HOW TO SOLVE A PROBLEM LIKE …
BMW S55 crank hub slip
BMW M’ s first turbocharged straight-six, the S55, opened up the brand to a whole new market, one that loved massive modified boost. But that audience will tell you the engine has an Achilles Heel, the crank hub
Where do the problems occur?
The 3-litre twin-turbo straight-six known as the S55 may have been the first inline six developed by BMW’ s M department, so the first boosted S-designated engine with six cylinders, but there’ s very little to indicate it is an early try. It’ s powerful, strong, tractable and characterful, and while it doesn’ t have a screaming high rev limit or ear-splitting induction noise of its naturally aspirated predecessors, it’ s easy to see why it has its fans. And there are plenty of them who love to modify the S55 and turn up the boost.
First launched in the F80 generation M3 and its closely related sibling, the very first M4, the early S55 produced 430hp. As more variations of the M3 and M4 were launched, more powerful versions were introduced; 450hp in the Competition models and 460hp in the CS versions.
When the M2 was upgraded to become the M2 Competition, as its N55 engine was replaced with the S55, a less powerful 410hp derivative came out. But to round off the S55’ s life, a 500hp version – complete with water injection into the inlet from the factory – was installed into the M4 GTS and DTM Champion Edition.
While there’ s a lot to applaud, and a lot that means the S55 can make high horsepower reliably, there’ s one element that is a known weakness: the crank hub. The sprocket for the timing chain can slip, causing the cam timing to change. Every version of the S55 uses the same design.
The bolt and front of the crank hub of the S55, the engine in the first turbocharged M3
What is the common fault?
Ben Koflach, the managing director of BMW engine specialist Hack Engineering, talks us through the S55’ s crank hub setup and why, actually, it’ s not the design of the hub that’ s the problem. Despite what is often reported.‘ BMW used a friction-reliant design for the crank sprocket. There are two sprockets on the front of the crank, one for the oil pump, fuel
52 The Engine Rebuilder Issue 05