Now this is a complicated area. Generally speaking, exhaust manifolds need to be matched to the camshaft profile. Primary pipe bore and length are both very important. Get it wrong and you loose power all over the place.
With 4-cylinder engines, you have options; 4-2-1 for better torque, 4-1 for top end power. With a 5-cylinder engine, you only have one choice, 5-1. Popular 4-cylinder engines also benefit from being, generally, tried and tested and thus information on bore and length for various cam profiles is out there. The only people who've done a 5-cylinder, naturally aspirated, race engine in the UK were TWR in the 90s with the Volvo 850 BTCC entry.
When making an exhaust manifold, no amount of software can tell you what dimensions will work - trust me, I tried quite a few and compared them to known power and torque outputs from known race engine exhaust manifold dimensions. Versus the real-world dyno figures, they were all way off. So the plan was one of averages. We knew what exhaust manifold dimensions worked best for the camshaft profile I was using but these dimensions were for 4-cylinder engines. Length is not quite as critical as bore diameter so we came up with a figure based on the 4-cylinder dimensions and decided to make the manifold adjustable in length so we could optimise it on the dyno.
The exhaust collector is probably one of the biggest expenses in exhaust manifolds and I wanted it to be right. Thankfully, I found an American company, Cone Engineering, who just happened to sell a pre-fabricated collector, used for the V10 Viper engines. I also ended up getting the rest of the pipework from them (thanks NCC!), all in mild steel, to allow us to make the initial "dyno pipes".
The easiest part of it all was the exhaust manifold flanges. I just sent an exhaust gasket off to a local laser cutter and got the flanges back :) The rest was a lot of fiddling and cutting by me and then copious amounts of welding by Mr. Exon. The result turned out quite well for a first attempt at anything like that. Proof will be on the dyno though...