Has anyone noticed a difference in oem pulley diameter between cars?
 

I first noticed a difference in pulley size when I looked at my friends alternator pulley versus mine. He has a 5 speed and I have an auto. Theres about a quarter inch difference in diameter. I bought his air pump off of him and there was also a noticeable difference in those pulleys as well. About a half inch in diameter difference. My car is a 93 and his is a 94.
I just recently received some greddy pulleys from fastrx7man(great seller)
Guess what they are also much bigger in diameter. Im really confused here.
Can anyone shed some light on this unusuall discovery. My pulleys are oem and so are his.
The same goes for the water pump pulleys as well.This is not some half cock eyes assumption either.I measure the diameters with a ruler.So im not just imagining things

Yes, there were different OEM pulley diameters and different belts specified for the auto and manual cars. I thought it was just the air pump pulley that was different, but its clear that the alt pulleys are different, too, from your report.

The GReddy pulleys are different sizes because they are underdrive pulleys. They are supposed to be different.

-Max

I thought the underdrives were supposed to be smaller in diameter and also lighter. Its apparent that the auto oem's are much smaller. Dont foget that the water pump pulley is also smaller.Im willing to bet that the crank pulley is also smaller.

Underdrive pulleys are bigger for the accessories (the "driven" components) and smaller for the drive pulley.

How to make an alternator spin slower: make the alternator pulley larger, or make the drive pulley smaller. Or both.

I know that various companies say that weight is a big part of the benefit of their underdrive pulleys (and it may indeed be true for many or most application), but that is clearly not the case on the FD. The main pulley is light and has a relatively small diameter (especially the center hub where all the mass is, and anyway the whole thing is MUCH smaller than a flywheel), so weight isn't going to do much. The stock PS pulley is lighter than any aftermarket pulley -- weight isn't a benefit of aftermarket pulleys here. The alternator pulley is small, though it spins fast so weight might help a little here. But holy crap, the other stuff attached to the alternator shaft has WAY more inertia than the pulley itself. The major effect on the alternator is from reduced speed of the alternator's internals, not reduced inertia of the pulley itself. I am guessing that the inertia of the pulley is also not the main resistance when accelerating the water pump from 2000 to 8000, but rather it is pumping the water. Verdict: the mass of the pulleys doesn't have much to do with the performance increases you get from installing aftermarket pulleys on an FD.

-Max

So then whats the benefit of having larger pulleys? wouldnt you want the waterpump to pump as much as possible? and as fast as possible?
I also dont see how underdriving the alternator makes power? less electrical load? Im a bit confused. If the two pulleys are bigger they would have to spin a lot less in order to power the accesories at the same constant.That part I get.
But isnt a bigger pulley harder to turn than a smaller one initially? I would think that this would place more stress on the main crank pulley. Pardon my physics if im wrong.lol

Preventing cativation at high RPM is the reason for slowing the water pump down i belive.

whats cavitation?

The pump spinning so fast its no longer working properly, IIRC.

Black97Vr4 gave you the short answer, here's a longer (winded) one...

Cavitation is what occurs when a pump (water, hydraulic, etc), or propeller like on a boat, creates too much suction resulting in air coming out of solution. In affect the water boils at a much lower than normal temperature because it’s under vacuum. This is just the opposite effect of raising the boiling point by using by a radiator cap to increase the pressure on our vehicle cooling systems.

Cavitation is a bad thing. When it occurs there is a loss of flow and excessive turbulence. If the situation is bad enough and/or lasts long enough, then overheating and even mechanical damage can and will happen.

In an RX7 you can probably see the coolant temperature rise as the water flow diminishes. The sensitivity varies by model year because of the different combinations of pulley sizes, etc. Under severe conditions water flow might almost completely stop resulting in rapid overheating.

Mazda, like all manufacturers, had to try and keep enough water flow at low engine RPM’s to maintain sufficient cooling capacity while simultaneously attempting to minimize cavitation at high RPM. Such challenges frequently mean trade-offs are necessary. Racers frequently alter the pulley sizes to both free up some HP while slowing down the pump to reduce the possibility of cavitation. They succeed because they aren’t concerned about low RPM cooling.

Turning accessories slower = less energy "wasted" on turning accessories.

A bigger pulley is easier to turn than a smaller one. Just like a long wrench is easier to turn than a short one. A large-diameter pulley gives you a long level arm to turn the shaft it is connected to. Note that this applies only for "driven" pulleys -- just the accessories, and not the main pulley.

The main pulley is easier for the engine to turn when it is smaller.

-Max

how will the greddy pulleys affect my car since its a street car? will the alternator be hurting? loss of electrical power?

I have run underdrive water pump and alternator pulleys, AND an underdrive main pulley. I never noticed any problems. I don't have a big stereo or anything like that, however.

-Max

any noticeable power gains?
Ive also come to the conclusion that the 5 speed alternator pulley is the same size as the greddy unit.Or damn close.Are there two different kinds of alternators ? one for the auto and one for the 5 speed? if so is it just the pulley or is the 5 speed alternator stroger electrically.Just a thought. I figure if theyre biotht eh same then the larger greddy pulley wont hurt anything. Plus a larger water pump pulley should help things as well.Win win situation. Hope im right.

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You're gonna have to show me those pulley's so I can see how they compare to the ones I have on my car.

There won't be any power gains until you replace the main pulley. Even then, the most that I've seen was 15rwhp (Jason did the dyno reported in an old thread) using an Unorthodox Pulley kit. However, I wouldn't say that is a baseline for all cars.

The Greddy kit is to remove the airpump, not for power gains.