# question ## Recommended Posts this may seem like a strange question, but would anyone know how fast the alternator pully spins compared to the crank pulley?, Is engine rpm equal to crank pully rpm? I thnk it is but am not sure.......

##### Share on other sites No, the alternator turns faster. Related by ratio of pulley size. Wc/Wa=Ra/Rc where Wc is the crank speed in RPM, Wa is the alternator speed in RPM, Ra is the radius of the alternator pulley, and Rc is the radius of the crank pulley. <BR>So if you crank pulley is 4 times as big as your alternator pulley, the alternator will spin at 4 times the RPM.

##### Share on other sites The alternator spins much faster. It is a straight ratio. Divide the alternator pully diameter into the crank pully diameter and multiply engine RPM by this fiqure. This gives you alternator RPM.

##### Share on other sites ....yes...the crank pulley is the same as the engine RPM. The formulas above are excellent. This is a "means and extremes" ratio therefor you can take the larger pulley RPM times it's dia. and divide it by the smaller pulleys dia.... and this will be the smaller pulleys RPM.<BR>example:<P>3" pulley @ 2300 RPM<BR>2" pulley @ ???? RPM<P> 3 : 2300 = 2 : ????<BR> ___ ____ ___ ____<P> dia (lg)RPM dia (sm)RPM<P>invert the dia. proportions graphically<BR>(on pulley calculations only)<P> 2 : 2300 = 3 : ????<BR> ___ ____ ___ ____<P> dia (lg)RPM dia (sm)RPM<P>now multiply the "means" (the 2 inside numbers) and divide that number by the extremes (the left side isolated number)<BR>...odd how the pulley ratios are inverted as when other calculations such as fuel to miles predictions and cubic inches of material to lbs estimates do not require inverse proportion.