With the bodywork removed it's much easier to see the type and size of the headers on this
You may have noticed on many race cars the headers are insulated or wrapped in special heat-resistant tape. While the reasons for the insulation may be varied, the best reason to insulate the headers is to keep the tubes as hot as possible. The hot pipe keeps more of the energy in the exhaust flow and in turn assists the headers' ability to work by keeping the exhaust gas moving and not giving up energy to the surrounding metal. We need to remember that the exhaust still has a significant amount of energy. In the form of latent heat, this kinetic energy is an important part of improving the power of the engine.
So, we have this wave of pressure and heat that is traveling down the pipe at a given speed that will be somewhat proportional to the speed or rpm of the engine (which varies with the power levels). We also have a wave of sound energy that is exiting from the exhaust port, but this wave is traveling at a speed that is significantly higher than the slug of gas and the pressure that accompanies the gas.
We need to remember that we have many different engine components that will have a marked effect on the exhaust flow-displacement, intake efficiency, combustion efficiency, exhaust port flow characteristics, camshaft profile, compression, and target rpms. These will all have an impact on the optimal design of the headers.
Viva La Difference. You will notice how different the headers are on this car.
From an engineering perspective there are many calculations and design work that can be accomplished to help you reach an optimum pipe and pipe length for a given rpm and combustion scenario. Unfortunately, racing engines, especially the poppet valve-type engines, operate over a broad rpm and load range. Achieving the optimum may be a great engineering exercise but we need a set of headers that will work over a broader range than the set we may design on our computer. Understanding how the headers work will help us to make more intelligent decisions when it comes to adjusting or tuning headers.
Mild versus Stainless
Let's look at the materials used to manufacture the headers. Will you use mild steel or stainless steel? For the majority of the Saturday night racers they will pick headers made from mild steel from purely a cost perspective.
From a power perspective, two headers with exactly the same physical dimensions, one made from mild steel and the other made from stainless steel will not show an appreciable power difference. There is nothing magical about stainless steel. It is more durable and better able to cope with the temperature and the physical demands of racing over the mild steel header, but if you are racing in a fairly restricted class like a Hobby Stock it doesn't make sense to have a set of headers that cost more than the car.
The primary tubes are larger and longer. The layout is very different. Notice how long the
Stainless steel headers will elevate your costs into the several-thousand-dollar range, versus several hundred for mild steel. In addition to raw material cost, stainless steel headers require different manufacturing processes which are traditionally more costly. So you have to employ some common sense when making material decisions. The use of headers fabricated out of mild steel for the Saturday night racer makes a great deal of sense, especially from a economic perspective.
Diameter And Length
When working on the pipes, you have to determine not only the diameter but the length. The goal should be to make the primary tubes as close to the same length as possible; this is sometimes difficult due to space in the engine compartment. Sometimes, the type of car the header will be in determines just how long the primary tubes can really be.
Other questions include, will they have a merged or a stamped collector? And how long will the collector be? It's not as easy or as simple as the completed product looks. All of these features can and do affect the power the engine produces and the shape of the power curve, which is what the racer will want to be able to affect by his adjustments.
Talk with your chosen header manufacturer about different options. You'll find that they are a wealth of knowledge when it comes to picking out or designing a set of headers.