Straight Pipe Exhaust System: Everything You Need to Know About Its Pros & Cons

The automobile industry has been working on reducing or eliminating the noise and pollution created by vehicles for decades. They have developed a variety of ways to improve the quality of the air in and around automobiles. Today, modern vehicles use mufflers, catalytic converters, and other exhaust system modifications to reduce the harmful exhaust gasses released into the environment.

The straight pipe exhaust system is a type of exhaust system that begins at the exhaust header. The straight pipe exhaust is also called a full-length exhaust. Straight pipe exhaust systems are more powerful than the stock system because they reduce back pressure. Back pressure occurs when the exhaust gases leave the cylinder and enter the exhaust system. The resistance created by the exhaust system causes a loss of power. A downside of the straight pipe exhaust system is that it’s a loud and very obnoxious way of adding horsepower to the engine.

straight piping exhaust in Honda Fit

Before exploring further about the straight pipe exhaust system, first, let me explain the concept of back pressure in the exhaust system of the engine. This is very important for you to understand this concept as back pressure is the reason people want to switch to the straight pipe exhaust system.

Related: White smoke from exhaust

What Does Back Pressure Mean in Exhaust System?

Back pressure in exhaust system is a measure of resistance to the flow of  exhaust gases when they flow through the pipes. When the engine is running, the exhaust system sends the gas from the cylinders to the tailpipe. The tailpipe is connected to the muffler. Back pressure is created by the catalytic converter, particulate filter( in diesel engines), bends in the pipe, muffler, and other components such as resonator through which exhaust emissions have to pass before passing through the tailpipe and entering the environment. The higher the back pressure, the lower the flow of exhaust gas.

There are the following important components in the exhaust system of an engine:

  • Catalytic converter
  • Resonator
  • Muffler
  • Tailpipe

The catalytic converter is installed after the engine and is connected with the exhaust header/manifold. The muffler is a sound attenuator, and the tailpipe is the exhaust outlet.

Here’s how a typical exhaust system looks:

how does exhaust system look like

The purpose of a catalytic converter is to convert harmful emissions such as oxides of nitrogen and carbon monoxide to less harmful gases i.e. nitrogen and carbon dioxide. On the other hand, the function of the muffler and resonator in the engine is to reduce the noise and frequency level of the sound waves. produced by the exhaust gases when they pass through the pipes at a pretty high speed.

Muffler basically reduces the volume of exhaust gases but does not change the sound. A resonator modifies the sound waves of the exhaust gases and reduces the unwanted resonance.

All those components I explained above act as restrictions to the flow of exhaust gases when they travel from the exhaust manifold to the tailpipe. In simple words, back pressure is simply fluid friction. Whenever a fluid (exhaust gas in our case) passes through an area of a smaller crosssection or faces any restriction in its path, the fluid friction is increased which is the cause of back pressure.

For instance, when passing through a pipe, a fluid or a gas comes across a bend or a turn in the pipe, this will act as a restriction and oppose the fluid flow. As an example, if there is a traffic jam at a signal, vehicles will find it hard to cross the road. The same is the case with the exhaust gases when they pass through the exhaust system.

So, in order to force the exhaust gases to pass through all those restrictions, the piston of the engine has to do more work against that back pressure. So, there is more load on the engine’s crankshaft. Due to this reason, some people feel the need for a straight pipe exhaust system.

Back Pressure Is A Myth!

Now, some people believe that back pressure improves engine performance and efficiency. People confuse back pressure with the exhaust velocity of the gases. For instance, if someone installs a 3” exhaust, he would wonder why his engine is losing power as he has tried to eliminate the back pressure (restriction to the fluid flow) by increasing the area. You will be wondering if a wider pipe allows more fluid and does not offer any resistance to the gas flow, then why a wider pipe isn’t better.

The constrictions in the gas flow increase its velocity (which is good for the fast removal of exhaust gases), but it also increases back pressure (which is bad for the gas flow). That’s why people often confuse the back pressure and the flow velocity.

Check out the below video for a better understanding.

Take an example of a garden hose w/o a spray nozzle. If you let the water just run unrestricted out of the hose, it flows at a rather slow rate. However, if you put your finger on the part of the opening of the hose, the water will flow out at a much faster rate. The smaller the area, the higher the velocity. So, the exhaust gases in the 2″ pipe will be traveling considerably faster than the exhaust gases in the 3″ pipe. 

back pressure and velocity of gases increase as the diameter of the pipe decreases

While selecting the right diameter of exhaust pipes, we need to balance the flow capacity of exhaust gases. No doubt, velocity is increasing with the smaller area, but we are also putting the restriction in the direction of the gas flow, which is increasing the friction, or we can say this is the energy lost from the exhaust gases due to the friction. To overcome this friction, the power of the engine is lost. Below, I have attached the snapshot of an equation I came across during my engineering studies while analyzing the effect of the diameter of a pipe on fluid friction.

equation defining the related between the pipe diameter and the exhaust gases frictional loss.

In the above equation, ‘d’ is the pipe diameter, ‘hf’ is the friction in the fluid (back pressure) and ‘V’ is the velocity of the gas. Ignore the other factors for simple understanding. Now, if ‘d’ decreases, ‘V’ increases. So, overall the friction in the exhaust gases will increase which will affect the horsepower of the engine. Coming to the point, a smaller pipe diameter will produce higher exhaust gas velocities at a lower engine RPM but will create unacceptably high amounts of back pressure at high engine RPM. Thus if your powerband is located at 2-3000 RPM, you would want a narrower pipe than if your powerband is located at 8-9000RPM.

