I have a 1987 RX7 5spd. It already has adrop in K+N with the stock cold air inlet, some 2.5" header exhaust that still has a cat and then splits back to 2 with mufflers (kinda like an X pipe.) I would like to know with this rotary engine being N/A does it still benifit me with Horse Power to remove the catylitic converter/mufflers? I know this is my first rotary and I have found in the other type engines that without the right back pressure from cats or mufflers you can lose HP. Also would a CAI kit be better than the K+N drop in?

 

If you remove the cats, the exhaust won't generate enough pressure to open your auxiliary ports, resulting in a major loss of HP above ~3800rpm. I believe the Racing Beat headers have a seperate pickup tube that can open the aux ports without a cat though. Might want to look into how their header is designed and possibly modify yours to work in the same way.

As far as a CAI goes, a proper one may be worth it, but the typical cone filter just hanging in the breeze will suck in hot air and probably lose you a couple HP. It needs to be sealed off from engine bay heat and have an outside source of air to be effective.

 

The issue about the secondary ports is real. They require about 2.1 psi to open completely; the more you reduce the backpressure, the later in your acceleration cycle they will open up.

I have heard of some owners who have modified the system to eliminate the use of backpressure to open the ports, and instead hooked it up to air from the air pump. I do not have the details on how that is done.

 

Thanks guys! Back to the CAI I am aware that you need to get the air from an outside entry away from the engine... My stock system is like a ram air setup but connected to the stock box with a K+N flat shaped drop in filter. Now I have bought some intake tubes and a cone shaped K+N filter...I am sure I can modify and get the tubes to put the cone filter outside the engine bay but my question is: Would it even be worth it? Or should I just keep my stock ram air system with the flat shaped K+N filter?

 

The air pump activated secondary ports can be done by splicing into the air pump outlet and then (depending slightly on if you removed the emissions or not) utilizing a typical air valve solenoid found on pretty much any RX7's emissions rack with the colored connectors and having the 2 inlets/outlets go from the air pump to your secondary port actuators, and then finding the colored connector that gets powered around 3500 and splicing that to your air valve solenoid thus making the solenoid to your secondary ports activate around 3500 (which is when they are activated on the S5's and when the exhaust would build up enough pressure on the S4 system anyway). Kinda simple but could take a Saturday to do

I've done that exact setup on at least 3 S4 NA's and its always turned out fine, maybe even noticeable because the secondary ports open up really quick instead of gradually do to back-pressure having to build up.

There are also other ways of making this happen but this is what I like to do. Hope this helps

 

Whether or not it is worth it depends a lot on your engine's output. If you just have the exhaust modification, then a custom intake will not help that much. If you have porting and other modifications, then a custom intake will be more worth it. The rule of thumb is that the stock intake is fine up to about 200hp.

Also, a simple pod filter and intake snorkel configuration really isn't optimal for a NA engine, anyway. If you are serious about performance, you should get a standalone EMS and individual throttle bodies, and use a round carburetor-type filter inside a round custom air box fed by a 4-6" diameter intake runner with an inlet near the front bumper. See the Mazdatrix EP convertible for an example:
http://www.mazdatrix.com/faqpics/roadrace/ep01123ib.jpg

 

Wow! www.mazdatrix.com/faqpics/roadrace/ep01123ib.jpg That is an amazing intake! Its fully custom, so you cannot just order it?

 

You can call and ask them if they will make you one, but I doubt it would fit unless you modify your engine bay in a similar manner, which would require a carb kit or ITB setup, as well as removing the light on the right side of the car. It is usually better to custom fabricate something like that yourself so you know that it will fit your particular engine bay and front fascia configuration.