There was an Australian magazine that did a test of 12 different stock airboxes. They tested some of them with and without filters. While none of them was the RX-8 box, there was some good info never the less. For all of you that think the filter is the most important thing, here's a direct quote from the article:

"It should be noted that the major determiner of flow isn't the filter element but the box design itself. As you'll see, well-designed airboxes have a flow only 10 per cent or so different with or without a filter anyway. To put it another way, we saw flow variations from box to box of 65 per cent - with exactly the same filter element being used in each of these boxes!"

These were all tested on a flowbench under the same conditions. Does anyone really believe that a new filter will make more power? It is the intake design that makes the difference, not necessarily the filter.

Basically I am saying this. If you are considering an aftermarket intake system, does it really matter which one it is if every system is the same basic design? NO! If you are using the stock intake system and decide to use an aftermarket "green" or "Mazdaspeed" etc... filter, will it really matter? NO!

 

cool. thanks rotarygod....that certainly saves some money for me!

 

i love my greeen filters.. got them in all my cars..

 

Changing the filter will make a difference, less resistance=more flow. I haven't noticed much of a difference on cars, but when you change the filter on a motorcycle you notice it immediately (probably because of the better power/weight ratio).

Changing the airbox design will make a larger difference though, I agree. More air volume, length and depth matched to the resonant pulses of the engine and intake length, pockets that air can be trapped in removed, pockets added to increase turbulence, etc. Witchcraft at work! I really respect intake/exhaust engineers. They work with something you cannot see!
Cheers

 

I installed a K&N, out of habit if for no other reason.

Roy

 

I installed a green filter, not because I wanted more power, but simply because I wanted a more reliable filter. Have you lookes at the stock filter?? Pretty scary...

I agree with rotarygod... and I also believe that the stock box in the Rx8 is very well engineered

 

This is only true if the most resistance is at the air filter. If the highest restriction is not at the filter, then changing the filter will not do anything. It's simple (ok, not simple, I had to take chemical engineering classes to learn it), material transport.

Increasing an orifice size/void volume through a system will only increase flow if it is changed in the area which has the lease void volume.

Has there been any study out there that tells us the specific location of the most resistant unit operation of the intake system? Those specific bottlenecks in the flow are the areas to target first.

 

The article showed several air boxes that used the exact same filter. The variation in flow was about 65% difference between the best and worst flowing box with the same filter! How does the filter necessarily serve as the restriction? Less resistance does equal more flow but only when more flow can effectively be utilized. The RX-8 box is relatively well designed but it can be improved. If a perfectly designed box can only flow 10% more air without a filter than with the average filter, this must mean that even a good filter will still have some restriction which will result in less than 10% more airflow than the stock filter. This is also assuming that the aftermarket filter does in fact flow much better than the stock filter, which they probably do. Just to throw out a number, if your intake flows 400 cfm of air and you get a 5% gain in airflow, you now only have 420 cfm of airflow. About the only place that will help you is on the extreme top end of the rpm scale and even then only a couple of horsepower. How much do cone fitlers honestly give and where is the gain at? The extreme top end and only a couple of horsepower isn't it? They only gain power based on flow which isn't much. Remember that the stock airbox total intake area is smaller that that of the maf or the throttlebody. Logically that would mean that there is a restriction at that point. Resonant tuning makes up for some of that loss and the smaller area tube nearly gives us the same power as an unlimited flow system. Now just design a high flowing unit with resonant tuning. This all makes perfect sense to me!

 

It is a standard pareto problem. Address the areas that will give the most bang for your buck first then move on to the next. By the time you are on your Nth improvement your tweaking the last bit of improvement out.

 

so...has anyone really designed an intake that will actually increase hp? has anyone made a better box?

 

no.