Exhaust Manifold Fabrication.

[twoqu]

Hello!
I would be interested in any information that would be useful in designing a fabricated exhaust manifold. I think I have got pictures of all the usual suspects including the factory stuff and the aftermarket. Unfortunately there appears to be substantial differences between all of them. Obviously some of this is down to packaging/costs.
So, assuming no constraints, I need to know some more about the following:
Individual flanges or one piece?
Tube size/length to support 500bhp.
Integtation into the collector. Should the pulses follow the firing order?
Material. If I was to use Inconel, what would the flanges need to be made of?
I feel this could be one for Marco! However please feel free to chip in.
Cheers!

[sorens2]

Hi

I used ordinary steel. Running for 2 years now. No cracking or other problems.
Mine is a copy of Dahlbäcks vildest manifold.
When you have welded it together don´t forget to heat it up to over 1000 degrees C. Then it won´t stretch when you drive it. And have the flanges machined as the last thing you do. I would say 45-50 mm measured inside of the tubing. The flanges shoulf be separated. And if you really wan´t to do it good follow the firing order in a circle.
Good luck....

Søren S2

[sorens2]

Another

[Wizard-of-OD]

What kind of turbo do you plan on running?
If you plan on using a Garret series turbo,then maybe you should contact Javad Shadzi at JShadzi@aol.com
he is who is building my manifold and he himself runs a 20V Turbo Quattro 80.

[Jeffer]

Hi,

A good site for information on header fabrcation is www.burnsstainless.com
A fellow S2forum-er and I are making our own tubular exhaust manifold. After reading the Burns site and investigating on materials we choose stainles steel 321 grade. For the flanges and the tubings. We used the same inner diameter tubing as the exhaust port. the flanges are 1 cm thick and CNC-ed. And the flange attached to the head is made out of one piece.

The welding is done with welds of grade 347 and welded with backing gas to avoid weldups inside the manifold.

HTH

Jeffry

[Jeffer]

pic

[Pisobiker]

@ SøernS2

nice EM! I might "copy" that layout for mine as well (when I'll have time and money to finish my engine project...) .
ID of tubings seems a little too big for the 500 Hp target, in my opinion. Unless your ported head has exhaust ports outlets of 45mm or more, I don't think getting a step bigger from the EM flange would give benefits. Does it?

@ Twoqu

EM design is really complex stuff. A lot of compromises have to be taken unless the use of the car is only for track competitions.
From a theoretical point of view, and as far as peak hp are concerned, best performances are obtained with an EM wich has the shortest and straightest possible runners that allow same lenght for all of them.
The reasons are:
1) the longer (and bigger in diameter) the runners the biggest the total inner volume, therefore increasing lag;
2) the straightest the runners the strongest the kinetic pulse reaching turbine's blades (each bend causes pressure drops and kinetik energy to be converted into heat and transferred to the EM itself);
3) the equal lenght runners allow for pressure and kinetik pulses to reach the turbine's blades at equally spaced intervals, therefore giving the steadyest pressure to the turbine as possible. The fact is turbines are very sensible to pressure variations, loosing a lot of efficiency while pressure decreases.
Turbine looses much more efficiency in the time where pressure decreases below average than what it gains when pressure is above average.

In my opinion points 2 and 3 become important when really wishing to squeeze the last few HPs from the engine, while, in mid tuned engine (450 to 550 Hp for our 5ers) the biggest gain is given by the use of less restrictive and smoother runners (tubular steel instead of cast iron, bigger diameter runners) keeping runners the shortest/smallest possible (no matter of equalizing lenghts) to minimize lag and improve throttle responce.

About runner diameters I think the best would be a runner same ID as exhaust port outlet, gently increasing while going to the collector (compensating the pressure drop caused by bends and internal drag). Of course such a design would be VERY difficult to build, and also gains compared to a constant diameter design might be totally negligible.
So my opinion is to use EM runners same ID or only max a couple mm ID bigger than exhaust port oultet Ø, using the EM flange as a smooth divergent to the increased section.


This is actually how I would make my own EM at the moment. Of course any other suggestion and/or feedback from already tested solutions is very welcome.


Ciao

Marco