U/D pulleys, alternator pulley

35th99gt · Sep 30, 2006

i heard of some people not even replacing the alternator pulley, is this something a lot of people do? or is it stupid to do this?

Gearbanger 101 · Sep 30, 2006

Some don't change it in order to keep the alternator spinning at a faster rate at idle and in lower RPM heavy traffic driving. The U/D alt pulley is a larger diameter than stock, combined with the smaller diameter crank pulley, it spins measurably slower at idle and all other RPM. Some owners believe that the slower spinning alternator can lead to battery charging issues since it's not seeing as many amps at idle as it would with the stock pulley. This was a regular problem for the old '87-'93 5.0L's, but not so much with the later model higher output alternators.

Use the stock pulley if you wish. The parasitic loss associated with the alternator pulley is nearly immeasurable anyway. The main reason the pulley is changed is to allow the user to maintain the stock serpentine belt length.

35th99gt · Sep 30, 2006

would i have to get a new belt if i kept it? what kind of belt would i have to get?

StangPro · Sep 30, 2006

The alternator will pull power (drag) as it needs to to charge the system regardless of its' pulley diameter.

The point of the UD pulley is to prevent over revving the alt on the track, and like metioned above, keep stock belt lengths. Odds are you can still use the stocker without changing the pulley though.

parkgoons · Oct 1, 2006

When I did the pullies I put on the new alt pulley that they supplied. I have no charging issues to this day (its been 10,000 miles since the swap).

sgarlic · Oct 1, 2006

I had the Steeda alt pulley on for a while, then put the stock one back on a few days ago. I noticed no power difference between pullies (obviously) and no visible affect on charging with either pulley. Then again I have nothing besides a few extra gauges that would cause extra power drain.

You can use the stock belt with either of the pullies.

markwsoil · Oct 1, 2006

Gearbanger 101 said:
Use the stock pulley if you wish. The parasitic loss associated with the alternator pulley is nearly immeasurable anyway. The main reason the pulley is changed is to allow the user to maintain the stock serpentine belt length.

+1 to that. I bought my MAC UDP's used from a guy who didn't even have the alternator pulley. He told me that the dyno tests he had seen indicated that at most an alternator takes maybe 2 HP, maybe even less. I was able to use the stock serpentine belt without it. It's really smart engineering when you think about it. The crank pulley is smaller, but the water pump pulley is bigger, so they even out lengthwise, while adding back HP. Ingenious.

GettinThere · Oct 2, 2006

sgarlic said:
You can use the stock belt with either of the pullies.

+1 I've had Steeda underdrive pullies with the stock alternator pulley and stock belt on since June 2004 with no problems.

BooWFO · Oct 2, 2006

I picked up my Moto blue UD from a stangnet memeber and he didn't have the alt pulley seen zero issues and was able to use the stock belt. Although I did swap to a shorter belt trying to elimante and annoying whine that turned out to be the header gaskets going. DOH

U/D pulleys, alternator pulley

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