Racing Exhaust System - Header Tech

In this instance, the test engine was a rebuilt 350 Chevy crate motor with a two-plane intake. The headers, courtesy of Hooker, had an average primary pipe length of 34 inches and dumped into a 14-inch-long collector. The pipe sizes tested were 1 5/8, 1 3/4, and 1 7/8. From Figure 2 we can see that the two smaller pipe configurations produced much better output in the 3,000-5,500 rpm range. The curves also tell us that in this instance (and in most but not all instances) an exhaust pipe that is too big, even by just 1/8 inch, negatively impacts the output far more than one that may be 1/8 inch too small. The 1 7/8 header did not match the output of either the 1 5/8- or the 1 3/4-inch headers until nearly 6,000 rpm. After that it only produced barely measurable increases in output. In this test, the curves from about 6,200 to 6,500 are virtually identical regardless of the header diameter used. This engine had a hydraulic roller cam. It could be that the valvetrain dynamics were marginal and may have set the limits on output, not the headers being tested.

As revealing as the test in Figure 2 is, it certainly does not tell the whole story. So here is some info other tests have produced. First, the crossover point from a good 1 5/8 system to a 1 3/4-inch system seems to occur around the 420-440hp mark. Another factor is that the higher the CR, the later the engine needs to go to the larger-diameter primary. For example, a 440hp 13:1 restricted engine may produce the best results on a 1 5/8, whereas a 9:1 440hp engine may be better on 1 3/4-inch primaries.

The relation of primary length to rpm range is clear cut with a four-cylinder engine. Here is a useful tip concerning headers and four cylinders. If the top end of the range used is less than 8,000 rpm, a 4-2-1 system is usually a lot better than a 4-1 system. How much better? I did a lot of testing on a 2L Pinto engine while doing a series of cam design exercises for a well-known cam company. Since they were paying the dyno bill, I could afford a lot of tests to establish which exhaust systems worked best with which cam. On a Pinto engine peaking at 7,200 rpm, the 4-2-1 system had as much as a 20 lb-ft advantage on a 4-1 system.

But back to V-8 systems. The test engine in this case was a 302 Ford, and headers in four lengths were tested. These had average lengths of 18, 29, 32, and 38 inches. Figure 3 shows the results. We can see that the engine really did not like the short primaries, but the change in output per change in length was not really that much when the length was extended into the 30-inch range. Although there is a visible trend from these tests (and others) in as much as longer favors low speed, we can say that anything from 30 to 45 inches works just fine in the main. We can also conclude that, unlike a four-cylinder engine, putting any real effort into equal lengths is unlikely to pay off to any measurable extent. The only conclusion here is that the uneven exhaust pulses cause one or more cylinders to get better when length is changed while others get worse. So output changes little with length changes. If ever there was a factor pointing toward the need for more research, the length issue is it.

Assuming the primary is close to the optimal diameter, a good starting point for the secondary or collector pipe diameter is to multiply the primary diameter by 1.75. Our tests in this round look at the effects of diameter on a nominally 475hp, 383ci engine. The collector length, which previous tests had shown to be near optimal, is held constant at 15 inches. For these tests, collectors of 2 3/4, 3, and 3 1/2 inches were used. Figure 4 shows the results. It does not take a great deal of studying to see that an overly large collector sacrifices output just about everywhere.

Assuming the diameter is correct, let's look at how output varies with secondary length. Amazingly enough, this is an area where many racers fail to appreciate the ease with which they can make dramatic changes to the engine's output with minimal cost or effort. The tests depicted in Figure 5 were carried out on a Street Stocker with very restrictive rules. The rpm band of importance was the 4,000-6,200 range with this engine. The lengths tested were 21 inches (red curve), 16 inches (green curve), and 12 inches (blue curve). As shown, the 12-inch length was best over the rpm range of importance. As the pipe length was increased, the low speed output came up at the expense of top end. A 60-inch secondary pipe works really well in the 2,500-rpm range but costs 10-15 hp at the top.

If shorter is better for top end (as we see from Figure 5), what happens when the collector length is made really short (say 3 inches or less)? Answer: The output goes down the tube everywhere in the entire rpm range. A collector length of 0-2 inches can cost as much as 20 hp at peak power rpm and 40 lb-ft at about a thousand rpm before peak torque. Collectors with zero or near-zero length are unacceptable. If you do not have access to a dyno, then use a 12-inch collector.