We are dealing with an air breathing heat engine here, so let’s start off with the air our engine has to work with. At what we call Standard Temperature and Pressure (STP) conditions, atmospheric pressure at sea level is 14.696 psi (1,013 millibars or, as measured with a mercury barometer, 29.9213 inches of mercury). At 70 degrees F the density of air at 14.696 psi is 0.074887 pound per cubic foot. Assuming the air is dry, we find that, by volume, 21 percent of it is oxygen and 78 percent is nitrogen. The remaining 1 percent is composed of trace gases. But in most parts of the world, the air is far from dry; it has a water vapor content. Since water vapor does not support combustion, an even lesser amount of the whole is oxygen available for combustion.
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Assuming we are not racing under less-than-normal conditions, the atmospheric conditions just described are about the best we are likely to be able to see. At this point, things can only go downhill from here, unless we take steps to the contrary and that is what we investigate next.
Assuming there are no unwanted side effects, the lower the inducted air temperature, the more torque (and consequently more power) your engine will make. Underhood air temperatures can reach the top side of 180 degrees F without any difficulty. If you live in a climate like the hot arid conditions of the Southwest, those temperatures can reach 240- plus degrees F while the outside (ambient) temperature may only be 105 or so. Under these conditions, your engine could be giving up as much as 13 percent of its output everywhere in the RPM range. Since this brings about a reduction in torque, it also means that your gearing for best acceleration is also likely to be off by a similar amount. The result is a sizable increase in the 0–60 and quarter-mile times.
Simple Cold-Air Induction
Most auto manufacturers pay only minimal attention to cold-air induction. They build induction systems to keep the air within a narrower temperature spectrum than the prevailing ambient temperatures. This makes it a little easier to meet emissions and, while this is good, what it does do is leave a lot on the table for hot rodders to exploit. In Chart 2-1, you can see just what a K&N cold air intake did for my GMC 4.8 truck, which I use mostly for towing. Just installing this cold-air kit produced, on a typical 75-degree day, an average increase of a little more than 8 ft-lbs and 7 hp, with peak power going up by 13 hp. At the track a few days later, under similar before-and-after test conditions, the cold-air-equipped truck went 0.13 seconds and 2 mph faster in the quarter-mile.
Extreme Cold-Air Measures
A cold-air inlet is a simple and effective means to achieving a little more output. Knowing it works, let’s apply the cold-air principle in a more extreme manner.
Let’s start the ball rolling here with a very important point. Inducing cold air is always of value but, to make the most of cold-air induction, it is necessary to prevent heat from subsequently getting into the cooler charge so that it arrives at the intake valve as cool as possible. Doing this can achieve some very worthwhile results. Tests, shown here (in Charts 2-2 and 2-3) with a 5.0 Ford Mustang, are very revealing. Also, these tests demonstrate that heat can get into the intake charge by means not always quite so obvious. The 5.0 Mustang intake is a good example because it is one of the worst socalled high-performance intake systems on the planet. The reason being: It has an extremely high surface-areato-volume ratio, and this makes it act like a heat sink that absorbs every BTU within 50 feet (well, not really, but you get the picture).
If you have ever watched a 5.0 Mustang owner who is intent on achieving the best quarter-mile time possible, you have noticed they are almost for sure packing ice on the intake before making a run. At the end of the run, the intake temperature often rises from a little above ice cold to more than 200 degrees F.
In an effort to improve the stock manifold, the first move made for these tests was to pull it from the engine and make any simple mods to improve the airflow. But these “porting” modifications also cut intake charge temperatures.
Several of the runners of the lower half of the 5.0’s fuel-injected intake have a severe “dog leg” just before entering the cylinder head port. By welding a little on the outside of the manifold, these dog legs can be straightened somewhat. After that’s done, I send the lower and upper half of the intake to be “extrude honed.” In this process, a highly viscous compound containing grit is forced through the runners. This produces a very smooth surface finish, and in so doing achieves two things: First, and most obvious, is that it smoothes out casting imperfections that may have snagged the airflow somewhat. Second, the smooth finish has far less surface area than a cast finish, so the charge picks up less heat from the walls of the intake.
