Straight pipes are a relatively common feature on Muscle cars and Hot Rod race cars. These pipes have no bends, or sharp turns, to direct the airflow through the pipe. The main advantages of straight pipe racing are that they have less friction loss and a more predictable flow rate. Keeping in view the props of the straight piping system, does straight piping increase MPG? Let’s find out in this guide as I have covered some important regarding the effect of straight piping on engine performance and fuel economy.
You can also learn about the effect of cold air intake systems on MPG.
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Does Straight Piping Increase Mpg?
Straight piping increase MPG, horsepower and the noise of exhaust gases flow. By eliminating restrictive muffler and resonator, straight piping reduces back pressure and allows the engine to breathe freely. So, the engine does not have to produce extra power against that back pressure. In this way, it increases fuel economy and engine performance. However, to hear the crazy sound of exhaust gases through straight piping, drivers just put their engine in the high RPM range by aggressively pressing the accelerator pedal, such as in racing cars. In that case, it can hurt your fuel economy and decrease MPG.
Here is what a user experienced after installing straight piping in the exhaust system:
“My Duramax engine didn’t come with a cat and I removed the muffler and replaced it with a straight pipe. I grabbed 1mpg, and it sounds way better.”
Keep in mind that too wide straight piping can be detrimental to your engine performance. This is because engineers have designed an exhaust system by maintaining a proper balance between the frictional losses and the velocity of exhaust gases.
A restrictive and less diameter piping of exhaust system can increase exhaust gases velocity, which helps in rapidly evacuating the exhaust gases from engine cylinder. But, on the other hand, it also increases the frictional losses, which offers resistance to exhaust gas flow. So, selecting a straight piping system with a proper diameter is quite important for your engine. You can check my guide on straight pipe exhaust systems for more details. I would highly recommend you first read that guide as I have discussed the concept of backpressure.
Straight Pipe Exhaust Benefits
There are the following benefits of straight pipe exhaust:
- More horsepower
- Better MPG
- Reduction in weight of the vehicle
- Roaring sound of exhaust gases, specially on V8 engines
Here is what the user says about the sound of exhaust gases through straight piping exhausts on V8 engines:
“My V8 turbodiesels sound incredible straight piped IMO. I love the sound of my Duramax spooling and coming off boost. Despite being straight piped it’s not too loud when cruising. I’ve done some 10-hour road trips when towing 10K+ lbs and didn’t get tired of the exhaust at all.”
In straight piping, there are no mufflers, resonators and catalytic converter, this will significantly reduce the vehicle’s weight. As a result, it will allow quicker acceleration, and there will be a slight improvement from a slightly less accelerator pedal position required for similar power production.
One thing I would like to highlight is that if you are also removing the catalytic converter from the exhaust system, make sure to reinstall the O2 sensor on the exhaust. On the upstream and downstream of the catalytic converter, there are O2 sensors.
O2 sensor measures the amount of unburnt oxygen in the exhaust gases and sends signals to ECU to adjust the amount of fuel being injected into the engine cylinder. If the O2 sensor is not installed, your engine can get P0171 code.
The first O2 sensor is mandatory. You should find a spot and install it on the straight piping system if you have deleted the catalytic converter. You can bypass the second O2 sensor as it is only emissions related. It could turn on the check engine light, but it will not affect the engine performance. However, if you want to get rid of the check engine light because of the secondary O2 sensor, you can use an O2 sensor spacer.
O2 sensor spacer works to pull back the O2 sensor away from the exhaust stream by roughly an inch and thus attenuates the signal sent back to the ECU, thus “tricking” the ECU into believing there is still a catalyst in the system.
To install a primary O2 sensor on a straight pipe, you can use weld bung. You have to drill a hole in your straight piping to install the O2 sensor in the weld bung. For the primary O2 sensor, you don’t need a spacer as we want exhaust gases to pass through the O2 sensor so that ECU automatically adjusts the air/fuel ratio and maintains the fuel consumption.
You will have to drill two holes to install two oxygen sensors. After welding the O2 sensor bung, you can directly install the primary O2 sensor in it. In the second O2 sensor bung, you will have to fit the O2 sensor spacer and then install the secondary O2 sensor.
Does Straight Pipe Waste More Gas?
Straight pipes do not use more gas. Instead, by reducing the back pressure and weight of the engine, straight pipe exhaust systems can improve horsepower and fuel economy, depending on how you install the system.
I have seen people complaining about a significant decrease in MPG after installing a straight piping exhaust. There can be the following reasons behind this:
- Straight piping diameter
- Not reflashing the ECU after changing the exhaust
- Removing the O2 sensor
- Aggressively pushing the gas pedal to hear the roaring sound of exhaust
Exhaust Pipe Diameter Importance
Straight piping diameter should be selected based on your requirements. A larger exhaust pipe diameter decreases back pressure, which is good for exhaust gases to flow easily. On the other hand, a smaller exhaust pipe diameter increases velocity, which is also important to enhance scavenging. That’s why it is important to select the right exhaust pipe diameter based on your needs.
A bigger diameter exhaust allows the engine to breathe freely but it reduces low RPM torque. At low RPM, there is less throttling i.e. intake air does not flow at high speed.
If the exhaust pipe diameter is large, exhaust gases will stagnant in the pipe and will not favor fresh ambient to enter the engine at a higher speed. Moreover, bigger diameter exhaust piping will also increase turbulence in the flow of exhaust gases. In short, the larger the exhaust pipe diameter, the lesser will be scavenging.
3-in diameter exhaust pipe gives maximum performance if your engine has a turbocharged engine. You will see no performance increase with a 3″ system. Unless you have a very large/high-performance engine. 2.5-in straight piping will work well in naturally aspirated engines.
ECU Tuning After Changing Exhaust
ECU is responsible for adjusting the air-fuel ratio of the engine. ECU can learn and readjust parameters accordingly after installing new exhaust piping. But it takes time. However, ECU will not learn the new parameters fully as with the tune. ECU tuning will maximize the performance gain after installing a straight piping exhaust. Without tuning of ECU, there is a risk that the check engine light is turned on and the engine runs on a lean mixture.
FAQs About Straight Piping
Does A Straight Pipe Work On All Cars?
It entirely depends on the exhaust system of your car. If it is more restrictive, installing a straight pipe system will improve horsepower and MPG. however, the pipe diameter of aftermarket exhaust should be optimal. If you have a traditional naturally aspirated engine, you should use a 2.5-in exhaust.
Do You Lose Torque With Straight Pipes?
A large diameter straight pipe can lose low-end torque due to a decrease in the velocity of exhaust gases. That’s why 3-in straight pipe exhaust works well in turbocharged engines and at high RPMs.
Do Straight Pipes Have A Catalytic Converter?
Straight pipe exhaust systems do not have catalytic converter resonators and mufflers. They are just straight pipes without bends. However, in the market, cat-back exhaust systems also exist that consist of high-performance mufflers that offer less restriction compared to the stock mufflers.