QUALITY AND INNOVATION FOR OVER FOUR DECADES
None! In Australia, headers were known as extractors through the 60's and 70's by the pioneering companies that developed them. In America at the same time they were coined headers (also spelt hedders). This was in part due to the fact that they (headers) are an extension of the cylinder head. In the 80's new Australian companies, along with the established industry, began to market extractors as headers. The main reason being that these companies intended to change the image of the humble extractor to more of a common vehicle replacement component. Some 'old school' enthusiasts still call them extractors today.
A 4-into-1 design is perhaps the most traditional design. This design is common to the US market and is well used within the drag race fraternity. The basic principle behind the design is that the scavenging effect within the collector cone effects the three remaining pipes (in a V8 half), instead of limiting the pulse to the very next pipe in the sequence. This generally results in a header that performs very well in the mid-range to top end of the engines performance.
In most cases around 10% is quite achievable. This may mean if you have, for example, a 4WD vehicle delivering around 70 kilowatts at the wheels you could expect possibly 6 to 10 kilowatts increase. If you have a V8 5.7 Litre commodore with around 160 kilowatts at the rear wheels you could expect to reach 170 to 180 kilowatts. This is just a generalisation and other factors do become involved. Cold air intake and open exhaust systems encourage the full effect of a well designed set of headers.
There are two main reasons, firstly like all things, vehicles are made to a standard and a cost. The cast iron standard manifold will always be the cheapest way of producing an exhaust manifold. However, car manufacturers do know the benefits of a tubular designed exhaust headers and indeed generally do offer this type of product on a sports related sedan, eg. HSV, Tickford/FPV, etc. These tubular manifolds are still better than a standard manifold but are still limited to cost, and as such are a compromise in design.
Secondly, cast iron manifolds can be cast into awkward and different shapes to accommodate steering boxes, etc, and can be installed onto the engine along with all the other necessary bolt on components during engine assembly and with the engine still able to be slotted into the body during the vehicle build. Headers however will generally take on a different proportion and often can wind themselves around steering components, etc, which in turn would slow down the vehicle build time process.
In most cases, yes. Due to the scavenging effects of an efficient design of headers, more clean air fuel mixture can be delivered into the cylinder bore resulting in a more powerful power stroke. Quite simply this means that the engine does not have to work as hard to deliver the same power. The biggest cause of excessive fuel consumption in this case would be driving behaviour. Quite often once a set of headers and a new exhaust system is installed the driver will tend to drive the vehicle a little harder than usual because of the new found audible experience along with the increase in power. This is when the vehicle can use more fuel. Fuel consumption will generally decrease once the vehicle is driven over a long distance and at a constant speed.