Calculating Critical Air Scoop Speed With A Forward Facing Air Inlet

For a race car or race boat, a forward-facing air inlet creates a ram-air effect above a certain speed. The air being pulled into the inlet has a higher density of air than in the surrounding atmosphere, which can create a significant boost in engine performance, as we discussed previously here.

For example, in Pro Mod drag racing, blown alcohol entries weighing over 2,500 pounds routinely reach speeds over 250 mph in the quarter-mile. To do that, the math says they need close to 4,000 horsepower. Yet, stationary dynamometer tests of these engines routinely only register 3,000 horsepower or less.

While that may seem like a mystery, in actuality, the extra horsepower is provided by air ramming into a forward-facing air scoop at high speed. With an appropriate adjustment in fuel, ram-air is reported to provide as much as an additional 800 horsepower.

Popular stack-type Hilborn mechanical fuel injection is shown on an early Ford Flathead V8 racing engine with an air scoop for added ram-air effect. This setup was considerably faster with the scoop than without it.

Inlet Area, Airspeed, and Engine Air Demand

Tuning for ram-air is usually trial-and-error and quite an extensive process. It is dependent on the relationship between the amount of air being rammed into the scoop and the amount of air the engine is actually consuming. Several factors affect the first factor.

The first factor is the size of the scoop inlet facing the air. A smaller frontal-area brings in less air with a reduced need for fuel enrichment. Conversely, a larger frontal-area brings in more air with a greater need for fuel enrichment.

The amount of air being rammed into the scoop is also dependent on the speed at which the scoop is moving relative to the air. A slower speed differential has less air entering the scoop with a reduced need for fuel enrichment. A faster speed differential rams more air in, resulting in a greater need for fuel enrichment.

Finally, the amount of ram-air pressure is dependent on the air consumption of the engine. Low air consumption from a smaller engine doesn’t empty the air scoop very much. Not much speed or inlet area is needed before horsepower is increased from ram-air, and there is a need for more fuel. More air consumption from a larger engine will empty the volume of the scoop more rapidly. So, a faster speed or a larger inlet area is needed for increased horsepower from the ram-air effect.

Many tuners compensate for the extra air at high speed with ignition retard while leaving the fuel system the same as the low-speed setup. Alternatively, an increase in fuel at speed will usually be a better alternative and make more power.

Critical Air Scoop Speed 

With a forward-facing air scoop at lower vehicle speeds, the engine is pulling air into the scoop. There is a small vacuum within the scoop from the engine (or supercharger), drawing in more air than is provided from the forward-velocity of the vehicle. This state generally requires less fuel.

Then, at a certain speed, there is a balance between the volume of air forced into the air scoop and the air consumption of the engine. That is the critical air scoop speed or CASS. Above that speed, the volume of air the scoop accumulates is greater than the volume of air the engine is ingesting. In this state, there is positive pressure in the air scoop. The pressure increases with vehicle speed, and the associated “boost” can require fuel enrichment, or as mentioned earlier, a reduction in ignition timing.

A fuel system may have differing mixture requirements for the various modes that can be numerically categorized by CASS. According to the Kinsler Handbook: “At a certain speed, extra air is deflected around the scoop. It may be more of a resistance to airflow than the power that is contributed by the positive pressure.”

A portable weather meter can be used to indicate the speed of the air from a fan into a stationary engine air scoop on a dynamometer test, which can then be used to determine a fuel system adjustment trend.

Secondary Effects From Different Modes

Consider a forward-facing mechanical fuel-injection hat on a methanol-fueled racing engine. At low-speed, the small vacuum is adding to methanol vaporization. The suspension of more methanol as vapor with fewer fuel droplets is beneficial to combustion, as is the additional cooling from the vaporization.

At the Critical Air Scoop Speed, the fuel injector behaves normally. That is, it is injecting a volume of fuel into an airstream that is essentially at atmospheric pressure.

At high speed — beyond the CASS — the higher pressure in the air scoop is retarding fuel vaporization and suspension of methanol in the air stream, leaving more droplets in the air stream.

Combustion is affected by the amount of fuel vaporization. Evaluation of the engine at low-speed would be different than evaluating it at high speed, just from the amount of ram-air.  The pressure differential’s effect on vaporization has a subsequent impact on the fuel mixture. This difference is difficult to evaluate on a stationary engine or chassis dyno.

