UnderDrive Pulleys - Lets talk Alternator RPMs
Thread Starter
Team Owner
Joined: Sep 2003
Posts: 20,179
Likes: 673
From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24
UnderDrive Pulleys - Lets talk Alternator RPMs
So the big question is; What is the minimum RPM required by the alternator to maintain a charge?
This is important when touring during the summer with A/C and headlights on.
Here some info on the pulleys & Pulley Speed Calculator
Stock Crank 7.125 dia
Under Crank 5.6 dia
Stock Alt 2.125 dia
Under Alt 2.75
Alternator speed at 1000 RPM (engine)
Stock Crank & Stock Alt - 3352
Under Crank & Stock Alt - 2635
Under Crank & Under Alt - 2036
Your thoughts and opinions? Thanks in advance
This is important when touring during the summer with A/C and headlights on.
Here some info on the pulleys & Pulley Speed Calculator
Stock Crank 7.125 dia
Under Crank 5.6 dia
Stock Alt 2.125 dia
Under Alt 2.75
Alternator speed at 1000 RPM (engine)
Stock Crank & Stock Alt - 3352
Under Crank & Stock Alt - 2635
Under Crank & Under Alt - 2036
Your thoughts and opinions? Thanks in advance
Le Mans Master
Joined: Mar 2001
Posts: 9,464
Likes: 1
From: Modesto CA
I've had the Underdrive Crank Pulley on my vette for a few years now. No real problems. The only time that it gets SORTA low (like 12.0 volts) is when I'm sitting idling, with my headlights on, holding the brakes (so the brake lights are on) and the cooling fans are running.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
I run the March under crank & over alt pulleys. Here the crank is of smaller than stock and the alt is also of smaller dia, in the same ratio. This drives the alt at the stock speed but all the other pulleys (AIR, water pump, PS, A/C compressor) at lower speed.
Melting Slicks
Joined: May 2003
Posts: 3,490
Likes: 68
From: Hewitt NJ
Most kits that I've seem "promise" a full alternator output by 900 RPMs. As others have stated the only time it will become a real problem is idling for long periods of times. As soon as you take off the alternator will be able to provide a full charge.
