The Exhaust Intermediary: Tacoma Mid-Pipes

A mid-pipe is the exhaust pipe that is positioned between the exhaust manifold(s) and the muffler. It is in the middle of the exhaust system, hence the name mid-pipe.

Depending on the application, the mid pipe can contain a few different elements. 
The first design is just plain tubing, containing nothing else. The mid-pipe can also contain resonator(s), or even secondary catalytic converters. The mid-pipe can also have flanges(s) on one or both ends, that bolt to complementary flanges in the system. Some also have fittings for an oxygen sensor.

The mid-pipe on a 4-cylinder equipped Tacoma is a single pipe connecting between the exhaust manifold and the muffler. The mid-pipe contains both the primary and the secondary catalytic converters.

The mid-pipe on a V6 engine truck is y-shaped. The front has two separate input flanges to connect to the exhaust manifold on each side, or bank, of the engine.

The two pipes merge into a single pipe to connect to the single muffler. The two secondary catalytic converters are incorporated into the mid-pipe, prior to the merge point.

Dual side exit exhaust systems are designed to work hand in hand with the factory mid-pipe. As long as the aftermarket mid-pipe is designed to bolt to the factory exhaust system, it will bolt up directly to any cat-back exhaust system.

The major difference with the side-exit exhaust system is that the pipe between the end of the mid-pipe and the muffler inlet will be much shorter than that of a rear-exit exhaust system.

The proper exhaust pipe diameter is crucial to your engine’s operation. When dealing with pipe diameter, there are two things to consider: backpressure and flow velocity.

Backpressure is the amount of atmospheric pressure pushing back through the exhaust system. If the piping is too small, there will be too much backpressure in the exhaust system. 

This will result in the exhaust gases not being able to escape the engine fast enough. Too much backpressure will cause the exhaust gases that are not able to escape fast enough to contaminate the cool air-fuel mixture entering the engine on the next cycle. This will reduce horsepower.

If you increase the pipe diameter, it will increase flow velocity. Flow velocity is how fast the exhaust flows out of the engine and through the pipe. By increasing flow velocity, it helps to pull the exhaust from the engine, as well as helping to pull the air-fuel mixture into the engine. This increases power and performance.

If you increase the pipe diameter too much, backpressure will be nearly eliminated, and flow velocity will be excessively increased. The result is the exhaust escaping too fast, and pulling some of the air-fuel mixture out through the exhaust. That creates extra pollution escaping the exhaust and reduced power.

The general rule for proper backpressure and flow velocity is 1 inch of exhaust tubing diameter per 100 horsepower. Therefore, if your engine is producing 300 horsepower, you would need 3 inches in diameter of exhaust pipe. The proper piping diameter can be obtained by a single 3-inch pipe, or dual 1 ½-inch pipes.

On 4-cylinder model Tacomas, both the primary and secondary catalytic converters are located in the mid-pipe assembly. With aftermarket mid-pipes, you can either eliminate the secondary converter or both converters.

You may also replace one or both converters with high flow units. If you decide to eliminate both converters, you will need some additional hardware to fool the oxygen sensors. Without these extras, the check engine light will remain illuminated on the dash.

On the V6 trucks, the two secondary converters are located in the mid-pipe, while the primary converters are built into the exhaust manifolds. An aftermarket mid-pipe can facilitate removing the secondary converters completely and adding resonators, or replacing them with high-flow units. These pipes have bungs to screw the oxygen sensors into them. This will allow the system to be utilized without having the check engine light illuminated.

If you are completely removing ANY catalytic converter, you should consider the consequences. These setups are designed for off-road use only. Removal of any catalytic converter can render your vehicle not valid for inspection in states that have a safety and/or emissions inspection program.