Header design
| Line 20: | Line 20: | ||
Velocity is created in the exhaust system from an exhaust pulse traveling through the primary tube and as rear part of the pulse cools, will create a vacuum. This vacuum will help to pull the next exhaust pulse out of the cylinder. This leaves a cleaner cylinder with less spent exhaust fumes and more room for the incoming air/fuel mixture. More air fuel = more power and torque. | Velocity is created in the exhaust system from an exhaust pulse traveling through the primary tube and as rear part of the pulse cools, will create a vacuum. This vacuum will help to pull the next exhaust pulse out of the cylinder. This leaves a cleaner cylinder with less spent exhaust fumes and more room for the incoming air/fuel mixture. More air fuel = more power and torque. | ||
| − | ===Equal primary tube length=== | + | ====Equal primary tube length==== |
| + | =====Argument FOR equal primary tube length===== | ||
If the length of the primary is part of the tuning equation, how well does an engine run with different primary tube lengths? Try and jet that carburetor without pulling your hair out! Most of the commercially available headers out there have a large variance in tube length. Check out a set for a big block mopar in a B or E body for an example. The variance between longest and shortest tubes on these units can be as much as 16". | If the length of the primary is part of the tuning equation, how well does an engine run with different primary tube lengths? Try and jet that carburetor without pulling your hair out! Most of the commercially available headers out there have a large variance in tube length. Check out a set for a big block mopar in a B or E body for an example. The variance between longest and shortest tubes on these units can be as much as 16". | ||
| + | [[Image:unequal.jpg|Big block Chevy headers. Note how the driver's side rear tube (yellow) must be about 10"-12" shorter than the next tube (in red)]] | ||
| + | |||
| + | In the header photograph above, the short primary tube would scavenge at a higher RPM and the long primary tube would scavenge at a lower RPM for the respective cylinder. Therefore the cylinder with the short tube will be running lean at low RPM and the long tube cylinder will be running lean at the high RPM and would require different jetting and timing than the others. How do you do that with a standard kettering distributor and a simple carburetor? That's why equal length is important: so you can tune your car. | ||
| + | |||
| + | =====Argument AGAINST equal primary tube lengths===== | ||
Equal length headers are good for a certain part of the RPM range of a typical engine. However, when buying an equal length header you are left with the length the manufacturer wanted to use, NOT the correct length for the engine you are building. How do you know if its the right length? | Equal length headers are good for a certain part of the RPM range of a typical engine. However, when buying an equal length header you are left with the length the manufacturer wanted to use, NOT the correct length for the engine you are building. How do you know if its the right length? | ||
Different primary tube lengths are not nearly as hard to tune. This type of header shown has proven itself for decades to be a well designed, good flowing header that will free up a good amount of horsepower compared to stock manifolds. | Different primary tube lengths are not nearly as hard to tune. This type of header shown has proven itself for decades to be a well designed, good flowing header that will free up a good amount of horsepower compared to stock manifolds. | ||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
"Equal length" is usually defined as the longest and shortest tubes being within 2 inches of each other (about as close as you can measure with a tape measure at the swap meet). | "Equal length" is usually defined as the longest and shortest tubes being within 2 inches of each other (about as close as you can measure with a tape measure at the swap meet). | ||
| − | There have been claims by some manufacturers that unequal lengths broaden the torque curve due to different cylinders performing better at different RPM. It is left to the reader to decide if flattening the torque curve is a good thing to be doing with headers. | + | There have been claims by some manufacturers that unequal lengths broaden the torque curve due to different cylinders performing better at different RPM. It is left to the reader to decide if flattening the torque curve is a good thing to be doing with headers. However, a flat broad torque curve makes better drivability, and a smoother power band than a peaky engine. |
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
| + | ==Collector== | ||
Now we get to the collector - ever see a car header with 1 5/8" primaries that had a 2 1/2" collector? Hooker makes them for trucks, but you don't see them for cars. But that is the optimum diameter for most street headers. Run that right into a 2 1/2" exhaust and you have a sweet system with lots of torque where you need it, and better fuel economy to boot. The collector should have a smooth, gentle shape from the four tube area down to the final diameter to keep things moving smoothly. | Now we get to the collector - ever see a car header with 1 5/8" primaries that had a 2 1/2" collector? Hooker makes them for trucks, but you don't see them for cars. But that is the optimum diameter for most street headers. Run that right into a 2 1/2" exhaust and you have a sweet system with lots of torque where you need it, and better fuel economy to boot. The collector should have a smooth, gentle shape from the four tube area down to the final diameter to keep things moving smoothly. | ||
Your Privacy Choices