Well said Ray! I have never heard of a 2.0L 914 that actually ran Djet and 48mm intakes and worked. Just too big a valve and problems with intake air speed.
I run a 38mm exhaust which helps the exhaust flow alittle more, but I also run ported heads.
As DD said with Djet its all in the cam, if you stray too far from the stock valve timing/duration it won't work at all due to the vacuum problems/MPS.
As for the 2.3l engine, I would say Jake Raby has the most experience trying to set up those engines. I recall that he had problems with the stockish cam and getting rid of the heat. Meaning the engine wasn't able to breath properly and the exhaust heat built up. His solution IIRC was a longer duration cam or split duration cam and adjusted the exhaust timing so the engine was able to exhaust the exhaust heat, as the Type 4 head exhaust design is it weak point.
Geoff