Mass Air Flow:

Mass Air Flow (MAF) is an engineering term used to describe the quantity and quality of air being consumed by the engine. The "quantity" part of this term identifies air flow, typically expressed in CFM. The "quality" of air is identified as air density and typically expressed in pounds of air per cubic foot volume. When these two terms are multiplied together, we get MAF expressed as pounds of air per minute. The higher the MAF going into the engine, the more air molecules are consumed per minute to produce higher combustion pressures for more power.

Aftermarket manufacturers have to work with the same Laws-of-Physics as the OEM in developing their air intake systems. A good aftermarket air intake design should make an improvement over the OEM on two very important functions that the stock air intake performs: 1) The air intake should reduce air flow restriction and 2) The intake should increase density of the air going to the turbocharger. Most air intake manufacturers will state that there intakes increase air flow and may show laboratory air flow test numbers, but these manufacturers will not mention what effect their intakes have on air density.

Reducing Air Flow Restriction:

Lowering air flow restriction through out the intake will typically require a manufacturer to use 1) A semi-open air box with up to 50% of the box open to allow air to easily enter the air filter; 2) A conical filter to obtain a large filtration surface area; and 3) An intake duct with a smooth inside surface and a larger diameter duct between the air box and turbocharger. Most manufactures of air intakes do very well on lowering the air flow restriction and proudly advertise their increase in air flow percentages (up to 40-50%) over the OEM air intake. Manufacturers use an air flow test bench machine to do their laboratory testing. Unfortunately this test data is very misleading. First; flow bench testing with an air intake in a free and open air environment does not duplicate the air flow once the intake is installed in the engine compartment and the hood is closed. Secondly; diesel engines do not need more air flow unless the fueling level is substantially increased to the point where the engine is extremely over fueled and heavy black smoke is pouring from the exhaust. A stock diesel engine typically ingests 30% more air than is needed for complete combustion. Therefore, a less restrictive good designed aftermarket air intake will usually provide a small power improvement (typically less than 7 hp) over the OEM intake when tested on a dynamometer. Only when a large turbocharger is installed or the OEM turbocharger boost pressure is significantly increased does a less restrictive air intake start to make 10 or more hp above the OEM intake. Beware of those high advertises horsepower numbers from most manufacturers.

Increasing Air Density:

Designing an air intake to provide an increase in air density to the engine is completely ignored by the aftermarket industry. Manufacturers will market their systems as a cold air intake, but not one manufacturer tests or advertises how much cooler air their intake system delivers to the turbocharger compared to the OEM air intake. PSM’s actual test data shows that the reduction in air density due to a poor designed semi-opened heat shield type air box has a larger impact on lowering engine power than any power increase because of its less restrictive air box design. Under real-world driving conditions with high ambient air temperature and heavy and continuous engine loading, the OEM air intake will outperform most aftermarket air intakes.

Why does the OEM air box outperform most aftermarket intakes in real-world conditions? The factory box receives 100% of the intake air from outside the engine compartment, through an inlet in the inner fender panel. Outside air between the inner and outer fender is drawn into the air box through a duct or opening that is sealed to the air box. This air is typically 40 to 80°F cooler than the air in the engine compartment when the engine is operating in high power applications. Most aftermarket intakes use a heat shield semi-opened air box which allows up to 40% of the under hood air to get sucked into the air filter. This combination of under hood air mixed with outside air is typically 30 to 40°F hotter than the intake air in an OEM air box. Hot intake air kills power as hot air is less dense. It is very important to keep hot air out of the air intake. Air is made up of molecules. Molecules that get hot expand and become larger than cool air molecules. Fewer hot air molecules can be packed into a given volume (say a 1-foot cube box) compared to the number of smaller cool air molecules. Since more cool air molecules can be placed in a 1-cubic foot box, the box becomes heavier. The heavier the box, the denser the air is inside that box. The more air molecules the engine digests in a cubic foot of air, the more power an engine will make.