Blowing air into the inlet with a fan may help to generate engine characteristics for higher speed. A portable weather meter can be used to indicate the speed of the air from a fan into a stationary engine air scoop on a dynamometer test, which can then be used to determine a fuel system adjustment trend. As an example, the common Kestrel 5500 meter is rated for up to 89.5 mph of wind speed, if you were able to generate that much airflow in a static environment.

Enderle Bug Catcher hat injectors are popular methods of feeding blown small-block drag racing engines. The throttle blade area for one injector is 32.08 square inches, less the throttle shaft cross-sectional area of 3.83 square inches. That equals 28.25 square inches of air scoop area. The front injector is facing significant air pressure at speed and may need speed-based enrichment. The rear hat injector, however, would get turbulent air due to being directly behind the front injector, offering less ram-air effect and requiring less enrichment.

Doing The Math

The critical air scoop speed can be computed as follows:

Common units are selected for each parameter.

  1. CASS would be in MPH.
  2. Engine air demand is usually in CFM.
  3. The air scoop area is most often in square inches.

The equation can be adjusted for these units.

This equation includes some simplified factors to get the 1.64 constant. The conversions to get CFM and square inches to produce an answer in miles per hour simplify to a constant of 1.64. If you started with different units for any of the variables, the constant would change.

As an example, we’ll assume a carbureted mountain motor of 700-plus cubic inches. Note the following:

  1. Engine air demand of 1,600 cfm at 8,000 rpm
  2. Air scoop frontal area = 55 square inches

Rounding up to 48 miles per hour as the Critical Air Scoop Speed — below 48 mph, there will be a small vacuum in the air scoop. That vacuum will increase the float level in a carburetor, which may cause the engine to run richer.

At 48 mph, the pressure inside the scoop is equal to atmospheric pressure. Fuel delivery is correct for the atmospheric air pressure.

Above 48 mph, there is positive pressure in the air scoop, and the pressure increases with vehicle speed. The pressure may reduce float levels in the carburetor, which in turn, may make the engine lean out.

To a certain extent, the carburetor venturi will compensate for increasing amounts of air going through the air scoop from increasing speeds. However, an adjustment may be necessary with higher air scoop pressures, if the demands are beyond the compensation range of the carburetor.

RPM Effects on the Critical Speed

After the transmission shifts and the engine drops to a lower RPM, the airflow demand of the engine is lowered. That lowers the CASS, which may affect the tuning in and of itself. It’s something to be considered when changing transmission shift points as that change in RPM can cause a domino-effect of changes that could leave you chasing your tail.

Similarly, a change in the differential gear ratio or tire size can all cause changes to the CASS. Raising the engine-RPM at a given wheel-speed with a higher numerical gear ratio or a smaller drive tire will increase the CASS. Conversely, a lower numerical gear ratio or a larger drive tire lowers the CASS. Again, the change in the CASS may affect the tuning all by itself. Without this awareness, the effect may be overlooked, and the required tuning may be ignored.

In a race boat, a propeller pitch or diameter change would affect the RPM, the CFM value, and the CASS. Raising the engine-RPM with a lower pitch or smaller diameter propeller raises the CASS while lowering the engine-RPM with a higher pitch or larger diameter propeller lowers the CASS. For the boat racing scientists, changing the propeller cupping usually would change the RPM, CFM, and CASS as well.

For racing boats with narrow power bands that need to slip the propeller to get up on plane, high-RPM slip would most likely be at sub-critical speed. CASS would not be reached until a level of speed where the propeller hooks up in the water.

Timer controlled fuel enrichment often used on drag boats may miss the transition if the point of hook-up is a variable. Fresh water is different than salt water and affects the point of hook-up as well as the planing speed. The CASS is an added awareness of numerical control tuning.

Although the air scoop is removed, RONS Dual Terminator Mechanical Fuel Injection throttle bodies are supercharged by ram-air at speed and may need more fuel at high speed. For open-loop EFI systems and some carburetor setups, that would be the case as well.

