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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > NA Engine (non-turbo) / Drivetrain / Exhaust Modifications > crankshaft pulley advise



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      06-16-2010, 05:09 AM   #1
q_jonblaze
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crankshaft pulley advise

so i was thinking of replacing my crankshaft pulley on my e93 and was wondering this....what are the gains or side effects of putting a lighter cranshaft pullley on a car in the short or long term.

any help with this is truely appreciated.thanks
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      06-16-2010, 08:11 AM   #2
onefastman
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There is a lot of debate about this

while there have been no ill effects it is too soon to tell


Give it a try
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      06-16-2010, 01:31 PM   #3
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Quote:
Originally Posted by q_jonblaze View Post
so i was thinking of replacing my crankshaft pulley on my e93 and was wondering this....what are the gains or side effects of putting a lighter cranshaft pullley on a car in the short or long term.

any help with this is truely appreciated.thanks
My guess is:

gains = nill
ill effects = lower money for beer

Typically, you're swapping pulleys to lower the spinning speed of your peripherals (alternator, water pump, power steering etc). By lowering their speed you free up HP. Downfall is things like the alt. might not charge well at idle.

If it's just a lighter pulley (no size change), I doubt you'll see any change in performance etc. Making spinning things lighter will reduce the rotating mass of the them, making the engine spin up faster. I doubt that the weight difference on that would be noticable.
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      06-16-2010, 03:03 PM   #4
Healey3000
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Quote:
Originally Posted by q_jonblaze View Post
so i was thinking of replacing my crankshaft pulley on my e93 and was wondering this....what are the gains or side effects of putting a lighter cranshaft pullley on a car in the short or long term.

any help with this is truely appreciated.thanks
Hi,

You do have to be careful when changing pulleys. I'm sure you realize that it is not just a pulley but is actually a harmonic damper. It serves a vital function in terms of damping the vibrations of the engine.

The catch may be that BMW balances all rotating masses as a set. That is, pulleys are not interchangeable. However, BMW may not balance engines at all so it's difficult to know.

I would ask the vendor of the new pulley whether they balance it and more importantly, how it is likely to affect your engine.

As others have said, if you are staying with the same diameter, there is little to be gained. The right way to get big gains is to use a lightened flywheel, not a crank pulley. In my opinion, the possible risks of imbalancing your engine far outweigh any gains you may see. Also, getting the crank bolt off is not trivial as it is probably torqued in the 350 lb-ft range.

Regards,
Adnan

Last edited by Healey3000; 06-16-2010 at 04:12 PM.
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      06-16-2010, 03:07 PM   #5
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      06-16-2010, 03:13 PM   #6
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My crankshaft pulley advise to you is to not replace the factory crankshaft pulley.

There are many other better ways to gain performance, one of which is of course getting an E92 instead.
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      06-16-2010, 03:22 PM   #7
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Theoretically the less rotational mass the more HP you will have, ie: light weight wheels and flywheel. The gains from a light weight crank pulley will not be noticeable though. I wouldn't replace it unless you had a supercharger and wanted to run more boost, in that case you would put on a bigger crank pulley to spin the blower faster. Also note what Healey stated about it being a harmonic damper. If you are doing it for the looks then just have the OEM pulley powder-coated.
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      06-16-2010, 05:01 PM   #8
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Unless you're trying to squeeze every last little bit of power out of your engine you possibly can, it's probably not worth it. If you don't already have downpipes, meth, FMIC, tune, and some good lightweight rims, there are still many other mods that will provide a lot more bang for you buck.

I think the general rule for rotational weight is

1lb of rotational weight = 10lb's of sprung weight.

So lets say the stock pulley weighs 10lbs and the new one weights 2. That is basically the equivalent of taking 80lbs out of the cabin, for what, 2-300 bucks? Not too bad.

Now think about the wheels. Say you replace your stock 189's (or whatever you have) that weigh about 26lbs, and you put on some Forgestar F-14's that weigh about 17lbs. The difference is 9lbs per wheel.

9x4x10=360lbs of sprung weight. If you want to feel the performance difference from that, just go find two average sized grown men and throw them in the back. I think you find the gains for the buck is much better.

Not sure what sort of setup you already have but I figured I'd throw that out there.

EDIT: This was in the general forum when I posted it. Disregard the comments about tune, meth, DP's, etc.
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Last edited by mechanixhorseman; 06-16-2010 at 05:39 PM.
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      06-16-2010, 06:27 PM   #9
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Worthless. Don't do it.
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      06-16-2010, 08:49 PM   #10
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I maybe be wrong but... didn't all the guys who did this pulley mod on the old e36 M3's find that after many miles... their oil pump (driven off of the timing chain) the drive sprocket nut would work itself loose! There by loosing oil pressure. SOme guys even used saftey wire to secure that nut. Ofcourse you had to drop the oil pan and subframe to do this repair!

I think the negatives outweigh any potential Hp or throttle response gain!
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      06-16-2010, 11:12 PM   #11
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do NOT do it...this has been beaten to death on this board...don't...
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