Our test engine is a Mopar...
Our test engine is a Mopar Performance 300hp crate. Equipped with a dual-plane intake and an extremely mild .385/.410-inch lift cam, the 300hp crate is as docile as a daily-driver engine can be.
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...
On top, we bolted a 750 mechanical-secondary Speed Demon carb. We actually found some linkage interference with the mounting pad of the intake, so an open-spacer was fitted beneath the carb.
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...
We moved up a notch from the early 318 iron manifolds to late 360 iron log-style manifolds. The 360 manifolds are nothing to look at, but the internal passages and outlet are significantly larger than those of the 318 pieces. Power was up a little, moving to 314 hp and 407.1 lb-ft of torque.
For our baseline test, we...
For our baseline test, we fitted the 360 with the lowest of exhaust low, a set of stock small-passage 318 iron exhaust manifolds. Bolted to the manifolds was a set of 2 1⁄4-inch head pipe extensions 24 inches long. So equipped, the 360 put out a respectable 311 hp at 4,600 rpm and 403 lb-ft of torque.
Our final test of factory...
Our final test of factory iron was a set of rare and expensive early 340 high-performance manifolds. With the 340 exhaust, the 360 inched up to 315 hp and 409.1 lb-ft of torque, which represented very little gain on this engine.
With the manifold portion...
With the manifold portion of our test complete, we moved on to tube headers. First was a set of Hedman shorties with a tube diameter of 151/48 inches. The shorties bolted up to the block nicely. However, in the '70 A-Body Duster we once tried them in, the collector exit was too close to the steering drag link to connect effectively. We haven't tried to fit them in other chassis types. The shorties did provide a good power increase when compared to all of the iron manifolds tested, recording 322 hp at 4,700 rpm and 416.7 lb-ft of torque. Spark-plug wire clearance was also good.
On the driver-side shorty...
On the driver-side shorty header, tubes 5 and 3 cross over for the purpose of separating them in the collector, since these cylinders fire consecutively.
Our second set of headers...
Our second set of headers was a pair of Hooker 151/48-inch Competition-series street headers. The passenger-side tubes bolted up with no trouble. They typically fit a Mopar chassis nicely.
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...
Left with the choice of banging clearance into the headers or clearancing the block, we pulled out the grinder and hacked away at the block. It took quite a lot of material removal to gain the required clearance.
Late-model truck blocks, on...
Late-model truck blocks, on which the crate 360s are based, are cast with large motor-mount pads behind the traditional mounting ears found with earlier production blocks. With the Hookers, we found tube interference on the driver side, preventing it from bolting to the head. This was not surprising since the headers were designed long before the change in block-mounting provisions.
The Hookers feature a layout...
The Hookers feature a layout similar to Hedman's full-length tubes, with the number 3 tube passing up along the valve cover, crowding the plug wires to cylinders 5 and 7. Also, the number 1, 5, and 7 tubes are routed low to go under the steering drag link in the chassis, hindering ground clearance. The 1 5⁄8-inch full-length headers provided quite an improvement in torque, with 423.9 lb-ft on tap. At 319, horsepower up top was down fractionally compared to the shorties. We would expect the results to be different if the engine was equipped with more cam duration, but at this mild spec, the tuning advantage of the long-tube header is largely lost up top due to cam timing.
We swapped the 1 5⁄8-inch...
We swapped the 1 5⁄8-inch Hookers for a set of tti 1 5⁄8x1 3⁄4-inch step headers. We immediately realized there was clearance for a standard box-end wrench to be used at the header bolts. The tubes also cleared the late-block's mounting bosses much better, bolting up without binding, although it was close.
The passenger-side tti header...
The passenger-side tti header will clear either a stock straight oil-filter arrangement or a production-style 90-degree oil-filter adapter.
The layout of tti's headers...
The layout of tti's headers is unique, with none of the low-hanging tubes of the other designs and ample clearance for the plug wires up top. Past experience has shown they have excellent chassis fit. Power output with the tti's was the best of all the systems we tested, clearing 327 hp and producing a healthy 425.5 lb-ft of torque.
With the positive response...
With the positive response in output gained by going to the step header, our next change was to determine if even bigger would be even better. We went to a pair of 1 3⁄4-inch Hooker Super Comp headers. The Super Comps are race-style headers with slip-fit collectors. With this 360's mild combination, the Super Comps proved to be overkill, with output lagging the step header, posting 321 hp at 3,700 rpm and 420.7 lb-ft of torque at 3,700 rpm. As a point of reference, we have seen the Super Comps add 10-15 hp to a standard 1 5⁄8-inch header in engines of the 450-500hp level.
Unlike the Hooker street headers,...
Unlike the Hooker street headers, the Super Comps tubes were routed clear of all spark-plug wires.
|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