Originally Posted by 300bhp/ton

Marketers just can't resist it. Ram air! The words themselves summon up images of rushing wild beasts, or of secret military aircraft operating on futuristic principles.

Unfortunately, on most perofrmance cars, ram-air is as functional as tail fins were on cars of the ’60s.

What is it? Ram air just means using a forward-facing air intake to gain some extra intake pressure. We have all, as children, felt the pressure of moving air on our hands when we held them out the window of the family car. When moving air is brought smoothly to rest, the energy of its motion is converted into pressure. Motorcycles went through a "ram-air" period in the early 1990s, during which street bikes were equipped with the forward-facing "rocket-launcher" engine air intakes seen on many road-racing machines.

While it's appealing to imagine the forward velocity of a car being converted into free supercharge, the actual air pressure gain is extremely small at normal 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 realizable automobile 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, ignored by the advertiser's busy pen. 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.

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 realized 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.

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Cliffnotes:

-Ram Air a myth? = NO
-Does it work on a road car = NO
-At 150mhp there is next to no gain.
-Significant gains arn't seen until 300mph+
-The air box is the key, not the ducting.
-When buying a CAI/induction kit look for the one that uses air box resonance


Edit: Additional Reading

Intake temperature is a whole different ball game.

The simple rule is:

'Cool for power (maximum charge density), hot for economy (minimum charge density to reduce losses due to throttling).'

Although in many cars the under bonnent temperatures are no where near as bad as many people beleive. This refers to a 5.3 liter Jguar XJS V12. So a big engine in a small engine bay.

The under-bonnet air temperature at idle can easily get up around 70 C but the faster the car goes the lower the air temperature falls - simply because the radiator is passing its heat to a much larger quantity of air per second - so at 80 mph. the engine is breathing air at around 45 C. That's still a bit higher than the ideal but not nearly so bad as many people think. Obviously the standard arrangement helps to maximise economy in moderate speed urban cruise without compromising top end power too much.

In setups that duct cool air from outside. The power gains from such a system are almost certainly attributed to the filter, and less restritive intake (meaning quite simply a bigger opening), and a form of air box resonance coupled with a 'cool air intake' from outside the engine bay. Sadly even at very high speeds (well over 100mph) I doubt that it has any form of 'Ram Air' effect. If you reconfigured the system to take air from the inside of a wheel arch it would produce the same results as having the intake ducts at the front of the car. The source of the air, not the location of the ducts is the important factor.

Remember the only way to get a greater volume of air into the engine is to compress it. This is what turbo and superchargers do. An air intake scoop either on the front of a car or on the bonnet will not compress the air at any speed most people are likely to travel at.

Taking the airbox resonance theory futher with the intake manifold itself by optimising the length and entry profile into each of the tracts to better exploit induced harmonic resonances in the air as it flows towards the cylinder. Any tube containing air can be made to resonate at certain critical frequencies in the manner of an organ pipe. Such is the case with the inlet tracts of an engine and if the natural resonance frequencies can be matched to the engine speed then a mild supercharging effect can be induced. Get it wrong and the reverse will apply, resulting in a loss of performance.

Originally Posted by chris_s14

theyre more like keyboard warriors