Ram Theory

In addition to discovering the best use for the fig, Sir Isaac Newton created three "law's of motion".
More commonly known as the "law of inertia", the first law of motion is:

An object at rest tends to stay at rest
an object in motion tends to stay in motion.

This law is the foundation for Ram Induction.

Visualize the intake cycle of the engine as air flowing through the intake manifold runner, past the intake valve, and into the cylinder. Everything is fine and dandy until the intake valve shuts.

Here is where the law of inertia comes to play -- because the air was in motion, it wants to stay in motion. But the air can't go anywhere because the valve is shut so it piles up against the valve like a chain reaction accident on the freeway. With one piece of air piling up on the next piece of air on the next on the next, the air becomes compressed. This compressed air has to go somewhere so it turns around and flows back through the intake manifold runner in the form of a pressure wave.

This pressure wave bounces back and forth in the runner and if it arrives back at the intake valve when the valve opens, it is drawn into the engine. This bouncing pressure wave of air and the proper arrival time at the intake valve creates a form of supercharging.
In order to create this supercharging, all of the variables have to be aligned so the pressure wave arrives at the intake valve at the right time. This combination of synchronized events is known as 'resonant conditions'.

The Math for Long Rams

Let's look at some numbers to see how the 30" long ram tubes were developed.

Our pressure wave travels at the speed of sound. A good estimate for the speed of sound is 1,125 feet per second.
For the F & G camshaft, the intake valve is open for 2680 of crank rotation.
The engine rotates twice (7200) for the intake to open once.
7200 minus 2680 = 4520 of crank rotation that the intake is closed.
At 2,800 RPM, 4520 = .0268 seconds. (See below)

2800 rev/minute divided by 60 seconds/minute = 46.66 rev/second
46.66 rev/second X 3600/rev = 16,8000/second
4520 / 16,8000 per second = .0268 seconds.

This .0268 seconds is the critical time factor. During this .0268 seconds that the intake valve is closed, the pressure wave is moving at 1,125 feet/second and travels 30.15 feet.
At resonant conditions, the pressure wave has to travel 30.15 feet to arrive at the intake valve when it is open. Since the pressure wave spends this time going up the runner AND going back down the runner, the runner length is actually only half of 30.15 feet, or 15.075 feet, which is equal to 181 inches.

Swell. Terrific. 181 inches. How does a 30" ram tube work if our equations say we need 181"?

The answer is: 6

Start the clock the first time the valve slams shut and the pressure wave is formed.
The wave bounces away from the valve and reaches the end of the 30" tube. Time now: .00223 seconds.
The wave bounces back down the 30" tube and hits the closed intake valve. Time now: .00446 seconds.
The wave bounces off the valve and repeats the trip every .00446 seconds.



.00446 sec


.00892 sec


.01338 sec


.01784 sec


.0223 sec


.0268 sec


At the critical time of .0268 seconds, our wave traveled the target distance of 181" up and down the tube. Although our pressure wave arrived at the intake valve five times to find it was still shut,
on the 6th bounce our wave arrived to find the valve open. The pressure wave entered the cylinder and we had ram induction!

This chart demonstrates the effect of ram induction using long rams:

The example above has demonstrated why ram induction works.
Now let's look at the limitations of ram induction.
The following parameters affect the arrival of our pressure wave at the intake valve:
  • Engine speed
  • The number of crank rotation degrees the intake valve is closed.
  • Length of the intake runner tube.
If these parameters are not balanced properly, ram induction doesn't work.
Let's go through the calculations again with the engine RPM changed to 1200 RPM:
The critical time factor calculates to .0528 seconds.
At .0528 seconds, our pressure wave has bounced around 11.8 times.
In this situation, the wave is only 80% through the intake when the valve opens and
the pressure wave arrives at the valve too late.

Change the RPM to 3600.
The critical time factor decreases to .02084 seconds.
At .02084 seconds, our pressure wave is midway down the tube during
its fourth bounce when the intake valve opens.
Once again, the pressure is not synchronized to the open intake valve.

In the illustrations above, we only changed engine speed.
What would happen if you also decreased the intake runner length and adjusted the cam timing?
You could once again synchronize the pressure wave to the valve opening
for 3600 RPM and you would have Short rams!


  • Why wasn't ram induction use before 1960?
    It was. Racing engines used tuned intake and exhaust systems to boost performance. Offenhauser and Hillborne produced some exotic tuned systems.