A few times in the past I have mentioned the problems with 90 degree V-6s particularly relating to tuning and vibration since a 90 degree V-6 does not fire evenly.

Buick had the same issue with the 231 V6 of the early 60s which later became the 3.8 as in Turbo Regals and the 3800 in Reattas. As I mentioned the only advantage to a 90 degree V-6 is that it can use the same tooling and assembly line as a V8.

In 1978 Buick solved that problem by offsetting the crank throws resulting in the "even-fire" V6 which could be tuned properly and resulted in some of the fastest cars in GM history, first with turbos and later with a supercharger in L67 form.

The was also the issue of Mercedes timing the dual spark plugs since there were only six coils.

Browsing through the FSM, I found this "The engine uses a split-pin crankshaft to provide even firing, and a balance shaft to compensate for the rocking motion which occurs with a 90-degree V-6." Sounds just like the engine in my Reattae.

Given even firing, the dual plugs (second firing 60 degrees after the first to scavenge any unburnt gas) now makes sense.

The exhaust system also looks remarkably like the one in my Pentastar Jeep. Only thing missing to bring it into this century is DOHC and VVT. And a 2.60 final drive. (Jeep has a 3.09 but also has 31" tires).

 

Also note that the firing alternates, A fires, then B, followed by B fires, then A
Supposedly this is to keep the wear even on the 2 plugs ....

 

What does "even fire" mean?

 

a) "even fire" means the cylinders fire 120 degrees apart (2 revs or 720 degrees in a full cycle). The CF firing order is 1-4-3-6-2-5 so banks sometimes alternate. With a normal crank and a 90 degree block this would mean some cyls would fire 90 behind the last and others 150 degrees. By offsetting the crank journals 15 degrees positive on one bank and negative for the other you can make the firing sequence even.

So what you have is cyl on each bank firing exactly 240 degrees apart. This means you can tune the intake and exhaust ports for laminar flow to maximize the gozinta and gozouta (not as important with boost).

This generally means you have torque building to a peak and then tapering. Exactly what this curve looks like is what tuning is all about and why modern engines with VVT I&E can have such broad torque curves (90% of peak from 1800-6400 rpm). Without, just how peaky an engine is depends on how wild the cam and the intake and exhaust runner length.

If you really want to know the physics invold fond a copy of Harry Ricardo's "The High Speed Internal Combustion Engine". I have a 1958 edition but if you really want to understand the theory, that is where to start. Obert is OK but I like Ricardo better.

b) If you want a quiet exhaust you give it time to cool a bit and join the two banks about 30" from the head (why two CCs, you want to maximize exhaust heat since they do not work under about 600F). This is why any "interesting" V8s have an X or H pipe behind the trans (se also Helmholtz tubes).

However if you really want it to sound like a D jag or a Corvair on trombones then you need to keep true duals from the cats back. Use a single pipe or duals with a crossover and it won't sound nearly as pure.

c) They may be alternate firing plugs but I would be surprised (anyone have a picture of the combustion chamber, the ones in the service manual show a single plug four valve head) if they do not fire sequentially for emissions purposes rather than power. Usually there is a single sweet spot in the head for even flame propagation, probably between the two intakes and as near center of the combustion chamber as possible (not going to get into CVCC and such if possible but other designs usually have a CR in the 7-8:1 region) and if serious you index the plug so the gap faces the exhaust valve.

The current Hemi uses dual plugs but the second is to try to keep emissions down and not for power.

Enough for now.

 

The plugs fire out of phase and alternate. Like ala_xfire said, A-B, then B-A but the ECU can shift the delay for the second by x amount of degrees. It compensates for knock before the knock sensor's even register anything. This was all said here before and there is a ton more to it. I want to say the second fire can very up to 15 degrees.

 

Great answer, thanks!!

 

Here ya go:

Mercedes-Benz Specification Site - Marcus Fitzhugh

 

Rumor has it both plugs fire when you floor it.

 

According to the (horribly wrong) service manual that's not true, so I would believe it lol. But I do believe the report I posted above which say's they fire at minimum 5 degrees out of phase period

 

Could still be true though.

I honestly find the M112 and M113 motors fascinating. They seem to be light years ahead of their time. They may not produce the fullest potential of their power from the factory but that's why we have Eurocharged.

 

Got one on the M112 specifically?

EDIT: Damn, link launched on page 3.

 

I haven't seen a strictly M112 dedicated section but they are the same. The V6 firing is alternating banks, but the V8 fires on the same bank twice per firing order. Everything else is the same though.

 

I' have always found the physics of the modern engines very interesting (you know: why more than two valves, how variable timing works and what effects it has, etc.)

The very idea that we can get so much power from an engine the would SEEM to use a lot of power overcoming it's design 'flaws' - like reversing the direction of pistons twice a revolution, and making a LOT of heat that is simply radiated to the surrounding air.

 

I know, I'm the same way. It's crazy to think about and it's crazy to see what manufacturers are doing to improve efficiency. The different ways of tackling the obstacles.

I find these engines nuts because of all the technology behind it. The valves in the intake manifold to change the tract length. The shift phase twin spark. All of it.

 

Dunno about who Marcus is but one part disturbs me a lot "That's because in an efficient, well designed engine, the size of the exhaust valve needs to be a bit smaller that the intake valve area. This is because the residuals from the combustion process take up less space than the incoming air fuel mixture. "

This is so rong it is painful. You start out with air carrying a small fuel charge. During the combustion process this adds a large amount of heat which raises the cyl pressure and it is this raised pressure that forces the piston down again.

Sometime around BDC the exhaust valve opens and it is the greatly raised pressure plus the piston movement which forces the exhaust back out but the "residuals" by mass is exactly equal to the intake charge and the higher temperature makes it greatly expanded. (gozinta must equal gozouta)

At the same time the piston is pushing the exhaust out which does not require as much area as it needs to pull the same mass in using vacuum.

