On the outside these 2 cylinder heads appear to be identical. The ports are the same size and the valves are the same size. Using a non turbo cylinder head may seem like a good idea as they are usually in better condition having not suffered any damage caused by excessive boost and or poor tuning.
- The valve stems on the Non Turbo engine are 6mm in diameter. (right image)
- The Valve stems on the Turbo engine are 7mm in diameter. (Left Image)
The extra 1mm in diameter can make a huge difference as the 7mm valve is 36% larger in cross section and 16% larger in circumference. The larger cross section of the valve stem allows for better heat transfer up the valve stem and the larger circumference or contact area with the valve guide also allows for better heat transfer from the valve into the cylinder head.
Whist there is probably no decernable performance difference between these two cylinder heads the bigger valve stems will help to keep the exhaust valves cooler and prevent heat damage and detonation should the exhaust valves get too hot.