The Under Drive Pulley debate

First off a few disclaimers:  This page deals ONLY with the 3000GT/Stealth cars and Unorthodox Racing’s under drive pulleys.  I welcome differing opinions and will post anyone comments if they request.  My only motivation for posting this is to give a heads-up to all people considering doing this mod.  It seems that every month there is another person asking about the UDP’s on the message board or starnet list and they need to have a clear view of the facts.  My opinion is also included in this page…. take it for what it is worth.  I don’t want to tell anyone what to do with their car, just present the facts so they can decide for themselves.

Definitions used in this discussion:

  • Every time a cylinder fires, the connecting rod hammers the crankshaft, making it twist as well as spin. After every twist, there’s a rebound as the crank unwinds. This twist/rebound cycle is known as torsional vibration and if not controlled, it will result in rapid bearing and crank journal wear and can lead to a broken crankshaft. ( Source: Fluidampr’s tech pages. )

    Harmonic balancer: The harmonic balancer, or vibration damper, is a device connected to the crankshaft to lessen the torsional vibration. When the cylinders fire, power gets transmitted through the crankshaft. The front of the crankshaft takes the brunt of this power, so it often moves before the rear of the crankshaft. This causes a twisting motion. Then, when the power is removed from the front, the halfway twisted shaft unwinds and snaps back in the opposite direction. Although this unwinding process is quite small, it causes “torsional vibration.” To prevent this vibration, a harmonic balancer is attached to the front part of the crankshaft that’s causing all the trouble. The balancer is made of two pieces connected by rubber plugs, spring loaded friction discs, or both.
    When the power from the cylinder hits the front of the crankshaft, it tries to twist the heavy part of the damper, but ends up twisting the rubber or discs connecting the two parts of the damper. The front of the crank can’t speed up as much with the damper attached; the force is used to twist the rubber and speed up the damper wheel. This keeps the crankshaft operation calm.
    ( Source:  AutoInsight training program )

    A few facts:


    The 3000GT/Stealth cars DO have a harmonic balancer.  Here is a diagram from the factory service manual.  The cross hatched areas are the rubber rings which absorb the torsional vibration.

  • dampener
    • Torsional vibration will destroy a crankshaft and/or bearings if left un-dampened.

    What the other side argues:

    • The 3000GT/Stealth engine is so well balanced that it does not need a harmonic balancer. My reply:  The balance of the engine has nothing to do with torsional vibration.  In a perfectly balanced engine ( which is impossible to achieve) there would still be torsional vibration and a need for a harmonic balancer.

    • This next quote is right from Unorthodox Racing’s FAQ page.  I have included by thoughts in red.

      Start quote:

      3) “Isn’t my stock crank/eccentric pulley a balancer, harmonic/torsional, or vibrational damper?”The factory pulleys on today’s late model cars (from 1986 to Present and sometimes even earlier depending on the vehicle) serve two functions. First and most importantly they are designed to reduce or eliminate the audible noises herd in the cabin of the car that the accessories (alternator, air conditioning, power steering, and air pump) make when the engine is running. This fact exhibits the factory fanaticism about making the car quiet for the occupants (i.e. the use of resonators in the intake to quiet intake noise, all aftermarket intakes eliminate these resonators). It has nothing to do with engine function or longevity.
      I agree that auto manufacturers have obsessed about cabin noise but the dampener will not stop noise from accessories…. if anything the drive belts will absorb most of the vibration before it gets to the crank pulley.  However you will hear noise from torsional vibration at certain RPM’s without a harmonic balancer, so I am betting that’s why they include them 🙂

      The second function of late model crank pulleys is torsional damping. Torsional damping is necessary due to the excessive diameter and weight of the factory crank pulleys.What?!?!?!  maybe the “engineers” at UR need to get a clue.  See the definition at the top of the page for the real cause of torsional vibration. The design of our underdrive crank pulleys eliminates the need for any torsional damping for two reasons. One, the diameter of our crank pulleys is smaller than the original designs, in almost all cases smaller even than the rubber torsional ring, therefore effectively reducing the force the accessories have on the crankshaft.So now they are saying that torsional vibration is caused by the accessories exerting force on the crankshaft??? Second and most importantly is that our pulleys are significantly lighter than their OEM counterparts (anywhere from 3 to 11 lbs.). This weight loss dramatically reduces the stress exerted upon the rotating assembly by the excessively heavy factory crank pulley.Increased mass will IMPROVE a pulleys ability to dampen torsional vibration so they are making the situation worse.

