Hey turntaker, couple things on cowl intakes, coldside intakes and crossover tubes to help you out as i did a fair bit of research before making mine.
swiss cheese air box the yanks talk a heap about but will suck hot air and many who have done it regret it and change it back, yes it makes noise but the other reason the yanks like it as they are restricted on cowl intakes because they drive on the wrong side of the car and the brake master cylinder is there
cowl intakes add between 5-10rwhp from what has been documented many times and seen from dyno days. save arguments lets split the difference and be generous say 7hp at the wheels. (barely noticeable on the "Butt dyno")
My cowl intake is cheap without the heat shield tape but not as effective, throttle response mildly better,
above 5,000rpm there is more pull than before no doubt but it is a small amount (air box snorkel restriction? no curves? colder air?), fuel consumption has improved. pod is enclosed so boys in blue wont give you grief.
down sides,
-
its loud with soft top up, louder with hard top on, no top its actually quite a nice growl. it is louder and deeper than a loch stewart cowl intake and similar to a mania but more cc volume due to the larger diameter i used.
-takes about 5 minutes to clean leaves out of the filter (like the mania). mine is garaged so not a big issue but if you park near trees it would really annoy you, clog up air filter quicker and reduce performance but this has been discussed many times on cowl intakes.
-bloody big hole in your firewall.
bottom line would i do it again- yes i would, my wife doesn't like the noise as much
.
crossover pipes,
My AFM will be gone soon and i intend to get an NB crossover tube to replace my NA8 one (better design, mazda changed it for a reason).
I expect that these will remove more of a restriction and net me a couple HP but will be hard to measure as the computer will be different.
i think the large volume crossover pipes on the market would benefit max HP at max RPM on a dyno but most if not all are metal and run between the radiator and motor so bonnet closed and longer running times heat soak will kill that small gain in the real world.
Documented many times on here and other forums is that the hermholtz (sorry on spelling) chamber on the mazda factory pipe does help mid range torque and would contribute to the shortside/large crossover pipe torque dip.
short coldsides have been shown to slightly reduce midrange torque and really there isn't that much to lose in the first place. air velocity would probably come into play here as well as the hermholtz.
real performance messurements of large crossover tubes and cowl intakes is probably a "what is your trap?" kind of question rather than a "make a difference on the dyno?" one.
this will take into account and show heat soaking of components and engine bay air temps. it will also show the benefit from cowl intakes of the pressurised area under the windscreen that isn't seen on a dyno, dont get me wrong it wont be 20 psi more like an 8th of a bees fart but might be something that explains why so many people think these make a larger difference than the dyno shows, i wonder at what speed there is enough pressure to actually makes a difference?.
anyway here is how i did mine and best of luck with it all.
viewtopic.php?style=3&f=29&t=72551&hilit=cowl+intake
if you need to hear it PM me and ill get it to you, the vid in that topic has expired.