You must first determine what is the lowest RPM your competition engine is being run at and what is the highest; where does the engine get pulled down to on the shift drop or each individual gear change, and how long will the engine be in any particular RPM range. As stated before, a shorter primary tube tends to make more power in the upper RPM range and a longer tube favors the lower RPM.
An example would be best stated that an engine spinning at 8,000 RPM might make its best power with a primary tube length of about 26", but this same engine while at its shift drop might make its best power with a primary tube length of 30". Therefore there is no true ideal length primary tube, as it's impossible to change this length as it's accelerating your racecar. This is where the trial and error method beats computer engine simulation in real world racing conditions.
The actual horsepower of a particular engine usually dictates primary tube size, and by the maximum RPM the engine is spun.
Engine basics teach us that an engine is nothing more than an air pump, the more air we put into the engine the more air must come out. Today's smaller high flowing intake ports require a high flowing exhaust port that can expel the spent gasses, which proves that it requires a certain amount of volume and flow to produce a certain amount of power. This same general rule of thumb can be used to dictate the size of the primary tube required to make a certain amount of power on a competition engine.
As previously stated, Performance Welding feels its best to use as small of a primary tube possible for your particular application, just as with the collector sizing.
Every Competition Engine does, although have its own inherit characteristics. Some engines use a small high flowing intake and exhaust port and some use a large high flowing port. Some classes require a stock intake port, and to obtain high volumetric effiency with the engine, excessive camshaft overlap and duration is required to help the competition header pull air through the engine, as the low flowing stock cylinder head cant perform this feat on its own.
So being very general, a low horsepower 350 CID V8 engine making only 1.5 horsepower per CID may only require a primary tube size of 1 5/8"stepped to 1 ?". This particular engine would only require a merged collector throat size of approximately 2 ?" with a 3" outlet. The primary tube length is dictated by what RPM the engine is run at to obtain these power levels, with the vehicle weight, transmission, torque converter, and type of racing playing a dominating role.
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