vw 16v the right engine to go in a mk1 but by far the least tuneable
In all fairness, the VW 16v cylinder head is a very poor design. If you compare the flow rates with that of an XE head - the VW head looks very silly. Did you know VW couldn't design a 16v head? They got an Italian company to do it.
The flow rates of the VW 20v(based on the Yamaha R1 engine) engine is far better than the VW 16v, especially in the higher rev range.
The initial batch of 2.0 XEs came from Cosworth. Vauxhall had overheating problems and gave the design to Cossie to sort out. They, basically, drilled one coolant gallery though the head and gave it back! lol! The first batch had all the cossie stuff - valves, guides, casting etc. The rest were duplicated by Vauxhall and aren't as good quality.