Charts 2-2 and 2-3 show the result this porting/extrude-hone exercise has on flow. This exercise produced positive results, which are shown above.
After dyno testing with the ported and extrude-honed manifold, it was removed and subjected to an intensive thermal-management program. This involved applying a zirconium-oxide coating over the entire exterior and base surfaces of the manifold. Along with that, the inside of the runners had a 0.005-inch thick, thermal-barrier coating applied. To minimize heat passing from the hot oil splashing on the underside of the manifold, a splash tray was fabricated and the gap between this and the manifold itself was filled with structural insulating foam. The throttle body spacer was also modified to eliminate the heat passage.
How did all this work out? Here the results about speak for themselves as Charts 2-4 and 2-5 shows. The chart’s baseline tests are for a system already equipped with a C&L cold-air intake system. The aluminum tube from the K&N filter to the mass airflow meter had the internal-diameter thermal barrier coated to cut heat transfer at this point. This system was initially worth about 8 hp, so the sum total gains from the porting/ extrude hone and the thermal management was substantial at 35 ft-lbs and 44 hp! In addition to track and dyno results, some temperature recordings with an infrared heat gun were also made. A typical 5.0 intake manifold tops 200 degrees F.But with the weather prevailing during these tests, it was more like low 190s at the start of a pass and mid to high 190s at the end. In our test vehicle’s case, the starting temperature typically was about 140, and it dropped to the mid 120s after one pass. It went as low as 105 after a second pass, which was done immediately after the first. For regular street driving, there is plenty of time to heat soak, but despite this, the temperatures rarely rose above about 145 degrees F.
From the forgoing, we can see that actively seeking the coldest source of air is a good move toward making more horsepower. There are, however, a few points you should be aware of to realize the best performance from any cold-air source. If the engine is fuel injected, then any possible air/fuel ratio error will more than likely be compensated for by the engine’s ECU.
If the engine is, as is so often the case, a carbureted unit, then a significantly cooler intake charge almost certainly needs some carb calibrations to make the most of it. The first point is that denser air almost certainly needs a little more fuel. Usually a carb has a certain amount of self compensation, but extremely cold air may require an increase in fuel to restore the optimal mixture ratio. The second factor here is the carb’s fuel-atomization function. For good ignition properties and initial flame propagation, a certain amount of fuel needs to reach the cylinder in the form of vapor. To compensate for reduced vaporization, it may be necessary to go with a booster design with greater atomization capabilities.
Ram Air: The Basics
We have seen how thermal management can have a dramatic effect on power output. Now it’s time to look at another part of the intake air density equation: ram air. The big air scoops, as seen on cars such as Formula 1, Pro Stock drag cars, and the like, epitomize this means of power boosting. If all we had to do was look fast, then these scoops succeed in grand style. But going fast and winning races is about function, not style, so the question we have to ask here is: Do they work? Sure they do, but let us not overlook that they are also cold-air intakes. That said, let’s get down to analyzing the possible gains that may be had by using the forward motion of the vehicle to increase the induced-air density.
In engineering terms, the increase in air pressure brought about by bringing a stream of air (moving in relation to an object) to a dead stop is known as the static pressure head. Two factors that affect this “ram air” pressure are relative air speed and air density. The ram air pressure goes up in proportion to the air density but changes as the square of the speed. This means: double the speed, and the pressure goes up by a factor of four. In Chart 2-6, you can see that, at the speeds most of us travel, the potential gains from ram air are slim to minimal.
To put things into prospective here, at 100 mph, the absolute best you could hope for is an increase in air density within the scoop of 1.2 percent. That is about the same as given by a 12-degree-F drop in intake temperature. If all this is starting to look like hard work for a small increase, you can rest assured; we have not reached the end of issues that are potential problems in making ram air work. The first is that the air is not brought to rest within the scoop, so the full effect of the vehicle’s speed is not realized. Remember, the scoop is being fed air at one end while the engine is sucking it out at the other. The scoop needs to slow the air after it has entered the scoop, thereby converting some of the kinetic energy into pressure energy. This is one of the reasons hood scoops on faster drag cars are bulbous in shape.
