Originally Posted by lnguyenh
Thanks for the answers. I sounds about right to me (2 intakes, 1 exhaust). I still cannot picture how a Single Cam (SOHC) works with 3 valves though. MB claimed to save on costs but a DOHC engine would have worked less and produced more TQs --> more HPs. Anyways, I am just curious because I used to have 2 SL500's of which one of them was DOHC and one of them was SOHC. The SOHC (5 speed tranny) has 13 HPs less than the DOHC (4 speed tranny) but ran faster! (Me and my friend raced it side by side).
A SOHC can control 3 valves per cylinder with rocker arms. The photo on the right is of an M113 head. You can see that the cam runs down the center of the head and the rockers ride on that cam. Using rockers, a SOHC can control 4 valves per cylinders.
If youâ€™re just looking at peak HP numbers, itâ€™s easy to be confused about why one car is slower than another. The M119 design had 4 valves per cylinders. The M119 was replaced by the M113, which was a 3 valves per cylinder design. The initial 4.3 liter M113 had 275 HP, the same peak number as the 4.2 liter M119. The difference between the two is the M113 produced more HP over a wider rev range. Itâ€™s easy to compare the two engines â€“ look at the 1997 E420 and the 1998 E430. Aside from the engine, those two are the same car. According to Mercedes-Benz, the zero to sixty times decreased from 6.7 to 6.4 with the M113 design. Initially, one might think the extra 100cc of engine made the difference, but the fuel economy increased as well. According to Mercedes-Benz, the M113 design gets 13% better fuel economy. Due to heat transfer, a three valve design also produces fewer emissions. Also, a 3 valve design has less mass, which translates to less drag.
This isnâ€™t to say every three valve design is superior. In some cases, a particular three valve design can be better than a particular four valve design. There is a page at my website
that discusses the M113 design.