Underdrive Pulleys
07-29-2001, 09:27 AM
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Underdrive Pulleys
Hey Random I read on a post earlier that you do not recommend these but did not say why. I have one on my car & I don't notice any ill-effects. Could there be something I'm missing. sad.gif
07-29-2001, 10:35 AM
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Vehicle: 2008 Toyota Prius 2006 Suzuki SV650S
There is on on going debate in automotive tuning circles as to weather or not Underdrive pullies on I4 or I6 engines do any harm.
There is a hell of a lot of techno babble involved in it, but I'll try to boil it down.
Your Crankshaft has 4 distict power "hits" to it, as each of the 4 cylinders fire off. These create momentary stress in the crank and accelerate the crank, then it "coasts on momentum" untill the next power pulse hits the crank.
As you can imagine...this causes the crank to speed up and slow down, plus it causes vibrations within the crank.
the stock pully on I4 engines is a sort of "dampner". It cancels out these vibrations, and becuase there is a dampner built into it, it keeps your accesories spinning at a fairly constant, vibration free speed.
Now let's install an underdrive pulley. It is a solid piece of metal, and does not have a dampner built into it. This drives your accessories at a slightly slower speed, plus the lighter weight of the pully alows the crank to accelerate faster, and with less drag on the pulley side, loose less velocity in the "coast" section while waiting for the next cylinder to fire. However, it does not dampen out the vibration.
As you know, a "stick" vibrates more at the ends than in the middle. Well, we just removed a dampner from one end. This is going to allow even more vibration....
Then end result CAN be claped out crank bearings...mainly..the bearing closes to the pulley.
I say CAN, as this theory has been proven to some, but not to others.
The main debate is that they DO cause damage, this is usually posted by engine builders and race teams who refuse to use them.
Then the manufacturers who make them claim that they cause NO damage...
Some people with underdrive pullies have had problems in as few as 30,000 miles, some have driven for 100,000 with no problems...
So the debate rages on.
The question you have to answer for yourself is....is it worth it for you to risk your engine for what ammounts to maybe 5 wheel Horsepower? I personally will not risk my engine, but...
This is a personal question that you must answer for yourself.
As a closing note...underdrive pullies that replace the accessory pullies, and leave the stock pully alone, do not cause damage, and are safe (as safe as any engine mod is!). As of right now, no one makes underdrive pullies for our accessories.
In korea, they have discovered electric water pumps. They replace the stock belt driven pump in favor of an electric pump. This free's up HP, and does not damage the system. The install is a Pain in the rear, and the gains are more like 3 HP, so I do not think they are worth it unless you are going to go all out.
There is a hell of a lot of techno babble involved in it, but I'll try to boil it down.
Your Crankshaft has 4 distict power "hits" to it, as each of the 4 cylinders fire off. These create momentary stress in the crank and accelerate the crank, then it "coasts on momentum" untill the next power pulse hits the crank.
As you can imagine...this causes the crank to speed up and slow down, plus it causes vibrations within the crank.
the stock pully on I4 engines is a sort of "dampner". It cancels out these vibrations, and becuase there is a dampner built into it, it keeps your accesories spinning at a fairly constant, vibration free speed.
Now let's install an underdrive pulley. It is a solid piece of metal, and does not have a dampner built into it. This drives your accessories at a slightly slower speed, plus the lighter weight of the pully alows the crank to accelerate faster, and with less drag on the pulley side, loose less velocity in the "coast" section while waiting for the next cylinder to fire. However, it does not dampen out the vibration.
As you know, a "stick" vibrates more at the ends than in the middle. Well, we just removed a dampner from one end. This is going to allow even more vibration....
Then end result CAN be claped out crank bearings...mainly..the bearing closes to the pulley.
I say CAN, as this theory has been proven to some, but not to others.
The main debate is that they DO cause damage, this is usually posted by engine builders and race teams who refuse to use them.
Then the manufacturers who make them claim that they cause NO damage...
Some people with underdrive pullies have had problems in as few as 30,000 miles, some have driven for 100,000 with no problems...
So the debate rages on.
The question you have to answer for yourself is....is it worth it for you to risk your engine for what ammounts to maybe 5 wheel Horsepower? I personally will not risk my engine, but...
This is a personal question that you must answer for yourself.
As a closing note...underdrive pullies that replace the accessory pullies, and leave the stock pully alone, do not cause damage, and are safe (as safe as any engine mod is!). As of right now, no one makes underdrive pullies for our accessories.
In korea, they have discovered electric water pumps. They replace the stock belt driven pump in favor of an electric pump. This free's up HP, and does not damage the system. The install is a Pain in the rear, and the gains are more like 3 HP, so I do not think they are worth it unless you are going to go all out.
07-29-2001, 10:35 AM
#3
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I think it was said that these pulley's are not recomended if you have alot of other things sucking up power, like fog lights, neon, stereo system, etc.... These pully's are lightweight and if you have alot of things taking up power, it going to be hard for your electrical system to supply enough power with these pulley's installed. eek.gif
*Edit...Damn...Random beat me to it.
[ July 29, 2001: Message edited by: 2000Lo&ShowTibby ]
*Edit...Damn...Random beat me to it.
[ July 29, 2001: Message edited by: 2000Lo&ShowTibby ]
07-29-2001, 11:43 AM
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Wow, I've never heard of the bearing argument/theory, I only knew of the electrical one. Hmm rolleyes.gif I may be considering the removal of my pulley, after all this is my daily driver/money maker.
Thanks guys smile.gif
Thanks guys smile.gif
