Richard Holdener
June 1, 2009
Can bolting on a set of headers really be worth over 100 hp?

It's no secret that there is plenty of power hiding in your exhaust system, especially with the right set of headers. But often the question is how much power is there to unlock. At times there can be as much as 100 hp or more. But all that from a simple header swap? Now that we have your attention, it's time to dig deep into some header theory, and then follow up by getting our hands dirty installing not one, but three, sets of long-tube headers on a 5.4L mod motor. Not on just any mod motor, but an all-aluminum monster chock full of modifications that (as our testing will reveal) made the combination very sensitive to changes in the exhaust system.

The idea was a simple one--run the 5.4L mod motor with the factory GT500 exhaust manifolds feeding a set of 2½-inch exhaust tubes, then follow up by testing a set of 1¾-, 1 7/8-, and finally 2-inch long-tube headers from American Racing. Not only would this demonstrate the worth of aftermarket headers in general over the factory exhaust manifolds, but also the specific gains offered by the different sizes. To make things even more interesting, we plan to follow up with the same test on a supercharged combination.

Our test engine consisted of a highly modified 5.4L mod motor. The combination started out life as an all-aluminum 5.4L block from the Ford GT. The aluminum block was stuffed with a forged crank, rods, and pistons that produced a static compression ratio of 10.0:1.

Oddly enough, the opportunity for this test came about from the buildup of this particular 5.4L motor. The guys at Dynatek Racing had the crazy idea to build what they call a GT1000. I figure this was a great opportunity to not only verify the four-digit power numbers of the GT1000 motor, but also run some header testing along the way. Would the headers offer more power once we installed the blower? On the surface, this seemed to make sense. After all, wouldn't more power require more exhaust flow? As we soon found out, the theories were all thrown out the window, but it must be said that all data is good data, even when it has you leaving the dyno scratching your head.

Before going any further, thanks to American Racing for supplying three sets of stainless headers for our modular application. American even went so far as to build a set of custom 2.0-inch headers for the 5.4L, but don't ask them for a set for your GT500, as fitment has not been confirmed, and the company has no intention of doing them as regular production headers.

To maximize airflow, the forged rotating assembly was topped with a set of CNC-ported Ford GT heads.

The first order of business was to get the GT1000 motor assembled. Since the goal was 1,000 hp, the best way to achieve a specific power goal on a supercharged motor is to start with a powerful normally aspirated combination. To that end (and to shed a few unwanted GT500 pounds), the GT1000 started life with an aluminum Ford GT block.

The GT block was treated to precision machining and stuffed with an array of forged internals. The stock-stroke, steel crank was run with a set of forged rods and pistons to produce a seriously stout bottom end. Unlike the factory GT500 and Ford GT 5.4L motors, this GT1000 was equipped with high-compression pistons.

While Ford saw fit to run the static compression of its pair of wonder mods near 8:1, extra power was available by increasing the static compression to a full 10.0:1. In addition to the increase in power (roughly 3-4 percent per each point in compression), a 10:1 motor will get better fuel mileage than its 8:1 counterpart. Not that someone running a GT1000 is likely to worry about such things, but it's always nice when you can combine more power with better fuel efficiency. The short-block was finished off with a GT500 oil pump and pan assembly, and was ready for the power producers.

The real reason for the huge power gains offered by the headers was the wild cam timing. The combination of aggressive specs (lift and duration) and cam settings (significantly advanced) resulted in plenty of cam overlap. It was the overlap that produced dilution of the intake charge with the stock exhaust manifolds. Since there was no overlapping of the ports with the long-tube headers, the power was up significantly.

While the hike in compression was a surefire benefit, the GT1000 motor needed even more power (compared to the standard GT500 motor). With maximizing efficiency in mind, the cylinder heads were treated to full CNC porting. Impressive flowing right off the shelf, the GT500 heads were further improved with a serious complement of valves, springs and retainers to go along with the full CNC porting. Next up came the custom cam profiles that were responsible for the huge gains offered by the headers on this normally aspirated motor. Not only did they sport over 240 degrees of duration (Dynatek Racing was naturally hush-hush about the actual specs), but also more than their fair share of centerline advance.

