Richard Holdener
August 1, 2008

With The modular Mustang world chock-full of Two-Valve motors, it's only natural that Mustang owners figure out the best way to modify them for increased performance. While it's true that the Four-Valve motors and big-inch Windsors ultimately offer more power potential, don't discount the Two-Valve motors, as they have a lot of things going for them, the most obvious being price and availability.

Production figures suggest that as many as 10 Two-Valve GTs were produced for every Four-Valve Cobra. Toss in the tremendous production of 4.6L and 5.4L truck motors, and you have an avalanche of Two-Valve motors from which to pick parts. Add to this equation that most '96-'04 Mustangs still on the road are housing Two-Valve motors, and it's easy to understand the appeal.

Lurking Inside Any stock 4.6L GT motor is a performance combination just waiting to be unleashed.

Given its continued popularity, we here at MM&FF decided to investigate the inner workings of the Two-Valve motor and offer some insight into its performance potential.There's always forced induction and nitrous oxide, but rather than take the easy way out, we decided to cater this discussion to the all-motor crowd, since that makes up the majority.

Though the topic is power production in normally aspirated trim, remember that all of this information will transfer to any buildup involving forced induction, as the best blower and turbo motors always start out with efficient, normally aspirated combinations. Additionally, what you'll learn will also translate to just about any internal combustion engine.

By Subjecting countless combinations and components to the rigors of the dyno, we've taken all the guesswork out of building a healthy Two-Valve combination.

Induction Function
For this all-motor discussion, we will begin with the throttle body and work our way back to the exhaust. Along the way, we'll cover intake manifolds, ported cylinder heads, and even displacement. Cam timing will also be covered, as well as compression and the scavenging effect of a set of headers.

In truth, the induction system begins with the air filter and includes the mass air meter and associated plumbing. When it comes to the induction system in front of the throttle body, obviously it's important to minimize any airflow restrictions. These include the filter itself, the mass air meter, and the induction tubing that joins the MAF to the throttle body. Gentle radius bends are important, not only for absolute airflow but also to minimize turbulence in and around the mass air meter. The orientation of the mass air meter relative to the flow is important, as air is not distributed evenly in a bend. If the bend precedes the mass air meter, then the meter will read only a portion of the actual airflow. If the element is positioned toward the short-turn radius, then the meter will read much lower than if it were positioned toward the long-turn (where a larger percentage of the airflow will be). Simply rotating the meter can alter the signal and therefore the air/fuel and timing specs of the motor. This can obviously have a dramatic effect on the power output and longevity of the motor. Meter orientation can be turned around using current SCT or other available software. Naturally the air-intake system should be designed to supply a source of cold(er) ambient air rather than running an open filter in the engine compartment or the more-restrictive stock airbox.

Running through the intake system, the inlet air will then come to the throttle body and intake manifold. As with the mass air meter, the throttle body should be sized to eliminate airflow restrictions, but not too big where it will cause a loss in intake air velocity, thus resulting in poor throttle response. For most normally aspirated Two-Valve combinations, the typical aftermarket 70-75mm throttle body and elbow combination should work well. There's a great deal of confusion about throttle bodies, however, as bigger is not always better.

If you have a stock throttle body that flows 600 cfm and a 75mm throttle body that flows 750 cfm, shouldn't the motor make more power with the larger throttle body? The answer is-it depends. If the stock throttle body doesn't represent an airflow restriction at the given power level, then it's likely the motor won't make any more power with the larger throttle body. The motor simply can't take advantage of the additional airflow offered by the larger throttle body. In contrast, a larger throttle body may feel like it has improved throttle response, but this is a simple function of the airflow versus throttle angle. If you open the throttle just 25 percent, the larger throttle body will offer more flow than the stock throttle body, so the motor will make more power at any given throttle angle-except wide-open throttle. This perceived power gain is what sells a great many larger throttle bodies to otherwise stock motors. On modified motors, larger throttle bodies have been shown to improve power-just don't expect huge gains on your stock motor.