Add boost and the issue can reverse so you need a larger exhaust surface than intake particularly for a turbo, but few do.

Now for large displacement engines at high RPM the charge can approach sonic & go into turbulent then choked flow which is Not Good. Fortunately that is not a consideration here.

All that said, the ideal ignition point is in the center of the chamber to minimize the distance the flame front needs to travel and reduce the likelihood of a second FF starting (two colliding flame fronts is what causes knock). Deviation from this ideal point raises octane requirements since it is harder to avoid detonation (second flame front forming spontaneously).

Now an ideal flame front varies in speed depending on the octane, cylinder pressure, and temperature but around 170-200 ft/sec is sorta kinda close for cruise, faster at WOT.

So if you have two plugs about 1" apart that is about 4/10,000 of a second or about 7 degrees at 3,000 rpm. This is the ballpark but computers are much better at it (and can accommodate many more variables).

Now you may have noticed that I really like the Chrysler Pentastar design with NA 290-305 hp at 6400 rpm & over 230 lb-ft of torque from 1800 rpm to redline on 87 PON. 4 valve/cyl & center spark plug in a pentroof design. Does require DOHC to do it but nothing new.

 

Makes sense.

When I was researching header designs they said one of the biggest factors to compute for was vacuum. To help draw the exhaust out. They were saying the size of the exhaust valve(s) could be smaller than the intake and perform just as well if the right diameter and length and all that could help draw the gasses out. Our manifolds are definitely not doing the job in that area. IMO the valve should be bigger or there should be 2 but they couldn't fit another.

I like DOHC 4 valve heads. VVT on I&E. The oppurtunity for better flow in and out. Center spark plug. But look at the difference between the M119 and M113 in that article. The real world outcome was intense to say the least.

 

Idea is to have the exhaust charge mass flow from the last cyl help accelerate the next cyl. You really do not want a vacuum because that could result in reversion.

One confusing issue is that US cars count cylinders on bent engines with odd cyl on one side and even on the other. MB seems to be different so for the crossfire it is 1-2-3 on the passenger side and 4-5-6 on the drivers so the odd-sounding firing order 1-4-3-6-2-5 is really alternating banks so each side has an exhaust event every 240 degrees (on an American engine it would be 1-2-5-6-3-4 still odd from a harmonic standpoint but more reasonable).

I agree, better than what went before but still last millennia.

 

Having the cylinder that fires, help the previous in acceleration, is what I meant lol. It's hard to put it in words when I've slept since I read it lol.

I hate that my firing order on the V8 is 1-5-4-2-6-3-7-8. It makes it a ******* to make good headers. I have to criss-cross to make them consistent.

 

Hi,

this graphic shows the firing order of the M113:



Source: OLDTIMER: Tipps für Mercedes Geländewagen 460/461/463 und 230 460 und 463

There are also other aspects concerning ignition:

For the following operations, a firing angle adjustment is done:

Catalyst heating, idle, fuel cut-off, transmission overload protection, ESP / ASR control operation, smoothness evaluation, knocking tendency, anti-knock control.

Catalyst heating: if the coolant temperature is between -10 and +40 degrees Celsius, in addition to the increased idle speed, the ignition angle for about 20 seconds are set in retarded ignition.

Idle: For faster control is not adjusted by the throttle, but the firing angle is changed.

Fuel cut-off: a torque shock is prevented after the injection valves actuated again, by moving ignition momentarily to "late".

Transmission overload protection: During the switching process (1-2, 2-1) is the ignition is momentarily moved to "late" - this reduces the engine torque.

ESP / ASR control operation: if the angle of the throttle is reduced wihile ESP/ASR is activate, the ignition is momentarily adjusted to "late" .

Smoothness evaluation with two knock sensors: Combustion misfires - the catalysts must be protected against thermal overload - to to this the injectors are no longer actuated.

Knocking tendency: At elevated charge air (> 35 degrees Celsius) and coolant temperature ( > 100 degrees Celsius ), the ignition angle adjusted to "late " to reduce the tendency to knock.

So there's a lot more stuff going on.

Regards
Markus

 

So basically, anything out of the ordinary results in a retarded spark. In American cars the fuel cut off (when you take your foot off the gas) usually results in an opened IAC (idle air control) to prevent the sudden change from triggering a puff of HC. As near as I can tell the CF engine does not have a separate idle air control but does have electronic throttle control to accomplish the same thing.

Pontiac also had problems with scattering 4T65E trans behind L67s until they started cutting the injectors during a WOT shift. Cars were slightly slower in a 1/4 mile but the warranty claims plummeted.

There are also various maps and offsets used on computer cars to adjust to various conditions but 100C (212F) coolant seems very low to be turning back the ignition. Mine does seem to run mainly in the 180-190F range. But most GM cars and my Jeep are happy at 225F (I'm not but they are).

Also it does not have VVT but does have dual intake paths, one for operation below about 3,000 rpm and tuned to torque and a shorter one for higher speeds.

Agree the stock exhaust manifolds are just an unturned log but with alternate bank firing, a tuned exhaust would not be difficult.

Valves are pretty mild, about a 1.94 intake equivalent (two 1.4s) and 1.6 exhaust but enough for about 6000 rpm.

V8s have always been difficult to tune well because you need crossovers. Ford fixed that in the Indy engine by putting the exhaust ports in the valley. This works in a rear engine car (just leave the pipes high) but is more difficult for front engine RWD unless you wrap the exhaust around the front of the engine.

But the key here is that you have to know not only the firing order but also how the manufacturer numbered the cyl. See http://en.wikipedia.org/wiki/Firing_order for a quick look at the many differences.