How to Calculate Exhaust Pipe Diameter and Muffler Size

If you are a construction engineer or math whizz, you would love to delve deep into this article. However, we will place the rub before you to grab the significant takeaways or nitty-gritty.  

The factory exhaust pipe and muffler manufacturers do all the math for you. If they assure you that these things work for you, you can pick blindly. 

However, if you do not want to trust the sellers, the only left choice is to calculate from the exhaust system size formula below easily. 

Narrowing Down the Calculations

We will not boggle you down under the cumbersome crunching of numbers. We will provide you with simple calculations, most scientifically. Let’s get down to the details. 

The quick-and-dirty math is all set to serve you:

Mass of air that the engine breathes in + mass of fuel = mass of exhaust gasses.

That was the quickest way to get to the conversion of mass. Right?

To figure out the volume the air engine takes in, we will have to multiply the engine's displacement by its revolutions per minute. Then divide the emerging number by 2 (we halve the figure as it takes two full revolutions for the engine to exhaust the entire air volume). Finally, convert the emerging figure from volume to mass.   

You will have to assume that the consumption is perfect for precise calculations. By-products prove as a party spoiler for spot-on calculations. So, imagine the ideal combustion scenario. Now go back to the actual numbers going by the same fact. 

As you suppose the perfect combustion scenario, it is straightforward to figure out how much fuel is added to the exhaust. 

Now you know the mass of the exhaust gas, you will have to figure out how much space that group would possess. Now, you have to tinker for expansion due to rising gas temperature. 

That’s all! However, you are pretty much convinced that a spot-on mathematical estimate requires a plethora of bleed and sweat. This is why we don’t bother you with harrowing numbers repeatedly.

Sure-Fire Exhaust System Math

  • The easiest way: The intake system needs to flow at the rate of 1.5-2.2 CFM/engine horsepower.  
  • The foolproof way: Pick engine RPM X Engine Displacement. Divide the emerged figure by two. You get the intake volume. Now use the same volume of air for the exhaust. Finally, tinker with the figure concerning thermal expansion.  
  • Exhaust pipe size estimate: The ideal straight pipe will flow about 115 CFM/sq.inches. 

Below is the quick table that reveals how many CFM each standard size will flow. The table will also provide you estimated maximum horsepower for each pipe size. 

Pipe Diameter in inches Pipe Area in in2 Total CFM (estimated) Max HP/Pipe Max HP
1 1/2 1.48 171 78 155
1 5/8 1.77 203 92 185
1 3/4 2.07 239 108 217
2 2.76 318 144 289
2 1/4 3.55 408 185 371
2 1/2 4.43 509 232 463
2 3/4 5.41 622 283 566
3 6.49 747 339 679
3 1/4 7.67 882 401 802
3 1/2 8.95 1029 468 935

The thing to consider: These numbers are mere estimates. For closer estimates, all pipes should be made of 16 gauge steel. While the perfect vehicle specifications should be 400 hp with a dual exhaust system. Anything larger is indeed overkill.