WTB underdrive pulley and/or high flow cat for n/a 1.6
09-27-2012, 06:55 PM
#5
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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.
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09-27-2012, 11:35 PM
#6
right i remember this bit of theory from a thread a while back. i'm an sts car though so i run barely more than stock power levels. have there been any reports of pulley's causing damage to low output miata engines like mine? the same theory was thrown around on the z forums that i used to frequent, yet there was no testimony of the feared damage actually occurring, and most people just used the under-drive pulleys.
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09-28-2012, 12:06 AM
#7
right i remember this bit of theory from a thread a while back. i'm an sts car though so i run barely more than stock power levels. have there been any reports of pulley's causing damage to low output miata engines like mine? the same theory was thrown around on the z forums that i used to frequent, yet there was no testimony of the feared damage actually occurring, and most people just used the under-drive pulleys.
You really can go to google and click the "I'm feeling lucky" button for: "shattered oil pump miata" and get an example of it happening. First result, figured it out in two posts. Boost or not, why would you?
[edit] Ok, so I see a lot of sts cars do it, but I don't think you could get me to.
Last edited by hankclaussen; 09-28-2012 at 12:17 AM.
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