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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:
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Torsional vibration: 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.
-
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. Read the
letter here.
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)
-Kevin
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