Words And Photos: Richard Holdener

Throttle bodies are simple devices, allowing airflow into the motor. If the throttle body is insufficient in size and/or flow for the needs of the motor, then additional power can be had from increasing the size and/or flow rate. This is true of any motor using a throttle body, including the 4.6L two-valve (2V) used for this series of tests. Where things get slightly more complicated is the inlet elbow (sometimes referred to as an upper intake manifold), as flow through the elbow must make a turn into the lower intake manifold. A straight shot through the throttle body is one thing but asking air to make a turn adds another dimension. Increasing the flow rate of a throttle body can be as simple as making it larger, though the Accufab engineers put considerably more work than sheer size in their throttle bodies. When it comes to the inlet elbow, simply bigger is not the same as better. Now add boost to the mix and the testing starts to get very interesting.

Rather than run a simple test on a stock 2V motor, we decided to step things up by running our comparison on a modified motor that could take full advantage of the additional airflow offered by the larger throttle body and redesigned inlet elbow. Not content with running the test on the normally aspirated motor, we decided to kick the test up a notch by adding boost to the equation. If the throttle body and elbow represented a restriction on the normally aspirated combination, would blowing through the induction system eliminate said restriction? Only one way to find out! The 4.6L 2V test motor combined a 1998 (non-PI) short block with a set of CNC-ported PI heads from Total Engine Airflow, a pair of COMP XE274H cams and topped with the stock PI intake manifold. To this we added Fel Pro head gaskets, ARP head studs and long-tube Hooker heads. Since tuning was critical, we ran the 42-pound injectors motor using the FAST XFI management system.

The first step was to run the motor with the stock throttle body and inlet elbow in normally aspirated trim. Equipped with the stock components, the modified PI hybrid motor produced 393 hp at 6,000 rpm and 383 lb-ft of torque at 4,800 rpm. After replacing the stock components with the 75mm versions from Accufab, the peak numbers jumped to 406 hp at 6,000 rpm and 390 lb-ft of torque at 4,700 rpm. The 75mm Accufab throttle body and elbow were worth 13 hp on this modified motor and those gains would only increase with a wilder and/or larger combination. This is also true of a draw-through, supercharged combination (like a Kenne Bell blower).

Now the question was would the factory components still represent a restriction in a blow-though, boosted application? Testing with the (Aftercooled) Vortech supercharger system, the stock throttle body and elbow produced 614 hp at 6,100 rpm and 539 lb-ft of torque at 5,600 rpm. Stepping up to the 75mm Accufab induction system increased the peak numbers to 629 hp and 553 lb-ft of torque. Though we limited our testing to 6,100 rpm, the power was increasing with engine speed and so too were the relative power gains with the Accufab components. I guess it is safe to say that whether normally aspirated or supercharged, the power gains from throttle bodies in motion tend to stay in motion.

As we would expect of changes in airflow, the power gains increased with engine speed. Replacing the stock 4.6L 2V throttle body and elbow with the 75mm Accufab units resulted in sizable power gains, increasing the output of the 2V motor from 393 hp and 383 lb-ft of torque to 406 hp and 393 lb-ft of torque. What this 2V Mod motor really needed now was a decent intake manifold.

We wondered if the throttle body and elbow would represent a restriction when blowing through them with a centrifugal supercharger. Adding the Vortech supercharger to the Ford mod motor resulted in some serious power gains (229 hp). Running the stock and Accufab components once again on the supercharged combination revealed that the gains offered in normally aspirated trim continued under boost. Run with the stock induction, the supercharged 4.6L produced 614 hp and 539 lb-ft of torque. These numbers jumped to 629 hp and 553 lb-ft of torque with the Accufab components.

Sources

Accufab
accufabracing.com

COMP Cams
compcams.com

FAST
fuelairspark.com

Lucas Oil
lucasoil.com

Total Engine Airflow
totalengineairflow.com

Vortech Superchargers
vortechsuperchargers.com