...if you look at the ash green V8S that is sold on PH it is a 1992 precat..in fact most V8S's I have seen have 400 so I not sure that this is the reason...
Would like to know if it is the same engine just a different plenum chamber or whether there are other differences
The only variations in the engine that I'm aware of are cat / precat. The plenum top and badges are common to all the RV8 powered TVRs and it may be they just used a different badge when it was available.
If it says 3.9 its a Range Rover plenum. If its blank with a window for a badge that says 400 etc then its a TVR one. As far as I'm aware there is no other difference other than the badge.