I currentlly have my airpump of my car. What would I gain by putting it back on?
And what would I lose by putting it back on.

 

Since you have no cats, no 6PI, I don't see the need for the airpump other than having more belt area for your waterpump pulley.

 

I think the only purpose for the air pump I think is for emmission.

 

i have the Racing beat downpipe pre-silencer set up and my air pump is still on. Although it did come with a blo0ck off plate witch when installed did nothing or at least i didnt notice it so i put it back on for pulley length.

 

The airpump on a series five drives the auxillary ports and the VDI.

 

Pros:
VDI would work again (gaining back power at the high end), as well as if the aux ports were re-installed you would gain torque at the low end, with a working air pump

Cons:
you can't run a dual pully alt.

 

With your current set-up I don't think there would be any pro's to putting the airpump back on, since you have removed the 6PI and VDI.

 

Get a dual alternator pulley if u take off the airpump and get two Gates belts, i suggest gates because if you get other brands the belts may vary in length and the belts wont be tensioned evenly (i know from experience)