The 9a's are found in Corrado's and are transverse. The 6a and ACE is from the Audi 80 sport 16v, and is longitudinal. I used a 6a in my Golf, purely because an 80 is such a cheap donor car, you just bolt all the mounts, brackets, pulleys and manifolds from any transverse 16v straight onto them (which all came off my dead KR that was in it in the first place), whereas the 9a is all ready to go. The ACE is the same as the 6a (but from the facelift Audi 80's) but apparently the oil filter and housing comes off at the wrong angle so you end up with clearance issues, but again, you can swap the whole housing over. The Audi donor I got, cost me £175 off ebay, and I made a large portion of that back selling all the bits off it