Regime|Life

Wondering what the advantages using SOHC over DOHC with all the AMG's, including the C55 AMG.

Also what's the deal with the twin spark plugs?

I've always wondered both these things...

Thanks,
Alex

Wiedermann

Mr. Regime|Life (aka Alex ):
Single Overhead Cams
This arrangement denotes an engine with one cam per head. So if it is an inline 4-cylinder or inline 6-cylinder engine, it will have one cam; if it is a V-6 or V-8, it will have two cams (one for each head).

The cam actuates rocker arms that press down on the valves, opening them. Springs return the valves to their closed position. These springs have to be very strong because at high engine speeds, the valves are pushed down very quickly, and it is the springs that keep the valves in contact with the rocker arms. If the springs were not strong enough, the valves might come away from the rocker arms and snap back. This is an undesirable situation that would result in extra wear on the cams and rocker arms.
On single and double overhead cam engines, the cams are driven by the crankshaft, via either a belt or chain called the timing belt or timing chain. These belts and chains need to be replaced or adjusted at regular intervals. If a timing belt breaks, the cam will stop spinning and the piston could hit the open valves.

Double Overhead Cam
A double overhead cam engine has two cams per head. So inline engines have two cams, and V engines have four. Usually, double overhead cams are used on engines with four or more valves per cylinder -- a single camshaft simply cannot fit enough cam lobes to actuate all of those valves.



The main reason to use double overhead cams is to allow for more intake and exhaust valves. More valves means that intake and exhaust gases can flow more freely because there are more openings for them to flow through. This increases the power of the engine.

Some of the advantages of DOHC over SOHC are:

More flexibility in designing the combustion chamber, because you have more flexibility to change the valve angles and spacing in the design and to have the valves directly actuated by the cams.

More flexibility to change valve timing, without having to regrind the cam for each valve.

The ability to use variable valve timing, ala Porsche, BMW, etc., where the intake valve timing is adjusted to improve torque across a wide RPM range.

Twin spark plugs.. there are single spark plug with dual heads, and true dual spark plugs combustion chambers... similar to the valves... the more spark, the greater... (no, it does not mean a mega super cylinder with a bizzilion spark plugs will have more power... though the concept says "yes" the math will give you a flat "no"... so there are ways to work around that, thus why MB is so successful on SOHC configurations... try to math that!... )

Cheers!

Wiedermann

lap

You're going to give Steve (aka SMGC32) some competition on "detailed" responses!!!
All kidding aside, I'm glad you guys are around.... even though I may not have the TIME to read all your points

Wiedermann

Mr. Lap:
Thank you for your comment. Although, I will leave Mr. Steve (aka SMGC32) on the throne on the detailed responses!!! (Mr. Steve (aka SMGC32) is da' man!)

Cheers!

Wiedermann

Regime|Life

I still don't understand why MB uses SOHC?? You gave a lot of reasons to use DOHC but I want to know why MB is so persistant with SOHC... So far the SLK200 and 350 uses DOHC with 4 valves but the 55 still uses SOHC with 3 vavles and dual spark plugs.

s4iscool

because they still can dominate. GM still uses, push rods and leaf springs...does that mean its bad? Dont tell that to Z06 owners.

Regime|Life

Yea but I'm looking for a more technical answer.

Wiedermann

Mr. Regime|Life:

Your posting/question did not ask "why..." but instead the "advantages...". I posted what the main configurations are for each set up (SOHC vs DOHC) and let you decide and have an idea what is best.

MB as a brand has been and is good at one thing: Engineering. Once the designed something, you will see it for a while, sometime decades (mainly due to design, and that is easier/better to improve an already good design rather than re-inventing the "wheel" --another design for that matter--). That is applicable to body styles, engine configurations, etc (there are many reasons for that, but that was not your thread question, or so that is my understanding).

As I indicated:
MB is now recently started to migrate to newer engine configurations and as new platforms are rolled out, more and more the new engine configurations will make their change as well.

Cheers!

