I have just purchased a 3.5 efi rover v8 to replace the 24v v6 cosworth in my capri . The 24v lump puts out about 200 bhp and returns about 27 to the gallon .
I am going to rebuild the rover and obviously I would like a bit more go so what sort of spec would you recommend to get over 200 horses without losing too much economy bearing in mind that it is my only car and therefore does over 1000 miles a month
If you wish to have 200 plus HP from your 3.5 litre Rover V8, then fuel consumption is going to much greater than 27mpg.
Any head work while lifting power also shifts torque, thus you use more pedal for normal city driving, hence more fuel consumed.
Higher lift cam, longer duration, same effect.
City driving, 15mpg would be good, country 20mpg close to best.
Where the Rover V8 benefits is in its torque delivery. You don't need to change gears as frequently, for the maximum torque is around the 2750rpm mark and it is quite flat overall. Of course, with changes to improve power, so the torque maximum, while also improved is shifteed higher up the rev range.
Out of a 3.5 that starts with about 155 bhp, 200 to 215 is quite achievable with a mild cam upgrade, Lightly ported and cleaned up SD1 heads, good headers and EFI (with an aftermarket ECU) or a 4 barrel carb and care putting the whole lot together (match all the ports to manifolds leave a slightly rough surface on all inlet tracts a good exhaust system etc), all this for relativly little outlay and would probably do the same MPG driven sensibly (especially if you go mapped EFI). The next 10 to 15 BHP will probably cost you the same again in cash terms, or about 15 years experience in building the rover engine.
The next 15 you are talking doubling your budget again, infact a suitable bottom end alone will probably cost you 2 to 2.5 k.
I think what I am saying here is that you can match the cosworth V6 on power fairly easily, probably beat it on torque by a bit but don't expect the rover to be a fire breathing monster unless you have about 6 to 7 K to spend on it (and that budget is quite a bit bigger than that for my 383 chevy which I expect to get 600 to 620 BHP).
Mike
So if you're on a budget there's not too much you can do without needing the ECU re-mapped or fit an aftermarket one.
I got about 230Bhp from my 3.5 doing the following:
Isky 270 cam,
Ported the heads
Bumped the compression to 10.2:1
H. duty pushrods and springs to match cam
Removed flapper air meter and replaced with a fuelling fudge box which took a throttle signal and lied to the ECU about where it was on the map so sorting the amount of fuel required.
I then switched to an Emerald ECU ( after fuelling box went pop) and managed to get about 27-29mpg on a run
.....a Capri with a 5.7 Chevy..... 
