ELI5-Why does your car lose power when you cut the exhaust


ELI5-Why does your car lose power when you cut the exhaust

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Cut it how? The exhaust is engineered as part of the motor,with back pressure and flow part of the whole process, so changing part of the exhaust changes the dynamics of how the engine and the electronics that control it work. (I Am Not an Engineer)

The exhaust from your car is burnt fuel, which needs to be removed from the engine so that there is space for new fuel. If you block the exhaust, the burnt fuel cannot leave, and when the engine tries to bring in new fuel, the space is already occupied by burnt fuel. Cars can’t run on this, so the engine stops running.

Exhaust isn’t a constant velocity flow – each time the exhaust valves open, high pressure exhaust enters the pipe, then there is a brief period of pressure drop until the next valve opens, making the pressure over time cyclical.

As new exhaust enters the pipe, it pushes against existing gases in the pipe, creating pressure which moves all the exhaust out. If the gases are moving quickly, there will be lower pressure at the exhaust valves, allowing for the next release of exhaust to enter the pipe more easily. If you were to replace your exhaust pipe with wider pipe, the exhaust would have a state of lower pressure with a resulting lower velocity. This means the last pressure wave of exhaust doesn’t move as far down the pipe when the next is released, creating higher pressure at the latter end of the valves and leading to the engine needing to work a bit harder to evacuate the cylinders.