I'm running underdrive pullys on my '95 LT1 with a stock alternator, but at idle the voltmeter goes way down. What size alternator are you guys running with your underdrive pullys and what brand do you recommend? Thanks.

 

I think you can get an overdrive (bigger) pulley for your alternator that will help your charging at idle if it bothers you that much. That way you can keep your underdrive pulley and you won't have to buy another alternator.

 

I have the ASP intergrated UD damper. ~25% UD IIRC. Stock alternator and pulley. Zero charging issues even with thirsty amps.

 

I had the ASP 24% U/D crank pulley and 15% U/D alternator pulley, at idle, in heavy traffic with the A/C and brake lights on, the voltmeter would head for the red. To me, the tiny HP gain is just not worth it. Go to an O/D alternator pulley, and that tiny gain gets even tinier. If he has an A4, its an even bigger problem, because of the 550RPM idle speed, compared to 800RPM for the M6.

 

Actually an overdrive pulley would be smaller, and yes they are available. I have a 25% overdrive on my alt, to compensate for the 10% underdrive on the crank.

 

In my experience, underdrive pulleys are a very very bad idea in general.

 

I put an underdrive pulley on my 97. The seat-o-meter told me I had more power, but I noticed the charging amps were low at idle (and it was more difficult to steer in the parking lot). I was idling at the track while they cleaned up a car that had decided to leave its tranny on the track. The battery died and I had to jump it to get it home.
I put the overdrive pulley on my alternator and never had another problem.
AFA dollar per horsepower, I'm not sure I would recommend one considering the cost, but they do work AFA increasing horsepower. You turn your power steering and A/C pulleys slower, so it frees up a few horsepower.
Long story short, if you want to run this underdrive pulley, you need both an overdive pulley (a bit smaller in diameter) for the alternator and a slightly shorter belt.

 

Please explain why other than not worth the price for the very tiny power gain? FTR I got my ASP for $50.

 

Why take a chance of discharging your battery - last thing you want for max performance out of an electronic fuel injection engine. The WS6 numbers only show 0.7 HP peak, for the U/D pulley by itself. Its going to be even less with an O/D alternator pulley. Yes, there is a bit more "average" HP gain, but its not worth the risk of running down your battery. JMHO

 

Why exactly would an underdrive pulley "free up power"?

The affected accessories are the alternator, power steering and A/C compressor.

The alternators load is determined by how much electricity is being called for, not by RPM. If no electricity is being used, then it's just a pulley with no load.

Power steering isn't really offering much load unless you're turning and its doing work. The spool valve doesn't allow for pressure to build up when you're not turning.

A/C compressor is on a clutch and isn't turning unless the A/C is on, and it turns off automatically whenever you floor it (in the tuning).

Underdrive pulleys don't do **** other than maintain your battery just above dead, and render your power steering and A/C useless at idle.
Even if they did give 10hp (which they just don't) it's simply not worth compromising the drivability of the car. Ask me how I know lol.

 

You are overlooking the fact that the RPM changing (engine is accelerating) is what consumes energy. The amount of energy absorbed in accelerating the rotating mass, like the pullies, alternator, A/C clutch and idler is proportional (either directly, or exponentially) to the mass of the piece, the RPM it is turning and the rate at which the RPM is increasing. Slow down the RPM, and the amount of energy consumed (HP lost) as the engine accelerates to redline is reduced.

Run the engine at a constant speed, and the only loss through the serpentine belt system is friction - negligible. But when acceleration is involved, energy is lost from the nose of the crankshaft, meaning it can't be taken off the back of the crankshaft to power the rear wheels.

Point is, it is a small amount of HP. If you look at the WS6 graph, the delta between the no U/D pulley curve and the with U/D pulley curve is greatest (~5HP) when the RPM is increasing fastest, and tails off to the VERY small 0.70 peak HP gain because the engine is no longer accelerating as fast.


http://www.ws6.com/mod-2.htm

 

Why exactly would an underdrive pulley "free up power more power"?
Because of lesser parasitic drag and this is indesputable. Now what amount it does on the LT1 is by most accounts is very minimal. I remember in the Mustang days you could underdrive a 302 and get an easy 10RWHP due to lesser parasitic losses that resulted in freeing up power. You are not factoring in that all pulleys rob some power due to accelarating the heavy rotor in the alternator, having to overcome some resistance in the power steering pump due to hydraulics and having to turn the massive clutch in the AC compressor even when its not engaged.

Claiming they dont do **** like you claim is just your personal opinion and results and I simply can not agree. My battery stays well charged. My AC works great and I know there is some kind of gain (amount up for debate) due to lesser parasitic losses and the simple fact my ASP UD pulley is several pounds lighter than the OEM pulley. I have zero issues with my $50 used UD pulley. I would not have paid the $200 or so ASP wants for it new for this engine. This is my opinion based off of my personal findings.

Totally agree.

 

You got 10HP on the Mustang because the water pump was belt driven.

 

Agreed. Seems like there was a smog pump too, its been a long time. Point is UD pulleys always free up some amount of power which varies based on application.

 

I would venture to say that unless you are buying the full set that includes a smaller diameter crank/dampener pulley and the alt. set you aren't going to see the full benefit of running just a smaller crank pulley. Just putting a smaller pulley behind the crank only adds a small amount of rotating mass to your crank and slows your all your acc which you risk not fully charging at idle at such- (I know...been there)

For the same $150-200 however I would recommend to someone wanting to free up the motor to get an electric water pump for the LT1. You will get better cooling at idle and also free up 'horsepower' throughout the RPM range.

You aren't going to gain gobs of power but should be a little noticable since the cam driven water pump is like a 'tiny engine dyno' bolted to the front of the motor.