H3 underdrive pulley
 

Has anyone installed any underdrive pulleys for their H3? If so what is the pros, and cons. Thanks.

Old school stuff, useless and counter productive on current GM PCM controlled motors.

Your H3 (I5 or Alpha) OEM alternator is weak! Slow it down and your headlights will dim, wiper motors will slow, etc....

The PCM controls the alternator output based upon electrical needs. When demand is low, the PCM will variably reduce output to reduce load on the motor driven belt. Underdrive pulley(s) do that mechanically at a fixed rate, which will then not correlate to what the PCM expects when it adjusts load.

If you want to take a load off, do an Efan conversion. Here is mine from PCMforless.
Attachment 16177

Your H3 (I5 or Alpha) OEM alternator is weak!NOT SO
 

undersize pulley is a waste of time,no benefit,BUT NOT BECAUSE THE ALT/GEN IS WEAK!

everytime a question of voltage comes up i fine tune my bs filter and go to work!
the last time it was lights dimming when stopping.i tried everything humanly possibly,while monitoring with a digital meter and saw zero dimming and at most 1-2 tenths of a volt drop.
same here;
i monitored the voltage on startup and running. the voltage went up a few tenths on warmup,always above 14.3,PLENTY.

I REVED TO 2000 RPM WITH NO LOAD AND DROPPED TO ABOUT 600-700 IN GEAR WITH HIGH BEAMS ON AND FAN AT MAX,AND CRANKED UP THE RADIO!!!

THE VOLTS NEVER CHANGE MORE THAN .2-.4 V AND SETTLE DOWN TO THE ORIGINAL,STABLE OF ABOUT 14.7

you have no way of slowing it down unuf to matter
this alt/gen is one of the best regulated supplies i ever saw and i build and repair and work with reg supplies for electronic/2 way radio and im real fussy about regulation.
ive built supplies that would go "UP!".1v when keying the transmitter,100 watt! i like that!

so if you underdrive your alt/gen the regulator will turn it up when needed and certainly,under the worst case of 600 rpm you will not discharge or it would be unnoticeable when driving at 1000 to 2000 rpm.

based on my experience with this h3,if u have ANY prolonged (like 30 seconds) dip in voltage its probably caused by someone messing with the wiring,connectors,or possibly just a no good battery.
i welcome any comments/questions on monitoring,testing voltage.

fine tune your bs filter!!,your stock gen is fine.

btw,at high speeds its likely better to have no fan,but at low speeds in HOT weather when u need a fan ,i would doubt that the dc motor driven fan could be more efficient than the stock fan since all dc motors have losses and to provide the cooling power of the big fan,takes a lot of juice.
dc power loads the engine just like the fan does,it aint free

none available
 

I don't think there is an underdrive crank pulley available. It's a shame because it's probably worth 5-8 hp increase at the wheels.

not likely
 

first,if you could get free horsepower by changing size of the pulley,EVERY car maker on the planet would make them bigger tonight.they are known to be cut throat to make/save money. they never would throw it away.
there are technical reasons also that i mentioned b 4.

oops bigger pulley huh!

As much as I hate to agree with tom, an underdrive pully would not be a good idea in terms of the I5 or the V8 without some kind of PCM/ECM/BCM (depending on the year) changes to the regulated output. Pretty much EVERY electrical function is controlled by the BCM on our HUMMERS. The BCM receives it's information from the PCM/ECM (once again depending on the year) and processes that information in nano-seconds to either boost the generator output or lower it.

Doc, I have to disagree with your comment "OEM alternator is weak!"! The "alternator" (actually a generator) has more peak power output than a stock H3 could ever need even with all of the options and OEM accessories running at once.

Well, once again, you are incorrect. You will benefit from underdrive pulleys, just maybe not in the I5...it's hard to tell since no one makes them, and that may be the reason that the don't. I personally have contacted March Performance Pulleys seeking some.
However, it is possible to see hp gains from using underdrive pulleys, some as much as 15hp but that is usually in vehicles that have the alt, a/c, power steering, water pump, fan, and the smog pump all under one belt. Also, I can say that Ford does install underdrive pulleys from the factory on some vehicles, mostly their "SVT's or other special vehicles.

Underdrive pulleys are a direct application of physics. If you use a smaller diameter crank pulley you will spin the accessories slower (water pump + fan, PS pump, alternator, etc.) which reduces the parasitic power loss meaning more power to the wheels. The power to rotate the water pump increases with the RPM cubed. So, a 20% underdrive crank pulley will result in about a 50% reduction in the required parasitic power to rotate the water pump alone.

Manufacturers aren't perfect. The 06 H3 has an an air box intake nozzle that is too small and they increased it for 07. Doc Olds provides a larger diameter replacement nozzle with his air kit. Opening up the air box nozzle nets free HP, so why didn't Hummer jump on it overnight to gain free proven HP?

Here is an underdrive pulley for the Alpha:

2003-2009 Hummer H2/H3 Underdrive Pulleys from Thunder Racing

You don't need BCM or PCM changes. Your PCM must be pretty fast to control the alternator output voltage in nanoseconds!

I wasn't clear. This was for the I5. There are UD pulleys for the V8.