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View Full Version : Under drive pulleys with a charger? Question.


Bliss
02-07-2007, 01:43 AM
Any physics majors out there?…help! lol

I had UUC under drive pulleys for the water pump and the power steering installed on my car previous to my charger install. Now…after installing an AA charger on the car which according to AA, I had to change my pulleys back to OEM because they claim that the charger will then create more boost this way “because of the longer length in belt required”, causing the charger to run improperly(too quick). Me and Nelson(Thinair) were talking one night and we were just stunned at the idea of the charger creating more boost with just having under drive pulleys? Next day I had called Rocco at RMP (since he’s good at performance upgrades and this kinda shit), and he told me the same thing we were thinking, and that it should not make any difference in boost (OEM pulleys or not). Unless I change the crank pulley, the charger should continue to spin at the exact same rate either way…or not? I’m a little confused as to why this is recommended by AA? I called them twice already, but im still kinda blurry on the explanation. Anyone care to shed some light on this?

BigD
02-07-2007, 01:51 AM
Nonsense. First of all there is no extra belt length (due to the smaller pulleys) unless you installed a longer belt. There is more slack with smaller pulleys but that's what tensioners are for. The only pulley that can affect the boost of the charger is the one providing the force - that being the crankshaft. It spins at a particular RPM and if you were to install a larger pulley, then the belt would move faster. A belt, unless it's slipping, is no different than if all the pulleys were gears connected together. The only thing that determines how fast the gears spin is how fast the drive gear is spinning and how large it is.

If the belt is not slipping (which would only cause you less boost), if the crank pulley did not change, the charger pulley speed will not either, period. They must have thought you have a smaller crank pulley as well... for whatever reason. That's the only possible explanation I can think of for their logic.

Bliss
02-07-2007, 01:56 AM
Nonsense. First of all there is no extra belt length (due to the smaller pulleys) unless you installed a longer belt. There is more slack with smaller pulleys but that's what tensioners are for. The only pulley that can affect the boost of the charger is the one providing the force - that being the crankshaft. It spins at a particular RPM and if you were to install a larger pulley, then the belt would move faster. A belt, unless it's slipping, is no different than if all the pulleys were gears connected together. The only thing that determines how fast the gears spin is how fast the drive gear is spinning and how large it is.

If the belt is not slipping (which would only cause you less boost), if the crank pulley did not change, the charger pulley speed will not either, period. They must have thought you have a smaller crank pulley as well... for whatever reason. That's the only possible explanation I can think of for their logic.

Exactly!...i dont understand the logic behind it. I made it clear that i dont have a crank pulley changed cuz thats what i thought they were mixing things up with, but i got the same answer from them.:confused:

330DTM
02-07-2007, 02:23 AM
Okay I did some research on some old threads from B/F. Here's some insight from what I've gathered:
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carcrazed: You're correct to an extent. The entire means by which an underdrive pulley works is an increased pulley diameter and to some degree lightening of the pulley.

Specifically, the UUC pulley changes the diameter of the of the power steering pulley and water pump pulley, but not the alternator pulley (its weight is simply reduced).

That said, you can't use the UUC pulleys with the AA S/C. Even though the S/C is crank driven, the different belt length(s) required by the UUC pulleys increases the operating RPM's of the AA S/C at any given engine speed. As such, the AA S/C will spin beyond it's maximum warrantyable speed, causing several issues:

1. Your S/C's warranty will be void.
2. Your S/C will likely fail, if not right a way, much sooner than a normal lifespan.
3. Your engine will be damaged since the AA software doesn't account for the additional boost produced by the S/C in these conditions. It's likely you could kill it altogether.

In short, the UUC pulleys ARE NOT a good idea with the AA S/C. They'd be fine IF AA used a different S/C capable of the higher RPM's and accounted for the extra boost with software, but they do not.

In any case, there's not a significant amount of power to be gained with the pulleys. I guess you guys will just have to save the $185 and "suffer" with 5-9whp less.
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Source: http://forums.bimmerforums.com/forum/showthread.php?t=112811&page=4&highlight=rotrex

330DTM
02-07-2007, 02:27 AM
However in this thread, AA says you can use the UDP's in conjuction w/the S/C kit long as you do indicate that you are running those pullies so they can get you the proper sized belt.

http://forums.bimmerforums.com/forum/showthread.php?t=114594&highlight=underdrive+pulley+supercharger

propr'one
02-07-2007, 03:18 AM
Ask furious. I believe he just calculated what size belt he would need, found that belt, went with it, and it worked.

SickFinga
02-07-2007, 03:36 AM
WTF?? Sc is crank driver. No matter how many pulleys and how long is the belt for ever crank revolution your SC will spin the same way. Just get a proper lenght belt and you are good to go.


That's the only possible explanation I can think of for their logic.

or they got their own underdrive pulleys now, that for some reason work with AA SC ;)

thinair
02-07-2007, 08:52 AM
First of all there is no extra belt length (due to the smaller pulleys) unless you installed a longer belt. There is more slack with smaller pulleys but that's what tensioners are for.
Underdrive accessory pulleys are larger in diameter then stock, hence the extra belt length (which is irrelevant for the compressor rpm anyway). The greater pulley circumference means less accessory rpm. A smaller crank pulley has the same underdriving effect though.

Adriano
02-07-2007, 11:23 AM
Too busy to explain but here it is:
Underdrive pullies WILL NOT effect the RPM of you S/C UNLESS you also have a underdive crank pully. That is the bottom line. Who ever told you it makes a variance to S/C RPM is blowing smoke.

Bliss
02-07-2007, 11:41 AM
So i guess there is no logical reason for it afterall. Thanks guys.

PedroBMW
02-07-2007, 12:13 PM
Adriano is right

just think of it like this,

Velocitybelt = Angular Velocity of Pulley (basically speed that the pulley spins) * Radius of Pulley

Now since the crank pulley drives the S/C pulley, u know that the belt speed is the same for both (assuming no slip),

now since withyour UUC UDP the crank pulley has not been changed at all in size (or angular velocity obviously) that means that the speed of the belt is the same as stock. And thus that means that the angular velocity of the S/C pulley is the same as stock (with no UD puleys i mean) by the little forumla I wrote above.