I'm just curious why MB/AMG has stuck with the SOHC 3 valve cylinder heads (albeit with direct injection and 2 spark plugs/cylinder)where all the engines in the rest of the range are 4 valve DOHC.

Does a 3 valve SOHC give the engine the characteristic they want in the S65/SL65, ie low RPM torque rather than the high revving nature of the S63/E63 engines.

 

No real need to over complicate things and redesign the whole engine that has relitively low production numbers ($$$$). The M275 was great except for the ignition system which they just refitted with the tried and true coils from the M112/113. It passes emissions as is, has plenty of torque and isnt fitted to vehicles that require a high revving/hi HP engine. They just tweaked a already reliable performer.

 

Lots of answers here.

I don't think there was any way in which the M275 didn't meet its technical, financial or marketing objectives, and if they switched to DOHC they would improve one aspect and make others worse, and I assume the downsides would outweigh the benefits.

The original driver for three valves was to improve emissions - MB wanted to reduce the surface area to volume ratio in the exhaust ports in order to reduce exhaust gas cooling, and speed up catalyst warm-up. That was the reason, but it doesn't explain why more recent engines have done back to 4 valves. The addition of the turbos further defeats that arguement.

Other V12's only had SOHC heads - Jaguar, BMW etc, and 48 valve engines aren't common. The disadvantages of V12's are that they're big, heavy, expensive, and have high friction - all things that DOHC heads would make even worse. They also have long crankshafts that aren't so happy with high revs due to torsional vibration, so there's less opportunity to benefit from better high-rev breathing from 4 valve heads.

V12's take up a lot of space in the engine compartment, and its difficult to package all the manifolds and ancillaries around them. DOHC heads make them bigger still, and there's less room still for the manifolds in particular. And there's not much point in improving the head breathing if the manifolds become the bottle-neck.

With the M275 being turbo-charged, the intake and exhaust breathing is less important than it would be with a naturally aspirated engine. Once the turbos are spinning, the engine will get all the air it can use. Granted, that doesn't apply when the turbos are off-boost at low revs, but then 4 valve heads don't usually help at low revs.

Finally, and on a more practical note, the V12TT engine compartment is so tight as it is, that it would probably be quite impossible to fit bigger cyclinder heads in there.

Nick

 

Is the 5.5 ltr twin turbo engine as used by the S63 a much smaller physical size....it has DOC 4 valve heads.

Why does the 6ltr V12 develop so much more power and torque than the S63 unit, given the relatively small difference in displacement. Could it be boost pressure?

 

Isn't the S63 a 4.7 litre?

 

5.5 has 4 less cylinders which makes a significant difference regardless of displacement changes

 

6.3 L 8 cyl

 

The 63 normally aspirated engine (M156 & M159) has 6.2L and the new Biturbo version (M157) has 5.5L