Underdrive Pulleys

Use this information and these links at your own risk.

An underdrive pulley replace the main pulley off the crankshaft, spinning the accessories slower and freeing HP for the engine.

Recent installations of the UDPs reportedly give impressive results with no significant side effects, possibly 5-8hp.

• UDP picture

Installation/ Review

Another view: 16Jun98 - I just finished installing my UDP earlier this evening (err... yesterday). Took about 3.5 hours total, including a 30 minute trip down to the local Pep Boys for a talk with a guy about pulling pulleys, lightened flywheels, superchargers, beating his Z28 off the line, etc.. I didn't really understand what people meant by shaving off the 5/8ths of an inch, but it became very apparent once I actually had the pulley off. Nothing that a grinding wheel and a dremel couldn't take care of. Unlike some people's install, my stock crankcase pulley came off very easily. Just took a bit of wiggling and it popped right out. The job could definitely have been done an hour faster had I had everything ready to begin with and didn't spend too much Time jawing with my buddies during the install.

After the install, the butt dyno reported that the car actually felt slower with the UDP. I didn't notice any increase in steering effort, lights didn't dim, the AC worked just fine, and generally nothing felt different. Of course, I'm not much for trusting the ass' ability to discern 5 extra horses, so hopefully I'll be able to squeeze in a dyno run today and have something to report. Somehow I have my doubts about the pulley. The accessories that the pulley is supposed to power spun rather freely with almost no effort to begin with. The only other thing I can think of is that since the stock pulley is so much heavier, the Unorthodox unit acts like a small-scale lightened flywheel. Since it weighs less, it will spin up faster simply due to less mass to overcome. In any case, the dyno doesn't lie.

11Jun98 - When I had Shawn from Unorthodox Racing do the UDP install on my 93 PGT, he used the following belts:

Gates KO40355
Gates KO40385

They're different from the ones listed on BAPOC. Shawn did mention that the belts would stretch a bit over time. -VL

29May98- Well, I finally installed the UDP today...and first the good news. I believe it was Dan who said it best (Woooooooooohoooooooo!!) Yes, you really can tell a difference. It is amazing. I wasn't expecting much. Was I surprised! In all gears, especially above 3000 RPM its like a different car.

Now for the not so great news....it took me 6 freakin hours!! First of all, thanks to everyone I asked advice of. The install was pretty smooth except for one part. Removing the stock pulley from the crank. I was under the impression it would come off with a little effort. Nope...wouldn't budge. Tried prying it..nothing (except broke two chunks off the OUTER (wrong) flange (no big deal). Had to run around town today to find a pulley/gear puller. Came right off with that. Imagine that!! Also I wasn't able to loosen the bolt with the tranny in 4th/5th and the brakes on. I had to use a trick someone mentioned here.....put on the socket with the breaker bar and lower the car so the breaker bar is on the ground, pointing FORWARD (very critical). Actually, first remove the fuel pump relay and let the engine die, then do the previous step. Next, give the starter a few quick bursts and presto, the bolt is off. Removing the mat'l was easy but laborious. I found a heavy duty cut-off wheel worked best. I was able to make a pretty clean cut and finish it with a sanding disc. I lubed the crank with anti-seize and installed the new pulley. Curiously, I was able to torque it to 120 ft/lb with the brakes on and tranny in 4th. The first belts I picked worked too (amazing for the way things went....). I used a 380K5 (5 rib) for the crank/a/c/alt and a 350K4 (4 rib) for the water pump/ps/crank. These correlate to 380K5 (Dayco 5050380, Goodyear 4050380) and 350K4 (dayco 5040350, Gates K040350 and Goodyear 4040350).

You can either rent the tool or buy one for about $25. It makes life alot easier. If you use the kind that bolt into the holes on the pulley, you'll also need to buy two 3/8 x 24 x 3 in bolts. The other kind has two claws that attach to a lip on the pulley. I rented the tool for free ($18 refundable deposit) from Pep Boys.

Impressions (with stock stereo, pretty much)

Idle is fine, temp is fine, voltage is a little low at idle but is fine at 1000 rpm. The lights dim a little at night, but so what. I didn't notice any change in the steering like some have. A/C was fine on all settings, and the voltage didn't drop any lower with EVERYTHING on high. Also, no post install problems with CPS. I couldn't remove it because it was blocked by the dipstick tube which I didn't want to take out. I unbolted it and covered the hole with tape. For those in doubt, go for it. Just remember to rent a puller!! -Kevin (BaPOC member)

Thanks to PALUCE3@aol.com (Jan98):

Underdrive pulleys: Unorthodox Racing- for the Probe (MX-6, 626) 2.5V6. $169+shipping.

The install took 1.5 hours and it cost me 95 dollars canadian. ($ 60 US) plus two new belts that were $20 each ($13 US). The mechanic that did it was excellent.

Here are the install instructions for a stick, auto is slightly different. Two people are needed. (The instructions that unorthodox will send you has them)

1. remove right front wheel
2. remove mud cover
3. loosen belts
4. remove pulley

So far this is all very simple.

5. Now you MUST shave off 5/8ths of an inch from the oil pump housing because the stock pulley has a recess on the back side and the UDP does not, The inner flange of the oil pump housing fits into this recess, that is the reason for it having to be shaved off. NOTE. it is a good idea to remove crank sensor before #5. If it is damaged you will will have HUGE problems. Removing the sensor takes 2 minutes and eliminates all worries. (The sensor is mounted very close to the inner flange)
6. Bolt on the UDP. 116 - 122 pounds of torque (he put 120 on mine)
7. Make sure you have the right belts!!!! I didn't... The pulley review I got was WAY off. The belt sizes you need are 0504 350 and 0505 375

To help understand these numbers the 4th number means the number of grooves in the belt (The review I got said they were both 4 groove, That's obviously wrong) And the last 3 numbers are the length.

8. Put on your belts
9. Tighten them up
10. Put the mud guard back on.
11. Put your wheel back on.

Now go take a drive. Like I said I didn't feel 1st gear yet. There's 1 foot of snow here in Windsor Ontario Canada. If I felt a difference in 2nd and 3rd gear, imagine what 1st is like!!! I felt a difference all though the power band, and especially at 5000.

The drawbacks ---

My lights dim a little bit more, (Hella's)(thay already did, even with an optima battery) and the power steering is a bit harder too. As for lights you won't have this problem unless you hit about 1500 watts (max) I'm currently at (2500 watts max) that's 14 speakers!!! 2-12's 2-8's 2-4's 2-51/4's 2-5x7's and 4 tweets. (All highs Kenwood, all subs Orion) 3 amps, Kenwood 846, Kenwood 625, and Precision Power 275 running at 1 ohm MONO!!!! (PPI all the way.) Kenwood 6007 Cd deck.

Also, at idle the volts coming out of the Battery are only at 13V As soon as the car hits 1000 RPM it's back at 14.4V.

One last note: as of Jan 1st, 1998 the UDP's are $169.00 US. (UP $10.00)

Another Source

In the US:

23Dec97 5/8 inches of aluminum from the oil pump housing needs to be cut away. Total installation cost $45. New belts needed: 345K4, 350K4. Be careful not to damage the crankshaft position sensor which is near the oil pump housing.