4 branch exhaust manifold questions

[Howard Wright]

Hi All

I have a 4 branch manifold on my special with which I am not completely happy.  The plate is too thin at 5mm and the pipe bends make tightening the manifold nuts difficult.  So I intend to fabricate a replacement. I have an 8mm plate ready (thank you Bellini) and now need to design the system.

Question 1…. The existing pipes are 1 1/8” OD. Is this ideal or would 1 1/4” be better?

Question 2….  Welding between the two central pipes looks difficult so is brazing an option?

Question 3….  What is the best “pattern” for connecting the pipes at the exhaust end. A single collector or a sequence of pipe into pipe?

Thanks in advance

Howard

[Ruairidh Dunford]

I bought a rather nice manifold from Stuart Palmer that is fitted to the Special, it was excellent value and works very well indeed.

He has worked out all the problems you identify.

[Steve Jones]

Agreed. I had one of Stuart's manifolds for my now sold Trials Chummy. Works well and the problems identified are solved. It lived a very hard life during my ownership but survived without problem.

Steve

[Malcolm Parker]

Howard, I have what I think is a genuine Cambridge bunch of bananas manifold that came with a pair of SU carburettors.  Photos below.   The four pipes appear to be 1" ID, just under 1 1/8" OD.  The outlet pipe is 1 1/2" ID            

[Howard Wright]

Hi Ruairidh, Steve and Malcolm

Thanks for the responses. I’ve also had one via email.

The Cambridge manifold shown in Malcolm’s photos is very similar to the one I have. I’m sure it is better made but I can’t be sure it will fit in the confines of my engine bay nor that it fully clears No2 stud (where my existing manifold fouls the nut on the stud no matter how long an extension I make).

In practice the existing manifold works for now and I would quite like to have a go at making one myself as an autumn project. If I make a pigs ear of it I can fall back on contacting Stuart or accepting Malcolm’s kind offer (if it’s still available then  ).

However it does seem that brazing is acceptable and that 32mm OD pipe works (at least on a racing car).

Further comments critique welcomed.

Cheers

Howard

[Ruairidh Dunford]

Go for it, Howard, it will be fun.

[Nick Lettington]

Have a go Howard. Braising is definitely a better option... I bought a big Hilmor floor standing pipe bender like this...

https://www.ebay.co.uk/itm/325274293700?...SwpvBiCnmC

Then fabricated a 4 into 1 based on a Speedex manifold. 


  Screenshot_20220811-205613_Gallery.jpg (Size: 261.1 KB / Downloads: 625)

It's probably a good idea to bolt the 8mm plate to an old block or at least a cast iron manifold when you start applying heat! Don't ask me why I didn't the first time... 
Work out the shape, bend the tubes, cut them to length then square the ends... 

I used a sharpie to reference the location, bodge tape and some roofing batten to hold the shape.

[Ruairidh Dunford]

Nice work, Nick.

[Nick Lettington]

Thank you Ruairidh... if I could only find time to do the rest!

[Spex]

Regarding the size of your tube.
A mechanic friend told me years ago that with unleaded petrol,
to increase size of your standard tube to the next size for more power.
Who knows. My 27 Coupe goes well.
Peter