The Mercedes-Benz AMG 63 V8
Times have changed. Mercedes-Benz AMG vehicles have been the reigning kings of torque for a long time now. The supercharged 5.5 liter V8 makes 516 foot-pounds of torque, while the 6 liter dual turbo'd V12 makes an Earth orbit altering 738 foot-pounds. These were fantastic engines. In 2003 and 2004, the supercharged 5.5 liter V8 and the 6 liter dual turbo'd V12 took first place in the "Best Performance Engines" category of the "Engine of the Year Awards". Building a replacement for the supercharged 5.5 liter V8 was an arduous task, but Mercedes-Benz did it.
The fact that the 6.3 has replaced the 5.5 in most of the lineup is old news, but many people don't know about the major differences and one of the big reasons for the change. One of the biggest reasons for the change was the transmission. The new seven-speed transmissions are rated for up to 542 foot-pounds of torque. The old five-speed is rated up to 796 foot-pounds of torque. The first thing that comes to mind is the seven speed transmission appears to be rated higher than the supercharged 5.5 liter. Mercedes-Benz rates the supercharged 5.5 liter as 516 foot-pounds of torque between 2,650 and 4,500 RPM. That's not the peak number. The second thing that comes to mind is this new engine produces more horsepower, but less torque. I'm not going to get into the "which is better, horsepower or torque" debate. Better is whatever you like. It's true that Carroll Shelby once said: "Horsepower sells cars, but torque wins races." On the other side of that debate, no one ever won a race they couldn't finish, and supplying customers with cars that break their transmissions is a good way to finish last in sales. Could Mercedes-Benz have built a seven-speed with a higher torque rating? Sure. The question then changes to "return on investment".
Regardless as to what "could" have been done, what Mercedes-Benz "did" was build new engines with less torque. Less torque doesn't have to mean slower. This is especially true when one has two additional gears to play with.
A new valvetrain design
These engines are completely new, "clean sheet of paper" designs. Although both the old and new V8's use aluminum blocks and heads, the new engines don't share any parts with previous V8's. Beginning with the heads, the new 6.3s, whose engine designation is M156, are a dual overhead cam, four-valve per cylinder design. This is a departure from the M113's single overhead cam, three-valve design. On the M156, the DOHC arrangement was a requirement due to the use of variable valve duration technology. Both intake and exhaust duration are varied over 42 degrees, based on engine load and speed. Changes in camshaft duration are electro-hydraulically driven, and controlled by the ECU. Those unfamiliar with the benefits of varying camshaft duration may wish to read my piece on Camshaft Basics at the Signal to Noise website . The M156 camshafts are driven by duplex roller chains and a pair of gears.
The new valve train is uses bucket tappets rather than the M113's roller rockers. In addition to being more space efficient, bucket tappets allow for more exacting valve control, which is required with the new 7200 RPM redline. Speaking of valve control, on the exhaust side, the 34 mm valves use conical valve springs because of vibration issues. On the intake side, the 40 mm valves use double valve springs because the valve mass is too much for single springs. Valve clearance is still hydraulically controlled, so there is no additional maintenance. Also, these new large intake valves are fed by dual 70 mm throttle bodies, versus the single 74 mm throttle body used on the M113.
Those who know what a conical valve spring is and what it's for can skip to the next paragraph. For the rest of us, all springs vibrate at various frequencies. These new engines rev to 7200 RPM. Somewhere in the rev range, the folks at Mercedes-Benz probably noted that the coils in a conventional valve spring were vibrating. That's not good, because on a conventional spring each of the coils are the same size and they're equally spaced. That means when they vibrate, all the coils vibrate together. When the coils in a valve spring all vibrate at the same time, the spring squats. When the spring squats, it's no longer at the correct height. Short valve springs cause valve float. On a conical spring, each winding uses a different sized coil. With different sized coils, the entire spring doesn't resonate at once due to the same frequency, so the spring doesn't squat.
