ERT Underdrive Pulley

The next engine mod was adding an ERT Underdrive Pulley. The underdrive pulley is a mechanical mod and therefore isn't prone to the "OBD-2-style" arguments of whether it is effective or not as shown by Chris P's Dyno (thank you Chris P). It is probably the most effective engine mod you can do. From the first pictures below you can see that the pulley makes power by "underdriving" the electrical components (not the A/C) and also improves "spool up" by lightening the rotating mass coupled with the crankshaft (like a lightweight flywheel). The amount of underdriving still maintains the required voltage of the system (click here). There is an article (click here) by Steve Dinan claiming the underdrive pulley can cause problems for an engine in two ways: tampering with the electrical system and increasing stress on the crankshaft. The problem is that Dinan never uses empirical data, just hypothetical examples. Sure, you can say that underdriving too much is a bad thing, but you can also say drinking too much water is a bad thing. I say, "Show Me The Data!" In truth, the above reference I cited shows there are minor electrical implications for the mod and ERT did a study (click here) which showed that a lightweight flywheel causes more resonance in the crankshaft then the underdrive pulley. And now Unorthodox has confronted Dinan directly in this open letter: Response to: The Danger of Power Pulleys. Dinan (bless his heart because he does do some great work on BMWs) sells a lightweight flywheel (for many times the price of the pulley) and doesn't sell an underdrive pulley, so do you think he is biased? You make the call. Suffice is to say that the flywheel and the pulley should never be mixed.

My M Roadster has the new double pulley design from ERT, but I also have one of older single pulley designs to put into the 328i. The original design from ERT is the single pulley design and is the same design offered by a few other companies such as '02 Performance. The single pulley offers the same performance benefit as the double pulley design... the only benefit of the double pulley it that it is easier to install. ERT closed the older design out for $150. The pictures below show how the system works. In the stock setup the accessory pulley and A/C pulley are separate pieces that sit nested on the end of the crankshaft. The single pulley replaces the stock accessory pulley with a slightly smaller pulley made of lightweight aluminum as opposed to the heavy stock pulley but retains the factory A/C pulley (which is already made of lightweight aluminum). The problem is that there is some variance in the pulley alignment such that at the factory A/C pulleys and accessory pulleys have to be matched on each engine. In order to work around this problem, the single pulley design sometimes requires shims to properly space the A/C pulley out from the accessory pulley. The double pulley replaces both the accessory pulley AND the A/C pulley so there is no needs for shims. The weight and percentage of underdriving remain the same, so there is not performance benefit. Also notice that because the A/C is driven off a dedicated pulley the A/C isn't effected by the underdrive pulley.  (I apologize for not getting the reference lines in the first picture more parallel, I was winging it in the garage).

Stock  Accessory Pulley Vs. ERT Pulley Stock Accessory Pulley Vs. ERT Pulley

A/C Pulley (front) A/C Pulley Nested In Stock Accessory Pulley

A/C Pulley Nested In ERT Pulley ERT Double, ERT Single, Stock A/C and Nested Pulleys Note: ERT Single + Stock A/C = ERT Double

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