The cams for the GT1000 motor were installed well advanced of the typical straight-up position. It was this combination of aggressive cam timing and placement (advance) that played havoc with the stock exhaust manifolds. Topping off the normally aspirated version of the GT1000 motor was (appropriately enough) the big daddy of all factory 5.4L intake manifolds, the Cobra R. Truth be told, we had only the bottom half of the Cobra R intake (borrowed from Accufab), which was topped by a fabricated aluminum upper plenum and flange to mount the inlet and throttle body from an '03 supercharged Cobra. It's not nearly as cool as a complete Cobra R, but we knew it had plenty of power potential.

Feeding the heads was a custom intake combination consisting of a Cobra R lower intake and fabricated upper plenum. The reason for all the monkey motion was that we didn't have access to the upper portion of the Cobra R intake assembly. (They're as rare as hen's teeth.)

Before getting to the impressive dyno results, we need to take a look at header theory, as headers do much more than just provide a flow path from the heads to the exhaust system. The first notion that needs to be dispelled is that power gains offered by headers come from improvements in the flow rate. The reality is that headers improve the power output of a motor through scavenging. We will go into detail on the different types of scavenging, but it's possible for long-tube headers to actually flow less than a set of stock exhaust manifolds and still offer substantial power improvements. From a flow standpoint, the longer the tube, the lower the flow rate. The increase in flow resistance or drop in flow rate comes from the increase in surface area exposed to the air stream.

Based solely on length, the short stock exhaust manifolds may offer improvements in airflow over a long-tube header. As it turns out, the absolute flow rate is also determined by the exit orifice and any irregularities in the internal flow passages. This means that if the exit of the stock exhaust manifold measures just two inches and the long-tube header has a 2½-inch collector, the smaller-diameter exit on the stock exhaust manifold may well be the restriction. This holds true for changes in direction and pinch points in the stock exhaust manifold designed to provide bolt access or gain clearance for some component in the engine compartment.

While absolute flow is important, the real power behind a long-tube header comes from the scavenging effect that not only improves exhaust flow out, but also enhances the flow of the intake tract into the combustion chamber. For our needs, we focus on two distinct forms of scavenging: the kinetic energy of outgoing gases and reflected pressure waves.

Next up was a set of larger 1 7/8-inch headers. The larger headers improved the peak power numbers to an even 500 hp at 6,500 rpm and 445 lb-ft at 5,200 rpm, but there was a slight loss in power below 5,000 rpm compared to the smaller 1-inch headers.

It can be argued that the most important mechanism for extracting residual exhaust gases in the combustion chamber comes from the kinetic energy of the outgoing gases. When the exhaust valve opens near the end of the power stroke, there is a sudden expulsion of high-pressure gases. This expulsion creates a pressure wave that travels outward through the exhaust pipe at the speed of sound. Actually, it must accelerate and decelerate, but we can use an average speed for this explanation. This pressure wave travels considerably faster than the outgoing gases pushed by the upward moving piston. On the front side of the pressure wave, there is a high-pressure area, but on the trailing side of the wave, there is a depression or low-pressure area. It's the low pressure created by the traveling pressure wave that helps scavenge exhaust out of and help improve intake flow into the combustion chamber. The critical element is that there needs to be sufficient tubing length to allow the pressure wave to leave behind the depression capable of extracting the stagnant gases. Conversely, if it's too long, excessive flow resistance will create backpressure that limits the scavenging process.

Installation of the headers did require notching the GT aluminum block, though the guys at American Racing insist they've run their headers on GT500s equipped with aluminum GT blocks.

In addition to the kinetic energy of the outgoing gases, reflected waves also help improve scavenging. We know from our discussion on kinetic energy that a pressure wave is released when the exhaust valve opens, and this pressure wave travels outward through the primary pipe of the header. We also know that the depression left behind helps improve exhaust scavenging, but the pressure wave is not actually finished when it leaves the end of the exhaust port and enters the collector. What happens when the high-pressure wave exits the end of the port is something called rarefaction. This technical term means that the high-pressure wave that was contained inside the pipe is allowed to expand rapidly in the collector. This expansion in all directions creates a depression (low-pressure area). The elasticity of the surrounding air will rebound toward this low-pressure area and reflect the negative (low-pressure wave) back toward the exhaust valve. Basically, a high-pressure wave is sent out and a low-pressure wave is reflected back. Time the arrival of this low-pressure wave correctly and you have additional exhaust gas scavenging, as well as improvements in intake flow into the combustion chamber.

