Andrew055

quick noob question, i'm still learning about some things

i've been looking around at underdrive kits and most of them say they wind down the power going to accessories by about 25%

so is that a noticeable difference? like battery charging slower, water pump running slower, less response in power steering, etc

or are these accessories just overdriven to where theres more power going to them yet it wouldn't make a difference if it was reduced by 25% like this kit says

i mean i'm sure underdrive kits wouldn't be as popular as they are among gearheads if they were severly affecting things like power steering handling and cooling of the engine done by the water pump

just need some clarification, thanks

MeentSS02

iTrader: (11)

The only issue I ever had with an underdrive pulley was due to an aftermarket high(er) output alternator (it was a 160 amp) - apparently, aftermarket alternators tend to have higher turn-on RPMs, so the underdrive pulley caused it to be below that set point at idle. Not fun at night with the dimming lights, etc. unless you gave it some gas.

It probably wouldn't have been an issue with the stock alternator, but it certainly was for that one. If you put an UD pulley on an auto car, the charging thing could be more of an issue since they already idle at a lower RPM than the manual cars.

Just something to watch out for...as for everything else, I noticed no difference.

Decadence75

iTrader: (17)

Installing an overdrive pulley on any aftermarket alternator would be able to correct that issue as well.

99Bluz28

iTrader: (35)

IMO, the best solution if you have charging issues with a A4 trans and a 25% UD pulley is to bump the idle speed up 25% over stock, with a M6 trans it shouldn't be a problem. I also wouldn't recommend a OD alt pulley unless you keep you engine below 6k, and you'll also lose some of that freed-up hp/tq by installing a OD alt pulley so it's counter-productive.

Bugsy

iTrader: (1)

I've noticed no detrimental effects whatsoever.

TheBlueKnight

iTrader: (4)

My brother has a 2001 M6 TA. He just installed an UD pulley. He says when he is idling at a stop light his dash lights and stereo get dim, that's about it.

RPM WS6

iTrader: (3)

The decrease in charging would definitely be more noticeable with an A4, as the in-gear idle speed is 30% lower than it is with an M6.

As mentioned above, you could overdrive the alternator or bump the commanded idle speed via tuning. Either would help restore proper idle charging.

The other accessories should still function fine so long as they are not weak to begin with (such as the water pump).