6.9 exhaust

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This was based on a minimum ID of 40.5mm (1.59”) for the 560. Minimum ID of 41.5mm (1.63”) for the 6.9.

 

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I was speaking to a mate who races a 450SE & his mods have been recommended by a well known racing team in V8SC. They specified an OD of 1 7/8” (47.63mm) and a minimum primary length of no less than 28” (711.2mm)!! Of course that’s in an ideal world with no space limitations.... So based on a wall thickness of 2mm that leaves an ID of 43.63mm. Not far off my calculations which were minimum sizes not taking into consideration size availability....

 

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I just measured the collectors of a Euro 6.9 , Aussie 560, 450 & 420. I can confirm they are all around 43mm!!

So basically the 420 has the same size outlets as the worlds fastest sedan (in 1975)!! And the 420 inlet (throttle plate) is actually BIGGER than the 6.9! The fact the 6.9 performed as well as it did seem to be purely down to CCs!

 

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I just measured the collectors of a Euro 6.9 , Aussie 560, 450 & 420. I can confirm they are all around 43mm!!

So basically the 420 has the same size outlets as the worlds fastest sedan (in 1975)!! And the 420 inlet (throttle plate) is actually BIGGER than the 6.9! The fact the 6.9 performed as well as it did seem to be purely down to CCs!

Note that figure is four into one (the collector) on each bank/side. Not to be confused with individual exhaust ports.... At not even 2” per side it can be seen that even changing an exhaust system from the manifolds back will not increase power or torque significantly unless the existing system is faulty or very poorly designed. The main result will be a change in note/tone/noise levels.

 

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By comparison, the Holley 750DP square bore which I believe was fitted to the XYGT used 4 x 1 11/16” (approx 43mm) barrels.
So the 6.9 uses an exhaust which has 1/2 the flow capability of a 5.8!!

 

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Something like this would be ideal however space is alway the enemy in this area!!

 

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Not much room with these headers!! Twin 3” on a mates car but the collectors been crushed a fair bit to get around the steering box!! Driver’s side is very cramped. Makes you wonder how the fabricator managed to get it all in there without complete engine removal!! Food for thought...

 

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On reflection the above example dispels my theory the reason MB used a smaller diameter twin system was due to clearance issues! So a single 3-3.25” should be easy...

 

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Finally I have the ability to add real figures to this thread! By my calculations the following should be adopted for optimal performance. Oh and these sizes are MINIMUM ID (NOT OD as pipes are normally specified). Include lots of bends (and not straight-through mufflers) in your design and you should go even bigger....

6.9

Twin system (ea): 59mm (~2.5")
Single: 70mm (~2.75")

560

Twin system (ea): 58mm (~2.25")
Single: 69mm (~2.75")

PS I had to take an average port size on the 560 since they're oval and not round. Hence there's not much difference due to the port sizing.

Add 2x2mm (4mm total) for sufficient wall thickness to arrive at my recommended OD sizes

 

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Interesting that I noted my AU V8 exhaust became more muted over time which I put down to a loss of compression but could also be explained by exhaust deposits coating the inside of the system. If true it could also warrant deciding to go with a slightly larger diameter pipe. Given it seems there’s no availability for 3 1/4” pipe, you only have a choice of 3” or 3.5”.
Pipe wall thickness appears to be commonly 1.6mm and not 2mm.
So to recap 3” = 76.2mm OD
Subtract wall thickness 2 x 1.6mm = 73mm ID. Being 70mm is what I would consider absolute minimum ID with no bends or deposits, perhaps a 3.5” single system could be justified especially on the 6.9

 

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Add 2x2mm (4mm total) for sufficient wall thickness to arrive at my recommended OD sizes

Looks like 2mm wall thickness was for stainless & not mild steel which is commonly 1.6mm

 

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This mob have been recommended so I’ll give them a try on Midnight Express....

https://www.hitechexhaust.com.au/