How much amperage can an alternator support? no really...

[Vittorio Bares]

Thinking through this I'm reminded in seeing some competitors start
stages without all their lights on and only turning them on after about
100 feet into the stage. At the time I assumed they were just forgetful
or being extra cordial not to blind the starter - but looks like they
might have been using a different pulley, and waiting to be up to revs
prior to turning on the juice suckers...

As many of you know, but perhaps many do not - in stage rally there's a
lot of idling before a stage start - esp. if there are delays. The only
time the big lights are on is in competition mode - we also shut them
down as we're coming into the finish control.

One of the other aspects I try to maintain is to allow myself the luxury
(?) of using parts that I can source fairly easily, and due to the fact
that I'm not independently wealthy, try to keep costs down where I can.

So - as far as I can tell - based on the what I have from the list so
far:
Fuel Pump = 10a (always on)
CIS = 10a (always on)
Ignition = 10a (always on)
Rad Fan = 20a (always on - at least for now)
Heater Fan = 15a (always on in the winter ;)
Brake Lights = 5a (on a lot)
Computer, gauges, etc = 5a (always on)
Head lights low = 55w x 2 = 110 w = 10a

So given this set - looks like I need 85amps (in the winter). Do the
numbers look right?

However, as soon as I add my lights (4 x 100w, 2 x 35w = 470w) = 40amp -
I'm way over the capacity of the alt - needing 125amps :o Adding 20%
would put me at needing 150amps.

If I converted everything to HID, the lights would be 210w (6 x 35w) or
17.5a - bringing the draw to 102.5a - a 110a alt would give a 10% margin
and possibly a larger pulley might buy some additional efficiency w/the
lights on. I could also get rid of 2 lights - but as always more is
better ;)

So Cody - any suggestions on pulleys? And also what dimension does the
belt go to?

I have a 110amp alternator from the 200 aqt - (oh, and by the way - if
any of you were following that thread - regarding lack of charging -
after replacing the instrument panel, several rounds of diagnosing, etc
- it turned out to be a loose alt belt - argh !) I could throw in there
- probably will need to make some change to the mounting...

2 more HID kits = $150 + better lighting. Performance alternator - a bit
more than that - no change in lighting...but potentially more reliable?

Vittorio -


-----Original Message-----
From: quattro-bounces at audifans.com [mailto:quattro-bounces at audifans.com]
On Behalf Of cody at 5000tq.com
Sent: Tuesday, February 05, 2008 7:25 PM
To: quattro at audifans.com
Subject: Re: How much amperage can an alternator support? no really...

You might consider either changing the pulley on the alternator to spin
it slower or even if you can afford it buy a racing alternator.

The OE setup is designed to be able to keep everything running when the
engine is at idle, while your engine really only needs it to start
really having good output over something like 2500rpm. Changing the
pulley ratio to spin the alternator slower will move it most efficient
RPM range up to more like where your engine is usually running at, and
the improved drive ratio will help pull less torque draw on the engine.

Likewise a racing alternator will be designed to run at much higher
revs. We've got one on the V8 944 we are campainging in NASA and it's
pretty killer. It charges about 12.5v at idle, but comes up to 14v
arround 3,000rpm. We dyno'd the car with and without the alternator and
came up with a dead wash - no power loss, actually a slight improvement.
The torque required to spin the alternator is made up for by the
stronger ignition and quicker injector opening times with the extra
voltage. Our car has no lights except brakes though, so in our case we
used a 50amp Denso unit (see below link - they go up to 120amp). Not
very expensive, right around the same cost as a stock replacement
really, but it will require some fairly basic fabrication to mount.

http://www.powermastermotorsports.com/racing.html


-Cody Forbes

_______________________________________________
quattro mailing list
quattro at audifans.com
http://www.audifans.com/mailman/listinfo/quattro
---
Watch this space for ads :)