Our test engine is a Mopar Performance 300hp crate. Equipped with a dual-plane intake and
One of the key decisions in setting up a Mopar street car concerns the exhaust system. Different requirements and different preferences are what make our cars unique. For some guys, getting noticed is what it's about, so chamber-style mufflers and the calliope sound of exhaust pulses firing down long header tubes are the ticket. Others come from another school, looking for discrete, quiet, and low-maintenance operation.
These are the ones who will spurn headers for a set of factory manifolds and a modest-sized set of duals blowing through factory-style mufflers. A big part of the decision-making process is predicated upon the balance of power. For the first group, more is always the goal, with little consideration for anything as mundane as practicality and quiet cruising. For the more conservative, the choice revolves around how much power is being sacrificed with a given setup.
Exhaust headers are among the most significant power enhancers in a high-performance engine. The higher the power level, the more relevant headers become. On a 600hp street or strip terror, unbolting a big-tube header in favor of stock manifolds could cost upwards of 100 hp. Few enthusiasts in that realm would consider factory iron manifolds. On the other hand, plenty of guys building modest street drivers are torn by the decision, wondering how much difference it would make. Before we can answer that question, it's worthwhile to understand why a header does what it does.
On top, we bolted a 750 mechanical-secondary Speed Demon carb. We actually found some link
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 (or individual cylinder) doesn't just blow exhaust like a leaf blower; it fires in pulses. Once during every other crank rotation, a given cylinder has an exhaust stroke. Actually, the exhaust valve opens while the piston is still going down on the power stroke. This is significant in that the sooner the valve opens during the power stroke, the greater the cylinder pressure is. With stock short-duration cams, the piston position is toward bottom dead center (BDC), and the cylinder volume is near maximum.
Now, imagine a radical racing cam, with the exhaust valve opening much earlier as the piston moves down on the power stroke. In this instance, the gas pressure is still quite high, and opening the valve gives an immediate escape path, much like opening a valve on a high-pressure bottle. We can see from the above discussion that the pop of exhaust into the exhaust port (and ultimately into the header) is much stronger with a long-duration cam that opens earlier. The energy, or this pop (frequently referred to as blowdown) is carried into the header. As the exhaust pulse travels down the tube, it carries momentum (mass and velocity), helping to scavenge the cylinder.
So far, we see interesting things happening in an exhaust header, but it gets better. Upon reaching the end of the pipe in the collector, a low-pressure wave is reflected and travels back up the pipe at the speed of sound. If it gets to the exhaust valve while the valve is still substantially open-at around top dead center (TDC)-it will impart a low pressure condition to the cylinder. This helps draw the remaining exhaust out and also pulls fresh air/fuel mixture in through the now-open intake valve. This is the primary scavenging effect of a full-length tube header. Although it is dependent upon sufficient cam timing (duration), a radical race cam is not necessary in order to receive a benefit from the scavenging effect of headers.
We moved up a notch from the early 318 iron manifolds to late 360 iron log-style manifolds
For our baseline test, we fitted the 360 with the lowest of exhaust low, a set of stock sm
Our final test of factory iron was a set of rare and expensive early 340 high-performance
With the manifold portion of our test complete, we moved on to tube headers. First was a s
On the driver-side shorty header, tubes 5 and 3 cross over for the purpose of separating t
Our second set of headers was a pair of Hooker 151/48-inch Competition-series street heade