RCI equal length long tube headers. That's what they are called but the reality is that the primaries are not all the same length. Close maybe, but they are still not equal length. It is pretty much impossible to find room in a standard production vehicle (like a Mustang) to have all the tubes come out the same length. The four primaries start in different locations on the cylinder head, and have to end in a single location (the collector). And they have to miss various components (chassis, engine mounts, steering, etc.) along the way. If there were no collectors, making the primary tubes equal length would be easy, but if there were no collectors, there would be no way to create a full exhast system, with mufflers. Life is full of compromises.
The benefit of having all the tubes the same length is pretty much a myth. What we have found out on the engine dyno is that making all the primaries the same length doesn't give you enough difference in power to measure, particularly a vehicle with a full exhaust system. Most of the theories surrounding "tuned" headers are just that, theories. Shorter primaries tend to favor power ar higher RPM's and longer primaries favor power at mid range RPM's. If a mythical header (let's say one side of a V8 engine) had two shorter primaries that favored 7,000 RPM and two longer primaries that favored 5,000 RPM, what you would have are headers that are "tuned" for 6,000 RPM, or so the theories go.
A typical vintage Sprint Car with a big inch Small Block Chevy engine. A Sprint Car is a good measuring tool; Spint Cars are drag cars that have to turn at the end of the straight aways. As you can see, designing equal length headers on a Sprint Car would be very easy, there is really notrhing that would get in your way. Even so, most Sprint Cars have headers simular to the one above, with no thought about keeping the length of the primaries equal. Don't think they haven't tried equal length headers, they have. They just found on the track what we did on the dyno, which is it's not that big a deal in the howepower department. Camshaft specs, cylinder head and intake manifold designs are much more important when it comes to building an engine to favor more top end power versus more mid range power.