Basic Concept: header design3 300x275 Header Design Calculator

The two main aspects to our header design calculator are flow capacity and resonance tuning.

Flow capacity:

Flow capacity has to do with how much back pressure the header creates for the exhaust gasses coming out of the combustion chamber. In general we know that airflow velocity in a tube is highly related to the pressure differential between the tube inlet and outlet. Undersized diameter header pipes may promote higher gas flow velocities at lower rpm, but as the amount of air that the engine flows increases, and as the amount of exhaust gas that needs to be cleared from the combustion chamber increases, we find that an undersized header primary tube diameter becomes overwhelmed.

As the pipe becomes overwhelmed, the exhaust gas reaches a saturation velocity and the result is that the pressure in the exhaust system starts to build rapidly (some people have logged as high as 41psi of back pressure on a restrictive set of TD04 turbo exhaust manifolds approaching red-line).

This rise in back pressure acts as a restriction to exhaust flow and the engine runs out of ‘breathe’ early, as it struggles to exhaust the already burnt exhaust air sitting in the combustion chamber , thus contaminating the new intake air charge on the next engine revolution.

header design4 300x215 Header Design CalculatorA great example of this is the high flow manifolds for the supercharged AMG 55 series engines. These high flow manifolds have runners that are too short to be optimized for resonance tuning at any usable rpm, however by just up-sizing the diameter of the primary pipe (which kind of acts like a collector in this strange design), then we are able to dramatically reduce the exhaust back pressure and improve cylinder scavenging. The result of this ‘crude’ yet effective upgrade is anywhere between 35 to 50 dyno proven rear wheel horsepower depending on the car, modifications, and boost level.

The trade off is as follows: smaller primary pipes promote higher torque values but may cause the engine power to saturate early due to reduced scavenging and spiking back pressure in higher rpm. A larger primary pipe promotes better high rpm performance but may reduce low rpm scavenging requiring higher compressor, more timing advance, different camshaft timing, or higher boost levels at lower rpm to improve drive ability.

Resonance tuning:

Resonance tuning uses a tuned length header pipe that creates a unique condition where the exhaust pulse from on cylinder travels down the primary pipe to the collector and gets reflected back up to the adjacent cylinder, reaching the exhaust valve just as it opens, creating a temporary 2 to 5 psi vacuum to help scavenge the exhaust gases.

Depending on cylinder combustion timing, and depending on engine configuration (in-line, boxer, Vee or rotary) and depending on the crankshaft configuration (single or dual plane) and the exact rotation angle between each combustion event and the next…

depending on all those factors, it may be possible to maximize engine performance using resonance tuning by coupling cylinders that fire at 180 degree intervals with respect to each other using equal length piping. The piping, having a length tuned to have a length related to the wavelength defined by the frequency of our target RPM range allows us to specifically time this scavenging effect to happen at our chosen RPM range, weather it’s our launch rpm, our cruise rpm, or red-line depending on the application.

The most apparent application of resonance tuning can be observed on in-line 4 cylinder engines that typically have a single plane crank where cylinders 1 and 3 can be coupled together and similarly cylinders 2 and 4.

The power calculator builds on these concepts of header design by calculating the following:

  • Primary pipe diameter
  • Primary pipe length for long tube headers (targeting your chosen RPM range) or for shorty headers (targeting your engine’s red-line)
  • Primary and secondary pipe length and primary and secondary pipe diameters for tri-y headers targeting two different rpm ranges (1- red-line, 2- your choice)
  • Ideal collector and exhaust cut-out diameters for good merge flow
  • Dual mode exhaust design for nitrous applications with an up-sized collector and exhaust cutout for on-spray performance, and a smaller street sized header-back exhaust size for off-spray response.

For more details on 4-2-1 header design please check out the article below:

4-2-1 header design using the horsepower calculator


The article below was written for an older version (version 3.1) of the header calculator. Since then, in version 3.3 and above, the power calculator is able to do all forms of header calculations, including:

  • Primary and secondary pipe lengths and diameters
  • Shorty, tri-y, and long tube header designs
  • Targeting a specific RPM range for your long tube or tri-y design for maximum launch power or best street cruising

The first part of the video below is still informational because it goes through the concept and theory of a 4-2-1 header and why it is beneficial on street applications. Part 2 of the video which covers using the old version of the header calculator is now obsolete I’m afraid.

Be sure to checkout the link at the bottom of this page to the free trial of the power calculator where you can calculate the dimensions for your intake, shorty header and exhaust for FREE

JDMC Accord5zigen4 2 1header 299x300 Header Design Calculator 

I received a question in email last week about using the power calculator to design a tri-y or 4-2-1 header for a 4 cylinder application.


The car in question is going to be used during a hill climb competition, and in this type of racing a consistent average power figure is more important than a single peak number as the car manages complex terrain, a wide rpm range, and the difficulty of losing power as air density gets lighter during the hill climb.

Although the power calculator defaults to giving you a 4-1 recommendation optimized for peak power, it is possible to use the power calculator to design more complex headers such as 4-2-1 systems using the following procedure.

Watch the videos here:

Part 1 – Theory



Part 2 – Calculations




Street header Primary Secondary Pipe Collector
Diameter 1.32 1.4 2
Length 22.25 17.37
Race header
Diameter 1.52 1.67 2.36
Length 15 14.5

Use the horsepower calculator for your header design