I don't know how I missed this thread originally.
Props for giving it a go. Although 'm of the opinion that you won't notice any improvement over an off the shelf set.
Extractors, to be 100% effective, only work to their maximum efficiency at the rev range they are designed for. Even then, on a dyno, changing the length of the primaries makes very little difference on a stock engine.
Building the extractors to suit your rev range is one thing, as soon as you bolt up an exhaust system to them, you negate all that work. Not only do you have to take into account exhaust pulse, but also the effect of heat loss. You will need to keep the extractors as hot as possible to aid flow, and the further the exhaust gets from the engine, the cooler it gets and the less volume it has. Even as little as the length of the primaries will affect flow due to heat loss. The merge collector and reduction at the flange are the best compromise to this as it helps keeping the gas velocity up. What you really need to do though is measure the percentage loss of heat from the outlet of the exhaust port to the merge collector at your specified rev range and match the pipe size accordingly. EG if the exhaust loses 15% heat, the pipe size has to be 15% smaller to maintain velocity.
You also need to take into account intake temps and velocity. A change in intake air temp will affect exhaust velocity as well. As the temp rises, the exhaust velocity will reduce as there will be less bang due to less a dense O2 charge.
Vehicle engineers have very specific software to calculate all this to make the engine run as efficiently as possible on the street, race applications are completely different. What you are doing is more of a race application where the revs will be consistent, on the street this will never happen.
Well done on a good exercise though, they will look impressive.
