We tend to know a lot about our vehicle’s engine horsepower, but we never know our exhaust size. When it comes to exhaust systems, size does matter, and bigger is not always better. When getting a custom exhaust system for your vehicle, it is crucial to understand how car manufacturers size those components. Piping, mufflers, reducers, and tips are usually sized differently from one another to ensure compatibility and fitment.
The general rule we use to measure the exhaust size is against your engine’s horsepower; take your engine’s horsepower, multiply it by engine displacement, and then divide by two. Choosing the right exhaust size for your car is crucial as it affects the engine’s performance. You can size up or down; however, diameters that are too large can consume the engine’s torque and horsepower at lower engine speeds, while too small diameters can choke the engine with waste fumes from combustion. Tube size is what limits flow as in CFM or a volume over time.
How to Calculate Exhaust Size
To calculate the exhaust size, you need to measure the outer diameter and calculate it against your engine’s horsepower. We have explained below how to calculate exhaust size in relation to horsepower and torque.
The Outer Diameter and Inner Diameter
Outer diameter is the distance from edge to edge, including the thickness of the exhaust tubing. The inner diameter is the distance from edge to edge that is the space is inside, excluding the thickness of the tubing. The internal and external diameter are important as some parts of the exhaust system have to fit into or onto other parts.
Exhaust and mufflers use the inner diameter since they fit over the exhaust pipe. Most exhaust pipes use the outer diameter because they slide inside the muffler and tip.
Tools Required
- Tape measure or ruler
- One pair of dial calipers
How to measure the outer diameter(OD)
- Take the exposed end of the exhaust pipe that is not covered by a tip or fitted into a muffler, use a tape measure or ruler to measure from the outer edge of one side to the outer edge on the opposite side, across the center point.
- If the exposed end of the exhaust pipe is blocked, use a pair of dial calipers to measure the same distance for more accuracy.
- If you do not have a pair of dial calipers, you have to do some Math to get the external diameter. Measure the circumference of the exhaust pipe using a tailor’s measuring tape. You can also use a string and a ruler. Then divide the exhaust’s circumference by Pi, 3.14, to get the diameter.
- When taking your measurements, take note of the unit of length, that is, millimeters or inches.
How to measure the inner diameter(ID)
- If the end of the exhaust pipe is exposed, use a tape measure or ruler and measure the distance from the inner edge of one side to the inner edge of the opposite side, across the center point.
- If the end of the tailpipe is not exposed, you’ll have to cut the pipe to find the accurate diameter measurement.
After calculating the exhaust pipe outer diameter, match the diameter based on engine displacement and the optimum engine horsepower. The exhaust diameter chart below will guide you on the appropriate exhaust diameter for your engine.
| Engine Displacement | Horsepower | Single pipe diameter | Dual pipe diameter |
|---|---|---|---|
| 150-200 CID | 100 | 2 | 2 |
| 150 | 2.25 | 2 | |
| 200 | 2.5 | 2 | |
| 200-250 CID | 150 | 2.25 | 2 |
| 200 | 2.5 | 2 | |
| 250 | 2.5 | 2.25 | |
| 250-300 CID | 200 | 2.5 | 2 |
| 250 | 2.5 | 2.25 | |
| 300 | 3 | 2.5 | |
| 300-350 CID | 250 | 3 | 2.25 |
| 300 | 3 | 2.5 | |
| 350 | 3.5 | 2.5 | |
| 350-400 CID | 300 | 3 | 2.5 |
| 350 | 3.5 | 2.5 | |
| 400 | 4 | 3 | |
| 400-450 CID | 350 | 3.5 | 2.5 |
| 400 | 4 | 3 | |
| 450 | 4 | 3 | |
| 450-500 CID | 400 | 4.5 | 3.5 |
| 450 | 4.5 | 3.5 | |
| 500 | 4.5 | 3.5 |
Exhaust diameter in inch = Horsepower / 100 for a single exhaust pipe: The formula for calculating exhaust size is;
- Exhaust diameter in inch = (Horsepower/100) ÷ 2 four a dual exhaust pipe.
The easiest way to find the appropriate size for your engine is to follow this rule; for every 100 horsepower output from your engine, you need one inch of total pipe diameter in the exhaust system. For instance, a 500 horsepower engine will require a 2.5-inch dual exhaust.
Using this formula will give you a rough estimate, but you can use an online calculator to get a more accurate size of exhaust. To determine the right exhaust size for your engine, you need first to find the maximum RPM your engine can produce. In the online exhaust size calculator, enter the exhaust pipe diameter and number of pipes that will give you the maximum horsepower, then input the horsepower and the number of exhaust pipes.
What is the relationship between horsepower and exhaust pipe width?
For performance vehicles, the exhaust system plays a more significant role in the engine’s performance and power. Your engine’s horsepower depends on the complete combustion of the air and fuel mixture and the efficient removal of the exhaust fumes. Our engine needs to increase combustion by first increasing more air and fuel to produce more horsepower. Consequentially, for your engine to generate optimum horsepower, it requires a greater exhaust size to support the combustion cycle.
Your vehicle can easily inject more fuel for combustion. However, air flows naturally into the combustion chamber through the air intake using pressure difference. Here is where the exhaust pipe size comes into play. The primary role of the exhaust system in your vehicle is to remove exhaust fumes produced by the engine after combustion. Air flows into and out of the combustion chamber when the intake and exhaust valves open and close. But if your exhaust’s width, the exhaust gases can spill back in due to backpressure in the piping. It reduces available oxygen and decreases horsepower.
Combustion of fuel and air produces exhaust fumes that have to exit quickly; otherwise, they will occupy space required for new air to enter the combustion cycle. If the exhaust tube is too narrow, it will create too much back pressure, restricting the exhaust fumes from leaving the combustion chamber. Backpressure refers to the air that pushes down on your exhaust system, which can negatively affect your engine’s performance and power.
What does a larger diameter exhaust do?
When car users customize their vehicles, they choose large diameter exhausts to increase horsepower and increase sound. When you have a wider exhaust pipe, the back pressure will reduce, increasing the flow of exhaust fumes out of the combustion chamber.
You require a quicker flow of exhaust fumes from your engine to achieve maximum horsepower. A wider exhaust pipe allows for faster exit of the exhaust fumes and creates low pressure in the combustion chamber. The low pressure draws out exhaust fumes at an extremely high rate, creating a cycle for more engine performance. The combustion chamber now has more space for air to occupy, leading to more combustion and engine horsepower.
Though larger exhaust pipes have benefits, an exhaust tube that’s too large will reduce engine power and car speed because the exhaust fumes leave the combustion chamber too slowly. A little back pressure helps to create high exhaust flow.
Conclusion
To determine the size of your exhaust, you need to measure the outer diameter of the exhaust pipe and match it against your engine’s power. The correct exhaust size should strike a balance between low back pressure and high exhaust flow velocity to produce the optimum power from the engine.