The flagship M Roadster may well grab all the headlines, but we think that its 2.5-litre understudy is the best Z4 in the range. It is very fast, sounds great and is even fairly economical to run. Enthusiastic drivers will love the newcomer - especially in the latest Sport trim, which adds plenty of M-badged components to sharpen the handling. Only those who are after a more comfortable driving experience will be happier with the softer SE specification.

BMW might not have had the Formula One success to back up its hottest road cars, but when it comes to performance brands, the company's famous M Sport badge is guaranteed to set pulses racing.

However, the new £43,000 M Roadster is out of the reach of most buyers. Thankfully, two new 2.5-litre six-cylinder models are on the way. Coming with either 218bhp or 177bhp, these are priced at £25,650 and £28,365. So, do they represent good value? Auto Express grabbed the keys to a 218bhp Z4 2.5si to find out.

Speeding from 0-60mph in 6.5 seconds and with a 149mph top speed, the 2.5si is nearly as fast as the more costly 265bhp 3.0-litre Z4. Equally impressive are the sharp throttle response and flexible power delivery, providing plenty of pull at low speed, plus strong punch right up to the 7,000rpm red line.

As well as the new engine, our car also had Sport trim. This spec adds M Sport 18-inch alloys, stiffer suspension, a 15mm lower ride height and BMW's DSC+ traction control system. Inside, there are M Sport seats and a three-spoke leather steering wheel.

Corner quickly and the Sport model is more agile than the standard car, has less body roll and even more grip. The pay-off is a firm ride and the additional £2,360 on the list price.