I've done some searching and can't find the info i'm looking for. I'm wondering what exactly do underdrive pullys do, how do they make more power and are there any trade offs? I have used programmers with out much luck and would rather keep my stock tune, however if i can put some pullys on for a couple hundred bucks and not effect the driveability of my truck then i'll do it.

 

They spin your accessories slower, therefore less parasitic drag. Can possibly cause overheating at idle and give you less alternator output at idle. You can help with the heat issue by changing to E-fans, but this can become an issue with the lower alternator output. Can give you other idle issues such as power steering and A/C.

Overall, a good thing, but you need to be aware of these possible issues. If there were not the possibility of these issues, they would certainly put them on at the factory.

 

I don't know a whole lot about these pulleys, but the topic has come up several times on the forum for sure. I just ran a search using this --> Summit pulleys

Brought up a good list, including one titled "Will someone tell me about underdrive pulleys?" Probably what you are looking for. Here is the link to my search that I hope will work for ya. Otherwise, ran that same search and you should get it.

https://www.f150online.com/forums/se...archid=1549261

Good luck.