How (may you ask) do you time the arrival of these pressure waves? The answer is with primary tube length. Since the pressure waves travel at the speed of sound, timing their arrival for optimum scavenging is based on the opening of the exhaust valve (relative to crank angle), the extent of the overlap period, and (most importantly) the length of the exhaust tract. It should be noted that the positive pressure wave is reflected back as a negative pressure wave. When the negative pressure wave arrives at the combustion chamber, it is again reflected back as a positive pressure wave, and then again as a negative pressure wave; so it goes until the next exhaust cycle.

Like the smaller headers, the 1 7/8-inch headers were run with a collector extension. The merge collector measured a full 3.5 inches on the 1 7/8-inch headers.

These multiple reflections naturally decrease in amplitude, so it's important to time the first reflected wave since it offers the lowest pressure at the combustion chamber. The ideal situation is to time the first reflected wave to arrive at combustion chamber when the piston has just passed TDC at the end of the exhaust stroke. This means the exhaust pressure wave must travel from the exhaust valve to the end of the primary tube (to the collector) and back during a crank interval of roughly 120 degrees. Timing this is not terribly difficult, but the difficulty is that if the scavenging effect is timed in this manner, it will be timed to be optimized at a given engine speed (much like runner length on an intake manifold). At other engine speeds (high or lower), the scavenging effect will be less pronounced. Therefore, it's necessary to compromise to provide a system that will provide power gains at a variety of engine speeds. Not surprisingly, a short stock exhaust manifold (whether tubular or cast) does not provide sufficient primary length to offer any scavenging effect at normal engine speeds.

With all the talk about changes in primary tubing length, what effect does changing the tubing diameter have? After all, we did test a variety of different diameter (and not length) headers. The discussion on runner length was primarily to distinguish the difference between a long-tube header and the short factory exhaust manifolds. Changing the tube diameter actually increases the surface area exposed to the exhaust gases, so there is an increase in surface friction on larger-diameter headers.

The 2.0-inch headers also featured 3.5-inch merge collectors

This is a nearly insignificant variable. The real effect of larger diameter tubing is to shift the optimized VE point. Basically, the larger diameter header (assuming not change in length) will want to make peak power (and torque) at a higher engine speed than the smaller one. The elevated engine speed will usually come with a drop in power elsewhere (lower in the rev range), but there are a number of other variables that determine what happens to the curve that our space here does not allow us to cover (books have been written on the subject). Much like an intake, the header configuration can be optimized for specific engine speeds, even on a given combination. The header can be tuned to maximize power lower or higher in the rev range.

The tuning effect is not huge, but it is definitely possible to optimize a combination for a given application using the exhaust system. In the case of our test motor, the change in primary diameter had much less effect than the difference between the long tube headers and the stock exhaust manifolds.

Enough theory--let's get on with the test. The normally aspirated GT1000 motor was installed on the engine dyno at Westech and made ready for testing with a FAST management system and a set of Autolite plugs (thanks to Accufab's John Mihovetz). Thanks also go out to Mark Sanchez for once again coming to the rescue with the necessary connectors and E-DIS module, without which there would have been no test.

The first order of business was to establish a baseline with the stock exhaust manifolds. The factory GT500 cast-iron manifolds were run with a set of 2.5-inch pipes to simulate a free-flowing after-cat exhaust. Naturally, no cats were run on the dyno during any of these tests.

Initially we were disappointed by the power output of the 5.4L. No matter what we did, we couldn't make any more than 366 hp and 364 lb-ft of torque. We changed to a different management system, swapped spark plugs, drained the oil, bled the lifters (thanks again Mihovetz), and even ran the motor without the upper plenum. Crazy Tom Habrzyk stood in the dyno room and held down the plenum while we went to full throttle. He then simply lifted the plenum off and allowed the motor to run at WOT as an individual-runner X-ram. This improved the output, but it was clearly not the answer to our missing power.

The 5.4L didn't respond well to the larger 2.0-inch headers, though they did manage to match the 500 hp produced by the 1 7/8-inch headers. Compared to the 1¾-inch headers, the larger 2.0-inch headers lost power all the way to 6,200 rpm. For power above 5,000 rpm, the 1 7/8-inch headers would be the way to go, but the 1¾-inch headers offered the best power below 5,000 rpm (where we spend most of our time driving). Next month, we'll put boost to the GT1000 and see how the headers perform on a supercharged application.

