But that's the point. The diesel does better, while they both are 2.0 liter engines. The diesel engine works more efficiently, and it will get better laptimes. We tested this a bit in GT4 also.
(That game has both, as did the BMW GT demo that was released for the 1-series launch in Europe)
Difference may not be so big yet, but who would have expected this to happen 10 years ago?
I agree, the 120i/120d isn't a case that helps my argument much... then again, the 4 cylinder from BMW isn't exactly cutting-edge either. Also, there might not be an "i
s" version in the 1 class BMW, but there is one in the 3 series BMW. Both 2 litre engines, the 320i
s gets around 170bhp. Compared to the 320d, it's quicker too.
And as Ostepop pointed out: The diesel engine is forcefed, the NA isn't. We can look at a 335d vs 335i comparison if you wish though (ff vs ff). 286hp against 306hp... hmm...
When we look at the 6 cylinder versions, as in the 330d vs 330i (230hp vs 258hp), it's the latter that is significantly quicker, eventhough the former has 200Nm more torque available as low as 1750rpm. On the other hand, the diesel will be quicker in in-gear acceleration, though how important is that on a race-track?
When looking at just about any diesel vs regular fuel cars - one thing becomes quite apparent. All in all, the diesel is always around 100kg heavier. For one, it's the heavy engine (which on the other hand gives it good tuning potential), but also heavier/stronger parts from the clutch to the drive-train that needs to cope with the power. In lighter cars, this will become a factor even more so.
And weight is certainly a big factor when it comes to drivability and how well the car corners. You don't want to have more weight than necessary in the engine bay. The diesel is significantly more efficient though, not only in fuel consumption but is also a lot more environment friendly. On the other hand, more torque means higher wear of components which doesn't come cheap...