As insensitive as the primary pipe lengths are for power, the opposite is true of the secondaries. Here we find that just a few inches can make the difference between winning and going out in the quarter-finals. You often see headers that have no secondary length of any consequence, just the short stub on the collector section, and that’s a mistake. For such a seemingly trivial piece of pipe, this can be the loss of a fair chunk of power. Just how much this can be in a relatively normal situation can be seen from tests I did by adding 12 inches of length to a typical stub collector. In that test an extra 12 hp and 10 lb-ft (peak to peak) were gained.
When longer secondary lengths are used, the effect on power and torque is even more dramatic. The chart below demonstrates the difference among 21-, 16-, and 12-inch secondary lengths (as measured from the small end of the four-pipe merge). Obviously, longer secondary pipes are fine for the street, where low-speed torque is as important as anything, but if you’re racing only the top 1,500 to 2,000 rpm is used. If we take an average of the output over the range from 5,000 to 6,750 rpm, we see a very substantial difference in output, all from paying attention to a simple thing like the exhaust secondary length.
The last dimension we have to look at is the secondary/collector diameter. This is a pretty simple aspect to deal with. For a street/strip or race application, a good rule of thumb is to multiply the primary pipe diameter by 1.8 and then select the nearest available pipe size. A little oversize won’t hurt much, but it’s worth noting that often selecting something a little undersize is better than a bigger increment oversize. For example, if your calculated secondary pipe size works out to be 1/16-inch more than one standard pipe size but 3/16 less than the next size up, then opt for the smaller pipe as results are likely to be much better. If mileage and street performance is your goal, then a secondary up to ½-inch less than predicted here can work well.
Most readers are likely to get their headers and systems from a company like Hooker. This is fine, but if you want something special you may want to calculate as much as possible the optimum system for your engine. In this regard, I’ve been reviewing a program by cylinder head and exhaust specialist Larry Meaux (www.MaxRaceSoftware.com) that so far has correlated well with what I have found works on the dyno. This program also computes the requirements for a muffled system.