CASS With Mechanical Fuel Injection

For a mechanically injected application, we’ll assume the following for a supercharged Top Alcohol V8 drag race engine:

  1. Engine air demand = 4,200 cfm (9,000 engine rpm with a 14-71 supercharger,  50-percent overdriven)
  2. Air scoop area = 65 square inches

Below 106 mph, there is a small vacuum in the air scoop with reduced air intake. As with the carbureted example, the fuel mixture is richer. At 106 mph, there is atmospheric pressure in the air scoop. Fuel delivery is operating at atmospheric air pressure. With no fuel system changes from the launch, the fuel mixture will lean out.

Above 106 mph, there is pressure in the air scoop. That pressure increases with greater vehicle speed. Up to 3 psi pressure increase in the scoop was reported at 250 mph in this scenario. Without any adjustment from the low end, the fuel mixture leans out. At this point, fuel enrichment from a jetting change is usually necessary. The traditional high-speed bypass jet common in mechanical fuel injection is usually in the wrong direction as pressurized air is added via the scoop.

A pressure increase of 3 psi in the inlet of a supercharged engine can equate to as much as 8 psi more boost in the manifold. That is the inlet pressure times the supercharger boost ratio (boost pressure divided by atmospheric pressure) as follows:

However, oftentimes, that much of an increase is not measured from on-board data recorders, because of cumulative changes in manifold heating and increased blower leakage during the run.

Effects of RPM-Based Air-Demand Changes

Both of the previous examples are for one engine-demand level, which occurs for one specific engine speed. In most cases, the engine is operating over a varying RPM range. To illustrate, the actual airflow demand of a blown alcohol V8 engine would change from about 3,700 to 4,600 cfm through the typical engine speed range.

When setting up a tuning plan for sub-CASS, CASS, and above-CASS fuel demand changes, you need to take into consideration the effect the engine’s changing air volume demands have on the critical speed of the scoop as both the vehicle and engine are accelerating.

Large air scoop inlet on this  NHRA carbureted ProStocker saw ram-air at higher speeds. Approximately 2 psi of air pressure from ram-air occurs on a scoop this size at 200 mph. 2 psi translates to 4 inHg of barometric air pressure increase. That can result in as much as 10-percent more horsepower at the top-end if properly accounted for.

Tuning Considerations with CASS knowledge

In many of the top ranks of drag racing and other racing classes, timer-controlled bypasses are used to achieve multiple fuel mixtures during a run. However, a throttle interruption will change the run profile if the timer-controlled events are independent of the throttle. This changes the relationship between engine RPM  and run time, and any hiccup in the run complicates the fuel mixture needs. The hiccup can also affect CASS events and performance, as well.

Most motorsports tuners use trial-and-error to dial-in a combination, often reading the spark plugs for primary tuning indications. A spark plug reading is a summary reading from the run, and these intermediate changes may not be visible from the spark plug reading alone. With all of these CASS considerations, this method is very time-consuming. Any change to the vehicle’s gearing in the transmission, rearend, or even tire size can change the engine consumption profile vs. vehicle speed and affect the CASS.

Effects of Headwinds and Tailwinds

As stated in our previous article on engine tuning for ram-air, a head-wind or tail-wind changes the fuel curve adjustment. A vehicle traveling 200 mph with a 30 mph head-wind can now be considered to be going 230 mph for the purposes of ram-air adjustments, as well as tuning with CASS. Conversely, a vehicle traveling 200 mph but with a 30 mph tail-wind can now be considered to be going 170 mph for ram-air adjustments.

The measurement of air-fuel ratios, the CASS, and the performance effects are all science. However, the decisions for what to test and implement, such as from the CASS effects, might be considered an art form. Thus, motorsports tuning is a constant task throughout the lifetime of the vehicle.

Article Sources

About the author

Jennifer Szabo

Jennifer Szabo is the creator of AirDensityOnline.com, an online resource that assists racers in tuning their engines. The site aims to help racers get the most power and consistency from their engine setups. Jennifer is a professional web developer and the daughter of drag racer, Bob Szabo. Bob is a retired welding engineer and author of 6 books about mechanical fuel injection engine tuning at RaceCarBook.com. Bob and Jennifer maintain a blog at their websites in addition to co-writing articles for various auto industry resources.
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