Curiosity on underdrive pulleys

[EvasiveEF9]

Ok, I understand all the advertising on "underdrive pulley's free up tq and hp!" "extend accessory life!" etc etc

But I've never fully understood HOW they do it. Are they lighter in weight? Do they use a different type of belt? Is the circumference different?

Just curious...

[Scotsman]

lighter weight laddie!

[Motivation]

Good question... guess I never really thought about it to be honest...

[DecoyOctopus]

the pully is bigger than the original or smaller i forgot...its like having your accesories all run on a shorter gear. like you know how its easier to accelarate in 1st gear rather than 2nd. its a gearing thing. some of them are lighter in weight than the original so they have less unsprung weight but thats not whats responsible for freeing up the hp n torque its the size of the pully that does it.

[Motivation]

reps to u sammy. I dunno so I am just guessing that the pulley's would actually be smaller which would make them able to spin faster? but like I said I don't know

[DecoyOctopus]

i wanna know how well they work. i hear about alot of mustang guys doing that but not so many imports. i wanna see RESULTS!!!!!!!!

[wwli]

Keep in mind that when you replace any crank pully take into consideration of the harmonic distortion that an engine crank produces.

If just using any smaller CP without the Harmonic damper you can destroy your oil pump or damage your crank.

A good fluid damper can help you greatly. Here is a little reading Damper read

[EvasiveEF9]

the pully is bigger than the original or smaller i forgot...its like having your accesories all run on a shorter gear. like you know how its easier to accelarate in 1st gear rather than 2nd. its a gearing thing. some of them are lighter in weight than the original so they have less unsprung weight but thats not whats responsible for freeing up the hp n torque its the size of the pully that does it.

I see...but I just don't see how a smaller/larger would free up power if the weight of the pulley (due to its size) is changed.

The only way I could see that making a difference is rotational-weight; such as the same purpose of getting a lightweight flywheel. And on a 4banger, I don't see how that LITTLE amount of change in weight could cause the advertised power gains (granted, we know with advertising, they like to bump the real readings). However, on a v8 (like my latest car) I would think that as much weight as extra four cylinders has, I wouldn't think the weight change on THAT pulley would make the advertised changes, either...

I think I'm probably going to stick with my OEM pulleys after reading the posted write-up...but I still wanna know how/why they can even think to make the advertised power gains...

[cornercarver78]

it's either a smaller pulley on the crank, or a larger one on the accessory. This "under-drives" the accessory, or drives it slower, robbing less power from the engine. The pully should probably be lighter. It's just changing the mechanical advantage so less energy is going to the accessories.

[EvasiveEF9]

Sounds good. I wonder if I can find a kit where it changes all but the crank...who knows, maybe for my car I can find one from the its manufacturer.

[DarKStaR]

What kind of car are you looking to put it on? they have lots of different kits like just a/c, just p/s, just crank etc...

[grady156]

If this helps here is a picture of a crank pulley on a B16 I have. I dont think I would use it.

[The Youngn]

i wanna know how well they work. i hear about alot of mustang guys doing that but not so many imports. i wanna see RESULTS!!!!!!!!

People off CaymanClub did underdrive pulleys and dyno'd it with results averaging at about 8whp and 5wtq gains