Header design
(→Argument for larger collectors) |
(→Primary tube length) |
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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 collecter, 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 causign 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 | |
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| + | L = 120V/rpm | ||
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| + | For | ||
| + | L = pipe length, less port length in head, in inches | ||
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| + | and | ||
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| + | V = velocity of sound in hot gasses. Values of 1300ft/sec to 1700 ft/sec are common. | ||
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| + | Using V = 1700ft/sec the equations simplifies to | ||
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| + | L = 204,000/rpm. | ||
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| + | There have been various permutations on this basic design like tri-Y headers, stepped tubing size, etc. Each takes advantage of modifying the pressure pulse arrival time at the instant the exhaust valve closes to achieve a scavenging/ higher volumetric efficiency/ more torque result. The good is that you can achieve a very significant torque increase at the design rpm. The bad is that you likely will also achieve less torque at other RPMs. | ||
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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. | ||
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| + | 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 insde the typical smoothie envelope. "Performance" headers are desinged to look zoomie I guess because I can't figure out any other design criteria when I study them. | ||
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| + | 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 lenght 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! | ||
====Equal primary tube length==== | ====Equal primary tube length==== | ||
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