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.

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.