If you upgrade your vehicle’s engine, don’t forget to enhance the exhaust system. Otherwise, the increased exhaust gases produced by the improved engine might overwhelm the existing exhaust system’s capacity.
You’ll need new straight exhaust pipes with wider diameters to handle the higher output. But it’s not as easy as installing wider pipes and calling it a day. Excessively wide diameters can reduce the increases in engine performance.
When choosing a new exhaust pipe, the most important factor to consider is engine power. In many cases, you can determine the best exhaust pipe size solely on the horsepower generated by the engine.
So, how can you determine the exhaust pipe diameter based on the engine horsepower? Every 100 horsepower requires one inch of total pipe diameter. The rule applies across the entire exhaust system, from the headers to the mufflers.
Here is a helpful exhaust sizing chart with the recommended general diameter sizes for the exhaust pipes based on the engine’s horsepower:
| Power (hp) | Single Exhaust Outer Diameter (in) | True Dual Exhaust Tubing Outer Diameter (in) |
| Up to 200 hp | 2.5″ | — |
| Up to 250 hp | 3″ | — |
| Up to 300 hp | 3.5″ | — |
| Up to 425 hp | 4″ | 2.5″ |
| Up to 500 hp | — | 3″ |
| Up to 700 hp | — | 3.5″ |
| Up to 850 hp | — | 4″ |
| Up to 1,000 hp | — | 4.5″ |
The contents of the exhaust sizing chart apply to straight pipe exhaust systems. These systems usually have a single exhaust pipe that runs straight through them.
Other exhaust types have multiple exhaust pipes that bend several times. They’re more difficult to calculate.
Are you trying to squeeze every bit of performance out of the exhaust system and the rest of your vehicle? You must get more precise measurements. Instead of increments of 100 hp, use smaller increments of 50 hp.
Furthermore, you also have to account for the engine’s size. There are many engines in the same size range, which produce markedly different horsepower. Conversely, there are also many engines of significantly different sizes despite having roughly the same power output.
The engine’s internal dimensions can affect the amount of exhaust gases produced by combustion. Therefore, bigger engines will require exhaust pipes with wider diameters because they produce more exhaust.
Below is the recommended exhaust sizing chart that uses both engine size and horsepower:
| Engine Size (CI) | Power (hp) | Pipe outer diameter (in) | |
|---|---|---|---|
| Single Exhaust | Dual | ||
| 150-200 | 100 | 2 | 2 |
| 150 | 2.25 | 2 | |
| 200 | 2.5 | 2 | |
| 200-250 | 150 | 2.25 | 2 |
| 200 | 2.5 | 2.25 | |
| 250 | 2.5 | 2 | |
| 250-300 | 200 | 2.5 | 2 |
| 250 | 2.5 | 2.25 | |
| 300 | 3 | 2.5 | |
| 300-350 | 250 | 3 | 2.5 |
| 300 | 3 | 2.5 | |
| 350 | 3.5 | 2.5 | |
| 350-400 | 300 | 3 | 2.5 |
| 350 | 3.5 | 2.5 | |
| 400 | 4 | 3 | |
| 400-450 | 350 | 3.5 | 2.5 |
| 400 | 4 | 3 | |
| 450 | 4 | 3 | |
| 450-500 | 400 | 4.5 | 3.5 |
| 450 | 4.5 | 3.5 | |
| 500+ | 4.5 | 3.5 | |
In the past, people who customized their vehicles for high performance always chose the widest exhaust pipes available that could still fit on the engine. After all, the larger the exhaust pipe size, the larger the volume of exhaust gases that can flow through them.
This practice draws from the scavenging process in an internal combustion engine. Scavenging refers to how the engine expels exhaust gases from the combustion chamber, freeing up room for a new charge of air-fuel mixture that enters at the same time.
There are several different scavenging methods but they all benefit from wider exhaust pipes. Wide pipes reduce the restriction on exhaust flow, allowing the gases to move faster. Higher exhaust flow increases the vacuum effect that pulls exhaust gases out of the engine.
You also have exhaust back pressure to consider. Exhaust back pressure is the pressure that accumulates in the vehicle exhaust when something reduces the movement of exhaust gases leaving the engine.
However, increasing the exhaust pipe size only works until a certain point. If the exhaust pipe is too big for the engine, the exhaust gases will spread out and flow more slowly. The reduction in exhaust flow reduces the effectiveness of exhaust scavenging.
An excessively wide exhaust pipe also increases exhaust back pressure. High back pressure reduces power, raises the temperature in the combustion chamber beyond safe levels, and interferes with exhaust scavenging.
There are some things you should always remember if you’re installing larger exhaust pipes in your vehicle’s exhaust system. These tips can help make the job easier.
Make sure the inlet and outlet sizes of the muffler and catalytic converter are compatible with the exhaust pipe size. If the sizes don’t match, you might have issues like leaks that reduce the effectiveness of exhaust scavenging.
Minimize the number of times the exhaust pipe must bend in the exhaust system. The straighter the pipe, the more efficient the exhaust flow, reducing exhaust back pressure and ensuring scavenging works efficiently.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.