Richard Holdener
January 1, 2009
For our street motor, we selected an Xtreme Energy XE274HR cam, with a 0.555/0.565 lift split, a 224/232 duration split and a 112-degree lobe separation angle.

The mathematical equation for horsepower and torque is HP = TQ x RPM/5,252. What makes this equation so interesting and applicable to the choice of induction system is that it's possible for a motor equipped with a single-plane intake to produce the same peak torque figure as the same combination equipped with a dual-plane intake, yet still offer more peak power. How can that be, you ask?

Horsepower and torque are related and that relationship is based on engine speed. Given our formula for horsepower (HP = TQ x RPM/5,252), we see that horsepower is simply a function of the torque production at a given engine speed (rpm). Thus, we can have two motors that both produce 350 lb-ft of torque, but do so at different engine speeds. If the dual-plane intake combination produced the peak torque of 350 lb-ft at 4,500 rpm, the power output at that engine speed would equate to 300 hp. By comparison, the single-plane intake combination might produce peak torque of 350 lb-ft at 5,000 rpm, which would equate to 333 hp. According to this formula, it's evident there are two ways to improve horsepower. The first way is to increase the torque production, but the second (and focus of this intake comparison) is to increase the engine speed where peak torque occurs. As the power curve is raised in the rpm band, however, the vehicle will require more gear (numerically) and a looser torque converter in order to let the engine reach its powerband efficiently.

Topping off the 306 was a set of AFR 185 heads. The CNC-ported heads ensured plenty of airflow. What we like about the Airflow Research heads is that the airflow numbers always translate directly into power numbers. The AFR 185s are sized perfectly for a healthy 306.

Mathematically, we demonstrated that it's possible to increase the power output without increasing the peak torque output simply by increasing the engine speed where peak torque occurs. It's also possible to increase the power output by simply increasing the torque output without any change in engine speed.

Another example works well here to illustrate this point. Suppose you have a 302 Windsor motor that produces a peak of 300 hp at 5,500 rpm. Using our formula, and a little cross-multiplication, we see that our peak power output of 300 hp is equal to 286 lb-ft of torque (at 5,500 rpm). This was calculated by multiplying the horsepower value of 300 times 5,252, then dividing by the rpm (5,500). If we somehow improve the torque output (using an increase in compression, a larger carburetor, or improved cylinder heads), it will result in an increase in horsepower. Plugging 296 lb-ft into the equation, we see that Hp=296 x 5,500 rpm/5,252. The increase in 10 lb-ft resulted in a gain of right at 10 hp. Playing with the formula at different engine speeds will illustrate that changes in the torque output will have a greater effect on power above 5,252 rpm.

Comp Cams also supplied a set of 1.6 ratio roller rockers for our AFR-headed test motor.

Shifting the torque curve seems easy on paper, and as luck would have it, it's not terribly difficult in the real world. In fact, it's a simple matter of installing a single-plane intake in place of a dual-plane manifold.

This test illustrates the changes in the power curves offered by both intake designs. For our needs, we chose an Edelbrock Victor Jr. and Performer RPM as our single- and dual-plane intakes. In theory, and in practical application, short-runner intakes are efficient at filling the cylinders at high rpm, while long-runner intakes provide better (more efficient) fill at lower rpm. Dual-plane and long-runner EFI intakes will normally have much longer runners than single-plane and short-runner intakes. Picking the right one is critical in determining where (at what rpm) the engine will make the most power. Knowing this will help you select your gears, tire size, torque converter/clutch, and so on. It all has to do with ram-tuning and the velocity of the column of air and gas as it travels down the intake path toward the cylinder head and the open intake valve.