Richard Holdener
March 27, 2019

After the performance depression of the 1970s, which was caused by tightening emissions and fuel mileage concerns, the advent of the fuel-injected 5.0L (302ci) small-block Ford restarted a performance revolution that continues to this day. Some may argue that the mod motors and current Coyote are in a class by themselves compared to the first fuelie Ford, but it was that original 5.0L that got the performance ball rolling again. That the Fox Mustang continues to be the platform of choice for all manner of street, strip, and drag race vehicles is a testament to the original design and popularity. The reason for the prodigious popularity, both then and now, is the number of available performance upgrades and how well the little Fox responds to them.

Compared to the 400hp-plus muscle mod motors of today, the 225hp, fuel-injected 302s seem a little anemic, but the Fox Fords made up for the lackluster stock output with potential. Coaxing extra power from the 5.0L was a simple matter of replacing the restrictive cylinder heads, cam, and intake manifold. Of course, there was always boost from a supercharger or nitrous oxide, but even basic bolt-ons went a long way toward improving the factory power output. Endless articles, and even books, have been written on upgrading the Windsor engines, but despite the tireless testing and influx of information, there is still a great deal of confusion about how well individual upgrades actually work. In truth, this test is less about the actual cam and cylinder head upgrades as it is about the order in which they were performed. When it comes to dyno-testing things like heads, cam, and intakes, we all want clear-cut, black-and-white answers as to just how much power each component is worth. Nothing provides that information better than direct back-to-back testing, but there is more to the equation than the results of any single test might suggest.

What do we mean when we say there is more to the performance equation than test results? Well, the power gains offered by any individual component (heads, cam, or intake) are a function of not only the test engine itself but the order in which they are tested (if testing multiple components individually).

Suppose we upgrade the cylinder heads on an otherwise stock 5.0L Ford engine. Even if we install a set of cylinder heads with airflow numbers that double the factory flow rate (155 cfm), the head swap will only offer minimal gains because the test engine does not have enough camshaft (or intake manifold) to allow the engine to take full advantage of all that extra flow. It doesn’t make any difference that your CNC-ported heads flow enough to support over 600 hp when the rest of the test engine struggles to produce 300 hp. The same can be said of a cam or intake, as the power output of any engine is a function of the combination as opposed to any individual component. Were we to upgrade the camshaft after installation of the heads, the cam would show impressive gains, but only because the cylinder heads paved the way.

This point was illustrated recently when we ran a test on a 306 (0.030-over 302). The test mule featured a factory 5.0L hydraulic roller block equipped with a forged steel crank from RPM, a set of forged (5.40-inch) rods from Speedmaster, and forged flat-top pistons from Probe Racing. It is common knowledge that the stock block becomes the limiting factor in terms of absolute power production, but we planned to get nowhere near the limit with this mild combination. When combined with the stock 61cc chambers in the factory E7TE heads, the flat-top pistons produced a static compression ratio of 9.35:1. To establish a baseline, the factory heads were combined with a stock 5.0L cam, a GT-40 intake, and Accufab throttle body. The test engine was also equipped with a set of Hooker 1¾-inch long-tube Fox-chassis headers, a Meziere electric water pump (for ease of dyno testing), and a FAST XFI management system. Sufficient spark was supplied by an MSD billet distributor and 6AL ignition amplifier, while additional niceties included Lucas 5W-30 synthetic oil and a set of FAST 36 lb/hr injectors. The stock iron heads had been previously modified to accept a set of screw-in rocker studs, which allowed installation of a set of 1.6:1 ratio, guided (self-aligning) roller rockers. The fortified 5.0L also featured a double-roller timing chain, hydraulic roller lifters, and 6.25-inch hardened pushrods courtesy of Comp Cams. Though the stock heads had been upgraded with springs and rockers, the ports, chambers, and valves all remained stock.

Other than the GT-40 intake and long-tube headers, our test engine was essentially a stock 5.0L with a forged internals. After dialing in the FAST XFI management system, the 0.030-over 5.0L produced peak numbers of 279 hp at 5,300 rpm and 323 lb-ft of torque at 3,900 rpm. As we have come to expect from the stock stuff, the H.O. engine produced plenty of torque, but it was decidedly lacking in terms of power production. We hoped to rectify that situation by upgrading the cam and cylinder heads, starting with the camshaft. As indicated, we had previously upgraded the valve springs that allowed installation of an XE274HR cam. The Comp Xtreme Energy grind (PN 35-518-8) offered a healthy 0.555/0.565 split lift, a 224/232-degree split duration, and 112-degree lobe separation angle. We have used this cam successfully on NA, blower, and turbo 5.0L applications. After installation of the new cam, the power output of the 5.0L jumped to 312 hp at 5,400 rpm and 349 lb-ft of torque at 4,000 rpm. The cam improved the power output of the 5.0L combination through the entire rev range, but we have seen even greater gains offered by this cam on wilder combinations with better cylinder heads and intakes.

