Just thinking out loud...

Why are most VVT engines only variable on the inlet cam and not both the exhaust and inlet cams? Old man's Beemer for example has VANOS on the inlet cam only and that's the same as the VVT on the ST170. The M3's and M5's however run double VANOS so they run on both the inlet and exhaust cams. I'm assuming that's how the duratec Vi-VCT runs as well (only on the inlet cam). How comes it isn't introduced on the exhaust cam as well then on the ST170 for example? Also why isn't it used a lot on a turbo cars? The Evo 9 and 10 though has the MIVEC system I think.

Cheers!