383 Chevy Small Block Engine - Make 59 LB-FT and 33 More Horsepower For Almost Free!

This test was performed on the same 383ci small-block Chevy, but it was configured with a larger Comp Cams XR292 mechanical roller cam and a smaller 750-cfm Street HP Holley carburetor. The headers for this test were also different, employing a set of Hedman 1 3/4-inch chassis headers.

What's It All Mean?

There are tons of car crafters who uncork their exhaust because their car runs quicker at the dragstrip with open headers. If your engine is equipped with a big cam with lots of overlap, adding a set of collector extensions will probably improve the midrange torque. The reason is that when the exhaust pulse from the open exhaust valve arrives at the collector, a reflected wave of low pressure travels back up the collector and all the way back to the cylinder. If this low-pressure wave arrives at the cylinder during overlap, when both the intake and exhaust valves are open, it creates an additional "tug" on the inlet side of the engine, filling the cylinder. The timing of this reflected wave at a given rpm is determined by the design of the exhaust system. Adding collector length requires the exhaust pulse to travel farther before hitting open air. Shortening the overall length of the collector changes the timing of that reflected wave's arrival at the cylinder. It means less scavenging of the exhaust during this time, which means more residual exhaust gas in the cylinder and less power. This is why the torque increased with the longer collectors between 3,100 and 4,500 rpm.

When we tested the 3-inch muffled exhaust system on this engine, it unfortunately was not the exact same engine configuration. Perhaps most importantly, the headers were of different design and length. But we can still draw some thought-provoking conclusions. For example, adding a muffler creates a small amount of backpressure that certainly reduces the effect of the reflected wave, since it is not as intense as it would be if it were to exit into the atmosphere. Plus, our 3-inch exhaust pipe leading to the muffler was roughly three times longer than the 18-inch collectors used on the open-exhaust test. However, it appears that most 450-plus-horsepower engines will respond with positive torque gains when upgraded to a 3-inch exhaust system. The next step would be to experiment with H- and X-pipes to see what effect these designs have on torque and horsepower. Keep in mind that this was an experiment on just one engine and, as the saying goes, your results may vary.

By Jeff Smith

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