Mrmopartech
Well-Known Member
Headers are now and have been one of the most common performance modifications. Thirty years ago, the installation of headers on V-8 engines netted an instant performance gain. Today, the installation of headers on otherwise stock carbureted cars doesn't always result in a gain except at the racetrack itself. The headers haven't really changed the way carburetors were set up by the factory sure has.
Typically, pre-1970 carburetors were richer in fuel-air ratio than their later emission-era counterparts were. Headers always lean out the fuel-air ratio of all engines. They do this by scavenging exhaust out of the ports more efficiently. Creating this 'extra room' in the exhaust port while the engine is running means that the otherwise unchanged engine will flow more air. Think of it as the opposite of how EGR works. But, flowing more air only helps if the fuel system can compensate with additional fuel. If not, the air-fuel ratio is not maintained, it is driven lean. With the extra fuel available in richer, older engines, the headers scavenged exhaust out of the mixture level to time point that the engine was still running well and the lower backpressure available with headers resulted in an increase in horsepower. More air was matched by the available fuel in the fuel system.
The emissions laws since 1970 forced production carburetors and inlet packages to have almost perfect fuel distribution, but with much leaner fuel mixtures. The better fuel distribution is a plus, but the leaner mixtures leave no room for modification. The installation of headers on to these "emissions" engines leans them out too far. This means that the gain the engine got for the decrease in backpressure was canceled because of the engine output loss due to the too lean fuel mixture.
The solution is quite simple: enrich the carburetor. This usually requires richer jets to be installed. This allows the fuel air ratio to be corrected. The headed with the richer carburetor will then result in better output (horsepower).
Typically, pre-1970 carburetors were richer in fuel-air ratio than their later emission-era counterparts were. Headers always lean out the fuel-air ratio of all engines. They do this by scavenging exhaust out of the ports more efficiently. Creating this 'extra room' in the exhaust port while the engine is running means that the otherwise unchanged engine will flow more air. Think of it as the opposite of how EGR works. But, flowing more air only helps if the fuel system can compensate with additional fuel. If not, the air-fuel ratio is not maintained, it is driven lean. With the extra fuel available in richer, older engines, the headers scavenged exhaust out of the mixture level to time point that the engine was still running well and the lower backpressure available with headers resulted in an increase in horsepower. More air was matched by the available fuel in the fuel system.
The emissions laws since 1970 forced production carburetors and inlet packages to have almost perfect fuel distribution, but with much leaner fuel mixtures. The better fuel distribution is a plus, but the leaner mixtures leave no room for modification. The installation of headers on to these "emissions" engines leans them out too far. This means that the gain the engine got for the decrease in backpressure was canceled because of the engine output loss due to the too lean fuel mixture.
The solution is quite simple: enrich the carburetor. This usually requires richer jets to be installed. This allows the fuel air ratio to be corrected. The headed with the richer carburetor will then result in better output (horsepower).
