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.
Intake Test: Stock Pi Vs....
Intake Test: Stock Pi Vs. Reichard Racing
Were You to just look at the peak numbers offered by this comparison, the clear winner would be the intake manifold from Reichard Racing, thanks to its additional 15-17 hp. If you take a closer look at the curve, you'll see that the short-runner intake from Reichard Racing only offered more power than the stock PI intake once the engine speed exceeded 6,200 rpm. From 3,000 rpm (and below, had we tested there) to 6,200 rpm, the stock PI intake offered significantly more power and torque. For street use, always choose the intake that offers the most average power.
A Set of quality long-tube...
A Set of quality long-tube headers will really wake up a GT motor. These headers from Hooker increased the power output by over 30 hp on our test motor.
Increasing The static compression...
Increasing The static compression is always worth additional power. This can be accomplished by installing the PI heads on a non-PI short-block or by installing a new set of forged pistons.
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.
Porting The PI heads will...
Porting The PI heads will unearth additional power. Adding massaged PI heads and matching intake from Total Engine Airflow resulted in a gain of nearly 30-70 hp over the non-PI components.
The 4.6l GT motors respond...
The 4.6l GT motors respond very well to changes in cam timing. A number of manufacturers offer performance cam grinds. The more aggressive profiles will likely suffer losses lower in the rev range, but look for gains of 30 hp or more over the stock cams at peak rpm.
Naturally, The stock exhaust...
Naturally, The stock exhaust manifolds limit power production. The major culprit is not actually flow, but the short primary length.