Originally Posted by KasbeKZ
slows down the water pump too much? been wondering how significant that problem is on the miata.
The principle objection to these pulleys has nothing at all to do with the speed at which they turn the water pump.
It is a common misconception that the stock engine pulley is a pulley. Yes, it has some grooves machined onto the outside of it which happen to turn a pair of belts, but that can almost be considered to be a secondary function.
It's necessary to look in depth at the construction of the pulley itself. Stories abound about pulleys "slipping" such that the timing marks around the edge no longer line up with the engine, and there's a very good reason for this. The engine pulley is actually a three-piece assembly. There is a solid metal inner hub, an outer metal ring, and a rubber ring which sits between the two halves and joins them together.
Why such an odd construction? The pulley is a harmonic damper.
In fact, in the world of American Iron, they don't even call them crank pulleys. They actually refer to them as crank dampers or harmonic balancers.
So, WTF then?
A piston engine doesn't actually run all that smoothly. You have individual power pulses separated by period of very little happening. As the cylinders fire, the crankshaft itself actually deforms by a nearly imperceptible amount, and since this happens repeatedly in a predictable timing, it causes oscillations of various frequencies as the crank is repeatedly bending and then snapping back and then bending and then snapping back over and over and over again.
At the rear of the engine, you've got a big heavy flywheel to soak up this vibration, and very little in the way of delicate machined parts to be affected by it, so it's no big deal.
At the front of the engine it's a whole different story. There's a lot less mass, and more importantly, there's an oil pump. One made from delicate little gears of sinstered metal wrapped directly around the crank itself.
Thus, the harmonic balancer. Its job is to soak up these oscillations and counter-balance them by its own elastic nature, thus damping out the vibration of the crank and, among other things, preventing the oil pump gears from literally shattering.
There is a documented history of this occurring on the B-series engine. It tends to happen more frequently at extremely high power levels (where the loads exerted upon the crank are greatest, and thus the amplitude of its oscillations are highest), but as a general rule, it's kind of stupid idea to remove the one device whose primary job is to prevent this, and whose removal nets you virtually nothing at all in the way of measurable gains.