Header design
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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 | ||
| − | L = 120V/ | + | L = 120V/N |
| − | + | where | |
| − | + | ||
| − | + | L is the pipe length, less port length in head, in inches; | |
| − | V | + | V is the velocity of sound in hot gasses, in feet per second (ft/sec). Values of 1300 ft/sec to 1700 ft/sec are common; |
| − | + | and, | |
| − | L = 204,000 | + | N is the engine rotational speed, in rotations per minute (rpm). |
| + | |||
| + | Using V = 1700 ft/sec the equations simplifies to | ||
| + | |||
| + | L = 204,000/N. | ||
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. | 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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