If the engine is carbureted, then another issue can raise a specter. If only the throttle body experiences the ram air pressure, the pressure difference that draws fuel from the float bowl to the booster is reduced and causes the mixture to lean out. To avoid this, the float bowl must see the same ramming pressure that the carb venturis see. Even though this may be done, it is not a complete fix because the ramming pressure is more effective on the float bowl than it is on the venturis. If the engine is fuel injected, then, as long as the manifold absolute pressure (MAP) sensor is doing its job, the mixture remains within working limits.
Practical Density Issues
So much for theoretical issues involved with achieving high-induction densities. Now let us consider how best to apply what we have learned and how to avoid some of the most common pitfalls.
In 1979, I wrote an air filter feature article for Popular Hot Rodding magazine that started by claiming that air filter technology was not widely understood, even among professional racers and car builders. As I write this book, it seems that the situation has improved and many have a better understanding of air filter technology, but they still have a long way to go, especially among drag racers. Hopefully, what you read here gives you a far better understanding than you had before.
So far, we have discussed the reasons for picking up cool air and possibly ramming it into the engine. All this is of little use if the air entering the engine is dirty and proceeds to carry in grit that will quickly wear out the rings and bores to the point where they don’t seal. The air obviously must be cleaned by an air filter.
Significant Pressures and Flow
To determine where losses may occur in an air box/filtration system, it helps to analyze the box/case and the filter element itself as two separate but inter-related entities. To do its job effectively, a filtration system must perform in three areas. First, it must remove micron-sized grit particles from the air. Second, it must do this without incurring any significant restriction. And third, in the case of a carbureted engine, it must suppress booster buffeting. To see how all these sometimes conflicting requirements can be met, let’s start by investigating various filter elements and their effect on flow.
An air filter element’s flow capability is dependent on both its size and the characteristics of the material from which it is made. The physical size of the element is an easy assessment to make but the possible characteristics of the element are not. The chances are that the stock setup you have on your street machine is too small for the job. Inadequate flow in this area can be attributed to the case, the size/type of filter element used, or both. You are usually correct if you assume “both.” If you prefer to establish beyond doubt just how much of a restriction the case and element may be, it’s not hard to find out. All that is needed here is to make a pressure tapping into the case and measure the depression with a sensitive pressure gauge or manometer, as shown in Chart 2-7.
The technique for testing is simple. Find a reasonably steep hill and power it up in, say, second gear, from a relatively low speed. Have an assistant check the manometer readings at certain key RPM points such as 2,000, 3,000, 4,000, 5,000, and 6,000 rpm (more if the engine is a small high-revving unit). At each point, note the total inches of difference between the two legs of the manometer U-tube. When testing most stock cases with a new, regular paper filter installed, you find that an 18-inch manometer doesn’t suffice much past the mid range due to restriction. To determine the proportion of the restriction caused by the case and caused by the element, repeat the test with the pressure tapping in the case, but outside the element. Be aware that some cases are good and some are abysmal, while most paper filters are so-so.
Remember, a manometer check on a typical factory 4-barrel carb setup (as described here) may not show whether a filter case is too close to the carb mouth or not. For the record, a typical case lid needs to be a minimum of 3 inches from the air horn or the mouth of the carb.
If power is the principle criteria, all this testing begs the question: What is the maximum acceptable pressure drop within the case and filter assembly? In a perfect world, the answer is zero, but achieving that is nearly impossible. However, improving flow into the system for more power follows a strict law of diminishing returns. In practice, we find that if, at peak RPM, the measured pressure drop is no more than 11 ⁄2 inches of water, then you can assume for all practical purposes your no-significant-loss goal has been met. At such a minimal restriction, the effect on output is barely measurable, even on a very high-tech dyno
The fact that a filter is present in the system can, if it meets the flow demand of the engine, be of benefit in terms of output. This can occur to the extent that the engine actually makes more power with the filter in place than without it. If a carb inlet is hanging out in a high-air-speed environment, we find that the boosters can be buffeted by the large-scale turbulence they are likely to experience.