An increase in air density above what the OEM intake can provide to the turbocharger is very difficult to accomplish by the aftermarket industry because the OEM intake received 100% of the intake air from outside the engine compartment through a sealed air box. Most if not all aftermarket air boxes fall short of providing cooler air to the turbocharger because of their air box design. Recently many manufacturers are starting to offer a semi-sealed air box, but will often call them “sealed”. These boxes are typically not sealed to the inner fender air inlet and either have the bottom or front of the box cut away to permit air to reach the air filter. Since the air box is always under a negative air pressure while the engine is running, the higher air pressure from under the hood will always cause the hot engine compartment air to get to the air box, no matter how fast the vehicle is moving.

The benefit of increasing air density is that for every 5.5 degrees of cooler intake air the engine consumes, a 1% power improvement will be produced. Other benefits in providing cooler air to the engine are lower EGT and lower engine compartment temperature. For every 1 degrees of cooler intake air, the EGT decreases by 1.5 degrees and under hood temperature actually lowers 2 degrees. Faster engine cool down will also occur since there is less under hood heat to dissipate.

High Horsepower Claims:

Some air intake manufacturers make no claims of power improvement with their intakes while many other companies make claims of 40 or more horsepower. How can this be? Is one manufacturer’s intake that much better than another’s? To find out, Diesel Power Magazine in their October 2006 issue ran an independent dynamometer test on air intake systems from 5 major manufacturers. Their test vehicle was a ’04.5 Dodge/Cummins. The final results were that not one air intake kit made more or less power than the OEM air intake system. From PSM’s dynamometer testing, a properly designed/engineered air intake can make 4 to 6 hp throughout the rpm range over the OEM intake at the stock fueling level. Manufacturers who claim higher numbers than these are just overstating their data, if they actually perform the tests, in an effort to increase sales.

Actual dynamometer graphs are only accurate if the operator and driver do not vary their test procedures when evaluating different power enhancement products. A common technique that is use to overstate a performance of a product tested on the dyno is to start wide-open-throttle application earlier in the RPM so the engine is making more power when the dynamometer begins to record power numbers at the pre-established mph or rpm. This deceptive technique will produce an artificial high power gain in midrange rpm that will taper off as the engine approaches peak horsepower rpm. The proper dyno test between products will have all runs starting at the same rpm/mph and all runs making the same power at that starting rpm/mph. This type of testing is hard to find in magazine articles or in manufacturer’s advertisements. A final note of dyno graphs is if the lines on the graph are smooth with uniform curves, the graph is not real, it is a created picture, and little validity should be given to the information contained in that advertisement picture.

Dyno Power vs. Real-World Power

Depending on how the vehicle is used, the power numbers realized on the dynamometer will never be duplicated or be available on the street. Testing a vehicle on a rear wheel dyno is similar to running a laboratory test, where environmental conditions and test procedures are closely followed and optimized to produce the most power from the vehicle. Typically, 1) The hood is up to vent engine compartment heat away from the engine. Raising the hood may allow the air filter to be more exposed to cooler and less restrictive air above the filter. 2) The engine is brought up to operating temperature just prior to the test run. 3) The turbocharger, intercooler and other engine components are given adequate time to cool down between runs to eliminate heat soaking. Performance products tested using this method will perform much differently on the street than on the dyno because of higher intake air and under-hood temperatures and the effects of heat soaking from engine components that are run continuously, sometime at high loads for extended time periods. It is not uncommon for an aftermarket air intake on a moderate to highly modified engine to make 10 to 20 more horsepower than the OEM air intake on the dyno, only to make 60 to 90 less horsepower than the OEM air intake on the street.

