I have sen these mentioned a few different times in different forums. What exactly are these under drive pulley's and what do they do for the vehicle?

 

underdrive pullys are basically different sized pullys that go on in place of your crank pully, alternator pully, and power steering pump and i think you can get one for the A/C compressor as well... the basical princpal of them is that the different size reduces the ammount of energy needed to operate those accessories so they supposedly free up more power from the motor. do they work? i dont kno, i do not have them but might get them. i have an R/T with the 360, so if i notice a difference, i will let you know. hope that helps.

 

Well, at least in Mustang-Land, It is an OVER-sized Crank pully, and UNDER-sized Alternator and ... umm... something ... Anyway, these accessorires require power to run, power that comes from the motor .. if you Over-size your crank pully, the belt gets more "turns" with less work on the engine, and if you Under-size the alternator and other pullys, they turn less and rob less power from your engine... Yes they do work, I noticed a difference on my Mustang ... Good "bang-for-the-buck" Mod... easy to do also.

[sm=icon_cheers.gif]

 

Something to keep in mind is that if you change the RPMs of the alternator or AC or whatever, in relation to engine speed, you'll affect the performance of those accessories. I would NOT recommend an underdrive alternator pully, for example, if you have a high-powered stereo or auxillary driving and fog lamps. With the smaller pully, your alternator will be making less electricity. Same with any other pulley-driven component.

As for the over-sized crank pulley, I'm not sure your description is entirely accurate. With a larger crank pulley, you do get more turns of the belt for the same engine RPM. However, this must come at the expense of the engine working harder to make it turn. Simple physics. Think of gears on a 10-speed bike. Bigger front gear makes you go faster, but you need to pedal harder to get moving. Does the engine working harder get offset by different ratios making it easier to spin the alternator, etc? Maybe. As with any gearing, there's always a "sweet spot".

 

I thought the other devices would lose power. Thanks cbsaint. Would it still make a difference with just changing the crankshaft pulley? I have seen one kit for the truck that only comes with this pulley.

Hey cb, btw, where do you come up with your screen name from? I keep thinking something to do with cb radio's.

 

That, I can't answer. I've never used the underdrive pulleys. I was speaking about general mechanics and not personal experience.

Not CB radios. My first motorcycle was a Honda CB-750 Custom.

 

Mmmmm - I'm not 100% possitive - I'm sure there is some sort of 'sweet-spot'. I haven't looked at the underpully kit in a while, and my Mustang is in storage.... But yeah, the accessories do produce less power, but on average they produce more 'power' than you really need anyway. Also, alternator pullys are kinda hard to get off, so I never messed with that seems like little difference for the amount of work you need to get it off. But there is a difference to be had. And it's a safe mod as long as you get the kit designed for your application.

 

On my mustang with underdrive pulleys, the stock alternator wouldn't charge unless the engine was over 1000RPM or so. I corrected that problem by putting a Ford 3G alternator on. So, as far as the alternator goes most of the time they will generate the same amount of power at 1500 RPM as they would at 4000 RPM. On my underdrive pulley set, my crank my crank pulley was lots smaller then the stock crank pulley.

 

Eehhhh, what do i know, I"m just a drunken bafoon! [sm=partyparty.gif].

... so all the pullys on your kit were smaller?

 

As far as I remember, but that was several years ago. I do know the pulley on the crankshaft is actually smaller than the harmonic balancer.