I think this explanation from Lemans_Blue_M is the best I've read. I copied and pasted it for you to read.
Power pulleys are not the same as a CRANK pulley.
A number of aftermarket companies build individual accessory pulleys, that will under drive certain engine components like the power steering pump, the water pump, or the alternator. These are called "Power Pulleys".
A crankshaft pulley is different, since it replaces your existing OEM crankshaft pulley, to under drive all the belt driven accessories bolted to the engine. (to reduce the parasitic drag losses)
Power pulleys may yield 2-3 hp improvement over stock, but an aggressively spec'd aftermarket under drive crankshaft pulley (16%) will yield 10+ hp under ideal conditions.
I hope this clears up any confusion.
Also, I would like to clarify some things that Steve was talking about in that tech article...
In 2002-2003 when this article was written, the E36 & E46 M3's both used variants of the same 3.0-3.2 liter straight-six powerplants from the S50, S52, & S54 family of BMW Motorsport engines.
Now keep in mind, the state of the art "M" engine in those days was the S54B32 in the E46 M3.
That's an important detail...because the S54B32 uses a harmonic dampener to keep the engine vibrations in check. BMW straight-six Motorsport engines are very smooth and well balanced, but they require very precise crankshaft dampening to prevent damage at high rpms.
What Steve was talking about in this article, was the fact that if you remove the main CRANKSHAFT PULLEY (not the smaller individual accessory pulleys) you could potentially damage the engine bearings (or the engine crankshaft seal) over time, since the engine would no longer have that factory harmonic dampener protection in place.
The Dinan under drive pulley on the E92 M3 does not modify the factory harmonics or dampening capabilities, and it safely under drives the belt driven water pump, alternator, AC, and power steering pump, to reduce parasitic power losses on the M3 engine.
When Steve originally wrote that article, he was also railing against those modifications because several aftermarket companies were simply selling CNC'd under drive pulleys that were not perfectly balanced, nor did they retain the factory harmonic balancer. (which can lead to engine damage) There was little or no R&D done on most of these parts to ensure OEM-type fitment...that is crucial to safe, reliable performance.
These poorly engineered under drive crankshaft pulleys would lead to all the problems he explains in the article, and that's why Dinan never offered an under drive crank pulley of their own for the E36 M3 or E46 M3. (fact)
For the S85B50 V10 in the M5 & M6 and the S65B40 V8 in the E9x M3, Steve's engineering team talked him into it building a crankshaft pulley for these engines, after they were comfortable they could manufacture an OEM quality part for his clients (customers) that was perfectly safe.
You know, it really took me a long time to realize something...
On rare occasions, you have to make compromises when you are selling parts to the general public.
When you own a retail automotive parts business, you don't always get to pick and choose what parts you want to sell ...and parts you don't.
Let me explain...
If enough people within your loyal customer group (repeat customers) request that you build a particular part, then as a businessman it's really tough to say NO to them. It's not a good business move to deny a large percentage of your core customer demographic something when they really want it. So at the end of the day, you have to give in sometimes to keep your customers happy. That's called customer service folks.
It's not always easy to sit on your throne and say "I'll NEVER.........."
So I'm sure the Dinan haters will call him names (which they would do anyway), and I'm sure he realizes that criticism is coming...
Yes, his philosophy on this subject has changed a bit over the years, but that's actually a good thing. When presented with a way to do something he originally thought was just a bad idea, the man reconsidered his position on this issue...and made a course correction.
That's an admirable trait, and it takes guts to do something like that...especially when your KNOW people are going to criticize you for modifying your original position.
Anyway, that's the whole back story to the article on Dinan's website.
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