Racing car exhaust selection and specifications ?

[124CC]

I am ready to have a custom made exhaust system for my mild racing 124 sport coupe CC.
The engine is a 1585cc one with twin 40 IDF carbs, Colombo Barianni 1st stage cams, Lancia Beta pistons
(Compression around 9.8 ) and Ritmo 105TC cylinder head.

The problem is that I haven't got the knowledge of what would be the best solution for my setup (having in mind that noise is not such a big issue for a racing car) and I've heard so many different opinions from the local exhaust shops that I am confused.

I have a 124 AC/BC - 125 special (cast in number 4151276) manifold to use which is supposed to be the best among the OEM ones.

Regarding the material, I have two choices.
Either a budjet iron, press bent,  pipe or a mandrel curved inox one.

When the iron, press bent, one gets curved to the desired angle, the internal diameter at the curved section is reduced whereas with the inox, mandrel curved one this doesn't happen.

Does this affect the performance so much that it's worth spending the extra funds for inox?

Also what pipe diameter should be good and how many boxes in order to maintain some back-pressure (if back-pressure is necessary) for my volume of exhaust gas and for good flow?


Thanks!

[TOAD]

Back pressure is not required this is a fallacy.

You say Iron. But should be mild steel if it can be bent.

Iron will not be bent.

Do not use that cast iron manifold from fiat.
Have a manifold made from the head.

Use mandral bends in stainless steel it matters if the tube
Is pinched. Don't use this it is a waste of money.

Size of tube depends on projected power.

I'm on this silly phone thing so come back to me if you want more.

  regards
      Roger/Toad
  

[124CC]

Hello Roger.

Thank you for the usefull advice.
I guess I meant mild steel when I wrote Iron.
I will go for the stainless steel, mandrel bent one then.
Since backpressure is not mandatory I guess one free flow box for reducing noise would be enough?
Projected power should be around 125-130 HP according to Guy Croft.

[TOAD]

Do you have a bore size for the manifold and the system.  The Janspeed one is for up to 150fw. And is made well.

Roger.....

[TOAD]

Compression ratio seams low. Is engine assembled?

[124CC]

It has just been assembled.
Lancia Beta pistons do not have the biggest dome there is (Like the ones from Ritmo).
We did the volume measurements and the math before assembling and the compression is near but not above 10-1.

My manifold is approx 36mm internal port size whereas most of the other Fiat cast manifolds are 33mm or less...
For the rest of the system I will have to decide...

[TOAD]

When you build it next. Depending on valve to piston clearance you can up the compresons more.   What have you done to the head?

[124CC]

Well, when we did the dry build, the piston of cylinder 3 was slightly touching the valve so we used the thinnest gasket possible to avoid this issue.
The head is a refurbished Ritmo 105TC big inlet valve head with polished ports.

[TOAD]

Would that be a thicker gasket?  Hope you've got room for rod stretch.

[124CC]

At first I was thinking of having a metal one fabricated but after the dry-build, we went for the thinnest OEM one so there should be room for rod stretch.
The head was only skimmed for the new gasket and not to increase CR.

Any ideas about pipe size?

[TOAD]

But if you use a thin gasket won't that keep the valves close to the pistons like that.

[124CC]

What I am trying to say is that we used the minimum thickness which eliminates possible contact of the piston with the valve.

[TOAD]

I see ,    I'll have a look at the manifold I have and get back later.

[Thotos]

Rough rule of thumb for exhaust pipe size is 1 inch per 100bhp so 1.5inch internal diametre should be enough for you. I would go with a 2 inch pipe to be on the safe side. Guy Croft used to supply a single silencer exhaust system thet would be ideal for you but I don't know if he still does them.

[124CC]

I just came back from the exhaust shop.
They recommended 54mm (2.12 inches) outside diameter which is around 50 internal (2 inches).