Jardine Headers Theory
We are passionate about performance in the modern internal combustion engine and suspect you are as well. Below you'll find some of our ramblings on header design and theories. You can find alot of different ideas on the Internet about headers and the right way to build them. Some are close and some might be way-off. There are so many different combinations to take into account that there is really no perfect Header you can buy off the shelf. It's all a give and take. If you plan on building a max effort engine we strongly suggest looking at having a custom set built that fits all your needs.
The Tri-Y design has been around for a long time ever since Jerry first started building them in 1958. They work by simply merging cylinders together earlier in the header according to firing order. This creates a bigger signal earlier in the rpm range creating vacuum which in turn helps the cylinder evacuate spent gasses. A Tri-Y has to be built with your firing order in mind. For instance, in a Chevy with a firing order of 18436572, you would want 1& 5 y'ing and 7&3 y'ing. You want the two y'ing cylinders to fire as far aprt as possible. You also would want one short tube and one long tube in y'ing together. Esentially you're timing the pulses to the collector so they come into it stacked up. In this instance cylinder #1 would be longer than #5. How much longer? It all comes back to your particular combo but a general rule of thumb would be about 3-4" longer.
Collectors come in a wide variety of styles and sizes. No single one is the cure all and every header will react differently. A merge collector is expensive and works well if they are sized correctly with the right degree of taper and length. Simply putting a merge onto a header will not magically make power. A lot of variables go into collector sizing. Too often a collector is too big. Imagine a pulse traveling down the header tube at X amount of velocity that reaches the collector typically by itself which can be a big chamber filled with low pressure air. The pulse loses it's velocity instantly which now has to be pushed out of the way by the next pulse. People often will put a bigger collector and see an improvement but in reality all that is being accomplished allowing the first pulse space to get out of the way and eliminate any restriction. If you use a smaller size the pulse will retain its velocity and happily exit the collector on its own helping the next pulse do the same. If the header and collector are correct, the outlet should be a little bigger than one primary tube. A 2.250" header should have a 2.5" collector and so on. A lot of power can be made and lost in the collector but they get very little attention.
A 4-1 header is by far the most popular header on the market and is best suited if you are looking for peak redline horsepower. Below redline these headers typically lack power because the pulses have to reach the collector in order to start working and creating the vacuum needed to help scavenge the cylinders. This has to be taken into consideration when building a "Tuned 4-1". The header will reach this point at a particular RPM. Below this RPM the header is too big and is too small above it. A 4-1 is far from ideal in an engine for a heavy street car or truck that needs the full RPM range.