Pully Size?

[Slant]

I reecently purchased the AEM underdrive Pully for my car, and I noticed that it is actually larger than the stock one. I haven't installed it yet, but I understand that it has less rotational inertia b/c it's lighter and has holes(you know what I mean)
What I don't understand is, "why is it bigger?" If I remember highschool physic correctly, rotational inertia is directly related to the mass and the radius. I= M*R^2, something like that, right? So having a greater radius results in higher rotational inertia, which is the opposite of it's purpose?
I could see that if it has a larger circumference, it doesn't have to turn as much to get the same displacement (or whatever, I can't think of the correct term) but at the same time, it takes more torque to turn the damn thing if it has greater I.
torque = rotational inertia * angular acc. (is that right?)
Can someone clarify this? or am I totally making this stuff up in my head again?


edit: Or I could see that mass of the AEM is so much less than mass of the stock one, but is it enough to outweigh the increased radius? I mean the radius grows exponentially.

<FONT COLOR="#0000ff" SIZE="1" FACE="Verdana, Arial">This message has been edited by Slant on November 07, 2001 at 10:55 AM</font>

[autxr]

The gains from doing pulley conversion are not so much from changing the weight, they are from changing the parasitc losses created by what the pulley turns.

In this case, teh alternator is now spinning slower, so there is less frictional loss through the spinning alternator.

You spin accessories slower with bigger accessory pullies, or get a smaller crank pulley to make everything else spin slower (since the crank pulley is the drive pulley).

Scott

[vvtlikick]

Rotational inertia grows linearly compared to the mass (2

[Slant]

Holy **** that was quick. Do you guys just wait for someone to ask a question? http://www.newcelica.org/ubb/smile.gif j/k

Rotational inertia grows linearly compared to the mass (2?the mass equals 2?the rotational inertia). Rotational inertia also grows quadratically compared to the radius (2?the radius equals 4?the rotational inertia). You misspoke when you said exponentially.

What I mean by exponentially... I=mR^2, so the radius is squared, and the other thingy is...you're right, I misspoke, but you know what I meant, right?<IMG SRC="http://www.newcelica.org/ubb/smilies/cwm10.gif" border=0>


Ok, I understand what you guys are saying. So the alternater spins it at a slower speed now? For some reason, I thought it's at a constant speed, and it just takes less force to turn it the same speed --> enter my "rotational inertia horse **** theory"

Thanks again, you two freaks who know way too much about cars than I do.

[ALANlll]

Just to add....The fact that the pulley has a larger diameter means it is easier to turn. Kinda like when your turning a wrench. The bigger the wrench, the easier it is to turn the bolt. In turn, this makes it easier for the crankshaft to turn the pulley, thus freeing up power. (All be it a very little amount) In my opinion, if your gonna buy aftermarket pulleys, buy them for looks, not for results on a dyno. You'll be very dissappointed. Hope this helps in addition to what everyone else said.

[tool462]

Originally posted by Slant:
Holy **** that was quick. Do you guys just wait for someone to ask a question? http://www.newcelica.org/ubb/smile.gif j/k

What I mean by exponentially... I=mR^2, so the radius is squared, and the other thingy is...you're right, I misspoke, but you know what I meant, right?<IMG SRC="http://www.newcelica.org/ubb/smilies/cwm10.gif" border=0>


Ok, I understand what you guys are saying. So the alternater spins it at a slower speed now? For some reason, I thought it's at a constant speed, and it just takes less force to turn it the same speed --> enter my "rotational inertia horse **** theory"

Thanks again, you two freaks who know way too much about cars than I do.

The belt moves at a constant speed, however v = w*r where v is the velocity, w is the angular velocity and r is the radius, so since the new pulley has a larger radius, w gets smaller. And this in turn reduces the amount of work that the alternator is able to do. The work it can produce is roughly the kinetic energy of the wheel which is 0.5*I*w^2, so the kinetic energy reduces quadratically with w, which is where the savings comes in. The alternator saps most of its energy through generating electricity, as opposed to frictional losses. By reducing the radius, you reduce how much work is used by the alternator, so more is available to turn your wheels.

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2000 Liquid Silver GT-S
Mods:
AEM CAI // TRD Exhaust // RMM Front Strut Bar // TRD Rear Strut Brace // B&M Short Shifter

Coming Soon:
APC Clear Corners

[RexPokinghorn]

Apart from the physics of it, can you *feel* any difference with AEM in?