# Learn me on underdrive pulleys **URL:** \ **Category:** NYSpeed Automotive\ **Created:** [June 28, 2009, 7:13pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808 "2009-06-28T19:13:11Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![dirtye30](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/dirtye30/32/7333_2.png) [@dirtye30](https://forums.speedlife.net/u/dirtye30)\ **Post date:** [June 28, 2009, 7:13pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/1 "2009-06-28T19:13:11Z")
dont know much about them. pros/cons? ---
**Author:** ![ultradriver10000](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/ultradriver10000/32/7397_2.png) [@ultradriver10000](https://forums.speedlife.net/u/ultradriver10000)\ **Post date:** [June 28, 2009, 8:09pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/2 "2009-06-28T20:09:43Z")
I was always told that the ones without harmonic balancers adversely affected your engine long term. Otherwise I have one on my vette, and with a large cam my lights dim at idle. If you have a huge stereo you might have a problem. ---
**Author:** ![38stang](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/38stang/32/7479_2.png) [@38stang](https://forums.speedlife.net/u/38stang)\ **Post date:** [June 28, 2009, 8:22pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/3 "2009-06-28T20:22:37Z")
It turns your accessories at a slower speed in theory giving your motor 4-6 more hp. I don’t think they are ever needed on any car, don’t use them if you have a system or a lot fo electronics bec underdrive pullies barley drive the alt. enough to run all the foctory items on the car ---
**Author:** ![newman](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/newman/32/7569_2.png) [@newman](https://forums.speedlife.net/u/newman)\ **Post date:** [June 28, 2009, 9:06pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/4 "2009-06-28T21:06:17Z")
don’t replace a factory harmonic damper with an undamped pulley ---
**Author:** ![brent\_strong](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/brent_strong/32/7203_2.png) [@brent\_strong](https://forums.speedlife.net/u/brent_strong)\ **Post date:** [June 28, 2009, 9:21pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/5 "2009-06-28T21:21:39Z")
I definitely wouldn’t do one on the F20c. ---
**Author:** ![dirtye30](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/dirtye30/32/7333_2.png) [@dirtye30](https://forums.speedlife.net/u/dirtye30)\ **Post date:** [June 28, 2009, 9:26pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/6 "2009-06-28T21:26:03Z")
i should specify this is for a small block chevy. 383 4 bolt .030 over on :tspry: ---
**Author:** ![LZ1](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@LZ1](https://forums.speedlife.net/u/LZ1)\ **Post date:** [June 29, 2009, 4:06am UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/7 "2009-06-29T04:06:31Z")
I had one on my LS1 it was after market and was fluid dampened…Seemed to make it quicker. ---
**Author:** ![ubengineering](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/ubengineering/32/5171_2.png) [@ubengineering](https://forums.speedlife.net/u/ubengineering)\ **Post date:** [June 29, 2009, 5:30pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/8 "2009-06-29T17:30:20Z")
got an unorthodox udp on my neon, def felt a small difference, and i bought it used for 50$, also got better gas mileage cons: doesnt apply to me, but doesnt turn alt as much fix: buy an overdrive pulley for the alt verdict: 50 bux for 2-4hp is a good deal ---
**Author:** ![nytalonTSi](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/nytalontsi/32/5399_2.png) [@nytalonTSi](https://forums.speedlife.net/u/nytalonTSi)\ **Post date:** [June 29, 2009, 6:33pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/9 "2009-06-29T18:33:26Z")
On DSM’s i know an UD pulley is bad. There would be no balance, nothing better then a oem harmonic balancer. That way you won’t risk it. Or a expensive fluidampr harmonic balancer. ---
**Author:** ![Studderin](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/studderin/32/7197_2.png) [@Studderin](https://forums.speedlife.net/u/Studderin)\ **Post date:** [June 29, 2009, 7:47pm UTC](https://forums.speedlife.net/t/learn-me-on-underdrive-pulleys/69808/10 "2009-06-29T19:47:52Z")
a stock LS1s is fine with a UD. There only balanced at I forgot, low 800ish rpms for smoother idle. And its the fully assembled motor, by drilling some 1/4 holes in the balancer. Witch has no keyway or way to indexing it to repeat it, on the crack snout if you ever remove it. They only show a 0-5whp gain on a dyno but their always a least 1tenth 1mph faster at the 1/4.