Underdrive pulley

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Yeret

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I'm curious if anyone has ever swapped an underdrive crankshaft or even a crank + alternator pulley(ies) onto a Magnum engine and if so, what kind of results you had. Looking around, they're a pretty cheap modification, but I'm wondering if they offer any real enhancements. Or maybe underdriving the accessories could be detrimental to overall reliability/drive-ability?
 

marine6212

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https://en.m.wikipedia.org/wiki/Underdrive_pulleys

Poorly engineered underdrive pulleys can cause unwanted side effects; this is due to not spinning the alternator, power steering, and/or air conditioning fast enough. This leads to low alternator voltage, weak/no power steering assist, and weak/no air conditioning effectiveness, especially at idle/low RPM. The most commonly seen result is lighting may dim, or the stereo may cut out. Too much underdrive for a race car is not much of a concern due to the high RPMs they run at, but for daily driven vehicles it can lead to a dead battery if too much time is spend at idle or low RPM.



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monteholic

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not to mention it slows the waterpump and can cause cooling issues too!

the benefits are so small that the side effects out weight the gains, and you will never notice the gains
 

dudeman2009

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EDIT: I got underdrive and overdrive pulleys backwards. I see no reason to put an underdrive pulley on our trucks, they just don't rev high enough to blow things out due to over speeding them.

The crank pulley should not be reduced in diameter, only increased if increased idle rpm is desired on all components.

The reason you would want to increase idle rpm on components like the alternator or waterpump, etc. is to get higher performance from individual components at low speeds or idle.

For example, plow trucks can benefit from an overdrive pulley on the alternator as a large demand is placed on the electrical system if an electric hydraulic pump is used. Or in cars with large stereo systems, lots of incandescent perimeter lighting, inverters, or and DC load really. When a majority of use occurs at idle RPM.

Available current from an alternator is directly related to its rotational speed, the faster it turns the more current it can output, to an extent.

In hot climates it can be desireable to overdrive the water pump pulley in order to increase idle coolant flow and fan speed. Power steering is usually not overdriven unless there is another device on the system, such as a power steering drivin plow.

However care must be taken to not choose too small a pulley and over speed the components when the engine is redlined. Even if you never redline it. Because someone else may, or you might in an emergency. Generally pulleys are not changed in size by more than 10-15% max, at least that i've seen, except on specialty applications.
 
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dapepper9

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These engines have shown that underdrive crank pulley is fine but doing accessories like alternator and power steering are a bad idea. Alt isn't able to properly charge the truck and you lose steering ability from the pump
 

dudeman2009

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These engines have shown that underdrive crank pulley is fine but doing accessories like alternator and power steering are a bad idea. Alt isn't able to properly charge the truck and you lose steering ability from the pump

I've thought that an underdrive pulley was smaller in diameter, resulting in more rotations of that pulley per single revolution of the belt.

Well looked it up and I was wrong. An underdrive pulley is supposed to reduce rotational speed. I got it backwards then. I'll update my post accordingly.

Thanks for pointing that out dapepper.
 

dapepper9

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They reduce the power required to spin the pulley system. Made of lighter materials. Larger diameter of alternator pulley for example creates more tq to spin the accessory but also reduces rotational speed. Result is more power freed and less charging ability and low rpm.
 

RAMTRPR

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So is the Moes Performance underdrive pulley worth it for a daily driver or no?


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dapepper9

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I think so but many don't. Running just the crank pulley doesn't do a whole lot of good or bad.
 

RAMTRPR

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I have headers and full exhaust, CAI, trinity with hemi fever tune as well as upgraded coils and soon 87mm TB. :) hopefully it would be worth it with all that.


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Dave B

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The ATI superdamper that morning’s sells has a 15% under drive pulley. If you have a lot of aftermarket mods, then it’s like an insurance policy on your engine as it dampens vibration and balances your engine. As far as the 15% underdrive, it you have a tuner and you set your idle up at even just 100 Rpm’s more, that should cover the 15% underdrive by the pulley. I have heard will Free up 10-12 ra hp, but from what I have seen, it will free up about 4 hp to the crank. I haven’t put one on yet but mine is on order. So I can only tell you what I looked into, not from my own experience.
 
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