6.9 exhaust design

[s class]

Trusty rusty is presently running with the original spec 6.9 exhaust borrowed from the red car.  The time has come for trusty rusty to get its own system now. 

I've done detailed measurements and design, and I provide them here for anyone else who may be interested.

Some calculations show that for a 6.9 euro, intended for street use, around 52.5mm ID is ideal (two of).  For track use, something a little larger could be used. 

The original piping is 50mm OD, which benefits low end torque at the expense of ultimate top end power.  I intend to go to the next size up, being 57mm OD, 53mm ID - very close to ideal.  (This is quality thick wall (2mm) 308 series stainless). 

I have measured the original silencers, and modified the designs slightly to accomodate 57mm tubing. 







I was under the impression that 6.9's had no balance pipe or X box, but this diagram from the EPC clearly indicates an X box. 



Can any 6.9 owners confirm if they have X boxes?

Last comment - as is well known, the 6.9 header arrangement is far from ideal.  Given the space constraints, there's not a lot that can be done about it.  The ID at the outlet of the manifolds is 42mm, well below ideal. 

[koan]

Good stuff, do you intend to slice them open and duplicate the internals?

My AUS delivered 6.9 doesn't have any balance pipe or X box, engine pipes go straight into the first muffler. Just the same as the original system removed some years ago.

Currently the exhaust is a Timevalve stainless steel system imported from the US about 10 years ago. Not too happy with a couple of aspects of it, one of the engine pipes (RHS) doesn't sit well, it is a RHD pipe and the bends are in the right places but not quite the right angles. Same with the rear muffler, bends in correct places but not enough or too much.

koan

[s class]

Sorry, I'm not going to slice them open - my samples belong to the red 6.9 and they are original, in perfect order.  My red 6.9 is euro delivered, and as far as I can tell, the exhaust pipes up to the first box appear to definately be original, and they lack the X box, hence my surprise at the EPC picture.

I have ordered the silencers now as per my sketches.  they will be ready in a week or so for me to bring the car in. 

[Big_Richard]

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[s class]

PB, the X box as they are called here is essentially the free-flow version of a balance pipe.  Basically the way the aftermarket exhaust places make them is to bring the two pipes up against each other for a short distance and open up a hole in each where they touch, and weld them together.

I'm interested that your car had such a device, as its the first report I've heard of a 6.9 with one. 

I have no first hand experience of a 6.9 with and without the device, but the theory is that it should make the car idle smoother.  It serves to balance the exhast pulses which alternate between sides of the engine.  It also apparently smooths out the gas flow away from the engine, and thus improves waste gas scavenging and performance.  The smoothing of the pulses also smooths out the sound, and so it does provide some silencing effect. 

[koan]

6.9 mufflers have two pipes in and out but I doubt there is any separation inside the mufflers. The X Box box/balance pipe arrangement may be of some advantage on a true dual system where left and right don't combine.

The firing order is 1 5 4 8 6 3 7 2, left bank 8 and 6 fire consecutively as do right bank 2 and 1, which complicates exhaust system design.

Have you specified the internals of the mufflers?

The Bosch Automotive Handbook briefly covers the subject in a couple of pages but worth a read.

koan

[TJ 450]

My 450 has an aftermarket system, as per original, with the X pipe. The 6.9 had the original system without the X pipe. I have since installed an OEM Eberspaecher system on the 6.9 and it does not have any sort of crossover either. Front pipe is as the car left the factory.

Tim

[sparky527]

It seems, from all the recent reading on x-boxes, that the X-box scavenges the exhaust through impulse and that it serves to silence the harshness of the sound some. So, I'm planning to put in and X-box and run all the way to the muffler w/o the intermediate mufflers.  I like a little rumble---but not so that it destroys the elegance. Some of the aftermarket suppliers claim a measurable bit of power gain from the X-box. After all, that 6.9 can use all the help it can get.

[s class]

Update - I received the custom silencers a while back and had the system built up this week.  I decided that the consensus was, no X-box.  So I did not have one fitted (in the interests of originality) though I agree, in theory it should give a useful performance benefit. 

I'll put up piccies this weekend.  I'm very happy with the large standard size silencers.  The final outlet is a pair of polished 63mm pipes.  Sound is deep but subtle at idle, and is almost dead quiet above 1500rpm - as with the standard system. 

Interestingly, no real performance difference that I can tell, though admittedly I haven't been out on the freeway yet. 

New update - I had the car up to speed this weekend - I'm now even happier with the system.  The engine revs nice and clean to high RPM.   THis is the most sucessful custom freeflow exhaust I've ever had fitted to any of my cars. 

[s class]

'tis a thing of beauty.

[TJ 450]

Nice work, s class.
It fits very nicely.

Tim

[s class]

Thanks TJ - the pictures don't do it justices.  Its a masterpiece actually.  As any 6.9 owner knows, on the driver's side, the space to get the downpipe through is very tight between the steering box, bellhousing and cross-member.  Its tough even with the standard 50mm pipe.  These guys got 57mm through there, and there is no touching or rattling. 

[oscar]

yeah, that's impressive sclass, quality workmanship, congrats.  I'm warming up to the idea of straight exhaust tips too.  They look great.

[s class]

I think its what I've grown up with - the locally built 116's were originally all delivered with straight outlets - so it looks 'right' to me.  We only started seeing the droopy 'need-viagra' style on phase 2 W123's. 

[Big_Richard]

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