How credible is BMP's claim that underdrive pulleys add +12 HP and +8 lb/ft of torque across the RPM range? BMP claims in its catelogue (ed.# 16 pg 44) that by increasing the diameter of pulleys for alternator, water pump, power steering and A/C compressor, the reduction in drive ratio transfers power from these accessories to the crankshaft. Any experience or objective test results you can offer will be appreciated. Thanks.
I have a set of UUC Underdrive pullies and in my opinion, they did jack squat. My last dyno was with chip, intake and exhaust-haven't done one with my pullies. If anything, I THINK it might've helped to let the engine wind out a little quicker, but honestly, I think it's more of the placebo effect.
A crank pulley will most likely yield better results than accessory pullies (like what you mentioned, UUC's, Evosport's, Rogue Engineering)-but there are people that are not big fans of them.
Your comments are much appreciated. You just saved me several hundred dollars.Originally Posted by Jed
I don't know about accessory pulleys, but I curently use a crank pulley on my 98 M3 and it's made quite a difference.
Unlike a cat-back or CAI, you won't have to convince yourself that there are any power gains...the difference is tangible. It almost feels like a lighter flywheel, and you really feel the pull.
Well I've had both kinds. I had the UUC pullies (acc) and bought those because I thought I was going to get a LTW Flywheel. The flywheel would be too much money right now, so I went with the ERT pulley (crank). The crank does yield more power.
I put a set of EvoSport under drive accessory pullies on our 525 iT. They did improve the performance of the car, but not as much as I had hoped. The first thing I noticed after putting them on the car was that the power band felt like it shifted down about 200 rpms and the engine seemed to rev a little freer. I also did before and after 0-60 runs but the times improved by only 0.1 seconds. Not very dramatic.
The other thing I did was to weigh the stock pullies vs. the EvoSport pullies in order to determine how much "lighter" they were. Result: about 5 ounces. Some of this was probably due to the fact that the water pump pulley on our car was plastic.
The most dramatic change that I have noticed is that my gas mileage improved by about 1 mpg. Before the mod, mileage ran about 18 mpg city / 28 highway. After the mod, mileage runs about 19 mpg city / 29 highway. So with current gas prices, they might pay for themselves over the life of the car.
2001 330i (my car)
2004 Audi Allroad (wife's car and family truckster)
Think about it...would the world's techiest car company (BMW) not use an underdrive pulley if it gave more horsepower, more mileage, cleared acne, etc., etc.? As for underdrive pulleys giving more mileage, well, obviously ymmv but the EPA would beg to differ. Car manufacturers would sell their souls (oxymoron, they have no soul) for a tenth of a mpg...if underdrive pulleys were the answer they'd be on the cars from the beginning. Cost issues, I don't think so. A smaller pulley is cheaper to mfr (less material), yet they keep putting these big pulleys on the engines. What do they know that we don't?
Sure, everything is a tradeoff, but IMHO the modern M3 is pretty well wrapped out of the box. Even brittle if what I understand about overspeeding it during a missed downshift (like from 6 to 2) or somesuch.
As an example, BMW's come with things like the Clutch Delay Valve that slow the engagement of the clutch so that people can drive smoother. It happens to wear the clutch out faster because it basically slips the clutch during engagement. Pulling this valve brings an immediate improvement in the performance of the car. But to make the car more people friendly, BMW includes it out of the box.
A crank pulley isn't installed on stock cars for a number of reasons, but those reasons really don't mean anything to a more performance minded owner.
And the reason it's easy to over rev the motors during shifting is because BMW made their transmissions shift into all gears smoothly using multiple syncros. It was over engineering on their part... again to make things more people friendly.
Being an engineer myself, I will tell you it is all a matter of building a system (a car in this case) that will handle all the user requirements. When BMW designs a car, they have a long list of stress related requirements that the car must meet before it can be put into production. The stock pulleys were sized to meet these requirements regardless of the additional load on the engine. The peddlers of the underdrive pulleys are basically allowing you to relax some of the requirements in exchange for a little extra power.
The stock alternator pulley was sized to ensure that sufficient current would be produced to power whatever 500+ watt aftermarket stereo that the customer might install. If the car did not produce enough current, then the audiophile community would be merciless in their criticism. However, if you do not need the current, then a larger (slower) pulley can be used with no ill effects.
The stock water pump pulley was sized to ensure that customer could drive their car loaded with 3 passengers and a trunk full of luggage at 70+ MPH across Death Valley in August with the A/C on without having the car overheat. Again, if it overheated, the customer would not be happy. If you do not intend to do this, then again a larger pulley could be used.
The stock power steering pulley was sized to make sure that there would be no increase in steering effort when the car is loaded with 3 passengers and a trunk full of luggage, so a 65+ year old female driver car still turn the wheel while the car is standing still. Again, if you don't mind a little extra steering effort, then a larger pulley could be used.
There are other examples like this on our cars. Most aftermarket free flow intakes and exhausts produce HP and gas mileage improvements in exchange for a little extra noise. Your average BMW customer wants their car to have smooth quiet power, so BMW designs their intake and exhaust systems to keep the engine as quiet as possible without severely impacting performance. But if you do not care about the noise, then a different intake or exhaust can be installed that has been optimized for power.
2001 330i (my car)
2004 Audi Allroad (wife's car and family truckster)
I highly doubt that BMW cares about 500+ watt custom stereo, especially given the fact that every new car comes with a nice sticker on the windshield stating that installation of aftermarket stereo components may result in electrical problems and void the warranty.
-- Jason
It's funny you mention this. I remember when the E36 325i came out, customers were not happy with the lumpy idle (the 92s had hotter cams as well as a beefier head) and from what I recall some brought them back to have them "fixed." An example of performance that many BMW owners and customers (as opposed to enthusiasts) may not want as a trade off to comfort or what not.
I think that BMW puts that disclaimer on there to protect themselves from the idiot who fries his electrical system by splicing a 500 watt aftermarket amp into the first 18 gauge wire carrying 12 volts he finds. It is kind of like the labels on hair dryers that tell you not to use them in the bathtub.
2001 330i (my car)
2004 Audi Allroad (wife's car and family truckster)
You can pretty much say the same thing about most any bolt on part, engine or suspension.
The vast majority of clueless owners, versus those of us you will compromise in other areas for a better performing car.
Heck people buy E46 M3s and then complain tha the ride is not super smooth. Imagine if BMW really made a all out car?
I imagine even Lotus is getting compaints about the Elise.
Actually improving 0 - 60 by 0.1 seconds is pretty significant for a couple hundred bucks, a bit of time, and very little downside.
Terry Carraway
02 Topaz E46 M3 SMG
00 Dakar M Roadster
95 Alpine E36 M3 LTW
SCCA Spec Racer Ford Red/White
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