When I put together an intake for my AE92, I didn't have much flexibility in terms of routing but I took a lot of time to pick out the best components - I ended up going with an AEM Dryflow filter with the prefilter bag, and Spectre modular chromed ABS pipe. the filter offers good filtration and is washable (even with unofficial soaps 
) and the tubing is very lightweight and good at keeping engine bay heat out of the intake tract.
In reply to mad_machine :
The original papers by the originators of the NACA ducts are online, mid 1940's. TL:DR, NACA ducts are Terrible for throttled intakes (Any varying intake volume), and only a few serious experimental aircraft turbine engine intakes ended all attempts. (Engine flame out on landing approach due to high angle of attack ruining intake flow is intensely demotivating.) Aviation NACA ducts are for constant flow to full time air flow uses, often cooling, but also cabin HVAC. They are overly sensitive to variable angles of approach air, which racing cars excel at riding on pneumatic tires which only develop side forces at notable slip angles. The turbulence following other racers is also poison to an efficient NACA duct flow. NACA ducts perform best in high pressure areas. The image of the MGA is almost the worst possible placement. Optimum locations are found with water balances at speed, but almost always follow the same principles. Miata hoods are well researched, and the info is available. In contrast, the location shown (where it's normally most convenient for interior piping.) usually tests as enough of a low pressure area to be ideal for radiator air EXIT (See most LeMans prototypes, and newer Lotus), or just releasing hot engine compartment air. All current pro racing teams I'm aware of use Ram air, with an exterior scoop in most cases, if legal. All NASCAR teams I'm aware of use cowl induction, being the highest pressure area available that doesn't take damage in a 'rubbing is racing' league. Formula One is about ideal, an 'isolated from body flow' ram air duct with almost straight air path from opening to intake or turbo.
Note on flex tubing for performance sensitive airflow: Flow tests on accordioned tube show ID needs to be roughly 25% greater cross section than smooth tube to flow as well. This also strongly suggests transitions from smooth to accordioned, or accordioned to smooth tube are excellent opportunities to improve over almost all commonly available transitions. Expect resonance effects from each change in cross section Either way (Losses, unless well mated to engine RPMs), which is well understood in intake and exhaust runners, so minimized when it can't be used productively.
Please advice
Ive been working on my Volvo P1800 , swap engine from a Volvo S40 , bigger turbo , now I found these article very interesting but would like some advice regarding how to imporve my Cold Air Intake for the turbo .
Please advice
In reply to CapiDurazno :
With the turbo - you have a hot air intake there taking air directly behind the hot radiator and just above the exhaust in a tight engine compartment without much air exit. Best and easiest thing to do would be to extend the intake straight forward, to the front of the radiator in the grille opening. Don't kniw how much room you have between radiator and side bulkhead though.
In reply to CapiDurazno :
Hood scoop
So 30* is 1hp, but how much does air temp affect knock? What is the relative reduction?
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