i am get my cambelt changed and seen lots of times when people have changed their pulley for a UD one but i cant really find anywhere where it is explained what it does (apart from you loosen your belts to tighten them afterwards!).

 

Can someone explain to me what a UD pulley is and how it works how the theory of it gaining bhp works out. What are the down points or problems with them because there seem to be some dissenting voices in the ranks.

 

I have noticed it never mentions UD pulley in the insurance mod list so is it sounds like a rare bhp increase without being hit for it with the insurance.

they are just lighter and smaller versions of whats already on the car.

i am get my cambelt changed and seen lots of times when people have changed their pulley for a UD one but i cant really find anywhere where it is explained what it does (apart from you loosen your belts to tighten them afterwards!).

 

Can someone explain to me what a UD pulley is and how it works how the theory of it gaining bhp works out. What are the down points or problems with them because there seem to be some dissenting voices in the ranks.

 

I have noticed it never mentions UD pulley in the insurance mod list so is it sounds like a rare bhp increase without being hit for it with the insurance.

 

A U/D crank pulley is a smaller and lighter version of the stock crank pulley. As there is less weight for the crank to spin around it requires less power to spin it, this is where you gain a few bhp. As the diameter of the pulley is smaller it will spin around slower than a stock sized pulley (basic law of physics which states something to the effect that any point on the circumference of a circle will spin slower the closer it gets to the centre point). Because it spins slower it will also cause the other pulleys to spin slower (ie, the water pump will push less water through during any given time unit which could potentially lead to overheating problems). This is a bad thing because it puts undue stress on the various components within the water pump. The way around this is to also install an Over Drive (or O/D) water pump pulley. Because the diameter of an O/D water pump pulley is larger than the stock water pump pulley it will spin faster. And because the U/D crank pulley spins slower and the O/D water pump pulley spins faster the resulting rate of spin of the water pump is identical to the stock rate of spin.

 

I'm sorry if the above sounds like gobbledegook - it took me some time to understand it as well.

 

Basically, if you are considering going down the route of installing a U/D crank pulley to "free up" some bhp then please also seriously consider installing an O/D water pump pulley. Furthermore, when changing to a U/D pulley you will need to change the belts for different sized belts. I've got a list here which shows every pulley itteration possible and the corresponding belt sizes. NB - JDM and UK spec cars have different sized stock belts so this complicates it even more!

 

As for the insurance issue, anything that increases the bhp of a car - regardless of whether or not it is listed on their modification index - needs to be declared. Failure to do so could potentially invalidate the policy.

 

HTH

 

Dan

Otherway round Dan - the smaller diameter reduces the drive to the other components, the OD water pump pulley is to increase the revs of the water pump to maintain the cooling requirements ;)

i get the lighter rotating part reducing the losses of the rotataion energy thus letting more power flow from the engine to the wheels and the diameter of the pulley thing but i dont get what the actual "under"drive part means and what its benefit is? Also if the pulley is spining at a different rate are other things affects that is runs off, i am thinking mostly about the valve timing but also the tolerances of the alternator, A/C bearings and pulleys etc.

And i am also assume these pulleys are also stronger because if they are lighter and spinning fast the material would have to cope with alot more strain.

 

How can the OD pulley be fitted? Is it a whole different type of water pump or can you simply just change the water pump cog for a larger one? Is it necersary to do this is you believe your cooling system to be up to the extra power increase through upratred rad, water wetter, etc????

Under drive is because it is driving the auxiliary components slower (the gear ratio is lower). The valve timing runs off the lower cambelt, NOT the pulley so the timing is unaffected. The water pump, alternator, PS pump and aircon compressor are driven slower but not enough to cause any problems. The OD pulley just requires removing the viscous fan and swapping the pulley behind it :)

 

Cheers,

 

Andy

Otherway round Dan - the smaller diameter reduces the drive to the other components, the OD water pump pulley is to increase the revs of the water pump to maintain the cooling requirements ;)

 

See, I told you I had trouble understanding this the first time round! I'll go and make the necessary ammendments now and then get my coat ...

 

Thanks Andy - I usually hate to see people put up incorrect technical information and treat it as "law", now I'm one of those people as well ... :(

 

Cheerio!

 

Dan

See, I told you I had trouble understanding this the first time round! I'll go and make the necessary ammendments now and then get my coat ...

 

Thanks Andy - I usually hate to see people put up incorrect technical information and treat it as "law", now I'm one of those people as well ... :(

 

Cheerio!

 

Dan

 

LOL don't worry about it Dan, it was what you believed to be correct :) If everyone kept their gob shut in fear of posting the wrong info then this forum certainly wouldn't be the place it is ;)

 

Cheers,

 

Andy