Richard Holdener
January 1, 2009
What's the best intake for your engine, single- (left) or dual-plane? The intake manifold has a dramatic effect on the shape of the power curve.

One of the many choices you're forced to make when building your dream motor is selecting your intake manifold. No big deal, you say-how difficult is it to select the induction, and just how important is it anyway? Won't just any old intake do the trick and allow the fuel and air to get into the combustion chamber? Besides, how many different combinations can there be for a 302- (or 351W) based combination?

Naturally, your search will include leafing through the pages of MM&FF, as well as some time spent on the Internet, to find a suitable candidate for your carbureted or injected Ford. What your exhaustive search will unearth is that there are many different intake manifold choices for your little Windsor. Everything from stock cast-iron two-barrel manifolds to ultra-exotic individual-runner intakes are there for the taking.

To illustrate the effect of the changes to the power curve, we assembled a 306 test motor. The short-block came from Coast High Performance.

While it's difficult to rule out all possibilities, most carbureted motors will run one of two different intake designs: the single-plane or dual-plane manifold. For EFI users, the choices really boil down to long-, medium-, or short-runner intakes. There are IR (individual-runner) intakes, but they're rarely used in street applications.

For the purpose of this article, we narrowed the search to single- and dual-plane carbureted intakes. However, you can apply the same theory to EFI intakes, as a single-plane intake will act as a short-runner unit, and dual-plane intakes will have characteristics of a long-runner unit. Remember, though, there are still many different manufacturers to choose from with different runner shapes and sizes, but for the most part, the theory, as it relates to runner length, remains the same. The problem now becomes, which is the best manifold for your specific application?

Your specific combination and intended use will naturally dictate the ideal intake manifold for your engine. In truth, a little forethought on your part should go into the manifold selection before the combination actually goes together. The manifold should be chosen to work in conjunction with the camshaft profile and cylinder-head flow in the desired rpm range-the key being the desired rpm range. A street motor will require a different manifold, cam, and heads than a dedicated race motor. Your manifold choice often makes a dramatic difference in the power curve of the motor. Choosing to top your street motor with a dual-carb tunnel ram will look cool, but likely be an exercise in frustration and poor driveability. Likewise, limiting your race motor with a two-barrel intake will not likely produce the desired results.

The overall power production of the motor is a function of the heads, cam, and intake you choose.

The reason behind the importance of selecting the right induction system is that it helps dictate the effective operating range of the motor. That is to say, a motor equipped with a single-plane intake will be optimized at a different engine speed than its dual-plane counter part.

One of the biggest mistakes made when selecting an intake manifold is choosing it based solely on peak power numbers. Generally speaking, a performance motor will produce a higher peak power number with a single-plane intake (or a short-runner EFI intake) than a dual-plane (or long-runner type), but the extra power at the top of the rev range often comes with a penalty in the low- and middle-rpm ranges. This loss in torque production is especially important for a daily driver, as the loss in midrange torque also means a loss in torque at part-throttle applications. Despite all peak power numbers, what makes a street car fun is tip-in throttle response. You'd hardly notice 20 less horsepower at 6,000 rpm, but you'd feel the difference if you lost 20 lb-ft of torque at 2,800 rpm.

The fact is, enthusiasts spend much more time running at cruise and part throttle than they do running full throttle (and high rpm). This means the benefits of the extra torque production offered in the low and medium engine speeds can be enjoyed much more often than the peak power gains offered by the single-plane intake.

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.

Hooker supplied the necessary long-tube Super Comp headers for our test. All the airflow into the motor must also find its way out.

The test motor was a bored 302 short-block from Coast High Performance equipped with an XE274HR Xtreme Energy (hydraulic roller) cam. Ensuring plenty of power was a set of 185 heads from the flow technicians at Airflow Research (AFR). The CNC porting offered by the AFR 185 worked well with the combination of cubic inches (306), cam timing, and compression of 9.8:1. Also present was a set of Hooker Super Comp headers, an MSD ignition, and Holley 750 HP carburetor. As indicated earlier, the combination is important, as the induction system must be chosen to work in conjunction with the optimized engine speed of the remaining components. Match the components correctly and you have a winner. Miss with just one component and the whole combination suffers.

On the dyno, we relied on a complete MSD ignition including a Digital 7 ignition amplifier.

The AFR-headed 306 was first run with the dual-plane Performer RPM. Equipped with such, the small-block produced peak power numbers of 427 hp at 6,100 rpm and 394 lb-ft of torque at 4,800 rpm. The dual-plane intake offered a healthy torque curve of more than 350 lb-ft from 3,200 rpm to 6,000 rpm, and more than 375 lb-ft from 4,200 rpm to 5,900 rpm.

After swapping the single-plane Victor Jr. intake, the 306 produced 436 hp and 391 lb-ft of torque. Despite the fact that the peak torque numbers were so similar, the overall power curves of the motor equipped with the two different intakes was decidedly different and would feel like as such from behind the wheel. The reason is that the peak of 391 lb-ft came at 5,300 compared to just 4,800 rpm for the dual-plane. The effective shift in the power curve meant that the dual-plane intake improved torque production up to 5,300 rpm over the single-plane, in some cases by as much as 26 lb-ft. From 5,500 rpm to 6,300 rpm, the single-plane offered minor improvements in power, the largest gain of 12 hp coming at 5,900 rpm.

The question on the table now is, which is more useful, an extra 20-25 lb-ft through most of the midrange or the slight advantage in high-rpm power? For this particular mild application, the dual-plane would be much better suited for street (and occasional track) use. Ideally, the single-plane intake would be combined with more of everything, more compression, displacement, and even wilder cam timing to maximize power production higher in the rev range.