Exhaust Blowout on a Mopar 300hp Crate
Running the numbers on a mild 360
From the November, 2010 issue of Mopar Muscle
Photography by Steve Dulcich
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.
Our test engine is a Mopar...
Our test engine is a Mopar Performance 300hp crate. Equipped with adual-plane intake and an extremely mild .385/.410-inch lift cam, the300hp crate is as docile as a daily-driver engine can be.
On top, we bolted a 750 mechanical-secondary...
On top, we bolted a 750 mechanical-secondary Speed Demon carb. Weactually found some linkage interference with the mounting pad of theintake, so an open-spacer was fitted beneath the carb.
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 themanifolds was a set of 21/4-inch head pipe extensions 24 inches long. Soequipped, the 360 put out a respect- able 311 hp at 4,600 rpm and 403lb-ft of torque.
We moved up a notch from the...
We moved up a notch from the early 318 iron manifolds to late 360 ironlog-style manifolds. The 360 manifolds are nothing to look at, but theinternal passages and outlet are significantly larger than those of the318 pieces. Power was up a little, moving to 314 hp and 407.1 lb-ft oftorque.
Our final test of factory...
Our final test of factory iron was a set of rare and expensive early 340high-performance manifolds. With the 340 exhaust, the 360 inched up to315 hp and 409.1 lb-ft of torque, which represented very little gain onthis engine.
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.
With the manifold portion...
With the manifold portion of our test complete, we moved on to tubeheaders. First was a set of Hedman shorties with a tube diameter of 15/8inches. The shorties bolted up to the block nicely. However, in the '70A-Body Duster we once tried them in, the collector exit was too close tothe steering drag link to connect effectively. We haven't tried to fitthem in other chassis types. The shorties did provide a good powerincrease when compared to all of the iron manifolds tested, recording322 hp at 4,700 rpm and 416.7 lb-ft of torque. Spark-plug wire clearancewas also good.
On the driver-side shorty...
On the driver-side shorty header, tubes 5 and 3 cross over for thepurpose of separating them in the collector, since these cylinders fireconsecutively.
Our second set of headers...
Our second set of headers was a pair of Hooker 15/8-inchCompetition-series street headers. The passenger-side tubes bolted upwith no trouble. They typically fit a Mopar chassis nicely.
Late-model truck blocks, on...
Late-model truck blocks, on which the crate 360s are based, are castwith large motor-mount pads behind the traditional mounting ears foundwith earlier production blocks. With the Hookers, we found tubeinterference on the driver side, preventing it from bolting to the head.This was not surprising since the headers were designed long before thechange in block-mounting provisions.
Left with the choice of banging...
Left with the choice of banging clearance into the headers orclearancing the block, we pulled out the grinder and hacked away at theblock. It took quite a lot of material removal to gain the requiredclearance.
The Hookers feature a layout...
The Hookers feature a layout similar to Hedman's full-length tubes, withthe number 3 tube passing up along the valve cover, crowding the plugwires to cylinders 5 and 7. Also, the number 1,5, and 7 tubes are routedlow to go under the steering drag link in the chassis, hindering groundclearance. The 15/8-inch full-length headers provided quite animprovement in torque, with 423.9 lb-ft on tap. At 319, horsepower uptop was down fractionally compared to the shorties. We would expect theresults to be different if the engine was equipped with more camduration, but at this mild spec, the tuning advantage of the long-tubeheader is largely lost up top due to cam timing.
We swapped the 15/8-inch Hookers...
We swapped the 15/8-inch Hookers for a set of tti 15/8x13/4-inch stepheaders. We immediately realized there was clearance for a standardbox-end wrench to be used at the header bolts. The tubes also clearedthe late-block's mounting bosses much better, bolting up withoutbinding, although it was close.
The passenger-side tti header...
The passenger-side tti header will clear either a stock straightoil-filter arrangement or a production-style 90-degree oil-filteradapter.
The layout of tti's headers...
The layout of tti's headers is unique, with none of the low-hangingtubes of the other designs and ample clearance for the plug wires uptop. Past experience has shown they have excellent chassis fit. Poweroutput 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. Wewent to a pair of 13/4-inch Hooker Super Comp headers. The Super Compsare race-style headers with slip-fit collectors. With this 360's mildcombination, the Super Comps proved to be overkill, with output laggingthe step header, posting 321 hp at 3,700 rpm and 420.7 lb-ft of torqueat 3,700 rpm. As a point of reference, we have seen the Super Comps add10-15 hp to a standard 15/8-inch header in engines of the 450-500hplevel.
Unlike the Hooker street headers,...
Unlike the Hooker street headers, the Super Comps tubes were routedclear of all spark-plug wires.