October 5, 2009

When the 30-year evolution of the late-model Mustang hobby is assessed solely from a bolt-on-parts perspective, there is no arguing that a lot of progress has been made over the course of those three decades. Although literally thousands of high-performance small-block (and modular) Ford engine parts have been developed and improved on as the years have passed, there are only a select few pieces that forever will be cited as essentials for making big steam with a Mustang's powerplant. (For the uninitiated, "big steam" is big horsepower.)

Ask any hard-core, performance-minded 'Stang enthusiast for a breakdown of engine upgrades that should be made over the course of a 5.0- or 4.6-powered Pony's lifetime, and we're pretty sure that aftermarket aluminum cylinder heads--with their larger ports, bigger intake and exhaust valves, and sometimes-radical combustion-chamber profiles--will rank near the top of the assessment. As we've explained in previous reports on engine performance, air volume is a critical piece in the big-steam puzzle. Building Mustang engines with larger cylinder-bore and crankshaft-stroke sizes is a common practice, as it enables a bullet to take in and compress a larger volume of air, and thus create more power and torque.

Although they're important, of course, heads don't do everything. The real deal is they work in conjunction with intake manifolds, valvetrain components, and camshafts to round out an engine's "top half," a collection of performance hardware that we've evaluated extensively on 5.0-liter engines over the last four years using products from various manufacturers. This report is the first offering in our series on aftermarket cylinder heads for 5.0-liter powerplants. We begin our review by featuring aluminum upgrade heads for 'Stangs that fall in the street category, meaning they provide marked horsepower improvement over stock, are smog-legal (if available), and are direct bolt-ons.

Heads in this class are affordable (and in most cases, usually available with provisions for pedestal- or stud-mount rocker arms for less than $1,500 a pair). They are perfect for 'Stangbangers seeking improved performance from the bone-stock engines in their Ponys without having to deal with deciding on camshafts, rocker-arm ratios, or other engine reconfiguring. Although, stepping up to aluminum rockers is highly recommended with any head upgrade. Also, non-Twisted-Wedge-style heads with 2.02-inch intake valves require pistons with deeper valve reliefs for clearance in their pursuit of more power.

Air Flow Research, Edelbrock, Floo Tek, Patriot Performance, and Trick Flow offer CNC-ported castings that we think fit nicely in the Street class. The details on these entry-level upgrade heads can be found in the sidebars and captions. (MSRP prices listed are per pair.)

Horse Sense: Despite the 5.0-liter Mustang's lineage that dates back to 1979 and carbureted induction, the EFI GTs and LXs of 1986 and later have been the Ponys we've focused on when it comes to detailing potential upgrades. While the cylinder-head options detailed in this first report of our three-part series are categorized as bolt-on, "street" heads that don't require elaborate internal changes for an engine, it's important to note that most of the heads presented in this story are not compatible with the OEM flat-top pistons in the 5.0-liter engines of '86 Mustang GTs (unless the pistons are notched for proper valve clearance), and are not recommended for engines with camshafts that have more than 0.550-inch lift.

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Supporting Cast
The days of finding absolutely, 100 percent bone-stock 5.0-powered Mustangs are pretty much gone (even though it still is pretty cool to come across super-rare, ultra stockers at car shows or waiting at red traffic signals with 80-year-old grandmas behind the wheel).

While most Fox-body and early SN-95 'Stangs are upgraded with (at least) cold-air-induction systems, underdrive pulleys and short-tube headers at this point, we strongly recommend adding an intake manifold, camshaft, rocker arms, timing gear, fuel injectors and mass air meter that are compatible with the cylinder heads you select.

We suggest you review past issues of 5.0 Mustang & Super Fords and check out our reports on several heads/camshaft/intake manifold combinations for 5.0-liter Ponys. Remember, selecting and installing heads and supporting components as a package is one of the best ways to ensure you'll see good performance results on the dyno, and, more importantly, on the street.

Ford Racing Performance Parts
Ford Racing's GT-40 series of small-block Ford cylinder heads has been around for what seems like, "forever," as we bet some of you can remember the original cast-iron heads that had us all going years ago. Today's X heads (PN M-6049-X306; 64cc chamber; 1.94-inch intake valve/PN M-6049-X307; 58cc chamber; 1.94-inch intake valve; $990.00) are improved versions of the original GT-40s, that flow approximately 240 cfm (intake) at 0.550-inch lift. The Turbo Swirl heads feature 1.94-inch intake valves and 1.54-inch exhaust valves, with port volumes of 178cc and 62cc on either side, and are 25 pounds lighter than the iron GT-40 castings.

Do You Know Flow?
Cylinder head flow numbers usually are (or end up being) the main point of discussion, whenever 'Stangbangers talk about heads in a comparative or inquisitive manner.

While the mantra "the heads flow X, at Y cfm of lift in 28 inches of water" usually is recited, many times the message sender and receiver are only familiar with the data on a surface level, having read the details in a magazine, and they don't understand what the numbers mean. When it comes to flow numbers, bigger certainly is better, for the most part, but it's important to understand exactly how the numbers are derived.

Flow benches are fairly elaborate test platforms that can measure the overall amount of airflow through a head's intake or exhaust ports. Basically, flow benches measure the total amount of cubic-feet-per-minute of air moving through the ports, when a specific amount of vacuum is generated, usually in 28 inches of water.

So, with the flow bench recognized as the best way to quantify just how well cylinder-head ports flow air and fuel, the question is how can flow numbers be used to determine the single value that most hard-core performance nuts are concerned about horsepower.

Occasional 5.0&SF contributor and dyno-tester extraordinaire Richard Holdener says the airflow/power formula [horsepower = max airflow (intake flow number) x 0.257 x number of cylinders] is one of the best tools for estimating the maximum-horsepower potential of a set of cylinder heads.

It's important to understand that the 0.257 "constant" in Richard's formula is derived from many years of evaluating heads on the flow bench, and it's used to predict what we consider "glory pull" horsepower (dyno), and not the type of number that is indicative of a daily driven, fully accessorized engine. So, while plugging the flow values (provided by cylinder-head manufacturers) into the formula may produce what seem like incredible power figures, those numbers, while still impressive and certainly more than the 329 maximum horsepower that factory E7 castings are capable of making, are a bit more inflated than what we believe would be the actual max horsepower when the engine is in full dress and/or installed in a Mustang.