Every engine component has its good and bad points. Manufacturers like to emphases the best aspects of their product and minimize or not mention the negative aspects. Air intakes are no different, and the best system for our trucks will always be a compromise. For example: The best air intake for the older Class 8 semi-trucks was to locate a huge air box on each side of the truck. The air box housed a filter with an enormous filtration surface area to lower air flow restriction as the air passing through the filter. The air box was located outside the engine compartment in the cool air stream alongside the truck to get the densest air possible to the turbocharger. Today, Class 8 trucks are now using under-hood sealed air boxes with a duct running to one side of the hood to obtain cold air. The trade-off is increasing fuel economy at the sacrifice of making less power. Removing that old air box on the side of the truck did lowered the truck’s air drag coefficient and improve mileage.

Since pickup trucks never had air boxes located outside the engine compartment, a balance between lowering air flow restriction and increasing air density must exist in order to get the best performing air intake. Open element air filters located in a semi open heat shield box lowers air flow resistance at the cost of introducing hot under-hood air into the engine. On the other hand, the OEM sealed air box with its limited access for getting cold air into the box guarantees the coldest air to the turbocharger at the cost of increased air flow restriction. There are trade-offs in either design. Neither system is the best approach for increasing real-world power from the engine. PSM has found that the lesser devil is to accept additional air flow restriction for a greater benefit of improving air density.

In order to keep the same air intake temperature (to maintain air density) to the turbocharger as the OEM intake, a totally sealed air box must be used. Any air path to the inlet openings into the box must originate from outside the engine compartment. The best designed sealed air box is the OEM air box. The disadvantage of using the factory box is that it only has one inlet opening to obtain cold air. Increasing the size or adding another opening that gets cold air from outside the engine compartment will reduce air flow restriction and allow the turbocharger to make more boost for more engine power. PSM has proven in real-world testing and on the dyno that adding another air inlet opening to the bottom of the OEM air box and ducting cold air to this inlet from under the vehicle is the superior method for increasing power and lowering EGT for pickup trucks.

Dyno tests have proven that the stock air box will provide sufficient air to allow the engine to produce about 500 rear wheel dynamometer horsepower and 450 real-world horsepower on the street. Tests have also shown that adding a 4-inch diameter air inlet opening to the OEM air box to obtain air from outside the engine compartment (like the PSM Cool Power cold air intake) will provide sufficient air to satisfy the needs of a larger single turbochargers that is capable of flowing 1100 cfm or more and permitting the engine to make 650 rear wheel dynamometer horsepower. In real-world driving, the PSM intake will produce over 600 hp on the street. In contrast, a semi-opened heat shield air box intake that allows the engine to make 650 hp on the dyno may only make 500 hp in real-world driving situations.

The final choice between which air box to use will depend on the intended use of the vehicle. A truck that is set up to only produce maximum horsepower on the dyno well above 650 hp will want to use a conical filter in a maximum “opened up” air box since under-hood temperatures will typically be cooler during the test. A truck that is used for towing, sled pulling and drag racing where hot under-hood air will build up should use a 100% sealed air box for best results. A street truck that does not see any racing or towing can use either type of air box as engine compartment temperatures are typically lower in little or no-load engine conditions.

What Is My Engine Air Flow Requirements?

Manufacturers, magazine writers and others will use phases like “an engine is essentially a giant air pump” or “the more air in and the more exhaust out equals more horsepower”. What most of the above individuals do not comprehend is that there is a limit to how much air an engine can digest. We can’t force more air into an engine; no matter how large the air intake, intercooler or exhaust system is, if the engine can not accept it. Engine air flow requirements are determined only by four factors. These are: engine displacement; engine RPM at maximum horsepower; engine volumetric efficiency; and boost pressure at maximum horsepower. The graph below shows the engine air flow needs for our 5.9 Cummins at various boost and RPM points.

For example, a ’06 truck that is factory boost limited to 30-31 psi boost will consume 820 cfm of air. The simplest method to increase air flow into the engine is to install a boost fooler to trick the ECM into allowing the OEM turbocharger to producing more boost. At 40 psi boost, the engine will now use 1020 cfm. Raising turbocharger boost pressure is the only means to increase air flow. Air intakes, exhaust systems or other aftermarket bolt on up-grades will not increase air flow.