hydraulic vs mechanical head - What's the difference?

[foxracer1]

You can shim the relief spring on v8 chevy engines.

I haven't had a VW pump apart to know if you can. A friend put the 36mm in his 1.8 gasser and he's had issues with oil filters failing in the winter. It would always have more than enough oil pressure.

Too much pressure takes HP from the engine. I've heard 10psi for every 1000rpm.

[burn_your_money]

There are two different vac pumps, but they are interchangeable.  

I think the 1.9 AAZ vacuum pump is a larger diameter so it will not work on a 1.6