This leads into the next diversion. Some people don't know what valve float is, or what it does. If you know, feel free to skip to the next paragraph, where I'll try to stay on track, but I can't guarantee it. Valve float is when the cam lobe is not in indirect contact with the valve (it's "indirect" because the tappet is between the cam lobe and the valve itself). When the cam lobe loses contact with the valve, it's usually some time after the lifter has passed over the nose of the cam lobe. That means the valve is closing. When valves are closed, they're designed to be set down on the valve seat (the valve seat is the part of the head where the valve sits while it's closed). If the combustion cycle closes a valve, the valve will be slammed into the seat with a lot more pressure than the valve was designed for. Also, after being slammed down, the valve will bounce back up. It's sort of like slamming a door without a latch. In addition, valves are cooled while they're sitting on the valve seat. If they don't spend enough time on the seat, like when they're bouncing, they will overheat and hopefully "just" wear faster than they should. If a valve actually breaks, the results are catastrophic.
A new intake system
The intake air from the dual 70 mm throttle bodies is fed by a magnesium intake manifold. The M156 manifold differs from the NA M113's magnesium manifold in that it has two electronically operated throttle flaps, versus the M113's eight. The purpose for this patented, dual-length intake manifold is the same as the earlier design; creating an environment in which the engine can breathe at maximum efficiency throughout the rev range. The valves are closed at lower engine speeds, directing the intake air across the long intake path. When the intake stream follows the long route, pressure waves develop that result in an improvement in low and mid-range torque. Unfortunately, this same long path becomes a detriment at higher RPMs. By opening the valves and directing the intake air across a much shorter route, high speed efficiency is dramatically increased. The valves are ECU controlled using logic based on the engine's load and speed.
The blocks and bottom end
Although I've spent a fair amount of space discussing the valve train, the rest of the new 6.3 is just as technologically advanced. The cylinder spacing is a unique at 190 mm. That means cranks and blocks from other Mercedes-Benz V8s are not interchangeable. The blocks are an aluminum closed deck design with cast-in steel reinforcements. If the term "closed deck" is unfamiliar to you, it means the block has more space filled in around the cylinders. The above left photo is a BMW open deck design. You can see the open spaces next to the cylinders. The photo that's above and to the right is an M156 block, which has just enough open space for proper cooling. Closed deck blocks are more expensive to manufacture and they weigh more than their open deck counterparts. The advantage of this design is dramatically increased rigidity. The photo to the immediate right, is an open deck block with a cracked cylinder. This is what happens when the cylinder pressures exceed the block's structural capabilities. Before cracking the cylinder, an engine will usually have started going through head gaskets. They go through head gaskets because the cylinders vibrate within the block. This breaks the head gasket seal. Neither changing gasket material nor increasing torque specs for head are of much use. With computer aided design, cracking a block is extreme example. Proper R&D has allowed Mercedes-Benz to know well in advance what the limits are. Having experienced this in the past, Mercedes-Benz Motorsports provided the closed deck design as a cure.
The cylinder bores themselves are also unique. The M156 is world's first production engine to feature cylinder bores with twin-wire-arc-spray (TWAS) coating. TWAS is a new process that creates a very low friction surface, which is twice as hard as conventional steel. The application involves using water at high pressure to roughen the cylinder walls. Afterwards, high voltage is run through two metal wires, which begin to melt. An atomized gas is then used to spray the metal particles from the meted wires on to the cylinder walls. Afterwards, the cylinders are honed.
The bottom end on the M113 was bulletproof artwork. Despite this, Mercedes-Benz managed to raise the bar with the M156. The old engines used four-bolt end caps with the three center mains being a six-bolt design. Building on the foundation of the upper block's closed deck rigidity, the new M156's forego individual caps altogether and use a rigid bedplate design. This is another design that's used by Mercedes-Benz Motorsports. With a bedplate design, the bottom is secure. There will be no cap walk and the main bearings should wear like an engine with a low redline.
But wait, there's more
All this new technology makes some of the carryover items almost seem commonplace.
The electronic functions such as fuel injection, ignition, valve timing, the variable intake manifold and the electronically controlled thermostat, are controlled by Bosch ME 9.7.
