What's the best intake for your engine, single- (left) or dual-plane? The intake manifold
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.
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
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.