THE BMW Z3 convertible earns more street credibility with the adoption of a 2.0-litre, six-cylinder engine as base powerplant replacing the 1.9-litre four-cylinder. It also gets a new look with a reshaped, better looking rear end. The straight six is no powerhouse but it sounds great and is delightfully smooth. The previous four-cylinder failed badly on all three points. So the Z3 becomes a much more desirable sorts car without losing any of the easy-driving characteristics of its more scrawny predecessor. The only real problem is the price. One 2.0-litre Z3 convertible costs almost as much as two Mazda MX5s.