I have been meaning to write this for quite a long time and several members have recently been pestering me to finally post the details.... Sorry for the delay!!
A lot of you will know about some of the peculiar problems with the N54 engine, namely the failure of ignition coils, rough running, drop in fuel economy, poor performance etc etc... Many of you however might not know about the problem of carbon build up on the inlet valves / intake system.
It is a fairly well-supported thought that part of the problem with the N54 engine is the fact that because the engine is direct injection, there is no longer petrol washing over the inlet valves and this therefore lets carbon build up. There is also a belief that the recirculating crankcase ventilation system allows oil vapour to be deposited in the intake system and this exacerbates the problem of carbon build up.
There have now been a significant number of cars that have gone into the UK dealer servicing network where it has been discovered that the cylinder head has become carbonised to the extent that it has caused substantial deterioration to engine performance. The solution has been to remove the head and send it away for a chemical cleaning process, costing around £1500. What hasn't been discovered is whether the build up of carbon has had any adverse effect on the rest of the engine.
I have had all this in the back of my mind and over the last few months I have been more and more concerned at the performance of the engine in my car and the fact that it was giving out ever increasing amounts of smoke out of the exhaust. The smoke has generally been just white in colour, with a tinge of blue on sudden full throttle openings. I have known for a while that my engine was going to be in need of a strip down and inspection and I suspected there was oil seeping past one of the piston rings and/or the head gasket was on its way out. The strip down of the engine would also give me the excuse to finally change the internal hardware (if ever an excuse were needed
I had previously contacted CP Pistons (www.cp-carrillo.com
), one of the world's foremost piston engineering companies owned by Pankl Racing in Austria, and had asked them to fabricate me a bespoke set of lightweight forged race pistons.
The standard compression ratio of the N54 engine is 10.2:1, whilst Alpina use a 9.4:1 (corrected) piston in their B3S. In basic terms, lowering the compression in a turbo engine allows the safer use of more boost pressure, with one of the downsides being an increase in lag. Working with the engineers at CP, we decided on a 9.5:1 compression ratio, using their highest-quality forged pistons complete with low friction thermal coating on the dome and skirt. Their race pistons are a complete work of art and are proven worldwide having been used in many of the winning German DTM race cars.
I also contacted another division of Pankl Racing called Carrillo (www.cp-carrillo.com
), who are one of the world's top two manufacturers of conrods, and commissioned them to make a set of bespoke forged conrods for the N54. Like CP, their conrods are used in motorsport across the globe, and the same material used in the rods for the N54 is used in top-fuel dragsters producing over 1500bhp. The Carrillo rods are considerably lighter than the BMW OEM versions, and combined with the new pistons will offer less reciprocating mass in the engine (exact weight data will be provided once the OEM pistons are cleaned up).
So, back to my engine...
The ever increasing smoke from the exhausts eventually persuaded me to take the car to Birds (who else?!) for the full strip down operation to begin. After a day's work, the entire engine and gearbox assembly had been dropped down from the car and the engine stripped down to its bare components. And then the problems were revealed.....!!
The guys at Birds discovered that piston #5 was on its way to destruction. The piston had lost a chunk of material on the corner and the rings were badly worn. Oil had definitely been seeping past and there was also a tiny speck of plastic sitting on top of the piston crown (where that came from is anyone's guess!) The top of the piston was covered in specks of carbon and tiny bits of aluminium. The other cylinders looked ok in terms of piston wear, but the cylinder bores were all significantly scored, much more than would have been expected from a 50,000 mile engine (which is what mine has done). You can see what I mean in the following pictures -
Quite why the bores are so scored is unclear, although it is most likely that the pistons became too hot and grew too larger for the bore diameter, therefore rubbing along the bore and shaving particles of aluminium off the crown. It is a little concerning because we don't know why the pistons would have become so hot. The engine has only been running between 14-15psi of boost so it is unlikely this is the cause, especially given Alpina run similar levels of boost in their version of the N54 engine.
It could potentially be an oil lubrication problem, what with the car having been massaged around the Nurburgring once or twice
The oil sump has been cleaned up with a view to getting a special oil pan baffle manufactured to prevent oil surge and ensure constant oil pickup.
The engine block required an overhaul, with the cylinders bored to allow for the one-oversized CP pistons. The rebore also has added benefit in that it removes the scoring from the walls and any extraneous debris that might be found. Serdi (www.serdi.co.uk
) are the company used to handle the machining.
You can see the state of piston #5 in the pictures below, and you'll also see a brand new CP piston as a comparison
The crankshaft also shows abnormal wear on the bearing, as can be seen in the pictures below. Again why there is abnormal wear here is unclear, but what is certain is that this wear is not a result of increased power from the engine. The only solution, apart from buying a new crankshaft, is to have the bearing ground down to suit the next undersized bearing shell. Fortunately BMW manufacture four different sizes of bearing shells just for this scenario.
More problems here, although this was largely expected. There has been a substantial build up of carbon on the intake valves, so much so that air flow is visibly restricted. The valves and valve seats seemed ok and only in need of a clean up. The valve stems would need a total strip down to get rid of the carbon, so in essence the head would have to be completely disassembled.
Given the state of the head and the process required to return it to a fit and proper condition, I decided to have the head completely overhauled with a full gas flow and port polish process to improve performance even more. The process involves a complete disassembly and cleaning of the head with leak, crack and pressure testing. Inlet and exhaust valves are then unshrouded and reprofiled to allow better gas flow, and rebalanced and blueprinted with the installation of new uprated valve guides and Viton valve seals. The valve springs are also overhauled and balanced. The net result is a really significant increase in performance with absolutely no drawbacks.
The pictures below show the cylinder head as it came off the engine, and the state of the valves. If you look closely you can see the carbon build up on the stem of the open valve.
I'll post up more pictures in due course to show the engine in the rebuild process and also the new cylinder head. In the meantime, there are a few more pictures of bits of my car on the floor...