So far, we see interesting things happening in an exhaust header, but itgets better. Upon reaching the end of the pipe in the collector, alow-pressure wave is reflected and travels back up the pipe at thespeed of sound. If it gets to the exhaust valve while the valve is stillsubstantially open--at around top dead center (TDC)--it will impart a low-pressure condition to the cylinder. This helps draw the remainingexhaust out and also pulls fresh air/fuel mixture in through thenow-open intake valve. This is the primary scavenging effect of afull-length tube header. Although it is dependent upon sufficient camtiming (duration), a radical race cam is not necessary in order toreceive a benefit from the scavenging effect of headers.
In fact, typical moderate street- performance profiles will readilyrespond to the tuning effects of headers. Primary pipe length is aconsideration, as well. Even at the speed of sound, it takes time forthe pressure wave to travel up the pipe. The primary has to be at alength to time the event for the overlap period, but the period of timebetween cycles varies with rpm. As a result, a given primary-pipe lengthwill tune to a given rpm. The optimal primary-pipe length will vary withthe operating rpm range of the engine. Primary lengths between 28 and 42inches cover the range from high-rpm race engines to tow rigs. Cast-ironexhaust manifolds and shorty headers are at a disadvantage here.
This whole theory has been proven in dyno results on high-outputengines, and now we know why. We wanted to explore the question ofheaders versus manifolds in a much milder setting, a pussycat of anengine if you will--the type of engine that would have a guy questioningwhether to use headers or manifolds. To find out, we brought a MoparPerformance 300hp Magnum crate motor to Westech to run some variations.The 300hp crate is a real puppy, with a new-car-like idle (to us) andover 19-inch Hg of vacuum. Its docile nature is the result of a mildcam, with minimal overlap and stock .385/.410-inch intake and exhaustlift, respectively. Spec'd to drop in and go with no hassles, the360/300 crate will happily pull a full package of accessories, powerbrakes, air conditioning, and a tightly-converted automatic while idlingalong effortlessly.
We brought along a wide range of exhaust manifolds and headers, runningthe gamut from puny stock 318 iron manifolds to the bigger log-type 360pieces, and even the highly-sought-after and revered 340hp iron units.We also brought a range of headers to see how things would compare. Wefound some of the results surprising, if not downright shocking. Read onfor the results of our exhaustive research.