Exhaust Blowout on a Mopar 300hp Crate

One of the key decisions in setting up a Mopar street car concerns theexhaust system. Different requirements and different preferences arewhat make our cars unique. For some guys, getting noticed is what it'sabout, so chamber-style mufflers and the calliope sound of exhaustpulses firing down long header tubes are the ticket. Others come fromanother school, looking for discrete, quiet, and low-maintenanceoperation.

These are the ones who will spurn headers for a set of factory manifoldsand a modest-sized set of duals blowing through factory-style mufflers.A big part of the decision-making process is predicated upon the balanceof power. For the first group, more is always the goal, with littleconsideration for anything as mundane as practicality and quietcruising. For the more conservative, the choice revolves around how muchpower is being sacrificed with a given setup.

Exhaust headers are among the most significant power enhancers in ahigh- performance engine. The higher the power level, the more relevantheaders become. On a 600hp street or strip terror, unbolting a big-tubeheader in favor of stock manifolds could cost upwards of 100 hp. Fewenthusiasts in that realm would consider factory iron manifolds. On theother hand, plenty of guys building modest street drivers are torn bythe decision, wondering how much difference it would make. Before we cananswer that question, it's worthwhile to understand why a header doeswhat it does.

While many assume that the benefit of a header is reduced backpressure,there is much more going on.

A primary aspect of header function is pulse-scavenging. An engine (orindividual cylinder) doesn't just blow exhaust like a leaf blower; itfires in pulses. Once during every other crank rotation, a givencylinder has an exhaust stroke. Actually, the exhaust valve opens whilethe piston is still going down on the power stroke. This is significantin that the sooner the valve opens during the power stroke, the greaterthe cylinder pressure is. With stock short-duration cams, the pistonposition is toward bottom dead center (BDC), and the cylinder volume isnear maximum.

Now, imagine a radical racing cam, with the exhaust valve opening muchearlier as the piston moves down on the power stroke. In this instance,the gas pressure is still quite high, and opening the valve gives animmediate escape path, much like opening a valve on a high-pressurebottle. We can see from the above discussion that the pop of exhaustinto the exhaust port (and ultimately into the header) is much strongerwith a long-duration cam that opens earlier. The energy, or this pop(frequently referred to as blowdown) is carried into the header. As theexhaust pulse travels down the tube, it carries momentum (mass andvelocity), helping to scavenge the cylinder.