Wiedermann

Steve Clark

To answer your question- (long but thorough)
When these engines were designed, about 10 years ago,MBZ probably felt that the twin intakes with a single exhaust valve and twin sparkplugs gave the best compromise between power and emissions.
I've no doubt that weight/cylinder head size and cost also affected their decision.The single exhaust valve results in more heat going to the catalyst for quicker cat lightoff,while flowing well enough to make decent power once warmed up.No catalyst works until lit off by engine heat,so this is an important factor in today's emissions-restricted world.
The twin sparkplugs make sense with this sort of valve arrangement:both are roughly equidistant from the cylinder wall and to each other,nestling neatly into the open area of the head between each intake valve and the single exhaust.If you only had a single plug and three valves,no single location would avoid a much longer combustion path...the ideal is a sparkplug in the middle of the combustion pocket,but two plugs are just as good or better for slightly different reasons...
One major consideration for the plug is cooling:were it in the small gap left between the 3 plugs,you'd need to have the valves downsized to have enough metal to insert the plug without over heating the small remaining web of metal.Bigger valves = better torque.
With two plugs the combustion front doesn't have to travel as far,so less spark lead is needed for good light-up of the fuel/air charge and lower emissions.Engines make the best torque with the least amount of spark advance that results in all the fuel being burned:more advance simply gives the burning fuel more time to put heat into the piston crown/head and less torque goes to the wheels.Apparently the two plugs don't always fire together at the same time,which further enhances combustion control.Due to the quicker lightup/shorter flame travel,you can also run a bit more compression for a given octane fuel.
Also-have you ever been around an older-style Chrysler Hemi?...While it could make really good power,that long flame path resulted in needing a lot of ignition advance,which then resulted in a serious thirst for high-octane fuel and unfixable (for those days) emission issues.The Hemi's combustion pocket resembled an orange rind if you ran high-compression pistons.
I know of several engines,both SOHC and DOHC, that made better power,had lower BSFC, and could happily use a lower octane fuel after converting to a two-plug head.
SOHC vs DOHC: A single camshaft costs less,has a less bulky cylinder head,but is slightly less efficient due to mechanical losses to the rockers.Unless you want to have really long rocker arms,valve location options aren't as liberal as with DOHC.
DOHC w/o rocker arms is mechanically more efficient due to lower friction losses,gives you more options for valve placement,and leave a direct,easy access to a centrally located spark plug if desired.It will be slightly heavier and bulkier than SOHC.
With MBZ going to adjustable valve timing,SOHC would not work as well as DOHC.
All engine designs are compromises to fit the restrictions in force when they are designed.Those rules change,and materials/knowledge/design philosophies evolve as well.
Last of all,I have a question...has anyone seen the head design for the new direct-injection 4-valve DOHC heads?.I'm wondering if they will have the injector in the middle of the valve group with two sparkplugs,each located outboard of the intake/exhaust valve bridges.I don't know,I'm just wondering.

ATS

Amazing responses guys.

AMGod

Sorry, had to revive this thread. Porsche developed SOHC, 3 valve, dual plug motors when they were more focussed on winning races. MB just watercooled first.

CynCarvin32

hes alive!!!!

wtf... his car was sold though... what u driving these days mario? 95 Saturn again?

CynCarvin32

The reason they made the 3 Valve motor was.... to save money. It wa 30% cheaper to make than the old M119 and M104.

RJC

MB went to the sohc 3 valve engines from the dohc 4 valve one's for cost reasons. The 3 valve engines cost MB almost 50% less to produce than the 4 valve dohc engines of the previous generation. The dual plugs in this case, were used for more of an emissions reason than anything else. The good news is that MB has gone back to the 4 valve dohc engines beginning with the new 3.5 V6.

EKaru

It's interesting how my Audi has 40 valves and the benz 24... I'm assuming that's one of the reasons the S4 makes good power out of its smallish 4.2 liter displacement. With direct injection, the RS4 will be making 414hp out of the same sized motor.. Audi technology ROCKS...
Eric..

RJC

Not to mention 500hp out of 4.3 for Ferrari

AMGod

Methinks Mercedes learned a lot from the Porsche built 500Es in the mid 90's.

E55AMG99

In what area, engines, assembly? The M119 engine came almost unchanged from the 500SL. MB only changed the intake and the injection system. As for assmebly, even Porsche doesn't use the rolling pallet system anymore.

Porsche's involvement in the whole prcess was basically to fit various parts designed for other cars to a 300E chassis. They widened the frame, fenders and driveline tunnel. They bolted on a bunch of 500SL parts and dropped in a slightly modified (by MB) 500 engine.

AMGod

My point is that Mercedes learned from their Stuttgart counterpart. Taking lessons from Weissach is not a bad thing. SOHC, twin plug, 3 valve motors came from Porsche first. Just a fun bit of nostalgia.

E55AMG99

Then I think Porsche needs to thank Ettore Bugatti for the lesson in 3 valve technology. The old Type 18 and type 35/37/39 race cars from the 1910-20's had 3 valve heads! I think they were anywhere from 1.5 to 5 liter inline 4 configuration with SOHC. Some even had superchargers. Now, where have I seen a SOHC 3-valve supercharged engine recently?

AMGod

Yup; SOHC, 3 valve, twin plugs came out with Mercede's neighbor in Stuttgart first. I like to see good technology shared, particularly between companies that have a long history of racing against each other.

E55AMG99

Sharing is nice but give credit where credit is due. You need to look much further back in history to find the first use of 2 spark plugs per cylinder AND 3 valves per cylinder. The French car company of Cottin and Desgouttes built a very successful racecar as early as the 1920's that used this technology. There were others as well. BTW, I believe it was Rolls Royce who first used twin spark plugs around the same time.

The point I'm making is that most of today's technology that is believed to be revolutionary actually was introduced almost a century ago. The early pioneers of the automotive industry were brilliant but for whatever reason, much of their discoveries were used very little if at all when they were first introduced.

AMGod

I agree. Back then, one of racecars used many brilliant mechanical engineering applications. My point was that Porsche used the technology in their massed produced products decades before Mercedes duplicated it in their current offerings. It is good to see solid engineering not forgotten, post patent expiration.

Chappy

Yep. Just like the first diesel engine that was designed to run on peanut oil....lately, B100 seems the rage with all the greenies :p

E55AMG99

Is it true their exhaust smells like french fries?