The throttle body provides airflow to one of the most misunderstood elements in the internal combustion engine, the intake manifold. Many enthusiasts mistakenly lump the intake manifold in the same airflow-device category as the throttle body, that is to say that more airflow is better. In truth, the intake manifold is not a simple airflow device but rather a sophisticated resonance tuning system that all but determines the effective operating rpm range of the motor. Runner cross section and length dictate where the engine will make power.

More than any other single component, the intake manifold determines not only the peak power potential, but the overall shape of the power curve as well. At the risk of oversimplification, intake manifolds are designed to produce efficient power at a given rpm range, with longer runners promoting power lower in the rev range than shorter ones (of the same cross section). What this means is just because one intake outflows another, it doesn't mean it's "tuned" to produce efficient power in the desired rpm range, nor does it mean it will even produce more power on your combination.

This is why most ultra-short runner intakes don't fair well in back-to-back comparisons with the stock PI manifold. The short runners were designed to optimize power high in the rev range, and the stock manifold has long runners for good low- and midrange power. While we may see elevated peak power numbers well beyond 6,000 rpm, the loss in low-speed and midrange power is usually not worth the trade-off, especially for a street motor. Remember, with the 4.6 we only need to fill 281 ci-not 302, 347, or even 408-so you don't need a huge manifold. In the case of the 4.6, the runners are especially long, and for the most part, they need to stay that way in a naturally aspirated 4.6-5.0 build, especially if the car is street driven.

Next in line are the cylinder heads. It's certainly true that the later Power Improved (PI) heads are better than their non-PI counterparts, but it's the head flow (or lack thereof) that really differentiates the Four-Valve motors from their Two-Valve brethren. Cylinder-head flow is a critical element in ultimate power production. Obviously, port volume and the attending air speed (velocity) come into play, but the airflow capacity of the Two-Valve head is the limiting factor in terms of ultimate performance.

Porting the stock PI heads is one way to greatly improve the power potential, but even in ported form, the best 4.6L Two-Valve heads flow less than an as-cast set of Trick Flow Twisted Wedge heads for a 5.0L application. With head flow at less than 250 cfm, there's only so much power to be had from even a wild Two-Valve combination. Our typical trick of shifting the torque curve to increase horsepower production at a higher engine speed is difficult if we are limited by head flow. While more power requires more airflow, producing the same peak power number at a higher engine speed also requires more airflow. Thus, the flow rate of the PI heads becomes the limiting factor in terms of peak power and engine speed on a Two-Valve application. As we will discuss shortly, one way to overcome this is to increase the displacement and lower the engine speed.

Camshafts Can Do It
Before getting to changes in displacement, we need to discuss Two-Valve cam timing. As with the cylinder heads, the early and late 4.6L Two-Valve (PI and non-PI) motors also differ in their respective cam profiles. Where the early non-PI motors rely on just 0.500-inch lift cams, the later PI heads will accept higher 0.550-inch lift cams. The additional lift improves average airflow past the valve, as flow numbers on the ported PI heads increase with lift. The higher lift values also increase the ramp (or opening) rate of the cams. That is to say that two cams with identical duration figures will differ in their ramp rates with changes in lift. The higher the lift, the more aggressive the ramp rate. The increased opening rate is what makes the Xtreme Energy cams from Comp Cams so popular and powerful. Other cam manufacturers also offer fast-ramp cam profiles designed to enhance power production.

As previously indicated, the flow rate of the heads will ultimately limit cam selection, as there's no sense in running a Two-Valve motor to something like 8,000 rpm, since the head flow will not support the power output at this elevated engine speed. Thus, we see cam duration (measured at 0.050) kept below 248 degrees for most high-performance (normally aspirated) street applications. Despite this limitation, adding a set of cams to your Two-Valve motor can be worth 30-40 hp (even more at the top of the rev range).

