Now, to a point, a header manufacturer can control the speed of the exhaust gas by changing the diameter of the primary tube.\u00a0 The smaller the diameter of the tube, the faster the exhaust will flow (keeping in mind the exhaust flow slows as it cools).\u00a0 A clever header builder will recognize that by varying both the length and the diameter of a given primary tube in a header, those tubes can be tuned to provide the largest amount of inertial scavenging. But there\u2019s something else that is equally important and it can have an effect upon the header configuration, and it also shows why a flow bench is pretty much useless when dealing with header sizing:<\/p>\n
Wave Scavenging<\/b><\/p>\n
Wave scavenging does not involve the physical movement of exhaust gases up and down the header tubes.\u00a0 Instead, it deals with the sound waves created in the engine.\u00a0 When an exhaust valve first opens, a pressure wave begins to travel — moving in excess of 18,500 inches per second\u00a0 <\/i>(more than the speed of sound, which at sea level is 13,514 in\/sec).\u00a0 This sonic wave is moving quickly \u2013 much quicker than the ever expanding exhaust wave.\u00a0 When the sonic wave arrives at the end of the primary tube, a negative shock wave is generated, and this wave travels backwards toward the exhaust port (that means it is “reflected”). What the header builder has to do is to time the arrival of this negative pressure wave to occur just before the exhaust valve has closed and while the intake valve is opening.\u00a0 If the negative wave arrives too soon or too late, the power potential of the next combustion chamber cycle will be “diluted” significantly.\u00a0 Obviously, a flow bench doesn\u2019t have this capability.<\/p>\n
Generally speaking the engine likes to see a lower pressure and a higher speed in the exhaust (the larger the tube, the lower the exhaust speed). Typically, a situation with a highly negative pressure in the exhaust during the camshaft overlap period (when both inlet and exhaust valves are open) will produce power.<\/p>\n
Equally important, an accurate set of specifications for a “tuned” exhaust system can\u2019t be created easily by way of mathematics either.\u00a0 Certainly there are formulas and software available (\u2026readily available, and in an upcoming segment, we\u2019ll provide you with examples) to provide basic header dimensions, but many experts will tell you that in order to accurately arrive at the exact header primary tube length and diameter, there is only one real option:\u00a0 Trial and error.<\/p>\n
Collector Length or Primary Length \u2013 What\u2019s Important?<\/b><\/p>\n
Collector length is important, but some racers tend to ignore it.\u00a0 Sure dozens upon dozens of sharp little guy racers take the time to adjust collector length for their given combinations.\u00a0 But the problem is, some racers think collector length is directly related to peak engine horsepower.\u00a0 This could very well be wrong.<\/p>\n
Collector length is determined, primarily, by the variables of the racecar in question, and is not entirely dependent upon the engine alone.\u00a0 That means the header data you collect from an engine dyno test could very well prove invalid when the motor is installed in the car.\u00a0 When tuning the headers (and the entire exhaust system for that matter) for a given combination, there is a huge cross section of variables.\u00a0 Things such as vehicle weight, rear axle ratio, transmission type and end use of the vehicle all come into play.\u00a0 Once the variables are all taken into account, the header manufacturer can then determine at what RPM level the peak torque curve should be developed.\u00a0 In contrast to what may have been discussed and written in the past (and perhaps incorrectly), the most important thing to consider when selecting headers is the primary tube diameter.\u00a0 It\u2019s something you can\u2019t change without buying new headers.\u00a0 But you have to remember that primary tube length can be used to move the torque curve in either direction (effectively moving the torque curve from one RPM level to another).<\/p>\n
We\u2019ll dig deeper into collectors in Part III.<\/p>\n
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