Quick question re. underdrive pulley on alternator.

[TERRA Operative]

If I fit an under drive pulley on my alternator, the current output will drop yea? Voltage will be fine?

Damn I feel stupid. I'm an electrician and all (I don't do power generation... )

[LeeRoy]

Current drops, voltage stays constant.

[Billzilla]

I ran a 1:1 pulley on one of my 4AGE's and it was just enough to keep the alternator light off with everything turned on at a 1000rpm idle.
I wouldn't go that slow again though, more like 115% engine speed for a typical road-oriented engine.

[TERRA Operative]

Sweet.

What's the alternators stock drive speed?

[Billzilla]

Sweet.

What's the alternators stock drive speed?

Dunno!
About double engine speed I think. I can't see the point of running them that fast.

[Billzilla]

This is how it ended up looking though ->

[Turdinator]

According to the Bosch Alternator catalog you can get characteristic curves for alternators that plot current output verses rpm. If you could get one of these you could calculate exactly how fast you needed to spit your alternator at any particular time...

[TERRA Operative]

Nice. My stock pulley is about 2" across! Way smaller than yours...

Now to look for those curves for a 4AGZE alternator...

[oldcorollas]

According to the Bosch Alternator catalog you can get characteristic curves for alternators that plot current output verses rpm. If you could get one of these you could calculate exactly how fast you needed to spit your alternator at any particular time...

yah, go by the bosch.. there is basically two types.. old style and "compact" type.. the compact type need to spin faster to make VOLTAGE, but also have higher speed.


ie, spinning too slow means you drop from 14 to battery voltage at idle, which is not a big problem, but means long idling drains batteyr, and also you may need to get your voltage correction for injectors pretty close to stop stalling... not big issues.

then again, an increased idle from say 700 to 1000 or 1100 makes a big difference to the min speed and voltage

[takai]

The big problem with the stock 4AGE alternator and stock pulley is that when it overspins it also over volts. I saw 15.3v associated with a 9700rpm spike once. Strangely enough my shift light stopped working that race.

[oldcorollas]

well, it is not exactly designed for 9700rpm... if you will see that speed regularly, then of course you will need to underdrive, or otherwise protect the alt..

but if you increase idle by 20%, and underdrive the pulley by 20%.. then the alts redline is raised 20%... 20% of 700rpm is nothing to 20% of 7000 or 8000.... upping idle 20% (or whatever) is a small price to pay for that top end 20%.. if you need it..

[takai]

Yeah, i know its not designed for 9700rpm, but then again not much else is stock anyway. Besides, if you are revving that high on a 4AG then your idle wont be particularly low due to the cams needed to breathe up there.

[TERRA Operative]

Where can I get an underdrive pulley for my alternator? I've had a search, but my google-fu is weak.
I could design and get one machined up, but that means a hassle.

[GTI-TRD]

I have an R1-Wurks one from the states, off ebay. meant to Underdrive approx 15-20% and I got a water pump pulley with it too. I got both here for under $100. not sure on quality but they look like they are machined well.

[TERRA Operative]

Cool. Do they sell the alternator pulley separate? I've already got a water pump under drive pulley.