For several decades now BMW, in common with other European volume car manufacturers, have followed a strategy of offering each model with a range of engines to attract buyers with different requirements regarding purchase price/insurance/economy/etc.
Thus, Z3's were variously available with:
Obviously, the smaller, lower powered cars are going to be slower, but not that much slower compared to the entry level six - even the humble 1.8 is only 1.5 seconds slower to 60mph and gets to within 8mph of the two litre's top speed.
Throw a slush box into the mix and things aren't so clear cut. A manual 1.9(16v), the most common (and cheapest) variant, is only 2.2 seconds slower to 60 and 11mph shy of a 2.8 auto's figures. What did BMW do with all that extra capacity and two extra cylinders?
Looking at the figures, they only managed to squeeze another 53
bhp out of a 900cc increase. This gives the 2.8 only 69bhp/litre while the 1.9 pumps out a far more healthy 74bhp/litre, so it's a more highly tuned motor than the wheezy old 2.8, which is clearly underpowered for its capacity.
If only the 2.8 was as well developed as the 1.9 it would make an easy 207bhp, which, coincidentally, is pretty much what Porsche got out of the 2.5 it put in early Boxsters, which are generally considered to be a bit slow.
Unsurprisingly, given its highly tuned nature, the 1.9 is actually half a second faster to sixty than the auto two litre, and there's only 1mph difference in top speed. So a manual 1.9 will easily get away from a 2.0 auto on a fast B road blast. Does that make the 2.0 auto shit?
Only if your sole criteria for making the judgement is speed. This would, of course, mean that the 2.8 is shit compared to the 3.0, which is shit compared to the 3.2M, which is, of course, shit compared to a TVR Cerbera at a similar price point.
I've done a bit of research on the much maligned 1.9 and it seems that its major issue is a heavy flywheel, which blunts its ability to pick up revs. That's it.
As a kit car builder there are other considerations when choosing an engine. I will be building a 250 at some stage in the not too distant future, and, believe it or not, tyre availability will have a major impact on the donor I choose. You see, the wheels I want to fit will be wearing 215/65/15 tyres. Right now, I can only find one 'V' rated tyre of that size while there are a dozen or so 'H' rated options, so I'd be reluctant to use a donor capable of more than 130mph. That limits me to the 1.8, 1.9's or a two litre six.
Having said all of the above, it will, of course, be...
A two litre six.
Call me a hypocrite if you like, but I rather like the noise it'll make.