Exhaust manifold design - check my rationale

[itiejim]

As per my previous threads on various aspects of my build, I am currently building a K series engine (1.8) with a ROTTREX supercharger from a Turbo Technics TT260 kit, originally fitted to an Elise, but going into my 1967 Spitfire. It should produce 10psi of boost and I have now elected to run 260 degree duration Ph2 cams from Newman. I have a target power output of 240 - 250 BHP.

I require a bespoke exhaust manifold in this installation and want to understand exactly the design process so that I can either build it myself or make sure that what I get fabricated genuinely suits my requirements. The car will be used for a mix of road and track but should remain flexible, with a focus on usable torque and driveability. For this reason I have opted for a 4-2-1 design.

Research on the net (with credible sources including Vizard) leads me to the following information / formula:

1. That primary pipe diameter is the key influencing factor in defining where the engine will make effective torque. The formula I have found to calculate this is:

Primary pipe area X 88200 / displacement of one cylinder (inches squared) = peak torque RPM

2. That primary pipe length has an impact on the shape of the torque curve around the peak torque point - shorter will favour torque after the peak whilst longer will favour it before the peak. The formula I have identified to calculate this is:

850 X exhaust cam duration (260 in my case) / target peak RPM - 3 = total header length in inches.

I find further references to Vizard indicating that he proposes that the main exhaust pipe should allow 2.2CFM / BHP and asserts that a straight pipe will flow 115CFM / square inch.

A similar installation to that I am building, producing similar boost from a centrifugal supercharger and running the same cams produces peak torque at 5800 RPM, therefore, in order to try to keep the engine tractable whilst at lower engine speeds I have tried to design the system with a torque target of 4800 RPM. Using the calculations above this gives me the following:

1.5" primaries
2" secondaries
2.75" outlet

The length of the primaries comes out at a total of 41.2", which I would propose to split as around 30" primaries and 11.2" secondaries.

So - before I start cutting anything (initially the footwell and floor of the passenger side floor to get it all in) I would really welcome the oversight of those more experienced than I, of whom there are many...

Specifically:

1. Does the formula that I am basing this on stand scrutiny?

2. Should I be aiming for the torque peak target that I choose (4800 RPM) or that which the other similar engine actually delivers (5800 RPM)?

3. Do the dimensions that I have come up with look sensible for the installation?