Simon - it depends what for. It really would require an essay to get across all my thoughts!

There are generally 2 parameters to an inlet manifold - runner length, and plenum chamber volume. The runner length takes advantage of something called "acousting supercharging" The idea is that by altering the length of the runner, you get can dramatically (it really is a lot) increase the VE of the engine at a certain RPM. You have to decide where u want the power to be boosted, and then decide the runner length. Have a look at some Honda inlets, they are a good example. Short runners, move power up.

However, this all goes out the window with forced induction. The speed of sound, and the reflection speed of the waves if u will, changes with air density. In practise, on a turbo car, its a bit like having variable length runners - its virtually impossible to tune for. Also, it can become pointless to tune for. You can get to the point that you are forcing that much air in, that it would be impossible to flow anymore through.

So, on a turbo car, the runners are made as short as possible without too much of a restriction.

Plenum chamber volume is next. On a NA car, ur looking at a volume aprrox equal to the engine capacity. On a turbo car - well, the bigger the better in terms of power. You should aim for a plenum equal to the amount of air consumed under boost. So a 2 litre car with 2 bar of boost, the plenum should be 6 litres...

Next, u have plenum shape. This should be done so that each cylinder is seeing the same airflow into it's port. The throttle body should never be directly infront of any cylinder. On a centre entry manifold, (Std RST,Fiesta) the air is designed to hit a flat wall. It then dissipates equally into the cylinders. Disadvantage is that you slow air speed. On a side entrance manifold, you can have much higher gas speeds. Probelm is gas distrubution. The cylinder closest to the entrance is going to have the highest entry speed. This is because as the air enters the plenum, to maintain the pressure, it slows down. To combat this, you can taper the plenum so it becomes smaller at the far end/last cylinder. By getting the taper right, you effectively increase the gas velocity, and equalise the airflow into the cylinders.

There are drawbacks into making the plenum too big though - namely response. The engine can feel laggy and lazy. This is because the plenum responds to any throttle changes before the engine does. So, if you boot it, the turbo has to fill up the plenum before it does the engine. Like with all tuning, it's a compromise.

I will say though, the Fiesta inlet is garbage. It is far too small for turbocharging. It will give good response though, ideal for the fiesta turbo which was never meant for motorsport. The S1 - which WAS meant for sport, haas the best manifold - ford arent stupid.

Jamsport have the right idea IMO. Use the short runners of the fiesta, and put a decent plenum on top of it....

There's loads more too manifold design, but ive simplified it to the important points i hope - if anyone spots anything wrong - let me know - im always learning

Rick.