Hello. I've just bought a eu model fb from 84 with the 12a. The cars previous owner had bought i new from the dealer back in 84 and kept it stock with 45000km on the odometer. I was curious why it has a 3rd smaller pipe sitting next to the other 2 exhaustpipes. It desn't seem to get hot. It is running alongside the normal exhaustpipe and seems to go into a thing above the exhaust manifold wich seems to connect to a pump (furthest right from the drivers perspective) connected to the air filter (info about what this pump do would also be good). I don' know if the mini snakepit of tubing into the exhauspipe after the manifold also could be related as all of the pictures i seem to see when i search look very different apart from them having the same bends.

 

its the air pump air exit, if it doesn't go into the engine.

 

First off, welcome aboard. Your decision to buy a 35+ year old vehicle is a testament to your patience and technical agility! Now, onto your question...

The exhaust pipe you described is the bypass from the air control valve which routes air from the air pump into either A) the exhaust ports inside the engine, or B) vented to the outside as the cooling air isn't needed in the exhaust. The air pump provides fresh, filtered air from the air cleaner assembly to this valve, which is vacuum controlled based on how the engine is running. When needed, this cool, fresh air is provided to the exhaust chamber in order to burn off hydrocarbons and clean up the emissions. When not used, that pressurized air must be vented somewhere and venting it under the hood is loud. Therefore, Mazda chose to run an extended, smaller exhaust tube alongside the primary exhaust, in order to vent this air at the rear of the car where it would be less noticed by the driver.

Many enthusiasts remove this pipe and replace the entire exhaust with various aftermarket options, some of which also remove the Exhaust Gas Recirculation valve (EGR), and install a block off plate in its place. This will remove the extra exhaust pipe, but also require an exhaust header designed to block the air injection ports covered by the valve. Most aftermarket headers have an oversized mounting plate to accomplish this. These changes may not be legal in your state or country, as removal of pre-catalysts and various plumbing and valves will definitely increase noxious emissions.

Hope this answers your question, and good luck in keeping it on the road,