Test/race pipe for 2nd gen 3? Compatibility help

I seriously doubt they would fit, but I can't say for sure.

Either way, bypassing the cat on a standard skyactiv 3 won't do much of anything for you other than make you fail inspection/emissions, and make the car impossible to sell in most places. I'm not even sure a test pipe alone will make you any louder, if that is your goal.

For a naturally aspirated engine - especially a direct injection engine running at 13:1 compression - removing air restrictions won't have much impact on power for a variety of reasons. In the turbocharged engine, the intake pressure produced by the turbo (ie, "boost) is directly related to the pressure gradient across the compressor fan at the exhaust. If you remove restriction on the exhaust, the pressure gradient across the fan is directly increased, meaning the intake can "suck" harder for a given engine load (load is what spins the turbine), and you can pack more air into the cylinder. In that sense, the engine is capable of using more air because it can be compressed into the same volume at a higher pressure. In a naturally aspirated engine, you don't have a compressor turbine, so the amount of air you can physically move into the cylinder is constrained primarily by the cylinder geometry. The piston only displaces so much air by itself, and it's velocity is tied to RPM. The only way to move more air is then to increase either the piston velocity or displacement, or to increase the ambient air pressure. Unless the engine is seriously constrained in this sense (and there are some good engineering reasons for having some restriction) than decreasing back pressure is just increasing some overhead that you cannot really use without extensive engine modification. In the worse case scenario, dropping the exhaust back pressure too much will drop the exhaust gas velocity, leaving combustion products floating around the cylinder and and manifold. This is mostly a good way to foul your injectors.

/r/mazda Thread