4 branch exhaust manifold questions

[Howard Wright]

Thanks All

I just hope my attempt will be as good as yours Nick. Cracking bit of work!

Cheers

Howard

[Greig Smith]

Howard, have a look at this thread, I posted from #12 on the manifold I built for our supercharged single seater.

https://www.austinsevenfriends.co.uk/for...s#pid85304

It's not actually such a difficult thing to do but does take time and of course lots of trimming , grinding & filing in between measurements

Aye
Greig

[Howard Wright]

Hi Greig

Thanks I remember that post now.  I’m leaning towards using mandrel bends rather than buying or hiring a pipe bender.  As the manifold needs to be in the engine bay and exhaust under the car mandrel formed bends give a tighter radius and constant pipe diameter through the bend.

Thanks for the encouragement 

Cheers

Howard

[Greig Smith]

Yes I bought mandrel bends as they are all the same size & radius and just set about measuring & cutting from there. I have to build a second one for our normally aspirated single seater, so will buy another handful of the same mandrel bends and build the next one. It really is just fiddling & experimenting with pipes and angles and then measure twice & cut once.

Aye
Greig

[frequentflyer]

I spent the weekend trying to boil myself in a boiler suit welding one up. I started with some bits I got from a kind local club member and then starting adjusting and bending some new bits. All weldied up with a mix of TIG and MIG. Topped it off with a dash of logburner ceramic heatproof paint which I hope will stay put. We ended up with quite a bit of wasted tube having had to use some longer bits to make the bends and then cut out what I wanted.

[Dennis Nicholas]

Another case where a bit of reading up on exhaust designs before attempting making one is a great help.  Years ago I read up various designer instructions, some going into and giving mathematical formulae for various size/configuration of engines.  Vaguely I remember the 7 required 1 inch internal diameter and (on a long wheelbase chassis) the end came out just in front of the rear wheel (though that may have been for a BMC A).  Ah I thought, that will also allow the flames coming out to warm up the tyre compound 

Dennis

[Howard Wright]

Once again, thanks all for the encouragement and advice.

Dennis you are right in proposing a good bit of research before starting, and that is what I am doing at the moment.  I’ve gleaned several formulae from the web and others. Most, if not all, are rather empirical  and some not even dimensionally correct, the constant in the equation hiding a multitude of corrections! However I’m not a fluid dynamics expert and can’t comment on their veracity.

However it would seem that to extract the best from the manifold and exhaust on a Seven the pipe exit should be around 3 to 4 ft from the block. A 4 into 2 into 1 seems to give the best mid range torque but a 4 into 1 better at high revs.

All this may be interesting but I have the constraint of a fabric bodied car with doors, meaning realistically the manifold and pipes must run under the car.  So at the end of the day my design may have more to do with what will fit…. than out and out performance.  There is also my perception of aesthetics and a huge gargle of pipes under the bonnet just doesn’t look “period” enough for me  .

I’m quite taken by the Ulster style where pipes each feed into a cone tapered pipe but I can’t find any useful information on performance.  Saying that anything must be better than the original cast manifold!

Cheers

Howard

[Malcolm Parker]

A friend who has 60 years of tuning experience (mostly bikes) once told me that exhaust manifold design is not that important until you are looking above 5000rpm.   With Austin 7's, I am of the opinion that the biggest advantage of a fancy exhaust manifold is that you are usually forced to also have a different inlet manifold, and that makes the biggest improvement.
My Pytchley had the complete set of Cambridge twin SU's, inlet manifold with balance pipe, bunch of bananas (as shown earlier in this post).  Yes, it went very well but the temperature under the bonnet was high and access was severely impeded.   I reverted to a standard cast iron manifold with the restriction bored out and a single 1" SU.   There is no noticeable difference in performance and the engine is just as lively and free-revving.  Accessibility is good and the fuel consumption is much better.   The car is used on the road, perhaps if it was a competition car things would be different.
Whilst on the subject I have a Dante cast aluminium exhaust manifold but the outlet is very close to the position of the n/s radius arm and would require a very sharp bend on the exhaust pipe.  Has anyone any experience of using the Dante manifold on a Box saloon or similar?

[Howard Wright]

Hi All

Well here is my attempt (closely modelled on Tom Coates cup one). Currently sporting a VHT flame proof primer it will eventually be black.

It will be a little while before I try it on the car as I’m fitting a new down pipe, silencer and tail pipe. The car is currently running well and I don’t want to try anything new until the “season” is over.

Cheers

Howard



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[Speedex750]

Well done Howard, that looks really good and 'the part'. You've worked out that the middle stud is redundant and cut off flush or the tapped block hole filled with a sealed grub screw. I find that instead of trying to get full sized 1/4BSF nuts on it is easier to use M5 brass nuts tapped out to 1/4BSF - the spanners or socket are that much slimmer!

Dave