RX7Club.com - Mazda RX7 Forum

The easy way to tell when looking for stock vs. aftermarket exhaust is to open the hood and look on the side of the engine (passenger side). At a glance, you'll be able to see whether it has the stock fixtures or aftermarket.

Stock; you'll likely see a lot of thin sheetmetal covering the stock exhaust components as Mazda was religious about keeping the heat away from the intake components directly above the stock exhaust. Since hot air rises, having some shields around your exhaust system are a good thing, and Mazda went out of their way to cover everything up pretty well. With 79-80 SA22's, you'll see a single large canister mounted to the passenger side, lower portion of the car. This is the 'Thermal Reactor' and was utilized to mix the exhaust gases from both rotors inside a chamber that would be pumped full of fresh air from the airpump. The thinking was that any unburned hydrocarbons would quickly be consumed with the fresh oxygen being supplied directly to the Thermal Reactor from the ACV mounted next to the airpump. This worked well, and was a cheap solution to the expensive platinum-mesh 'catalytic converters' to come in later years. Extending back from the Thermal Reactor will be a large header pipe adorned with heat shields that connects to your exhaust connector (to the muffler). This has a fitting which routes back to the bottom of the Thermal Reactor and pushes bypass exhaust further down the line to continue the burning of unburned hydrocarbons.

On the 81-85 FB's, you'll notice that they have the same heat shields installed, but additionally, there are either 2 or 3 additional 'catalytic converters' mounted between a much simpler exhaust box which is the mixing chamber. This chamber is mounted in the same location and simply allows both rotors to exit out into the box, mix the gases, and then gas is pushed out to the in-line catalytic converters which have platinum which stays hot and burns off any unwanted gases. This system is also fed by the airpump via the ACV, though fresh air is pumped into the exhaust ports by an internal passage in the lower intake manifold (right where the ACV is attached).

Inspection under the car will reveal if you have the stock systems in place, which tend to work very well for emissions control, and are very expensive to replace.

Aftermarket exhaust systems will simply be two distinct and separate tubes leading from the side of the engine down below the car, usually bypassing any catalytic converters and going straight to the intermediate pipe (to the muffler).

Two parallel tubes which come straight out of the engine and make the same turn, followed by a collector are 'PaceSetter' headers and are of thin, mild steel, and are to be avoided. Two tubes with the rearmost tube curving up and over the forward tube are the common Racing Beat variety and are very effective.