A few words about header selection for those of you wondering "Who has the best header". This is a fairly indepth explanation of how the exhaust pulse affects horsepower and torque and why choosing the proper header size, type, and design is crucial if you want to build optimum horsepower and/or torque.
Let's start with what the "Scavenging Effect" is and how it affects torque and horsepower. Here's a look at just what the "scavenging effect" is:
To start, the piston is moving upward through the exhaust stroke, and the EXHAUST valves are open to expel the exhaust gasses. When the piston nears the top of the bore, the INTAKE valves begin to open as well (so all four valves are open). Since there is a low-pressure draw from the exhaust header/manifold, this aids in pulling the next intake charge into the chamber and helps in expelling the exhaust gasses. This is the "Scavenging Effect" and happens when the exhaust is properly tuned for the application (and rpm).
Now to take a look at how the "scavenging effect" affects horsepower and torque:
Exhaust gasses exit the head as a pulse at near the speed of sound, with a high-pressure wave at the front of the pulse and low-pressure behind it. A properly tuned exhaust system utilizes that low-pressure end of the pulse to help pull out the next exhaust pulse (and in turn, help to draw-in the next intake charge). What this means is simple: When the exhaust is properly tuned (thus providing the scavenging effect) the exhaust gasses are more efficiently expelled and the intake charge is more efficiently pulled into the combustion chamber. This = power.
Longer tube lengths (we're talking about the header/manifold) and larger pipe diameters increase volume but reduces pressure at lower rpms. This means that at lower rpm's the exhaust pulse is slowed considerably and the low-pressure end of the pulse is weakened. This leads to a poor scavenging effect and reduced efficiency at these lower rpms since the intake charge is not pulled into the cylinder and the exhaust charge is not pulled out, HOWEVER, upper rpm power is gained because the scavenging effect is reattained at upper rpms when the exhaust gasses build more pressure in the exhaust system.
Shorter tubes and smaller tube diameters are just the opposite. They promote a better scavenging effect at lower rpms, but due to the lack of volume, the scavenging effect is lost to the upper rpms (and once again there is a reduced efficiency at the cylinders).
Header tube length, tube diameter, style, and collector type should all be considered when you choose a header for your application. 4-2-1 headers often help keep the lower rpm torque up better than straight 4-1's. As far as this and the header collector goes, there are a million combinations that assist with power (good and bad).
There are several things to look for in a quality header. One of the main points should be tube length tuning. We're not talking about same-length tubes, but rather tubes that are tuned to provide the next cylinder in the firing order with the proper draw (usually the tubes are either uneven lengths OR the tubes that need to be made longer are built to provide some restriction -like a sharp bend- to accomplish this). On a four cylinder engine this is not as essential as a "V" style engine, but important nonetheless.
A commonly overlooked area of the header is the collector. You will find that a long-tapered collector will give some benefits to the upper rpm range by reducing the restriction created by the sudden change from 3 1/2" to 2" that short collectors have.
So in summary, LARGER TUBES and/or LONG TUBE LENGTHS = UPPER RPM POWER. SMALLER TUBES and/or SHORT TUBE LENGTHS = LOWER RPM TORQUE.
If you want the VERY BEST header, then you will need to have one custom fabricated for your application, plain and simple.
I know that having a header built is not always an option, so here is option 2. Check with the manufacturers of the headers and get their specs on the engine that they designed their header to work with (any GOOD manufacturer will have this available and for 'knockoff' designs, go to the manufacturer of the original design). Very often a manufacturer will promote a header by saying things like, "10hp Increase!", but what they do not tell you is that the engine they got 10hp out of was a H22 with 12:1 CR, Stage 3 cams, 300hp fogger, etc. Beware of manufacturers that blow smoke up your a$$ by telling you that it was designed for a "stock engine". The Honda manifolds are extremely good, reliable, exhaust systems that are already designed to give a stock and slightly modified engine everything it needs and can use.