This excerpt is from a thread where Darin Morgan was asked this question:
"It depends on your RPM range, Engine size and intended use of the engine. For instance, an engine utilizing an automatic transmission will have a much smaller plenum than an engine using a five speed. An engine with a five speed behind it can use another 20% more plenum volume. You must be careful here! The dynamics of the race track vary slightly from the dyno and in this regard, they vary a LOT! On the dyno you can tune an engine to produce max power by adding a considerable amount of plenum volume. In some cases you can almost double the volume and see a considerable power gain. On the track the engine will lag against the converter or worse yet, just not accelerate at all. Smaller engines are much more susceptible to this than larger engines. We know from extensive track tuning that our Pro Stock engines will not take any more plenum volume but on the dyno, I could add more volume and see more power. In this case, equations take a back seat to experience and practicality.
Your total intake runner volume (head and intake) to your cylinder swept volume should be from 78% to 96%. Depending on the rpm range you’re looking for."
Darrin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines
here are some volumes that are associated with common small block Ford engine sizes:
|
Engine Cubic Inches |
Swept Volume |
Swept cc |
78% |
96% |
|
302 |
37.75 |
619 |
483 |
594 |
|
327 |
40.875 |
670 |
522 |
643 |
|
331 |
41.375 |
678 |
529 |
651 |
|
347 |
43.375 |
711 |
554 |
682 |
|
351 |
43.875 |
719 |
561 |
690 |
|
377 |
47.125 |
772 |
602 |
741 |
|
393 |
49.125 |
805 |
628 |
773 |
|
408 |
51 |
836 |
652 |
802 |
|
418 |
52.25 |
856 |
668 |
822 |
|
427 |
53.375 |
875 |
682 |
840 |
The plenum area is where the intake runners meet. There can be one plenum that all runners meet, or two smaller plenums with 1/2 the runners meeting in each. The plenum volume is a very importing tuning aid. As high velocity gasses flow through the carburetor or throttle body, the plenum give the gasses a chance to slow down, as the velocity is reduced the pressure rises. Higher pressure means that the air will be denser, and of course that means more power.
As rpm goes up you need a larger plenum, but a larger plenum will reduce throttle response and low-end power. A plenum also reduces peak air velocity through the carburetor (or throttle body). The induction pulses in an intake cause velocity to rise and fall with every pulse. The plenum helps to reduce them by acting as an air capacitor. Average velocity will remain the same, but the highs and lows will be closer together. Since you need a carburetor that will flow enough air at peak velocity, a larger plenum will allow you to run a slightly smaller carburetor without losing airflow, but it will also reduce the peak signal strength, which is why large plenums tend to reduce low-end power.