Many years ago, I conducted some booster signal tests on a 1,600-cc Ford Kent engine powering a Formula Ford racer. This was done at Silverstone in England. The measurements indicated a 20-percent variance in booster signal. From this, I deduced that the mixture suffered rapid fluctuations to the tune of some 10 percent or more. Just installing a K&N filter and case for the Weber carb showed a speed increase on the longest straight of some 2 mph! I played on the fact that most racers would assume the filter slowed the car by telling them we were going to take the speed penalty (which of course did not exist) because we had and could only afford one engine for the year. They all must have believed it because no one else used a filter for several years to come!
It won’t come as much of a surprise that there is a substantial difference between the best and worst filter elements on the market. The amount of bogus information is also sizable, although not as bad in 2010 as it was in 1976 when I decided to investigate filter performance in depth. What I found out, and more to the point publishing it, almost cost me a career. Because a then-small and relatively unknown company absolutely blasted the entire opposition at that time, I had magazine editors ban my work because they figured I must be on the take, and big filter companies’ called and told me my measurements must be wrong. Fram’s PR guy even called and told me such critical tests should be done by an engineer, not a journalist. He was not too pleased when I told him I was an engineer—not a journalist! The irony here is that some 25 years later Fram introduced the Air-Hog filter element, which is a direct copy of the K&N!
All this controversy is like stirring up a hornet’s nest. The work I have done to bring real-world numbers to performance enthusiasts, who want factual tech and not public-relations tech, has even embroiled me in lawsuits. I have written much on this subject and I feel that repeating it all here, to again prove a point, is unnecessary. If you want a stack of numbers, I suggest you read my book, How to Build Horsepower Volume 2: Carburetors and Intake Manifolds (published by CarTech, Inc.)
What I am going to do here is to show some flow tests to highlight the fact that the K&N cotton/oil filter elements are just about the RollsRoyce of filter elements (incidentally, Rolls-Royce mandates these for many heavy-duty applications on its diesel engines). Since 1976, many companies have copied the K&N design, and this has led to the production of a number of really effective filters. However, before you take this information and go buy just any K&N look-a-like, I need to tell you there are K&N look-a-likes that are almost indistinguishable from the real thing that neither flow nor filter! My advice here is: Unless you are very sure of the filter you are buying, stick to the purchase of a K&N. It may cost more than a regular replacement paper element, but it is semi-permanent so you won’t be buying more filters down the road.
Other than flow and filtration capability, a filter element has to combat clogging. For a regular paper filter this is done by virtue of what might loosely be called excess area. The filtration takes place by arresting any particle larger than the holes in the paper. Unfortunately, each hole becomes, to a greater or lesser degree, clogged by the particles it stops. This results in less flow area for the air, thus increasing the resistance to flow. Here, the K&N has an advantage in that the engine-induced pulsating air causes the ends of the oil-covered cotton strands to vibrate. This causes what can best be described as a sweeping of the air as it navigates its way through the multi-layered element. This action tends to collect the dirt on the cotton strands themselves, not in the voids between them. This mode of function leaves us with an element that can hold huge quantities of dirt before flow starts to degrade by any real amount. This is why most offroad race vehicles use K&N (or functional copies of K&N) elements and not paper ones, which degrade from the moment of start-up.
The Filter Case
Now let us consider the case, or what often serves as the case. Look through any performance magazine and you find many ads for cold-air kits, be it for a Honda, a full-size latemodel fuel-injected Ford, Chevy, or Chrysler truck. What isn’t so well catered to, short of a monster hood scoop, are V-8 engines often built by hot rodders employing a single 4-barrel carb. The good news here is that Air Inlet Systems in Hamilton, Canada, specializes in cold-air filter cases for just such applications (visit www.ramairbox.com to see many highly functional examples). This is a company that truly has a handle on filtered cold/ram air. Some tests run on a friend’s car showed an ET change from 11.51 to 11.36, and an mph increase from 118.52 to 120.11 in before-and-after tests run within 15 minutes of each other. Using standard quarter-mile horsepower equations, this indicates the Air Inlet Systems unit was worth about 18 hp on a nominal 450-hp motor.
Written by David Vizard and Posted with Permission of CarTechBooks