Cubes Count
With the head flow limiting engine speed and power potential, one way to improve the situation is to increase displacement. For the sake of argument, let's say that the head flow limits normally aspirated power production to 450 hp at 7,000 rpm on our hypothetical 4.6L Two-Valve motor. Assuming that 450 hp is the absolute power limit on the 4.6L, we then decide to increase displacement. Using basic math skills, we see that our 450 hp 4.6L achieved a specific output of roughly 98 hp per liter. If we apply that number to a 5.0L combination, we see that the new peak output should jump to 490 hp, but remember that we said the head flow was limiting the power output of the 4.6L to just 450 hp. The two-fold benefit of having the larger displacement is that we achieve both a higher average power output (more power throughout the rev range) and a potentially higher peak number based on the combination of specific output and engine speed. You see, where the 4.6L needed to rev to 7,000 rpm to achieve the peak power, the larger-displacement 5.0L stroker motor will make its peak power at a lower engine speed (assuming the same cam timing and intake design). This drop in engine speed will reduce the airflow requirement to reach a given power output and make it more streetable. Thus, a larger motor is able to make more power at a lower engine speed if limited by airflow.

Displacement Test-4.6L VS. 5.0L
This Test illustrates the gains offered by increasing the displacement from 4.6 liters to 5.0 liters. In truth, the 5.0L combination benefited from a slightly higher static-compression ratio as well as cam timing that offered 4 degrees of duration, but the difference in power is interesting between the two displacements. Note that the 5.0L combination improved the power output throughout the rev range. The power gains cannot be attributed to the compression or cam timing, as bigger motors simply make more power and torque.

Another area that can be exploited to enhance the power output of any Two-Valve combination is compression ratio. This is one of the reasons for the popularity and effectiveness of swapping PI heads onto an early non-PI short-block. The difference in the combustion chamber size between the two heads (51 cc vs. 42 cc) means the swap will offer a significant jump in static compression ratio (over one full point). So, in addition to the flow improvements offered by the PI heads, installing them on the early short-block also improves performance by upping the static compression ratio by roughly 1.3 points. Conventional wisdom (and dedicated testing) suggests that every point in static compression will increase the power output (throughout the rev range) by roughly 4 percent, thus the extra compression can be worth as much as 20 hp on a 400hp Two-Valve combination.

Testing for my book Modular Performance (Building 4.6L/5.4L Ford Horsepower on the Dyno) demonstrated that installing a set of ported PI heads (ours came from Total Engine Airflow) and a matching PI intake on an early non-PI short-block was worth over 90 hp. Imagine that; swap out the stock non-PI top end for a set of properly ported PI cylinder heads and the matching intake manifold, and you've got yourself an extra 90 hp. That's like adding a blower or nitrous, but with fewer hassles.

There are, of course, additional avenues for power, including the exhaust system. As with the induction system, exhaust systems are often misunderstood as well. Everyone's mindset seems to be focused on increasing exhaust flow, but like the intake manifold, (long-tube) headers were designed not solely with flow in mind, but with pulse tuning as well.

Headers operate on the same principles as the intake manifold, only in reverse. The reflected resonance waves present in the exhaust system provide negative pressure waves to help scavenge the exhaust from the cylinders, which in turn helps draw in more intake charge. This scavenging effect is determined by the length and diameter of the primary tubing being used, as well as merge points for Tri-Y designs. Just like the intake manifold, headers are optimized for particular engine speeds and displacement, but since the start of the reflected wave is determined by the exhaust opening, the effectiveness of any header design is dependent on the cam timing as well. What this means is that there's no one design that is universally correct for all applications. Change just one variable in your engine combination, and you might well have changed the "ideal" header requirements. Aft of the headers, the exhaust system is merely a subjective combination of flow and noise quality, though x pipe systems can further enhance the power and aural gratification of any Two-Valve combination.

In addition to the stars of our "Bigger is Better" gala, there's also the supporting cast. Things such as underdrive pulleys, electric water pumps, and synthetic oil can all improve the power output. Unlike performance intake manifolds and camshafts, underdrive pulleys don't actually produce power so much as they reduce parasitic drag and power losses associated with driving the accessories. Also, be sure to gear your car correctly so the engine spends the bulk of its time in the proper rpm range for the type of driving you do, and of course, be sure to have a custom tune when looking for maximum power. All the parts in the world won't be worth a darn if the air/fuel ratio or ignition timing maps are off.

A good performance 4.6L Two-Valve buildup deserves nothing less.