I'm very interested in knowing why the pulleys are the sizes they are.

I've noticed a typical ratio of 1:1 when looking at stock waterpump & main drive pulleys. It tells me that a common speed must be in mind when designing the diameter due to water flow and electricity/generator.

So does anyone have the 411 specifically for the FC/FD cars?

 

You're limited with what you can do because of the engine design. The manufacturer has to think of everything the car will be doing from sitting in traffic at 100F to driving on the highway at -10F.

First, the water pump has to move enough coolant at low rpm's. Our car's water pumps cavitate at around 7000 rpm with stock pulleys, it basically boils the water and doesn't move it very well. It also sucks HP when it happens. Racing we go with smaller e shaft pulleys and bigger water pump pulleys. If you look, you'll see you can't fit a bigger water pump pulley without a smaller shaft pulley.

On the alternator, it needs to spin fast enough to generate the power the car requires and no more. Manufacturer thinks of a hot rainy night in traffic, the car is idling with just about every accessory on and it can't drain the battery. So you end up with an alternator that spins about 3 times faster than the engine. At high rpm the alt is hating life and is capable of making 100's of volts, the voltage regulator cuts power to the field to a trickle so it only makes 13-14v. Racing, I make big alt pulleys that cut the speed to about 75% of the engine speed. With this set up the alt doesn't even make 12v until about 4000 rpm. Since we are anmost never below that racing its fine and it also frees up some HP.

So, on the street you are probably best to stick with stock or at the most run a smaller e shaft pulley and don't mess with the rest. Racing only; spin the accessories as slow as you can.

 

My goal is to replace the stock pulleys (eshaft, alt & water pump) with a gilmer pulley/belt setup and this info you've posted goes right along with the thoughts i had in mind.

i had in mind that Mazda had to keep in mind the typical usage of the car to ensure the car doesn't overheat while idling - so there is a min rotational speed needed from the water pump to maintain respectable coolant temps. the alt is also included..

thanks for explaining, helps clear the picture.