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Discussion Starter #1
hasnt been a new post here in a while so i figured id start one. had an underdrive pulley for my civic and noticed my engine was more responsive. currently have an udp on my 6cyl altima and noticed the difference. my xk has much more electronic bs than the two other cars and id prefer to keep the same diameter. would the weight from the iron pulley to the aluminum make that much of a difference? 2 to 3 lbs compared to 15 to 20. ive seen the kits for the other hemis, but not specifically the xk. new to the pushrod world so feel free to discuss the harmonic balancer issue also.
 

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It is not so much the difference in inertia as it is the reduced crank load due to the underdrive.
A graphic example would be, Stock pully is say the same diameter as all driven pullys (it isn't, this is an example). So, for every full turn of the crankshaft, every belt driven accessory also makes one full turn.
Now, the example becomes the underdrive crank pully and this time we'll visualize it as 1/2 the diameter of the original pully.
Now, for every full turn of the crankshaft, each belt driven accessory only turns 1/2.
So, you have doubled the mechanical advantage the crank has on the driven accessories, and it becomes that much easier for the engine to overcome the drag of the belt driven accessories.
Your small displacement, low torque engines in your past cars would show a rather dramatic response to this mod.
Low R.P.M. high torque v-8s don't show a lot of improvement as they have little trouble overcoming parasitic drag like this.
And since they are low operating R.P.M. engines, usually there are more downsides along the lines of poor a/c output, low amperage output, poor power steering response, etc. at low speeds.
And of course, if you're ever off road, you're not going 50 M.P.H. (I hope)
The Vibration Damper absorbs torsional vibration ringing through the crankshaft and prevents work hardening and ultimatly cracking of the casting/forging.
If there is a underdrive pully available, this would be incorporated into it.

Believe it or not, that was a short answer,
..........Rob
 

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Discussion Starter #3
not looking to reduce the diameter, just wondering if the the same size pulley, but in a lighter material would be beneficial. alt/pwr stering/crank. the whole rotational mass idea
 

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Well then, I can't imagine a change in inertia for that diameter would be very benificial on a V-8.
Let me try to explain.
Lets say we have a 20 lb sleeve that has 1/4 inch wall thickness and is 3 ft long.
Visualize it as a pipe attached to the crank snout pointed forward inline with the crank.
Now, we start the engine and with wide open throttle, see how long it takes to reach 6000 R.P.M. It generally takes a bit more than 1 second to accelerate the crank to that speed.
OK, now we exchange our sleeve with one that only weighs say,5 lbs, but is dimensionally the same.
We perform the same crankshaft acceleration process.
So now we find it took only 1 second flat to hit 6000 r.p.m.
Not much change for a 15 lb reduction.
Similar results will be obseved with a lighter crank pully only because the diameter is comparatively speaking, not very large and the high torque production has no problem overcoming any pullys inertial moment.
Now, if our discussion involved a manual transmission and aluminum verses iron flywheels,
well, thats where the much larger diameter and weight reduction of aluminium would be very noticeable.

Am I doing any good here or am I just muddying the water more?
........Rob
 

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not looking to reduce the diameter, just wondering if the the same size pulley, but in a lighter material would be beneficial. alt/pwr stering/crank. the whole rotational mass idea
The benefit would be nearly nothing. Not even measureable. As Robby pointed out, the benefit is in having your accessories underdriven. That's why they call them underdrive pulleys.
 

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Adding a ture underdrive pulley (smaller) will affect the alternator performance right?
 

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Correct me if I'm wrong robby, but those pulleys are usually used for racing because of the high rpms that the engine turns. If the alternator turns too fast too many times, it could ruin it. At least that's what I've always been taught.
 

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Discussion Starter #10
just playing devils advocate. With the lighter materials, in theory the engine would be more efficient possibly getting better mileage, as well as being more responsive. that 6000 rpm over 5 gears and tenth/s of a second each gear would add up? if we keep discussing this someone will buy a set and let us know. Would the pulleys synergize additional bolt ons also cai, chip, exhaust?
 

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Correct me if I'm wrong robby, but those pulleys are usually used for racing because of the high rpms that the engine turns. If the alternator turns too fast too many times, it could ruin it. At least that's what I've always been taught.
Underdrive pulleys slow down the rotation of your alternator, not speed it up. An underdrive pulley set usually consists of a smaller crank pulley (driving) and larger driven pulleys for the alternator and at least one other accessory. If anything, the alternator will not turn as fast as it did with the original pulley.
 

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just playing devils advocate. With the lighter materials, in theory the engine would be more efficient possibly getting better mileage, as well as being more responsive. that 6000 rpm over 5 gears and tenth/s of a second each gear would add up? if we keep discussing this someone will buy a set and let us know. Would the pulleys synergize additional bolt ons also cai, chip, exhaust?
The weight difference would be insignificant. Also keep in mind that once the engine is running, it never has to "use" HP to move the crankshaft pulley from rest because it is always already turning. If you really want to "free up" a few HP with rotational mass, go buy yourself a set of lightweight wheels/tires. Then you would have the true advantage that goes along reducing rotational mass and gaining a bit of performance.
 

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Discussion Starter #15
on that note, what material is our drive shaft made out of? QD2. Also what price would a lightweight pulley have to be to persuade you to buy it?
 

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on that note, what material is our drive shaft made out of? QD2. Also what price would a lightweight pulley have to be to persuade you to buy it?
A really popular mod for the new Mustangs ('05 and up) is to swap out the OEM driveshaft with a lightweight aluminum unit.

As for lightweight pulleys...I probably wouldn't be interested even if they were free, but that's just me. Besides, I think there is some risk to swapping the crankshaft pulley if the dampner isn't up to OEM specs.
 

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Underdrive pulleys slow down the rotation of your alternator, not speed it up. An underdrive pulley set usually consists of a smaller crank pulley (driving) and larger driven pulleys for the alternator and at least one other accessory. If anything, the alternator will not turn as fast as it did with the original pulley.
That's exactly what I was saying. Maybe I just said it wrong. Maybe I was speaking Cajun!!!
 

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Okay, I thought your concern was that the alternator would be turning too fast. But that's obvisouly never the case with underdrive pulleys installed. Actually, some guys end up with charging systems problems when they use underdrive pulleys because the alternator is making fewer RPMs. Sometimes the battery doesn't get enough recharging...
 

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Discussion Starter #20
thats my main concern from the first post. ive noticed a difference on my two other cars. my concern is having more electronic stuff in the xk ie, dvd, nav xtra ac. not sure how much it would affect the voltage regulator and cause problems. thats why i was wondering abount a normal size pulley/s, just lighter.
 
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