Before calling it quits, we decided to install the 1¾-inch headers from American Racing. All I can say is wow. After dialing in the fuel curve, the peak numbers jumped up to 492 hp and 442 lb-ft of torque. We were so stunned by the results that I immediately ordered the headers removed and the stock exhaust manifolds reinstalled and retested. Having never witnessed power gains like this from a simple header swap, we were convinced something else changed when we installed the headers. This allowed us to perform an A-B-A test that ensured the accuracy of our results.

After installing the exhaust manifolds, we were rewarded (or punished) with the very same 366 hp and 364 lb-ft of torque. Though the scavenging effect offered by long-tube headers is significant, this went well beyond a simple header swap. The reason for the huge change in power was the cam timing. The log-style factory exhaust manifolds allowed crossover between the cylinders during the overlap periods. The common log allowed significant dilution of the intake charge, which caused the very low power readings. Had we run even shorty headers in place of the stock exhaust manifolds, chances are the power would have been up significantly (not to match the long-tubes, but certainly better than the stock manifolds). You won't see this type of gain by adding headers to your normally aspirated 4.6L Cobra or 5.4L swap unless you combine the Cobra R intake (reflected waves from the induction system must work in conjunction with reflected waves from exhaust) and the type of wild cam timing used on the GT1000 motor. The significance of the intake manifold will come into play in Part 2, where we run the header test again after adding a supercharger.

Amazed by the results, we still had to run the remaining two sets of headers from American Racing. Equipped with the 1¾-inch headers, the 5.4L produced 492 hp at 6,500 rpm and 442 lb-ft of torque at 5,200 rpm. This compares to 366 hp at just 5,900 rpm and 364 lb-ft at 5,100 rpm for the stock manifolds.

Installation of the larger 1 7/8-inch headers on the normally aspirated 5.4L upped the peak power numbers to an even 500 hp at the same 6,500 rpm, while the torque peak was up to 445 lb-ft at 5,200 rpm. The theory was proven correct, as the larger-diameter tubing increased the power peak but traded some power below 5,000 rpm in the process. Compared to the 1 7/8-inch headers, the 1¾-inch versions offered an additional 10 lb-ft at 4,700 rpm.

The final test on this GT1000 motor was a set of custom 2.0-inch headers. Don't ask for a set for your GT500 as the 2.0-inch headers were done strictly for this test. Equipped with the 2.0-inch headers, the 5.4L produced the same 500 hp at 6,500 rpm, but just 442 lb-ft at 5,300 rpm. These 2.0-inch headers weren't ideal for this combination, as they didn't improve the peak power (compared to the 1 7/8-inch versions), they dropped the peak torque output by 3 ft-lb and lost significantly below 5,000 rpm (as much as 20 lb-ft compared to the 1¾-inch headers).

I wonder what's in store for us when we run the headers on the supercharged combination?

Power Horsepower Numbers
1¾-inch vs. 1 7⁄8-inch vs. 2.0-inch (Normally aspirated 5.4L)
RPM1¾-inch1 7⁄8-inch2.0-inch
3,{{{600}}}250249 253
3,800271268265
4,000302299294
4,200329324319
4,400341333325
4,600353344335
4,800379373366
5,000412411409
5,200438441438
5,400450454452
5,600458462458
5,800465468463
6,000474476470
6,200485488485
6,400492498497
6,600491497499
6,800485490492

As is evident from the power numbers, the 1 7/8-inch and 2.0-inch headers increased peak power slightly over the 1¾-inch version, but there was a trade off in power elsewhere in the curve. Compared to the 1¾-inch headers, the 1 7/8-inch versions offered less power below 5,000 rpm but more power from 5,000-6,800 rpm. The highest peak power was shared by the 1 7/8-inch and 2.0-inch headers, but the same 2.0-inch headers lost over 20 ft-lb down at 4,500. For most street applications, the smaller 1¾-inch headers would be best, but for those looking to maximize power past 5,000 rpm, the 1 7/8-inch headers would be the hot ticket. The 2.0-inch headers are simply too large for this normally aspirated 5.4L (both from a fitment and performance standpoint).