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Do Turbo Cars Need Back Pressure to Work?

Do Turbo Cars Need Back Pressure to Work

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How pressure works with a turbocharger is an automotive engineering concept that involves some complex science. I’m not going to lie to you: There’s some dense information to learn to understand what’s going on here.

When it comes to turbocharged cars, some believe that a little back pressure is good. While others believe back pressure should be eliminated at all costs.

Who’s right?

That’s what we aim to find out. We’re going to look at what back pressure is. And what causes it as we follow the path of exhaust gases from the combustion chamber to the exhaust pipe to determine if back pressure is good or bad for a car with a turbocharged engine.

Let’s start first with how combustion happens to gain a clearer idea of what back pressure is and how it happens.

1. Combustion

The act of combustion is what creates back pressure.

An internal combustion engine is nothing more than a big air pump where the act of combustion happens. The combustion chamber is the main area of an engine wherein pistons move up and down. Each piston is located within a cylinder.

In a modern fuel injected internal combustion engine, fuel is squirted into a cylinder above the piston where it mixes with oxygen to create combustion.

This small explosion forces pistons down. (Due to the way they’re mechanically designed, as one piston is forced down by combustion another piston springs up.) This up and down movement happens several times per second.

Without going into even more explanation, the up and down movement gets transferred to the wheels which gives an automobile the power to move forward or backward.

After the fuel and air are ignited, the spent gases have to go somewhere. But where? Keep reading to find out.

2. Engine Valves

Engines valves play a role when it comes to back pressure.

How do engine valves figure into this equation as it relates to back pressure and whether it’s good for a turbo engine?

Very prominently, in fact.

You see, the byproduct of combustion is exhaust gases which need to leave the combustion chamber for the next mini explosion to happen. Engine valves provide that means. They sit on top of each cylinder.

At bear minimum, there are at least a set of engine valves per cylinder: an intake valve and exhaust valve.

The intake valve opens and closes to allow fresh oxygen into the cylinder where it mixes with fuel to cause combustion which creates exhaust gases.

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After the explosion the exhaust valve opens and closes to let exhaust gases out of the combustion chamber. Once out, exhaust gases head into the exhaust manifold where they travel through the rest of the exhaust system.

Just like the up and down movement of the pistons, the opening and closing of engine valves happens several times per second.

3. Exhaust Flow

Exhaust flow affects back pressure, especially on a turbocharged engine.

To paraphrase the preceding sections: As combustion happens, exhaust gases pulse through to the exhaust manifold and through the rest of the exhaust system before exiting the tail pipe.

As combustion gases make their way through the exhaust system, they create exhaust flow which is the flow of the exhaust gases making their way to the tail pipe. For a car without a turbocharged engine, back pressure is only coming from the exhaust system.

In a car with a turbocharged engine, back pressure comes from the exhaust side of the turbo which is connected to the rest of the exhaust system. And the inlet side of the turbo connected to the exhaust manifold.

4. How a Turbo Works

Turbos add more oxygen into the engine, which has an affect on the amount of back pressure created.

This seems like a good point to talk about how a turbocharger works.

Remember, an engine is a big air pump. There are two ways to make more horsepower in an internal combustion engine: add more fuel or add more air.

Adding more fuel can be dangerous to the combustion process, not to mention expensive. Adding more air to the act of combustion is a way more efficient way of creating more power, which is why almost every performance modification works to add more air to increase horsepower.

And that’s just what superchargers and turbochargers are designed to do: compress more air into the combustion chamber.

But it’s not air so much as oxygen that an engine needs for combustion.

Compressed air just means that there’s a greater concentration of oxygen per unit of air, which is what a turbo works to deliver to the engine.

A turbo is made of two halves joined by a shaft. One half is the hot side which is where exhaust gases spin a turbine connected to another turbine on the cold side which sucks in and compresses air into the combustion chamber.

5. Back Pressure

When deciding if back pressure is a good thing for a turbocharged engine, what we’ve found is that terms like back pressure, exhaust scavenging and exhaust velocity often get confused in the explanation.

