I’m interested in a buying a used CitroÎn C3 Picasso and have seen cars with both 8-valve and 16-valve versions of the 1.6-litre HDi engine advertised. As my budget seems to cut right through the middle of the changeover (late 2010?), which of the engines is best? What are the differences, apart from the absence of eight valves, and which of the pair are more reliable? 

Bill Hart

Well Bill, the early 16-valve 1.6-litre HDi engines were available in both 90 and 110 versions, and as I understand it, with the advent of the Euro-5 units (From October 2010 in C3 Picasso), they then all became 8-valve ñ the same change that happened to the similar Ford and Peugeot engines, as you would expect. It’s easy to tell which car is fitted with which engine, as the early C3 Picassos had HDi 16 on the engine cover, whilst the later cars just had HDi. All three of the current BlueHDi 1.6 engines (75/100/120) are 8-valve engines, too.

As for which might be more desirable, the later 8-valve engines are more economical, and one might possibly suggest that the simplicity of eight valves is a bonus, not that I’m aware of any problems with the earlier 16-valve units. For me it’s all a bit puzzling, because generally 8-valve engines have better low-down torque and I’m not really aware of why these engines started out with 16-valves in the first place, and what visible benefits they offered at the time over 8-valve designs, other than perhaps perceived technical supremacy. The change to 8-valves seems to have been purely to enable better emissions (to meet Euro-5) and the latest Euro-6 Blue HDi 100 engines are cleaner still, and even more economical. I think it comes down to issues related to the swirl of the intake charge, which is improved on 8-valve engines, and facilitates the lower emissions demanded for the later engines. I don’t think whether a used car you look at has the 8-valve or 16-valve engine should be a game changer, but the superior economy of the 8-valve would probably be a bonus. Hope this helps.

Doctor Diesel