Alan here from sunny Barnsley lol
I'm making a mould for a air intake to the air box on my Ford Focus RS its an odd shape and hopefully you could help me decide how many splits on the mould I would need ?
I need to sand down the joints then cover with filler before flatting then I plan to spray it to get a really good finish 

Hi
You should be able to get away with one split. My, not dissimilar, air duct was made with a single split along the narrow edge.

Thanks Steve that's brilliant news as this is my first go with pre peg once I've made the mold but i'm going to take my time and try my best to do it right the first time hopefully .

No problem :-). The main trick is to make sure that you don't get a mechanical lock. There are a couple of ways to achieve this. The traditional one is to make sure that any parallel sides of the mould sections aren't exactly parallel, they actually move away fro each other very slightly. An alternative method that I have used is to make the mould sides (the sides that are bolted together)  slightly flexible when the mould parts are unbolted so it can be flexed, popping the part out. This method isn't usually recommended by experts or books, but it has worked for me :-) As long as the sides don't converge this works pretty well for certain parts. It worked on the above part but only just!

Having said that, i recommend that you go with the traditional method as I don't want to be blamed if you can't get the part out! :-) The taper can be minimal. If the sides are of a convex profile then there isn't a problem.

Thanks again steve I'll keep your information in mind when i make my moulds 

This is the shape of my plug from the side 

no mate High density polyurethane foam Block

I would make the join somewhere along the black line.

With the sides being quite short you would probably get away with parallel sides, as long as you use plenty of release agent. Just make sure nothing converges!