I stole this from mazdaforum… who stole it from somewhere else.
Source: http://mazdaforum.com/forum/showthread.php?t=13389
I found it to be a good read…
this is not my work, but the work of a guy on another forum. it is posted specifically for our cars since it is from a mazda forum, and it is all theory on naturally aspirated tech. i recommend that you read this whole post and not just skim over it. the guy that wrote this is an n/a god.
Disclaimer: As with any type of Automotive Upgrade, N/A is full of Grey, it’s never as easy as a Black and White answer. There is always an exception to any example. The information presented in this thread is just that, basic information. It is possible to go into much further detail on the subject, which is up to you in the future. When types and styles of upgrades are compared it is under the condition that they are all of similar capability. It is always possible to make statements false by taking them out of context or changing the basic parameters of the discussion.
Ok, so I thought I’d go over the basics of Natural Aspiration theory for anyone on the forum considering NA mods. There are a few stages in which to make power when you’re going the all-motor route. Most people are familiar with the basic bolt-on mods, Intake, Header, Exhaust. These are the most basic and commonly done to give you a small boost in power, throttle response and even gas milage. With a good system, designed to work with what you have, you should see 5-10% gain in power, with the only real downside being noise. The primary way an IHE set-up give you power is by freeing it up. It takes energy for the engine to suck in, and pump out air. So by minizing pumping losses, by installing a free-flowing intake and exhaust system, you free up power.
Some power is also achieved by some more complex actions. A header’s gains are mostly by reducing pumping losses. But there’s more to a good design than that. A 4-2-1 design, aka Tri-Y, is a space saving header design, designed mostly to minimize pumping losses, but can also give gains in power by maximizing low-RPM velocity. Velocity is important for low-RPM power and throttle response, to help pull spent exhaust gases from the combustion chamber, and to aid in scavaging. Scavaging is the action of spent exhaust gases creating a partial vacuum in the cylinder, which when the intake valve opens, pulls in a fresh air/fuel charge. This is the reason it’s recommended to keep a relatively small diameter exhaust(2-2.5") with an NA combination- to maintain velocity. With a larger bore exhaust, you have no velocity at low RPM and lose this effect. Unlike turbo exhaust, bigger isn’t better.
The other way to go, is a 4-1, or tuned length header. As well as minimizing pumping losses, and maintaining velocity, you can tune a 4 into 1 design to create even more power over a somewhat narrow RPM range. The key, is that all four primaries have to be stacked two on two to work. When an exhaust valve opens, it creates a pressure wave. Every pressure wave has both a positive and negative side. When this pressure wave first enters the primary at the speed of sound, it travels down the primary and into the collector , and back up the other primaries towards the exhaust valves. As it hits the closed valve and bounces back, the exhaust valve opens and the negative side of the wave helps draw out the spent exhaust gases. Because the speed of sound is the speed of sound(it can change with gas density though), it can only happen over a limited RPM range, say 1000RPM. So if a 4-2-1 design gets you 10whp overall, a tuned length header can get you those same results, plus another 3-4hp at it’s tuned RPM, usually at higher RPM, since the physical length of headers tuned to low-RPM are not viable in most applications.
Headers in general are affected by both the primary length and diameter. Like I mentioned before a smaller primary diameter favours lower RPM velocity, but can limit high-RPM airflow; thin diameter(say, 1.5") piping can be a restriction when you’re running high-RPM, high-power levels, say 170whp and 8000RPM where you’d want at least 1.75" primaries, but should be fine for most low-tune, general applications.
Although a header itself will give you the single best bolt-on gain from your IHE set-up, the exhaust from the header back is important too. There are the basic cat-back exhaust bolt-ons, which like I mentioned, mostly minimize pumping losses. A high-flow catalytic converted can also free up a couple hp. The same theory applies, where smaller diameter piping will aid in low-RPM velocity. A well designed 2" exhaust would be great maintaining torque, but would choke off power above 6000RPM. 2.25" exhaust is a great compromise, and should give you the best of both worlds. Decent torque and decent power potential up to at least 8000RPM and 150whp. I chose 2.5" exhaust because of my future goals of 170whp and 8500RPM redline. I actually lost torque below 3000RPM over my factory exhaust, but will allow for all my future mods to give me maximum gains up top.
There is a lot of misinformation and myths with intake systems. Most intakes work by minimizing pumping losses(again, by making it easier for the engine to suck in air), but there are other variables. The Cold Air Intake theoretically takes cold air from either a fender or behind the bumper. Cold air is denser, and denser air has more oxygen molecules per weight, therefore more potential to react with fuel to create power. The effect is debatable, as most CAIs are made of aluminum, which gets quite hot and heats the intake charge quite a bit. The wrong CAI on the wrong engine can have the wrong results too. Just as with exhaust, intake air velocity matters. Air has mass, which has momentum. As the piston moves down the cylinder, it pulls in the fresh intake charge. Even as the piston starts to move back up the cylinder, the intake valve doesn’t close right away, and the air wants to keep moving into the cylinder, since the mass of the air behind it doesn’t want to slow down. With a small sized CAI, you get that same low-RPM velocity, but just like exhaust can choke off power at higher RPM. If the CAI’s diameter is too large, you lose that low-RPM velocity, but may be able to gain it up top.
The other option is the Short Ram Intake, which take air from inside the engine bay. The advantage of this set-up, is that it allow you to run a smaller diameter, say 2.5" versus the 3" of most CAIs, to allow for torque gains. But because the piping is shorter, pumping losses may not be as bad at higher RPM, than a CAI which is longer and has more bends. Air doesn’t like to change directions, so the less bends you have, the better. That goes for exhaust too. The downside of course, is that it draws in hot air from the engine bay. From most dynos I’ve seen, the SRI has outperformed the CAI. What works in theory, doesn’t always work in practice, and some cars will like one more than another. Another factor is resonance tuning. Just like tuned headers, pressure waves are created when intake valves open. Those waves can disrupt the intake flow at certain RPM. So what makes one intake work over another, could have to do with intake length, just as much as diameter, and where it pulls in air.
Filter choice can also make a difference. If you stick with a stock airbox, don’t bother with the ‘K&N’ filter upgrade. Our engines don’t flow enough air to require a K&N filter, in their stock running condition. I dynoed the K&N back to back with a Fram paper filter and then with no filter. The power was the same on all of them. Where the filter matters is on SRIs and CAIs…the pod filter. Bad designs or poor quality filtering material cam limit power. Look for ones with an intake horn to allow air to flow smoothly into the piping, such as the APEXi unit.
What matters most, is what works on your car. Learn from others who have done the work before you.
Well, that’s the end of part one. Theory 102 will<
