BMW power figures and performance

[brenflys777]

I have now put 4500 miles on a '61' Z4 S-drive 23i M-sport manual, it has the 6 cylinder 2.5 engine and is supposed to have around 204bhp.

A previous '08 Z4 I had was a 2.5Si with 218bhp out of the same engine. The new one seems to have a better spread of power and is a much less buzzy, more relaxing drive, but it seems just as quick as the old one. The new car is heavier, the tyres bigger and yet the fuel consumption of 34 mpg avg is almost identical to the old one.

I'm just wondering if this is a particularly good example, or if the claimed power figures have become more conservative than previously?

[Mr-B]

brenflys777 said:

The new one seems to have a better spread of power and is a much less buzzy, more relaxing drive, but it seems just as quick as the old one.

I think this is probably the answer. Headline figures are just that, a top number quoted for bragging rights. You would need to look at the respective power and torque curves of the two engines and see how they differ in their delivery. You might find that the more powerful engine only gave its full 218bhp in the last 500 revs of the power band so you only rarely got to use all of the horses whereas the 204 bhp unit may give access to most of those horses all the way from 2500 to 6000 rpm. If the car feels more relaxed to drive I would guess that there is more power and torque available for a larger spread of the rev range and the 218bhp engine was a bit toppy and gave everything it had higher up in the rev range.

I remember looking at the power delivery of a Honda S2000 when I was looking at buying one and found that below around 5000 rpm it only produced something like 84bhp but the headline figure for the engine was around 240bhp which mainly came at the top end and 8000+ rpm.

So its not just the big number thats important its how it gets there and what it does on the way.