i have a 81 caddy and i turbod the 1.5 idi but will a tdi setup work on the 1.5 or do i need to up it to the 1.9
If a 1Z, AHU, or ALH, head will fit on it,
and you can find pistons for it then yes. But you will need to find a piston for it. The Audi Lupo 3L pistons work for the 1.6l IDI.
If you know your piston Diameter then see what the new 3 cylinder VW/Audi cars have, you just might find a match.
You can check this site for Part Numbers and sizes if you want.
http://www.vagcat.com/
the bore on a 1.5 and 1.6 are the same, the stroke is slightly different so you will need to confirm the compression height of TDI pistons for your bore diameter. as for the TDI head fitting on, I am not sure if the valves will fit in the bores as I have not tried it. I am sure someone else on here can help you with that. I can say that if you try it you are going to want head studs and the head bolts in a 1.5 are 11 mm, the TDI's are 12 mm if I recall so you will need to do something to center the head bolts up in the bolt hole of the head.
later
jkeiffer
it'd be alot of work to do it. you would need custom rods, if i was going to do it i would use a later 12mm engine. instead of a 1.5