To find out if the heads were holding back the combination, we replaced the stock E7TE heads with a set of RHS Pro Elite 205 heads. As the name suggests, the CNC-ported RHS heads feature 205cc intake ports, a 2.055/1.60 stainless valve package, and 62cc combustion chambers. The heads flow numbers are capable of supporting nearly 600 hp, or more than enough for mild little 5.0L. After installation of the RHS heads, the power output of the 5.0L jumped to 351 hp at 5,700 rpm and 378 lb-ft of torque at 4,300 rpm. As with the cam upgrade, the heads improved the power output from 3,000-6,000 rpm—always a good sign. Given the power potential of the RHS heads and Comp cam, we know the GT-40 intake was now the bottleneck, as additional testing with a new intake would push this combination near 400 hp. If we look at the results of this test, we see that the cam was worth 33 hp while the heads were worth 39 hp, but is that the whole story?

The reality is that the gains offered by the cam were limited by the stock heads and the gains offered by the heads were limited by the GT-40 intake. We have seen cam upgrades (like our XE274HR) add over 50 hp and head swaps (like the RHS Elites) be worth as much as 100 hp on the right application. The takeaway from this test is that the gains offered by each individual component vary not only with the combination, but also with the order of installation.

Graph 1: Stock vs. Comp XE274HR Cam
Replacing the stock 5.0L cam with the Comp XE274HR cam was worth a sizable jump in power. Despite the restrictive, stock E7TE heads and GT-40 intake, the cam upgrade improved the power output of the 5.0L from 279 hp at 5,300 rpm and 323 lb-ft of torque at 3,900 rpm to 312 hp at 5,400 rpm and 349 lb-ft at 4,000 rpm. The cam upgrade improved the power output of the combination from 3,000-6,000 rpm with gains increasing with engine speed. Had the engine been equipped with something other than the stock heads, the gains from the cam would be even greater. Had the cam been installed after the RHS heads, the gains from the cam would be greater than those from the head swap.

Graph 2: Stock vs. RHS Pro Elite Heads
After upgrading the cam, we decided to try new cylinder heads. Off came the stock E7TE iron heads and on went a set of CNC-ported RHS Pro Elite heads. Despite being choked off by the GT-40 intake, the head swap improved the power output from 312 hp at 5,400 rpm and 349 lb-ft of torque at 4,000 rpm to 351 hp at 5,700 rpm and 378 lb-ft of torque at 4,300 rpm. Though the heads increased the engine speed where the engine made peak power and torque, the swap once again improved the power output from 3,000-6,000 rpm. After replacing the heads and cam, the bottleneck in the system was now the GT-40 intake.

What is worth more power on your 5.0L, heads or camshaft?
Since we planned on subjecting this 5.0L to extensive testing, we upgraded the internals with a 4340 forged-steel RPM crank and matching 5.4-inch connecting rods from Speedmaster.
Replacing the stock slugs was a set of 0.030-over, forged, flat-tops from Probe Racing. The aftermarket pistons featured generous valve reliefs to allow for our Comp Xtreme Energy cam upgrade.
Getting things started was a stock 5.0L camshaft. Hardly the cam dreams are made of, but it did provide a jumping-off point to start our test.
In addition to our cam upgrade, Comp Cams also supplied a new set of hydraulic roller lifters.
Working with the cam and lifters was a double-roller timing chain from Comp Cams.
To facilitate our cam test, the stock 5.0L heads were modified with a spring upgrade, screw-in rocker studs, and guided 1.6-ratio roller rockers.
We are not sure how many GT-40 intakes Ford sold, but we know it was a lot, and they were a very popular upgrade to the 5.0L back in the 1990s and early 2000s. Feeling a touch nostalgic, we installed one on our test engine.
To ensure plenty of air to the 5.0L, we equipped with GT-40 intake with a billet 65mm throttle body from AccuFab.
Knowing we would exceed the power potential of the stock 19–lb/hr injectors, we installed a set of larger 36-pounders from FAST. The EFI engine was dialed in using a Fast XFI management system.
Run with the stock 5.0L heads, stock cam, GT-40 intake, and Hooker long tube headers, the stock(ish) 5.0L produced peak numbers of 279 hp and 323 lb-ft.
The first upgrade for the 5.0L was a Comp XE274HR cam. Healthy but streetable, the Comp cam offered a 0.555/0.565 split lift, a 224/232 split duration, and a 112-degree lobe separation angle. Run with the new Comp cam, the power output jumped from 279 hp and 323 lb-ft to 312 hp and 349 lb-ft. The Comp cam upgrade improved power everywhere!
Knowing the stock iron heads were less than ideal, off they came!
Replacing the stock cylinder heads was a set of CNC-ported RHS Pro Elite heads. Compared to the stockers, the RHS heads offer aluminum construction, CNC porting, and a spring package that was more than adequate for our XE274HR cam.
For this mild 5.0L, we selected the smaller 205cc Pro Elite heads. According to RHS, the intake ports on the 205 heads flow nearly 300 cfm, enough to support over 600 hp on the right application. That’s a little bit more power than usually will split a stock 5.0L cylinder block!
We replaced the guided rockers employed on the stock heads with a set of 1.6-ratio, aluminum roller rockers from Comp Cams.
After installation of the RHS heads, the power output jumped from 312 hp and 349 lb-ft to 351 hp and 378 lb-ft, but the mild GT-40 intake was now holding back the power potential of the heads. The gains offered by any single component are always a function of the rest of the combination!