'95 E300 om606 intake manifold diaphram
 

I finally figured out that the purpose of the EGR diaphragm is to let exhaust gas and soot into the intake manifold! (it is now blocked off) but what is the purpose of the diaphram under the intake manifold farthest from the engine?
I removed it to see what's inside and there is a small flap plate.

It changes the distance air has to travel through the manifold in order to improve torque at different RPMs.

My understanding is they change the engine note, not fitted to OM605 or 606 Turbo so removed both on mine. No change in performance, if anything marginally better power wise.

It is part of resonator system to provide higher torque over broad rpms.

It is vacuum activated.

Flaps!
 

There's also a flap in the crossover pipe; together the flaps 'tune' the apparent length of the intake manifold, as others have already noted. A third flap, just in front of the EGR valve, is designed to reduce the pressure in the intake manifold, thus sucking in more exhaust when the EGR valve is open. This "throttle" flap can be disconnected or even removed (two Phillips screws inside the intake pipe).

Note to 1996-97 W210 owners with the OM606NA engine: the throttle flap cannot be disconnected. The EGR electronics monitors the IM pressure and sets a code if the flap is not present and working.

Jeremy

These flaps are the poor man's turbo. The opening and closing of the intake valves cause pressure waves in the intake manifold. At a particular RPM, the standing pressure wave will have peak pressure right at the intake valves, so additional air is forced into the cylinder when the valves open.

The flaps in the intake manifold are either opened or closed, and they change the effective length of the intake manifold. A different length intake means the standing wave pressure peak occurs at a different RPM. The flaps allow three different combinations - three different RPM pressure peaks - and so force more air into the cylinders at three different points in the RPM band.

If you want lower torque and reduced performance, remove the flaps.

There was a similar discussion a while back which I followed here, but I have to say I am skeptical thinking this may be something just repeated on forums.

http://www.peachparts.com/shopforum/diesel-discussion/123366-m606-so-what-do-all-these-valves-do.html

"Although this OM606 is naturally aspirated (non-turbo) it has variable-length or 'tuned' intake runners. The valve is used to 'tune' the intake depending on lower or higher rpms to improve exhaust scavenging and combustion air intake, a poor man's supercharger... "

These are the comments further down the thread which tbh, I share:

"As of 6 month ago, my resonance flaps, as well as my EGR valve, stopped working all together. Could be electrical wiring problem related, or vacuum related, although I suspect electrical. Could be the EGR relay. I am waiting for a nice warm day so that I can remove the cross-over pipe and diagnose the problem.

I can tell you for certain, that my car does not appear to start or run any differently, with the the resonance flaps, EGR and who knows what else, totally inoperative. It seems to start and run perfectly. That's one of the reasons, that I was not motivated to fix it earlier."

The crossover valve acts at very low revs so not sure how that helps further up the rev range and secondly, the OM605 which is identical minus one pot doesn't have these at all.

Flap! Flap! Flap!
 

The 6-cylinder engine is quite different from the five. The six cylinders are 'paired,' which allows the use of resonance techniques. You can't do that with five cylinders, which is why the 605 doesn't have flaps. Additionally, with five cylinders, getting reasonable smoothness becomes more important than a few horsepower.

In the factory's 1995 "Intoduction into service" manual, the resonance flaps are described along with a graph showing the difference with and w/o the flaps. I suspect it would require a dyno to show the effect—the text and graph didn't have any numbers attached.

The flaps change state at ~2500 and ~3500 RPM. I have added LEDs to my speedometer so I can see and confirm they are working. The 1996-97 engines in the W210 models also have flaps; I suspect a malfunctioning or disconnected flap in those engines would set a code. The 1995 W124 is spared that foolishness.

Jeremy

You can, it would just look very different. Volvo NA 5cyl from the early 90s.
http://farm3.static.flickr.com/2030/...c3280d16_o.jpg
I suspect the 4 and 5cyls were built to a budget and they weren't encumbered with as much emissions equipment as the 6 shipped to the US.

Jeremy,

I'm all ears and that is one very convincing answer..

...however how do you go about 'balancing' six cylinders which share a common manifold with opposing cycles of induction??

The reason I ask is because this was something of great consideration when it came to water injection nozzle placement, how to feed all cylinders equally from a possible non central location.

Are you positive this is not some marketing blurb by MB, put it another way if you wanted to change the engine note due to lack of turbo what would be the easiest way (thinking trombone here)..

..retreats putting first leg into flame suit..:)

Typing from my bunker:

Resonance of a tube of air

The resonance of a tube of air is related to the length of the tube, its shape, and whether it has closed or open ends. Musically useful tube shapes are conical and cylindrical (see bore). A pipe that is closed at one end is said to be stopped while an open pipe is open at both ends.

Modern orchestral flutes behave as open cylindrical pipes; clarinets and lip-reed instruments (brass instruments) behave as closed cylindrical pipes; and saxophones, oboes, and bassoons as closed conical pipes.[4] Vibrating air columns also have resonances at harmonics, like strings.

https://en.wikipedia.org/wiki/Acoustic_resonance

I'll scan and post the relevant pages from the "Introduction into service" manual for the 1995 E300D. I'm not proficient in the technology and have just accepted what MB says. I don't think it changes the sound of the engine, which is mostly drowned out by the sound of the IP anyway. I would appreciate the opportunity to read your analysis.

Jeremy

Jeremy, I am not disputing what MB have written or yourself, just how effective it actually is.

Using a 300psi pump with Devils Own fine misting jet located centrally, there is insufficient vacuum below 2200 revs to scavenge the chamber.

Even if you take it up to 2500 where the flaps come into play, it's still pretty low due to the huge pipe.

Try it yourself, easy to put the flaps in open position (no restriction) then take it for a run, as i said before if anything there might be a minimal gain due to improved air flow?

Don't forget the MB blurb which accompanies diesel EGR systems, now how many folks have disabled that.

Generally the flaps in intake manifolds are there to improve emissions and round out the torque characteristics of a given engine. I'm not familiar with the NA 606 so I can't speak on it's behalf. I do know that it has a sort of "throttle" valve in the induction pipe. That flap's purpose is largely to help with the EGR system at lower RPM's. Some of the earlier NA 240 and 300D's had a similar system for similar reasons.

If there are flaps in the intake runners, they are generally there to improve swirl at lower RPM's. By adding a restriction in the intake stream you get a higher velocity intake charge which helps combustion and cylinder scavenging. The difference is slight, but it is measurable - usually with fuel economy. Seat-of-pants tests usually won't yield anything appreciable.

The OM606 Resonance Flaps work
 

My 1995 E300 performance is noticeably improved with the valves working.

The lower valve, as the housing plate wears from shaft vibration, leads to vane failure. I've replaced the vane twice on the original valve and now I've replace the vane in this new valve, however this time, I discovered a bushing that is listed in the EPC, which fit perfectly into the upper bearing hole in the manifold and tightened up the entire assembly. I'm optimistic it will mitigate vibration that causes the vane to break off as the miles accrue.

Once again, the car performs noticeably much better than it did before the repair.

We'll see how long it lasts this time.