Do Engines Really Need Back Pressure?

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Backpressure is the most talked about topic among car enthusiasts and whether or not it is required by the exhaust engine or not. There are also many misconceptions surrounding backpressure. Here is everything you need to know about it and its requirement in a vehicle.

What is Exhaust Back Pressure?

Engine back pressure or backpressure is the pressure produced by the exhaust system to overcome any kind of resistance that can be produced by the fluid running through the engine pipes.

This is the gauge pressure that is produced at the turbo outlet of your vehicle’s exhaust system. The engine basically after combustion pushes or pumps the exhaust gases out through the manifolds and then through the exhaust pipes.

The pressure inside the cylinder is almost 6 to 7 times more than that of the atmospheric pressure. So, the gases conventionally go out towards the low pressure. However, as they go out through the pipes a resistant force is produced named as back pressure which opposes their motion and thus apparently is not good for the total horsepower of your car.

Limits of Back Pressure

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Different manufacturers have different limits of backpressure for their engines. Backpressure is measured in kPa (kilo Pascal), a unit of pressure. The level generally depends upon the engine size which is considered as its horsepower. For example, the Ford Fiesta MK6 exhaust system has a power of almost 50kW so the back pressure limit for it would be around 20kPa. Any engine with power below it will have a limit of 40kPa. The limit for above 500kW would be 10kPa.

Do Engines Need it or Not?

Engines do need a little bit of backpressure for a process called scavenging. As the exhaust gases move out of the engine and the piston moves up, the pressure inside drops. That means some of the exhaust gases are still going to be present inside the chamber as the piston is not fully up. This will eventually affect the proper performance and efficiency of the engine.

Backpressure actually provides a backward impulse to the pressure inside that makes the exhaust gases completely go out in the atmosphere. It means that it helps in cleaning out the cylinder after the exhaust stroke ensuring the efficiency of the exhaust. It basically needs a lot of technical aspects to make the backpressure go inside at a particular time for the scavenging to happen.

Also, too much of it is going to restrict the flow of gases and will not let the cylinder get rid of them by pushing it backward. This will lead to a decrease in the power of your vehicle engine.

If your pipes are small in diameter, the velocity and the flow rate would be high but the resistance would be very high. Whereas if your exhaust pipes would be large in diameter, these would be very less to no resistance thus low backpressure. However, the velocity would below as well. So, ideally, you have to find the balance at which the velocity is not as compromised and the back pressure remains low as well.

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