      Our pulleys are so well balanced that when owners call us about how happy they are with the product they always mention their motor feeling smoother.  Lastly is the misconception that the crank pulleys on these vehicles are harmonic dampers. A harmonic damper is a unit bolted to the crankshaft snout that is completely separate from the belt drive system. An engine that uses a harmonic damper has the accessory drive crank pulley bolted to it, they are separate pieces that are attached to each other. Our cars have two rubber inertia rings (see above diagram) the inner ring is NOT attached to the pulley thus their argument falls through. Balance shafts, which are used by several manufacturers, are specifically designed to eliminate harmonic vibrations. None of the vehicles we manufacture pulleys for have harmonic dampers in the traditional sense. Ours do !!

      Owners who have engines that use balance shafts must understand that if they eliminate their balance shafts their engines must be balanced to 0 grams if they expect to have no long-term engine problems with or without the use of our crank pulleys. Our pulleys are made with 6061-T6 aluminum billet which is a very consistent material and the CNC machining process ensures that all our pulleys are perfectly true. This balance shaft elimination is rare and only happens on a few models that are modified for racing only (Eclipse/Talon/Laser/Galant VR4/Conquest TSI/Starion just to name a few). No comments on balance shafts as I am not familiar with the theory and they do not apply to our cars.

      End quote:

    • Any performance mod you do to your car will reduce the engine life:Let’s look at the most common modifications people do to our cars.

      – Boost controller:  Mitsubishi and all auto manufactures build cars with a safety factor.  They determine the best boost level for all driving conditions and include a safety factor for reliability sake.  Also government regulations come into play also. (why do EU cars have 13g’s?)
      Now we go and raise the boost level and take the risk into our own hands.  I for one recommend to people that they get a datalogger (if they have a non OBD-II car)  or stick with 14 psi if the can not determine knock.  If we monitor our boost levels and assure that we are not getting knock then there should be no problem with raising boost and engine life.

      – Air filter – Providing better filtering can only increase engine life.  Increased airflow will increase HP slightly but will not reduce engine life.

      –  Exhaust – like the air filter, exhaust improves the efficiency of the engine and allows it to not work as hard to make power.  This increase in efficiency extends engine life and increases gas mileage in some cases.

      – Larger turbos – Now we are getting into an area where engine life is starting to be reduced.  I would suggest that Mitsubishi did not design their engines to withstand 15G’s at 16psi for 200,000 miles.  With higher horsepower levels provided by larger turbos, you will start seeing premature bearing and piston ring failures.  It is at this stage that I would be more concerned about having a harmonic dampener on my car since the limits are being pushed and I would want the most protection possible.

      Most people asking about UDP’s have been NA cars which have little options for adding power as of now.  It is these people who need to know the facts about UDP’s.

    To sum it up:

    If you want to risk bearing failure or worse yet crank failure run a UDP on your car.  I am not saying it will happen next week, or even next year but it will happen.  In my opinion the extra 5-10 HP is not worth it. (note these numbers are guess’s since no conclusive dyno results have been posted)  Just the idea of removing something which is designed to provide protection to your engine is fundamentally wrong.  Engine builders across the country will agree with me on that one.  With other mods we do, it is a calculated risk and with proper care we can measure and control the power we add.  With UDP’s we have no way of measuring what is going on and that scares me.  Take my opinion for what it is worth and good luck.

    P.S.  After researching a little more I found a letter from Dave Buschur telling of his problems with UDP’s on DSM cars. 
    I should also note that Buschur Racing does offer a UDP that retains the harmonic balancer for the 3S cars as well as DSM’s. (~$350 with belts)