FAQ:  UnderDrive Pulleys

Overview

This discussion comes up several times a year on our Team3S owners' list.  Please search our archives for numerous past conversations on this topic.  Start at: www.Team3S.com/Search.htm.  Enter a search term like "Underdrive Pulley" (duh), "UDP", or (one spirited thread was), "Unorthodox Pulley Is Bad For Engine".  It has been hashed and rehashed a million times over, but here's my summary, followed by a number of comments from Team3S members.

  May be extremely dangerous to your engine.  The underdrive pulleys that Unorthodox Racing sells are not harmonic balancers.  Meaning, your engine may do things its not supposed to.  Such as..., spin crank bearings.

  Will spin all your accessories slower.  If they were meant to be spun slower, don't you think they woulda done it from the factory that way!?!

  Will give you on the order of 5 HP - yes..., I said 5.

  Are illegal for use in drag racing applications.

Comments

Our engines are designed with harmonic balancers.
Here are some comments I pulled from the archives,
most of which relate to the undamped Unothodox Racing UDP:

"I would not get one.  Though they are a PULLEY, they are not any longer HARMONIC DAMPENERS.  The original purpose of that heavy disc on the front of all car engines is as a harmonic dampener; Mitsu just added a serpentine belt pulley on its outside aspect as a space-saving effort, so it does 2 functions at once.  The original and more important function is still dampening crankshaft harmonics.

A crank is a long, rigid metal shape; if you tap with a hammer it "rings" (especially forged cranks; cast cranks have some internal dampening).  So what?  Well, a hammer hitting it is only ONE way to cause the crank to RESONATE and exhibit harmonic VIBRATIONS.  Another way is to RUN the engine!  The engine is a vibration factory (as we all know)...and if left undamped, crank harmonics can build up at certain (harmonic) rpms and these vibrations DON'T just disappear, they have to be DISSIPATED!  The best way to dissipate them is via the SPECIFICALLY-DESIGNED-TO-DO-THIS "Harmonic Dampener" hanging off the front of the crank.  It has an elastomeric (rubber) isolator between the hub and the HEAVY outer ring, and is as a unit, designed to damp harmonics at the critical frequencies seen by the crank.

The heavier the outer ring, the better it damps, and the higher its efficiency at damping.  NASCAR LONGtrack engines use HEAVY dampeners, to allow the bearings to live the long race length; the shorter the race, the more often the bearings get changed (after EVERY RACE), the LIGHTER a dampener can be used because lighter is a little faster to accelerate the engine due to lower rotational inertia.  You are trading bearing/crank life for speed.  Where do YOU want to go on this?  Are you lucky?

The OTHER way to damp harmonic energy (which MUST GO SOMEWHERE) is through the main bearings, into the main bearing saddles on the BLOCK.  This can cause premature wear/fatigue/flaking of main bearings, and cracks in the main bearing saddles on the block.   This is a poor tradeoff for "an underdrive pulley".

Does this ALWAYS happen?  No.  Some people may not push their cars very hard very often; some may not do longtrack racing; some may not keep their cars long enough to discover the bearing wear, or to crack anything; the underlying design may be robust enough to cover over this MISTAKE in parts selection.  There is a pretty big industry in harmonic dampeners, different TECHNIQUES for dampening (Fluidampr, ATI, BHJ, "The Rattler", etc) but they ALL DAMPEN HARMONICS because it is important.  There may be some FREQUENTLY REBUILT race engines (?CART) that are not using dampeners but IF you think their ENGINEERS have not taken crank harmonics into account in their multi-multimillion dollar engine design, THEN there are worthless bridges to be sold to you...and foolish pennywise-pound foolish _NON DAMPENING_ "PULLEYS" to be sold.

As a person who comes to the hobby from an enginebuilder's standpoint (not sales/driver/etc) this misinformation about "you don't need a dampener" or when that fails "our unorthodox pulley does the same thing" is sad.  It gets down to this.  Do ya feel lucky?  Well, do ya...?  [Clint would say "punk" but that isn't my direction, just a good line :) ]  This is an insult to your engine's health.  It MAY take it in stride; but it is an offense.  I respect my hardworking engine too much to burden it with the sometimes insane levels of harmonics that can occur without harmonic dampener protection.  Where else are you going to cut corners?  What other basic enginebuilding good practices is one going to spit on, and which will be the straw that breaks the camel's back...eventually?

This is a hotbutton issue with me.  I never thought having a harmonic dampener would be considered "conservative enginebuilding" by some...geez.

PS:  If anyone is interested in a REAL HARMONIC DAMPENER with underdrive for our cars, I am looking into having some made, because NHRA rules require an SFI-approved harmonic balancer for cars going into the 11's.  With all the IDRA etc drag groups out there using NHRA tracks, better to be legal BEFORE someone challenges it/you and disqualifies you...".

---   Jack Tertadian

"I once played with such pulley on the Camaro and some of us really saw a gain ... It was around 2-5hp.  This is within any tolerances on the dyno and of a few degrees on the ambient.  The drawback is now a much harder steering for daily driving, dimming lights during idling, and slowing down fans at idle as well.".

---   Roger Gerl

"The Buschur pulleys contain a torsional damper assembly (essentially a section of rubber connecting the two halves of the pulley).  This allows the pulley to act as a damper against the harmonic vibrations in the crankshaft caused by the cylinders firing.  It is very important to keep the harmonics damped in some fashion to prevent bearing damage.  Of course, the Unorthodox folks will say that the car's accessories provide this damping, which is completely false, as a counterweight is required to perform the damping action.  The counterweight is supplied by the rubber section allowing the pulley to twist and let the inertia of the outer pulley ring counteract the torsional flex of the crank.".

---   Matt Jannusch

Dissenting Opinion

"My car (93 RT TT) has had a Unorthodox Underdrive Pulley on it for about 2 1/2 yrs & 30,000 miles now, and my car spends A LOT of time at 6000+rpm, in facked, my car held 6700rpm in 5th gear for over 40 miles strait!  My partner (92 RT TT) also has had his pulley on for about the same time and miles with NO problems!  To add to it we have sold 100's of these with not even 1 complaint!  Every1 that I have talked to after they installed the pulley has told me about the same thing 'it does feel to have a little more kick' or something along those lines.  Every bad thing I have ever herd about these pulleys is from some1 that has never tried it, nor have I ever even herd of a bottom end failure that can be linked to 1 of these pulleys.  But like anything make up your own mined, but I can tell you the pulleys work and I know of know problems related to them.".

---  Matt Monett (Dynamic Racing)

Final Comments:

Anyway... just search the archives - you will find some positive quotes as well on the underdrive pulley, but I would stay away from them, especially for 5 HP, maybe 10...  It's just not worth it.  NOS is easier on an engine than an UDP without an harmonic balancer...  Caveat Emptor.

---   Cody Graham

For the SCIENCE of UNdampened UD pulleys, this is about the best article I've seen, by Steve Dinan.  Read it - understand it - THEN decide for yourself, and if you THEN get a UD pulley you can honestly ADMIT it IS damaging but you have accepted that level of engine unhappiness.
www.dinanbmw.com/html/danger_of_power_pulleys.htm

---   Jack Tertadian