Underdrive Pulley-Does It Need OD Alternator Pulley

**Eddie 70** · 09-25-2007, 05:43 PM

I am going to be installing an underdrive pulley on my LS motor before installing it in my vette. Am I going to need an overdrive pulley for the alternator to counter the underdrive crank pulley? I will be running a Spal Electric fan as my main source of cooling and it may run quite a bit.

Will the OD alternator pulley negate the effect of the underdrive crank pulley advantage?

Thanks for any help on this one guys.

**JonB** · 09-25-2007, 05:47 PM

You should be fine without the alt pulley, ls1 run electric fans so i don't think your spal will take much more power than a stock setup.

***Ed Blown Vert*** · 09-25-2007, 05:58 PM

If you are an A4 you may need it. M6 you can probably get away with not using one.

I have a 10% UD and needed a OD for my alt at idle in gear.

**Eddie 70** · 09-26-2007, 01:22 PM

I will be running an ASP underdrive. I think they are 25% IIRC. My car is a 5 speed by the way.

One more thing, what is the stock output for the alternator as far as amps are concerned?

**jag42420** · 09-27-2007, 08:46 AM

105 AMPs, I think.

**MikeSomething** · 09-30-2007, 06:08 AM

I have an A4 LS1 and run the ASP crank pulley w/ out the OD Alt pulley and it's fine. I even had a system in it for awhile w/ an AC Delco battery and it never dropped the voltage low enough to dim the headlights or anything. Now I took out the system and have an Optima Redtop battery and it doesn't even move the needle on the AMP gauge.

**jag42420** · 09-30-2007, 08:12 AM

I wanted an underdrive too, but worried about the amp drop. I have 2 large amplifiers (850W Kicker ZX850.2 and 720W Soundstream XTA720.4) that pull about60amps.

Going with an underdrive crank pulley would mean I'd need either an overdrive alternator pulley or a more powerful alternator. Going with an overdrive alternator pulley would reduce the gain from the underdrive crank pulley, and would still only get me stock amperage at best.

There are 150 and 180 amp aftermarket alternators out there. I'm thinking if I change to one of them, an underdrive crank pulley would be an option. My line of thinking is, if 102 amp stock alternator is reduced by about 25% underdrive, then I'd only be left with about 76.5 amps. If it were 150, I'd have 112.5 amps. With the 180, it would be 135 amps.

**Eddie 70** · 09-30-2007, 08:24 AM

I think for my purposes, I should be okay with the stocker and the UD pulley. I recently found out at a rod run, with my stock small block, that my stock alternator with March underdrive pullies, was not enough. I was setting in the cruising lane and the car just dies. I hit the key and it will just barely click the battery is so dead.

I just wanted to avoid a repeat performance of this when I get the LS motor in my vette. I already had my ASP pulley when this occurence happened. I had never had any problems before this with the vette on any trip.

**xzaero0** · 09-30-2007, 02:07 PM

I have A4 with an SLP pulley, stopped in gear at a stop light with all lights and stereo up it drops down to around 12.3 on my radar detector so I turned my idol up with the predator to run at 50rpm more than stock and now it stays above 13.2 constantly.

**Eddie 70** · 09-30-2007, 02:26 PM

Originally Posted by xzaero0

I have A4 with an SLP pulley, stopped in gear at a stop light with all lights and stereo up it drops down to around 12.3 on my radar detector so I turned my idol up with the predator to run at 50rpm more than stock and now it stays above 13.2 constantly.

What is your idle speed set at? Just something to file away so I can be for sure.

**jag42420** · 09-30-2007, 05:35 PM

Most UD pulleys run full alt power at 950rpms from what I've read on their websites.

**xzaero0** · 09-30-2007, 09:29 PM

Originally Posted by Eddie 70

What is your idle speed set at? Just something to file away so I can be for sure.

I think that its around 600-700 at idol. The Predator tuner only has increases by 50rpm at a time, so I just turned it up one setting.

**oneBADDz** · 09-30-2007, 09:55 PM

Once you get moving you're alternator will still reach it's full output though. It doesn't reduce the "105 amp" output, it just requires a little more engine RPM to get you there. The alternator puts out peak power pretty low, usually at around 900 engine RPM, so with a 25% UD pulley you could just figure that to be pushed back to 1125 engine RPM for peak output

**jag42420** · 09-30-2007, 09:56 PM

Applied across the whole table I assume?

**jag42420** · 09-30-2007, 09:58 PM

Originally Posted by oneBADDz

Once you get moving you're alternator will still reach it's full output though. It doesn't reduce the "105 amp" output, it just requires a little more engine RPM to get you there. The alternator puts out peak power pretty low, usually at around 900 engine RPM, so with a 25% UD pulley you could just figure that to be pushed back to 1125 engine RPM for peak output

Problem is stoplights, traffic, warm ups, etc...

**oneBADDz** · 09-30-2007, 10:25 PM

Originally Posted by jag42420

Problem is stoplights, traffic, warm ups, etc...

You are correct. Someone up there was subtracting 25% form the max output though. When upgrading that is something important to take into account. The larger amperage you choose when you upgrade, the later the amps usually increase to a significant amount so you might actually have less at idle. WHen upgrading I chose an alternator that put out less peak amps than others but significantly improved my idle output.

**jag42420** · 10-01-2007, 08:23 AM

Originally Posted by oneBADDz

You are correct. Someone up there was subtracting 25% form the max output though.

That was me, but I was talking about sitting at idle (traffic, town, etc). I also said from what I read on the manufacturer's website they reached normal operating amps at 950rpm. It's the below 950rpm I worry about with 2 amplifiers and an A/C, which is why I'm looking at larger alternators anyway.