My apologies to all about the length of time this may take to complete but it will give us all time to discuss the theory and pitfalls, and also flow bench results at the end.
Guy may also wish to add to this by post with his technical knowledge (hopefully correct me if i'm wrong) and information regarding previously tested headers that have failed test to show you all what NOT to do.
The GOLDEN RULE as far as we are concerned
For any bend in a header primary pipe there should be no bend tighter than a 2:1 ratio, this ratio is "Chord Radius of the bend : Pipe Internal Diameter".
see picture below:-
- Chord Radius to Pipe Diameter
- 2_1Rule.jpg (69.37 KiB) Viewed 1498 times
All calculations will be shown for my header, this does not mean that it will be exactly the same for your own, but you will be able to use the methods for your own calculations.
PRIMARY PIPE DIAMETER
To calculate the primary pipe diameter you need to think about the theory of what you are attemping to do. Too smaller pipe and you will have good low end torque but the engine will be strangled at higher RPM and not produce its full power potential. Too larger pipe and torque at higher RPM will be good but not at lower RPM's. The Ideal situation is to keep the exhaust gas speed the same from when it leaves the cylinder to when it hits the turbine or in the case of a non turbo engine the 1st collector.
Now we may all know that if you increase a pipes diameter the gas speed will slow down, and if you decrease a pipe diameter the gas speed speeds up so where do we start? Where do we find our pipe size? The answer lies in the head at the exhaust valve throat. The exhaust valve throat is most likely to have the smallest CSA (Cross Sectional Area) between the cylinder and the turbine or collector. So we measure the exhaust valve throat and calculate its CSA. As in my case a 16V engine we add the 2 throat CSA's.
Area of a circle = Pi x rad x rad
Throat diameter = Td
Throat radius = Tr
Throat Area = Ta
Td = 24.5mm therefore Tr = Td/2 = 12.25mm
Ta = Pi x 12.25 x 12.25 = 471.45 mm sq
Total throat Area for 16V Engine = 2 x Ta = 942.9 mm sq
So now we need to find the nearest availiable pipe size that has a CSA of approximately 942.9 mm sq so work backwards.
Pd = Primary Diameter
Pr = Primary Radius
Pr = Square root of (942.9 / Pi) = 17.32mm
Pd = 2 x Pr = 17.32 x 2 = 34.64mm
1.500" OD pipe has a 1.6mm wall thickness so 2 x 1.6 = 3.2
add the calculated bore size (Pd)
Pd + 3.2 = 37.84 mm
If you want to convert to inches 37.84 / 25.4 = 1.489 inches
It is acceptable to go to the next pipe diameter up from your calculation, so I will be using 1.500" OD pipework purely because the bends are easily available and the kit to model the primaries which you will see at a later date is going to be available shortly.