Header design
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That brings us to primary length. First of all, those "shortie" headers are not headers, just tubing manifolds designed for clearance -- not horsepower or torque. Although they look like they would flow better than manifolds (and probably do in many instances), unless you are running a supercharger, you need more than flow out of a header. The bothersome part of the "shortie" (other than length) is that the collector is so short and causes a lot of turbulence right where the flow needs to be smoothed out. | That brings us to primary length. First of all, those "shortie" headers are not headers, just tubing manifolds designed for clearance -- not horsepower or torque. Although they look like they would flow better than manifolds (and probably do in many instances), unless you are running a supercharger, you need more than flow out of a header. The bothersome part of the "shortie" (other than length) is that the collector is so short and causes a lot of turbulence right where the flow needs to be smoothed out. | ||
| − | Exhaust headers (and intake runners for that matter) can be tuned to length to give a power boost at a given RPM. Tuned length is a function of the speed at which the boost is desired and has nothing to do with diameter of the tubes. When the exhaust valve cracks open, there is a strong pressure pulse sent down the tube at the speed of sound in the high temperature gas. When this pressure wave reaches a larger diameter such as in a header | + | Exhaust headers (and intake runners for that matter) can be tuned to length to give a power boost at a given RPM. Tuned length is a function of the speed at which the boost is desired and has nothing to do with diameter of the tubes. When the exhaust valve cracks open, there is a strong pressure pulse sent down the tube at the speed of sound in the high temperature gas. When this pressure wave reaches a larger diameter such as in a header collector, there is a low pressure (slight vacuum) wave reflected back up the pipe at the speed of sound. The goal is to have this low pressure wave reach the exhaust valve just as it closes which scavenges the cylinder causing a better charge of clean air and fuel. Since it is a function of fixed velocity of sound in the exhaust gas, the slower the desired tuned speed, the longer the pipe needs to be. Interestingly, the shape of the pipe (turns) isn't critical so a basket of snakes header is just as effective as straight tubes. The diameter of the tube should be as small as possible which strengthens the magnitude of the pressure pulses. Velocity of the gas really has nothing to do with the tuned speed available in header design but is a negative when it comes to friction losses. |
The main advantage gained in equal length, independent primary header tubes is from the strong negative pressure pulse that is reflected from the tube end when the strong positive pressure pulse form the exhaust valve reaches the collector. Other pulses from other header tubes are of much smaller magnitude in the tube of interest and can be ignored. Thus tuning length is very easy to determine once you have an estimate of the speed of sound in the hot gasses. A useful equation is | The main advantage gained in equal length, independent primary header tubes is from the strong negative pressure pulse that is reflected from the tube end when the strong positive pressure pulse form the exhaust valve reaches the collector. Other pulses from other header tubes are of much smaller magnitude in the tube of interest and can be ignored. Thus tuning length is very easy to determine once you have an estimate of the speed of sound in the hot gasses. A useful equation is | ||
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There are other design theories like the Helmholtz resonator which are useful in designing systems with more than one degree of freedom than a single pipe/cylinder, i.e., Tri-Y. | There are other design theories like the Helmholtz resonator which are useful in designing systems with more than one degree of freedom than a single pipe/cylinder, i.e., Tri-Y. | ||
| − | As you can see from this discussion, most popular aftermarket headers are poorly designed for any performance purpose. Tubes are too short, too big, and all different lengths. Most "street rod" headers are not designed for performance, rather to fit | + | As you can see from this discussion, most popular aftermarket headers are poorly designed for any performance purpose. Tubes are too short, too big, and all different lengths. Most "street rod" headers are not designed for performance, rather to fit inside the typical smoothie envelope. "Performance" headers are designed to look zoomie I guess because I can't figure out any other design criteria when I study them. |
Incidentally, this is the principle that Chrysler used on the cross ram intake manifolds they put on big block passenger car engines in the late 50s. The velocity of sound in the cold intake gasses is much slower than that in hot exhaust so tuned length for a street intake is much shorter @ about 18" from the valve seat to the plenum. Thus they put a 4bbl carb on either side of the engine and crossed over long ports. Looked and performed great! | Incidentally, this is the principle that Chrysler used on the cross ram intake manifolds they put on big block passenger car engines in the late 50s. The velocity of sound in the cold intake gasses is much slower than that in hot exhaust so tuned length for a street intake is much shorter @ about 18" from the valve seat to the plenum. Thus they put a 4bbl carb on either side of the engine and crossed over long ports. Looked and performed great! | ||
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[[Image:unequal.jpg|right|frame|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).]] | [[Image:unequal.jpg|right|frame|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 to the right, 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. Not only do equal length tubes make the engine | + | In the header photograph to the right, 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. Not only do equal length tubes make the engine tunable, but make more torque in the RPM range for which they were intended. |
This is particularly true for V6 engines. In the case of the 173ci 3.1L or the 3.4L pushrod engines, the pulse of the air pressure wave, as well as the sound wave, need to dissipate at a particular length so that a proper negative wave travels back AT THE RIGHT TIME to the exhaust valve, aiding the exhaust flow. In the case of the GM pushrod v6's the firing order is sequential and therefore cylinders fire on alternate sides of the block on each compression stroke. Generally, those v6 engines operate best with 1.5" pipe and a primary length of 32" to 36" depending on the compression and torque curve desired. It should be noted that most headers used for the FWD and Mid-Engine v6's are not optimum in performance. | This is particularly true for V6 engines. In the case of the 173ci 3.1L or the 3.4L pushrod engines, the pulse of the air pressure wave, as well as the sound wave, need to dissipate at a particular length so that a proper negative wave travels back AT THE RIGHT TIME to the exhaust valve, aiding the exhaust flow. In the case of the GM pushrod v6's the firing order is sequential and therefore cylinders fire on alternate sides of the block on each compression stroke. Generally, those v6 engines operate best with 1.5" pipe and a primary length of 32" to 36" depending on the compression and torque curve desired. It should be noted that most headers used for the FWD and Mid-Engine v6's are not optimum in performance. | ||
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