for those of you who play with the holes in the air box to change the noise and fit snorkels to help with hot air problem, this article posted on another forum may be of interest:

The box in question is located approx midway between the air flow metering unit and the inlet manifold. It is approximately of similar volume to the primary airbox. It is molded to the side of the air intake pipe, so that airflow passes an opening in the side of the pipe which opens into to the rest of the inlet system. This box is in fact a Resonator, the functions of which I will try to explain, I certainly would not recommend removal!

The following is over long in order to provide background, it is only essential to read part 2 to get the drift.

Part 1. While it's appealing to imagine the forward velocity of a car being converted into free supercharge via for instance ram air feeds, the actual air pressure gain is extremely small at low speeds. For example, at 150 mph, the pressure gain when air is efficiently brought to rest is 2.75 percent.

Because this is a dynamic effect, it is proportional to the square of the air velocity. At a more realisable speed of 75 mph, the effect (again with 100 percent efficient conversion of velocity into pressure) will be only one-quarter as great - that is, just under seven-tenths of one percent.
In fact, velocity energy is not converted into pressure at 100 percent efficiency. A figure of 75 percent efficiency is usual, which reduces our notional ram-air gain at 75 mph to one-half of one percent.

Therefore, at normal speeds, ram air is a myth.

However, something much more interesting lies behind it, often ignored or not understood by the masses, that something is airbox resonance.

In order to implement ram air, the carburetors or throttle-bodies of our engine must seal to an airbox whose volume is large enough that the intake cycle of one cylinder cannot pull its internal pressure down significantly. Box volume is typically 10-20 times the engine's displacement. Then the forward-facing air intake is connected to the box. When this assembly is tested on the dyno - even without an external fan to simulate the high-speed rush of air past the intake - it is discovered that the engine's torque curve is greatly altered, with new peaks and hollows.


Part 2.
Why? The answer is airbox resonance. If you hold the mouth of an empty bottle near your open mouth as you loudly hum scales, you find that at certain "hum frequencies" the bottle reinforces your humming, which becomes louder. What is happening is that the springy compressibility of the air in the bottle is bouncing the slug of air in the bottle's neck back and forth at a particular frequency - higher if the bottle is small, lower if it is larger. Your humming is driving a rapid plus-and-minus variation of the air pressure inside the bottle.

The same thing happens inside a resonant airbox. The volume of air in the box is the "spring" in this kind of oscillator. The mass of air in the box's intake pipe is what oscillates. The "humming" that drives the oscillation is the rapid succession of suction pulses at the carb or throttle-body intakes. If the volume of the airbox and the dimensions of the intake pipe(s) are correctly chosen, the airbox can be made to resonate very strongly, in step with the engine's suction pulses. The result, when this is done correctly, is that the engine takes air from the box only during the high-pressure part of its cycle, while the box refills from atmosphere through its intake between engine suction pulses. This produces a useful gain in torque.

Using this idea, motorcycle engines have been able to realize torque increases, in particular speed ranges, of 10-15 percent. In race engines, it is usual to tune the airbox to resonate at peak-power rpm to increase top speed. For production engines, it is often more useful to tune the box resonance to fill in what would otherwise be a flat-spot in the torque curve, resulting in smoother power and improved acceleration.
Early resonant airbox systems used long intake pipes that terminated in forward-facing intakes. More recent designs do not connect the ram-air pipe to the box at all, but terminate it near the airbox entry. The actual entry pipe is a short piece of tubing with bellmouths on both ends.
This is done because (a) the potential gain from actual ram air is too small to worry about, and (b) it's easier to tune the airbox with a short tube.

Where vehicle speeds are very high, gains from ram air are significant. This was discovered by Rolls-Royce in the late 1920s as the company developed its R Schneider Trophy air racing engine. At speeds above 300 mph, it was noticed that the R's fuel mixture leaned out enough to cause backfiring. When the mixture was corrected for ram-air pressure gain, the engineers realised they had a "free" source of power. At 350 mph the gain from ram air is almost 15 percent. Similar mixture correction is necessary when ram air is used on drag-race and Bonneville cars and bikes.

Intuition suggests that a forward-facing intake made in the form of a funnel, large end foremost, should somehow multiply the pressure of the air, resulting in a much larger pressure gain at the small end. Sadly, intuition is wrong. In order to convert velocity energy into pressure, the air has to be slowed down, and this requires a duct that widens rather than narrows. Next time you fly on a commercial airliner, note that its engine intakes widen as the airflow approaches the compressor face. Such widening passages are called diffusers, and they are universally used in the conversion of velocity into pressure.

Language often plays tricks on us - especially when language is used by product advertisers. "Ram air" sounds much more appealing than "resonant airbox." Nevertheless, it is airbox resonance that actually generates a significant power gain. At normal speeds, ram air is just words.

 

Thanks for that but my PRM sounds so

 

Interesting. Thanks for that.

 

it was posted for interest only. i am certainly not suggesting that people out their cars back to standard on the basis of this article.

 

Very good explanation.

It's also worth pointing out that the area behind the grille is usually low-pressure, as most of the air is forced around the front bumper!

A NACA duct in the bonnet might be more effective!