HEAD AND CAMS
The top end of the YB engine is impressive in stock form too — after all it was designed to go racing! 350 bhp is no problem for a stock head, but at around 500 bhp the ports start to become a bit of a restriction. As standard, the inlet ports are 22.5 mm and the exhaust ports are 23 mm – opening these up to around 25 mm and 24 mm respectively will be good enough for 500 bhp while retaining standard valve sizes. For even more power, the port sizes can be increased to 26/27 mm and 25/26 mm, but you’ll also need bigger valves to make the most of them.
As for camshaft options this will usually depend on what the engine is being built for — there are many off-the-shelf profiles to choose from and many engine builders have cams ground to their specific requirements too. Top-end engines will be running custom spec profiles (closely guarded secrets), and some are so aggressive they actually require the head to be machined in order to fit them — the ‘lobes’ are so big they physically won’t rotate within the confines of a standard head.
But for 500 bhp engines there are plenty of options. Generally, a BD10 inlet with standard exhaust cam is seen as the first stage upgrade and works well on a variety of engines, but BD14 and BD16 combinations have also been used to good effect too, while some tuners swear by the profile offered by an AB07 cam. As with turbo choice, the cam profile needs to be chosen to work with the rest of the engine spec, so is often best left to the engine builder to decide.