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Permit us as we attempt to unravel this enigma: In the simplest terms, back pressure is an opposing force acting on a gas attempting to move through a confined space.

In an engine, that confined space is the exhaust system which begins at the exhaust manifold – since this is the component physically connected to the exhaust chamber – and ends at the exhaust tip usually located at the rear of a vehicle.

Specifically, back pressure is caused by components of the exhaust system that impede exhaust flow as gases pulse out of the combustion chamber. Restrictions to exhaust flow include:

  • the exhaust manifold,
  • up pipes and down pipes on a turbo engine,
  • catalytic converter,
  • resonator (if an exhaust system is equipped with one),
  • and muffler.

Gases flow more easily into low pressure areas. But all these components work against exhaust flow creating back pressure.

And when it comes to a turbocharged engine, these gases also have to deal with back pressure coming from the manifold on the hot side. And the exhaust system on the cold side (back pressure from the manifold side is inevitable).

Let’s now turn our attention to other elements of exhaust gases that often get confused with a need for back pressure.

6. Exhaust Velocity

Basically, exhaust velocity is a term used to describe the rate at which exhaust gases can evacuate the combustion chamber to make room for the next combustion cycle.

Restriction to air flow has a direct effect on exhaust velocity. How quickly exhaust can be expelled from an engine’s cylinders has a direct effect on the efficacy of a turbo – if the turbine on a turbocharger isn’t able to move fast enough it won’t be able to charge enough air to send to the engine.

Practically, this means that your engine will need to work at higher RPMs for the turbine to spool. This is why many tuners opt for performance exhaust systems.

While they still utilize components like a catalytic converter and muffler to stay road legal. These aftermarket exhaust elements are less restrictive than stock exhaust systems encouraging much better air flow and exhaust velocity.

Exhaust velocity also affects boost pressure in that the higher the exhaust velocity, the more condensed air the turbo can feed the engine.

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7. Exhaust Scavenging

This is another element that gets confused with a need for back pressure.

Exhaust scavenging is all about quickly removing spent exhaust gases from the combustion chamber so that the next combustion cycle can happen – that’s it.

Exhaust gases travel quickly as they evacuate the combustion chamber and make their way through the exhaust system.

Remember that gases flow easiest into low pressure areas: As each piston gets pushed up the exhaust valve opens to expel the spent exhaust gas (referred to as a high-pressure area). The gases then travel into the exhaust manifold (referred to as a low-pressure area).

The pressure difference between the combustion chamber and exhaust manifold causes exhaust gases to get pulled out. This is what’s known as exhaust scavenging.

Exhaust scavenging is a good thing because it reduces back pressure.

8. Boost Pressure vs. Back Pressure

To make some incredible power, the perfect setup would result in lower back pressure than boost pressure. But this is usually configuration seen in drag cars, which have the option of adding bigger turbos.

Street legal machines tend to have more back pressure than boost pressure. This is often due to the fact that the turbine housing for the turbo is smaller than the compressor size. Changing to a bigger turbine wheel or housing lowers back pressure and increases power.

9. Turbo Lag

Turbo lag is a regrettable outcome of turbocharged engines, which also has an affect on back pressure.

A four-letter word when it comes to turbocharging.

I’ve experienced turbo lag. It’s annoying especially when you’re expecting a big rush of torque and instead all you get is a vehicle puttering forward barely able to keep up with traffic.

Turbo lag is the slow throttle response you get right before the turbo kicks in when driving a car with a turbocharged engine.

Turbo lag is just the byproduct of how turbos work in street legal vehicles. The good news is you can reduce turbo lag.

Motortrend suggests a few clever ways you can reduce turbo lag from adding a wastegate, to adding nitrous, narrowing your powerband, increasing your compression ratio or you could consider sequential turbocharging.

Sources:

Motorauthority.com; Howstuffworks.com; Eagleridge.com; Low-offset.com