Paul. You did not specify that you were thinking in terms of the turbo engines. The 1.6 EcoBoost would not be a replacement for the 4/4 but would be, in effect, a replacement for the Roadster.
A possible EcoBoost replacement for the 4/4 would be the tiny 1 litre 3 cylinder in its more powerful version: 92 kW (123 hp), 170 N·m (125 lb·ft) from 1,400–4,500 rpm and 200 N·m (148 lb·ft) on overboost
Compare that to the current Plus 4: Max Power 115 kw (154 bhp) @ 6000 rpm, Max Torque 201 Nm (148 lb/ft)
By comparison the 4/4: Max Power 82 kw (110 bhp) @ 6000 rpm
Max Torque 131 Nm (97 lb/ft)
At the end of the day it is all about torque from low to mid range. In the real world that is acceleration, overtaking, getting up hills etc.
Having said all that it is difficult to get good torque from a small engine until the turbo cuts in (turbo lag). I know that it is better now than it was in the past but it still exists. It surprises me that more work is not done on small supercharged engines.
The Lotus Elise SC is a good example of what can be done: 218 horsepower at 7800 rpm and 153 pound-feet of torque at 5500 rpm from a 1.8 engine. It has a very flat torque curve but is a fairly high revving engine. An engine tuned for less ultimate power at lower rpm would probably produce good torque from lower rpm too.
You are quite right Peter, I kept the turbo thing up my sleeve, but I honestly think it could be the way to go. I drove a 2.3 Ford RS Focus recently and that had so much low down torque and the turbo lag was virtually non existent, at least, I couldn't feel it, especially compared with the Saab 9000 turbo I owned for 6 years some time ago, in which nothing happened until 3500 and it would then take off like a scalded cat, giving huge amounts of torque steer, especially if in 2nd and overtaking, crossing a white line in the wet! Very exciting!