Considering the above discussion, engineers try to keep the exhaust diameter wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. However, a too wide pipe diameter will eliminate the resistance to the flow, but it will significantly reduce the velocity of exhaust gases, due to which the gases will stagnate in the pipe and create more resistance to getting the exhaust out the other end.

What Is A Straight Pipe Exhaust System?

A straight pipe exhaust system is a non-conventional exhaust system that consists of a straight stainless steel pipe that is connected with the exhaust header without a muffler, resonator, and catalytic converter. So, after combustion, straight pipe exhaust systems let exhaust gases low freely from the engine’s exhaust headers, allowing them to go into the atmosphere directly through the exhaust outlet without passing through any equipment. The straight pipe has a specific diameter to fit snugly into the engine’s exhaust manifold. 

Car enthusiasts like straight pipe exhaust systems because of the sportier or roar sound and the enhanced horsepower as the components that restrict the exhaust gas flow are not present in the straight pipe exhaust system.

Advantages of Straight Pipe Exhaust System

Here are some advantages of a straight pipe exhaust system:

1. Increased Performance and Horsepower

Straight pipe exhaust systems are typically designed to maximize exhaust flow while minimizing back pressure by eliminating the catalytic converter, muffler, and resonator. Since the back pressure is reduced in the straight pipe exhaust system, the crankshaft does not have to do work against any resistance. Therefore, you’ll get more torque and power in the straight pipe exhaust system. Due to this reason, straight pipe exhaust systems are found in racing cars and high-performance vehicles.

You can also read my guide on does straight piping increase mpg.

2. Less Vehicle Weight

Since there isn’t any component like muffler and catalytic converter after exhaust header in straight pipe exhaust systems, the weight of a vehicle is significantly reduced in straight pipe exhaust systems. Due to this, the throttling feels slightly more sensitive when your car has a straight pipe exhaust system.

One person just removed a catalytic converter from the exhaust system of his Subaru Legacy GT 2.5 Turbo, which resulted in the reduction of 50lbs of the weight of his vehicle. Considering this, if you also remove the muffler and resonator from the exhaust system, you can reduce the weight of your vehicle by around 100lbs.

The weight of an engine is an important factor in racing cars. Racing cars have a very high power-to-weight ratio and low center of gravity. Racing cars are built to achieve top speeds. Racing cars must be light and agile to accelerate quickly. Racing cars are designed to be lightweight to improve acceleration and cornering speed. This makes them more responsive. The lighter the car, the faster they go.

3. Might Sound Cool on the Right Car

A straight pipe exhaust system, when installed on the right car, produces a cool sound that draws everyone’s attention within a 1/8 mile radius instantly with the push of a pedal. In this way, a straight pipe exhaust system makes your car more fun to drive as the roaring sound of an engine with unrestricted exhaust flow seems exciting.

In the video below you can hear the sound of an engine with a straight pipe exhaust system.

Disadvantages of Straight Pipe Exhaust System

Here are the disadvantages of a straight pipe exhaust system:

1. Terrible Sound of the Exhaust

On most engines, straight pipe exhaust systems produce a terrible sound from the exhaust that makes it a really bad sign for the car. In the states of the US like California, the straight pipe exhaust system is illegal as it produces an insane sound with an awful drone in the middle of the car when you go for a long highway rides, and your car speed goes over 50 mph. A good exhaust is a beautiful tone with just enough volume to hear that tone.

According to the experts, straight pipe engine does produce exciting sound, but it is not applicable to all engines. Usually, a straight pipe exhaust system sounds amazing on v8 engines. In other engines, you can adopt a different approach, and many people are applying this to get high performance and better sound from their engine.

That is, instead of removing the muffler, replace the stock muffler with a more open-style muffler that offers less restriction on exhuast flow and also produces a better sound without causing a drone in the engine and irritating the people in your surroundings. Magnaflow makes high-performance mufflers that produce a deep and powerful exhaust sound without allowing it to irritate people. You can check Magnaflow mufflers on amazon.

People call stock mufflers “suitcase mufflers”. It is a huge thing in the center of your exhaust system behind the rear axle.

2. Harmful To the Environment

Since there is no catalytic converter in a simple pipe exhaust system, it will not convert harmful exhaust gases that cause smog, acid rain, and global warming. Those unfiltered exhaust emissions will also smell bad which can cause several diseases.

3. Depreciation in Resale Value of Vehicle

No one wants to buy a vehicle that is altered with the aftermarket engine parts, especially those that are critical for the engine. Due to this reason, modifying your car with a straight pipe exhaust system will also reduce its resale value.

How Much Horsepower Can Be Increased in a Straight Pipe Exhaust System?

In a straight pipe exhaust system, horsepower can be increased by 8hp to 15hp. However, the increase in the horsepower of engine entirely depends on the stock exhaust system. If the stock exhaust system is highly restrictive, then your engine will experience a huge gain in horsepower after installing a straight pipe exhaust system.

Moreover, the gain in the horsepower engine also depends on whether you have installed a complete straight pipe exhaust system or have modified the cat-back exhaust system with a less-restrictive muffler and resonator. A catback exhaust system costs more i.e. around $400 to $1000 as compared to a straight pipe exhaust system, which costs only around $100 to $300.