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
In fact, typical moderate street performance profiles will readily respond to the tuning effects of headers. Primary pipe length is a consideration, as well. Even at the speed of sound, it takes time for the pressure wave to travel up the pipe. The primary has to be at a length to time the event for the overlap period, but the period of time between cycles varies with rpm. As a result, a given primary-pipe length will tune to a given rpm. The optimal primary-pipe length will vary with the operating rpm range of the engine. Primary lengths between 28 and 42 inches cover the range from high-rpm race engines to tow rigs. Cast-iron exhaust manifolds and shorty headers are at a disadvantage here.
This whole theory has been proven in dyno results on high-output engines, and now we know why. We wanted to explore the question of headers versus manifolds in a much milder setting, a pussycat of an engine if you will-the type of engine that would have a guy questioning whether to use headers or manifolds. To find out, we brought a Mopar Performance 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) and over 19-inch Hg of vacuum. Its docile nature is the result of a mild cam, with minimal overlap and stock .385/.410-inch intake and exhaust lift, respectively. Spec'd to drop in and go with no hassles, the 360/300 crate will happily pull a full package of accessories, power brakes, air conditioning, and a tightly-converted automatic while idling along effortlessly.
We brought along a wide range of exhaust manifolds and headers, running the gamut from puny stock 318 iron manifolds to the bigger log-type 360 pieces, and even the highly-sought-after and revered 340hp iron units. We also brought a range of headers to see how things would compare. We found some of the results surprising, if not downright shocking. Read on for the results of our exhaustive research.
Left with the choice of banging clearance into the headers or clearancing the block, we pu
Late-model truck blocks, on which the crate 360s are based, are cast with large motor-moun
The Hookers feature a layout similar to Hedman's full-length tubes, with the number 3 tube
We swapped the 1 5⁄8-inch Hookers for a set of tti 1 5⁄8x1 3⁄4-inch step
The passenger-side tti header will clear either a stock straight oil-filter arrangement or
The layout of tti's headers is unique, with none of the low-hanging tubes of the other des
With the positive response in output gained by going to the step header, our next change w
Unlike the Hooker street headers, the Super Comps tubes were routed clear of all spark-plu
|Westech Engine Dyno Superflow 901|
|RPM||318||360||340||SHT||1 5⁄8||tti||1 3⁄4|
|RPM||318||360||340||SHT||1 5⁄8||tti||1 3⁄4|
318: Stock '69 A-Body exhaust manifolds with 24-inch extensions
360: Stock '77 360 iron log-style exhaust manifolds with 24-inch extensions
340: Stock '70 340hp exhaust manifolds with 24-inch extensions
SHT: Hedman 151/48-inch-tube shorty headers with 24-inch extensions 1 5⁄8: Hooker Competition 1 5⁄8-inch ceramic-coated header tti: Tube Technologies Inc. 1 5⁄8x1 3⁄4-inch chrome step header 1 3⁄4: Hooker Super Competition 1 3⁄4-inch header; bare, uncoated