Underdrive crank pulleys

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Ajwalk2008

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Who's running them and are they worth it? Pros and cons? Will it effect the charging capability of my alternator?


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monteholic

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they're like a CAI, maybe a very, very slight gain

but they also slow down the alternator, waterpump and AC compressor, not worth it IMO
 

hemi_loredo03

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added summit underdrive pulley myself. just one piece of the puzzle to the grand scheme of performance. Did notice an increase in throttle response. and I didn't make any changes to my alternator and have had no issues.
 
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Ajwalk2008

Ajwalk2008

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Hemi_loredo03 you have the black and silver RCSB right?


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charonblk07

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I had no charging issues with my Summit one; it still ran my 2500W sound system just fine. I can't say about a performance gain since I did my swap when I did the cam since it was already apart.


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usaf2006

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Any truth to the rumors that underdrive pulleys can grenade your motor? First time I heard about them, I did some thread research, and a lot of people said you shouldn't run them really. For as cheap the mod is, if it did any kind of performance, I think it'd be worth it. But if it's going to mess things up, then I'd rather not.
 

charonblk07

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Any truth to the rumors that underdrive pulleys can grenade your motor? First time I heard about them, I did some thread research, and a lot of people said you shouldn't run them really. For as cheap the mod is, if it did any kind of performance, I think it'd be worth it. But if it's going to mess things up, then I'd rather not.



Depends if you're talking about just an underdrive pulley or a harmonic balancer. Both the Summit and ATI pulleys are harmonic balancers and help reduce the inertia load from the changing rpm that can twist the crankshaft. They have an elastomer or fluid core that will stretch/absorb the shock from sudden acceleration/deceleration. Solid pulleys used in the past didn't do anything but change the pulley diameter and typically weighed more than the oem cast part so there was a heavier weight on the crank snout that needed to be accelerated and could result in damage to bearings or the crankshaft itself.


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justin13703

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Any truth to the rumors that underdrive pulleys can grenade your motor? First time I heard about them, I did some thread research, and a lot of people said you shouldn't run them really. For as cheap the mod is, if it did any kind of performance, I think it'd be worth it. But if it's going to mess things up, then I'd rather not.

I'm not into them just because I believe keeping your tank 1/2 full vs completely full will give you more of a performance gain lol, but I have no idea how anyone would ever link a underdrive pulley to a destroyed motor. It does nothing internally it just spins your accessories slower. As long as it doesn't cause the vehicle to overheat, I have no clue how it would destroy one.
 

usaf2006

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Char, you're speaking rocket science to me, but I'll take your word for it.

Justin, I've never seen anyone use one, so I was just going off what I read on other forums and what not. Good to know tho.
 

charonblk07

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II have no idea how anyone would ever link a underdrive pulley to a destroyed motor. It does nothing internally it just spins your accessories slower. As long as it doesn't cause the vehicle to overheat, I have no clue how it would destroy one.

The diameter of the pulley dictates how fast the accessories spin, which is correct, but the pulley itself definitely affects the engine internals since its connected to the crankshaft. Older style aftermarket solid pulleys, regardless of diameter, are a historical issue for causing either front crankshaft bearing or seal wear/failure or stress fractures/failure of the crankshaft itself; if the pulley isn't designed properly it causes an unbalanced twisting on the crankshaft which is where a lot of the information regarding failures from underdrive pulleys comes from.

The weight of the pulley on the front of the crankshaft has to be accelerated/decellerated everytime the engine RPM changes and since crankshafts are not solid (they flex and twist as they rotate and are forced to spin by the connecting rods) when you have a large weight on the end of the crankshaft that needs to accellerate/decellerate at the same rate as the crankshaft and if it doesn't you twist the crankshaft. Ideally, the weight of the crankshaft pulley (front) compared to its diameter will match the weight of the flywheel (rear) and its diameter to create an equal-Inertia effect so no one end of the crahshaft is causing more twist than the other.

Newer harmonic balancers allow for a dynamic load transfer rather than a shock load transfer so perfectly matching the weight/diameter of the pulley to the flywheel isn't as critical and with the dynamic torque transfer of the torque converters they balance themselves very well these days which is why it's a lot less of an issue with modern engines.
 

R/T_Fire

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Depends if you're talking about just an underdrive pulley or a harmonic balancer. Both the Summit and ATI pulleys are harmonic balancers and help reduce the inertia load from the changing rpm that can twist the crankshaft. They have an elastomer or fluid core that will stretch/absorb the shock from sudden acceleration/deceleration. Solid pulleys used in the past didn't do anything but change the pulley diameter and typically weighed more than the oem cast part so there was a heavier weight on the crank snout that needed to be accelerated and could result in damage to bearings or the crankshaft itself.


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Thank you!!!!! you explained it much better than I did...
 

charonblk07

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Which one is the underdrive pulley? lol

It's any pulley that has a smaller diameter than the OEM one, this spins the accessories slower. Good for naturally asprated engines that want to reduce the accessory drag caused by the serpentine belt drag on the pulley; it's the same concept as removing the clutch fan to reduce engine load.

Conversely, an overdrive pulley is one that has a larger diameter than the OEM pulley and spins the accessories faster. Good for superchagers since the larger the drive pulley and the smaller the supercharger pulley the faster the supercharger can spin = more boost.

A harmonic balancer can be either an underdrive or an overdrive pulley based on its diameter compared to the stock pulley.
 

justin13703

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^^^ Lol :roflsquared:

Charonblk07, that makes sense. I was thinking of most newer underdrive pulleys being lightweight, so the weight issue would be minimal. But yeah some aftermarket companies probably don't balance them well and I can see how this could cause issues too.
 
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