# Motorsport Plumbing Systems: Sizing

## Sizing

### 07.13

00:00 | - With an understanding of how the system functions, what materials we might want to use and some considerations around routing and heat management, the likely question you might be asking is what size should the plumbing be? When we say size we're specifically referring to the internal diameter of the intake which determines the cross sectional area the fluid will flow through. |

00:23 | Unfortunately, this isn't just a case of whatever fits or bigger is better, at least if we want the best results anyway. |

00:30 | In terms of our intake and more specifically charge air plumbing, too small a diameter will increase the pressure drop or in other words be restrictive but too large a diameter will slow the transient response. |

00:44 | Simply put, the drivability or response to our throttle inputs will suffer. |

00:49 | For NA systems, this is less critical. |

00:52 | Let's consider a single throttle body arrangement that feeds an intake manifold from multiple cylinders. |

00:58 | Where the only plumbing is between the filter and the throttle body. |

01:02 | We clearly still don't want to make the intake too small as this will create a restriction. |

01:09 | The same size as the internal diameter of the throttle body should be the absolute minimum for our plumbing, preferably a size or two larger so we can be comfortable that this won't cause a restriction. |

01:20 | If we make the plumbing very large, this won't hurt our transient response but it'll be unnecessarily heavy and take up more space. |

01:29 | This is most critical for forced induction applications and more specifically the plumbing between the turbo or centrifugal supercharger compressor and the throttle body. |

01:38 | This plumbing is commonly referred to as intercooler plumbing as it connects the intercooler to the other components. |

01:45 | So in this case, how do we determine what size intercooler plumbing is suitable? While there are some equations we can use to point us in the right direction which we'll cover soon, it should be stated clearly that there are no hard and fast rules. |

01:59 | What's best will be on a case by case basis and just like our blow off valve positioning, this usually comes down to the execution of the setup with regards to packaging limitations. |

02:11 | With that in mind, let's cover some useful calculations to give us a good starting point. |

02:16 | The first thing to understand is that the flow velocity through a pipe is equal to the volumetric flow rate divided by the cross sectional area. |

02:24 | The cross sectional area can be found by multiplying pi, a constant equal to 3.14159 and so on, by the internal pipe radius, being half the diameter squared. |

02:36 | The volumetric flow rate is determined by our compressor and how hard it's working. |

02:42 | While this can be measured or found from a compressor map, there's a quicker way to get a rough approximation for our purpose. |

02:50 | As a general rule, most forced induction gasoline engines would generate around 10 horsepower for every pound per minute of mass flow rate. |

02:59 | So an engine with a max target horsepower of 500 will need around 50 pounds per minute of air. |

03:06 | Converting this to a volumetric flow rate depends on the air density which is related to the altitude and temperature. |

03:13 | But under standard conditions, the density of air is 0.0765 pounds per feet cubed. |

03:21 | So a 50 pound per minute mass flow rate is the equivalent of about 650 feet cubed per minute or CFM. |

03:30 | The final part to understand is the velocity of the air in the pipe. |

03:34 | For this, it's common to use Mach 3 as the upper limit, which is about 330 feet per second. |

03:41 | This number is important because this is the upper limit of incompressible flow. |

03:46 | Basically, below this number the change in air density is insignificant since the density is proportional to the drag or restrictive force on the fluid, staying below Mach 0.3 helps us avoid restrictions. |

04:01 | So if we can size our plumbing diameter and therefore cross sectional area to achieve a velocity under 300 feet per second for our required volumetric flow rate, we should be able to prevent significant restrictions. |

04:15 | At the same time, we don't' want the air to move too slowly as this hurts our transient response which is a bit more arbitrary but a velocity of over 200 feet per second is generally considered to provide good response. |

04:28 | Let's work through a quick example using our 500 horsepower engine to clarify before wrapping up. |

04:35 | This engine uses 650 CFM of air or 10.8 cubic feet per second. |

04:41 | We want to start by aiming for around 250 feet per second of air velocity in the intake. |

04:47 | Subbing these into our equation and rearranging, the cross sectional area equals 10.8 cubic feet per second divided by 250 feet per second which results in 0.0432 feet squared. |

05:04 | Dividing this by pi and then taking the square root gives us a radius of 0.117 feet. |

05:10 | Multiplying this by 2 and converting to inches, we get 2.8 inches. |

05:15 | Although not a standard size for aluminium pipes, this is a fairly reasonable size for intercooler piping. |

05:22 | Using a similar approach, standard 3 inch intercooler piping would give us a velocity of around 220 feet per second, or 2.75 inch piping would result in around 260 feet per second. |

05:35 | Both sizes give us an air velocity that should prevent significant pressure drop while still maintaining good transient response. |

05:44 | At this point we should consider packaging constraints that might force us to use the smaller option or future proofing for more power by using the bigger option. |

05:54 | The final thing to note is that although the velocity is within this range, this doesn't mean that we won't have any pressure drop. |

06:01 | Of course we still need to consider the routing of the plumbing, avoiding sharp bends and aggressive changes in the internal diameter. |

06:08 | A minimum bend centreline radius of about 1.5 times the diameter of the pipe is a good rule of thumb to work to here. |

06:17 | To save you the time calculating out, we've attached a list of intercooler plumbing sizes for a general starting point for standard conditions. |

06:25 | Essentially, if you're making 300 horsepower or less, then intercooler piping in the 2-2.5 inch range will be suitable but then you're probably working with the factory setup anyway. |

06:37 | Up to 500 horsepower, piping around 3 inch would be a good starting point whereas 3.5 to 4 inch piping is going to work better for higher power in the 700 to 1200 horsepower window. |

06:50 | In summary, if our intake plumbing is too small, it'll be restrictive, resulting in pressure drop but if it's too big the response will suffer. |

06:59 | This is more critical for forced induction applications and we can calculate the required pipe and the diameter by targeting flow velocity between about 200 to 300 feet per second. |