Join Date: Mar 2007
Location: San Antonio, Texas
David Vizard's comments on header length:
Headers -- Primary Pipe Lengths
Misconceptions concerning exhaust pipe lengths are widespread. Take for instance the much-overworked phrase "equal-length headers." More than the odd engine builder/racer, or two, have made a big deal about headers with the primary pipes uniform within 0.5 inch. The first point this raises is whether or not what was needed was known within 0.5 inch! If not, the system could have all the pipes equally wrong within 0.5 inch! Trying to build a race header for a two-planed crank V-8 with lengths to such precision is close to a waste of valuable time. Under ideal conditions it is entirely practical for an exhaust system to scavenge at or near maximum intensity over a 4,000 rpm bandwidth. Most race engines use an rpm bandwidth of 3,000 or less rpm. If the primary pipe scavenging effect overlaps by 3,000 rpm then it matters little that one pipe tunes as much as 1,000 rpm different to another. Since this is the case, then all other things being equal, pipe lengths varying by as much as 9 inches have little effect on performance. A positive power-increasing attribute of differing primary lengths is that it allows larger-radius, higher-flowing bends and more convenient pipe routing to the collector in often confined engine bays.
Feom Vizard's How To Build Horsepower
So far, we have discussed basic manifold configurations based on all the primaries dumping into a secondary. This gives us the commonly seen 4-into-1 system used on V-8s and high-RPM four-cylinder engines. If the engine is a four-cylinder unit, and is not likely to see the top side of 7,800 to 8,000 rpm, then a 4-into- 2-into-1 system is the way to go. Such a system has the advantage of fitting the confines of the engine bay easier and produces more torque up to about 6,800 to 7,000 rpm. At the lower speeds (2,000 to 4,000), such a system can be up by as much as 12 ft-lbs per liter (0.2 ft-lbs per cubic inch). Above 7,800 to 8,000 rpm, it’s a clear-cut advantage to go with a 4-into-1 system.
Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series
Last edited by 180OUT; 02-24-2019 at 04:21 PM.