only 15-20% of the gains seen from our pulleys comes from the underdriving. We achieve the maximum gain for each application/model by the singular use or combination of reduced diameter ("underdriving") and weight loss versus the OEM crank pulleys. This principle allows us to maximize the HP gain without causing any adverse effects in the general function of the vehicle.

The diameter of each of our crank pulleys is reduced specifically (from 20-30%) to the vehicles accessory needs. In other words we maintain all accessory minimums: air conditioning efficiency, power steering feel, and voltage (minimum allowable voltage 12v) even at idle with everything electrical on in the car. There are no adverse effects from the use of our pulleys whether Ultra S, Ultra R, or Ultra SS pulley sets are used. Bolt them on and forget about them, then just follow your vehicles normal maintenance schedule.