The countdown to the launch of McLaren’s long-awaited second supercar has begun. Early prototypes of the final design began shakedown tests in mid-April.Codenamed P11, the first of a number of versions of the new McLaren is expected to roll into showrooms in March 2010, although it’s expected to be unveiled in autumn 2009. The P11 will be followed by both open-top and hardcore versions of the base model.Insiders say the mid-engined car will be pitched as a rival for the £130k Ferrari F430. The P11 is also said to be noticeably lighter and more compact than the competition.It will be powered by a 550bhp V8 engine which some sources say will be built by Mahle (which bought out the old Cosworth business) in Northamptonshire. It’s thought that the McLaren engine will be based on an existing Mercedes unit.Insiders in Germany say that production of the Mercedes McLaren SLR will be closed down by the middle of next year. This will clear the way for the P11 production system to be installed at McLaren’s Woking facility.The Spider version of the P11 is expected to appear in March 2011. There’s scant information on this model, which is still in the development phase, but it is expected to have a proper folding fabric roof.The summer of 2011 should see the ultimate expression of the new McLaren supercar going on sale. Project P12, which should cost around £250,000 in today’s money, is said to be a much more hardcore, driver-focused interpretation of the P11. It’s also likely to be the version closest to the spirit of the iconic McLaren F1.According to internal McLaren documents finalised last year, a V10 engine delivering over 600bhp will power the P12. The rest of the car will be stripped down and re-engineered for ultimate weight saving.