The end result is 507 horsepower with 465 foot-pounds of torque. Even though the engine revs to 7200 RPM, ninety percent of the torque is available at 2,000 RPM. This makes for a wide powerband. That fat powerband is transferred to the rear wheels by a seven-speed transmission. These new transmissions employ close gear spacing in order to keep the engine in a rev range where optimum power delivery is available both before and after gear changes. Speaking of gear changes, AMG transmissions are equipped with SpeedShift. SpeedShift transmission are special high performance versions of the standard Mercedes-Benz seven-speed automatic. SpeedShift transmissions shift 35 percent faster than and have much stronger internals.
For some of us, the biggest question is, "Will it hold up?" Few things are worse than having a high performance car in the shop every week because something broke. Mercedes-Benz appears to have thoroughly tested these drive trains. In addition to development work having been performed on high-speed tracks such as Nürburgring, Nardo and Papenburg, these engines and transmission endured altitude development testing in Denver, Colorado (USA), Lesotho (South Africa), Mont Ventoux (France) and Granada (Spain). High temperature testing was performed in Death Valley, California (USA), Upington (South Africa), the Idiada proving ground in Spain and in Phoenix, Arizona (USA). Cold temperature testing was performed in Arctic Falls Sweden. Time will be the final judge as to whether it will hold up, but there is no uncertainty regarding the testing efforts
Addendum: Time is beginning to reveal a few secrets . . . . . . . .
Early M156s have serious top end issues. Early as in through engine number #60658 (those are the last five digits of the engine number and this issue impacts all 63 engines through the early 2010 vehicles). "Issues" as in plural.
Some engines are a little noisy due to cam gear lash. As long as the gears themselves don't show wear, too much lash is usually just an audible annoyance. However the next issue was somewhat bigger - valvetrain wear.
The camshaft buckets typically bleed down when the car is turned off. This allows a gap to exist between the cam lobes and the buckets. The gap results in a tapping noise until the oil pressure builds back up. The cam lobes are made of a softer material than the buckets and the cams wear the entire time the engine is tapping. The fix is to replace the M156 valve buckets with the M159 (SLS) buckets. The M159 buckets are an improved design that doesn't bleed down like the M156s. Some added bonuses are the M159 buckets have an antifriction coating to reduce cam wear, the parts are under $1K (as of this writing), and they are a drop in replacement.
Some may say "Drop in replacement? What do you mean 'drop in replacement'? Don't the cams have to be removed?" Well yes, that's true, but the cams had to be removed anyway. The defective cylinder head bolts can't be replaced with the cams in place.
Yes, defective. The heads of early M156 cylinder head bolts have been known to break off. When that happens, the head gasket can catastrophically fail. Normally, when a head gasket fails, it's a gradual process and coolant is eventually either pulled into the combustion chamber or the cooling system gets pressurized. When the head bolt breaks, the coolant seal can be instantaneously lost. If that happens, a cylinder can rapidly ingest coolant and hydrolock / bend a rod before the driver recognizes the problem. This isn't conjecture, it has happened more than once. The good news is; that's not always the case. A head bolt could break and only result in an oil leak. It's not likely, but that could happen. One last thing to consider about what goes on when the head on a cylinder head bolt snaps off; that chunk of metal typically bangs around under the valve cover until the engine stops running. Sometimes it gets wedged between expensive moving parts. If it causes the valvetrain to stop while the crank is still in motion, a full rebuild will probably be required.
So, how does one fix this?
The factory solution is to install the improved cylinder head bolts. The new design uses an external Torx head. In retrospect, the design change is a simple fix. Too bad this wasn't discovered during testing. The improved cylinder head bolts are part number 1560160769, and they have a list price of $10 per bolt.
The other option is to get an ARP head stud kit. The kit is available for under $1000.
Either head bolts or studs will work. However from a technical standpoint, the head stud kit is the better solution. In comparison to head bolts, head studs provide a more even clamping force. The head studs themselves are installed finger tight. When the head is installed and the head stud nuts are torqued, force will only be applied to the head studs in a vertical axis. When head bolts are used, the bolt itself is actually being twisted while being torqued. This exposure to multiple planes of force results in stretch. That is why many mechanics will not reuse a head bolt. Head studs have the added bonus of being reusable - not that anyone looks forward to pulling heads on a regular basis.