If your racing vehicle features a forward facing intake air scoop, tuning for very high rates of speed is a difficult prospect. The density of the air entering the air scoop increases dramatically at high speeds compared to when the vehicle is in a stationary position. Beyond a critical air scoop speed, the density of air becomes noticeably greater and has dramatic effects on the engine tuning.
As the speed increases, the fuel volume can be increased in an appropriate amount in order to burn with that extra air being forced into the scoop. As a result, more combustion energy is made. When a vehicle’s forward velocity passes 200 mph, inlet air pressure can approach 2 psi over the current atmospheric air pressure. That increased pressure at high speed can add over 10-percent to the intake air density of a naturally-aspirated engine. In a supercharged engine, that increase can be greater. The increase in pressure is multiplied by the boost ratio between the blower and the engine. A forward-facing air scoop with no obstruction from the body or frame protrusions results in the least air turbulence and best air intake conditions for power.
Atmospheric Weather Changes At Higher Speeds
When the racing vehicle is at the starting line waiting to begin its run, the amount of oxygen in the atmosphere is affected by the temperature, atmospheric pressure, and humidity. This amount is fairly stationary as the vehicle is not moving and the atmospheric pressure does not vary greatly from second to second. Air is pulled into the engine where it is used in the combustion process to generate power. You can learn more about how the effects of weather and atmosphere affect the tuning process in our previous articles: Explaining The Effects of Weather on Mechanical Fuel Injection and Elevation Changes and Engine Tuning.
As the race car or race boat goes faster, the forward-facing air scoop helps the engine to ingest more air than it would if the vehicle were stationary. Inlet air pressure increases with speed, although it does start to taper off. A pressure front will build up in front of the scoop. Some of the air will spoil around the inlet area as the speed increases, which has an effect on the intake of forced air.
Combustion Energy Gains From Other Sources
Due to the effect of boost ratio in forced-induction engines, greater increases in fuel amounts are necessary to compensate for the ram air into these engines. For tuners, this oversight can be a considerable cause of lean-out at high speeds. Provisions are needed for increased high-speed enrichment for supercharged engines.
When considering air/fuel ratio, the amount of air becomes greater at higher rates of speed. Therefore the amount of fuel needs to be increased in order to maintain the appropriate ratio.
Roots superchargers are known for building a lot of heat during a drag race run. That heat reduces the amount of mass airflow at a given boost level. The accumulated effects on boost from both ram air and higher blower temperatures complicates the decision for how much fuel to add for higher boost at high velocity. The correct amount of added fuel should primarily be based on the increased mass of air from ram air.
Combustion Energy Losses From Effects of Burning
Ram air increases the weight of air into the engine at high speed. Without fuel enrichment, that leans out the air/fuel ratio and eventually leads to less power from the engine. This is one reason that retarding the spark is often done in high gear instead. It is more of a band-aid to compensate for the increased flame speed from an unexpected lean-out.
Unfortunately, band-aiding a lean fuel mixture only works up to a certain point. Beyond that, further leaning of the mixture actually slows down the flame speed, which leads to reduced power. In these cases, retarding the spark is in the wrong direction. With this type of energy loss, tuning can be extremely difficult. Proper high-end fuel enrichment is an alternative that can provide a performance gain instead.
Better tuning occurs when considering the effect of flame speed.
Tuning For Air Flow Changes From Velocity
To compensate for ram air effects at increased speeds or gear changes with engines using mechanical fuel injection, nozzle or main bypass changes may be necessary.
On the other hand, open stack-type fuel injection may need additional leaning out at high speeds compared to lower speeds. This is due to a reduction in air intake from high-speed air flowing over a vertical open stack inlet. Due to the Bernoulli Effect, that horizontal flow of air over the inlet can actually pull some of the air away from the inlet.
Naturally-aspirated engines with fuel injection normally lean out the top end for reductions in volumetric efficiency beyond the torque peak RPM. At high vehicle speeds, they may require a reduction in that amount of lean out. That is from added air inlet through a forward-facing air scoop or air valve hat. This is due to air pressure from ram air into the air intake. There is almost a 5-percent boost in engine airflow into a forward-facing air scoop from ram air in clean air at 200 mph. Less engine volumetric efficiency at the high end reduces the fuel need. More air from ‘ram air’ at high speed increases the fuel need. The combination can present a challenging tuning problem.
Add or subtract a 20 mph wind, for example, and tuning becomes even more challenging. Racing into a 20 mph head wind can reach 6-percent inlet boost from ram air at 200 mph. The amount of enrichment from speed would need to be greater.
Racing with a 20 mph tail wind causes the opposite: a reduction in that boost down to about 4-percent would be the result at 200 mph with less fuel at the high end to maintain the same air/fuel ratio. The amount of enrichment from speed would need to be less to maintain the same air/fuel ratio.
Air Inlet Considerations
The location of the air inlet at a high pressure point or a low pressure point on a race vehicle body can affect the amount of boost due to ram air. One example came from a racecar that reported a boost from ram air that was considerably higher than what was expected. The increase was from a pressure wave coming from the body that was lined up with the scoop.
Air/fuel ratio profiling (from jetting changes) can be done to provide adjustments to the amount of fuel for the following:
- Ram air effects in forward facing air scoops with an increase in speed
- Reductions in air intake in open stacks at higher speeds. This is caused by the Bernoulli effect from fast-moving horizontal air flowing over vertical stack entries at those higher speeds.
- Ram air effects for gear changes from a sudden loss of RPM. This reduces the air consumption to the engine, bumping the boost from ram air in the hat or scoop until the engine revs back to high speed.
- Auto or boat racing on long straightaways against or with head winds
- Reversion from camshaft timing that appears and disappears at different engine RPM causing differences in fuel needs
- Effects from inlet ram tubes and exhaust pipe tuning influences.
Engine Tuning Considerations
For ballpark tuning considerations for ram air, we use a 200 mph standard. At that speed with no head or tail wind, the air pressure increase into a forward facing scoop or fuel injection hat is 2 psi of pressure and 4 inches of mercury. This represents pressure increases without considering the engine air consumption through the scoop or hat. When also considering the engine air consumption through the scoop or hat at 200 mph, we use 1 psi of pressure increase and 2 inches of mercury (or Hg) for the pressure increase.
This rough estimate would change for different engine air consumptions. For smaller engines in streamlined vehicles going 200 mph, the air consumption would be less and the pressure increase from ram air would be more. For larger engines in less aerodynamic vehicles or those in drag racing at maximum acceleration, the air consumption would be more. As a result, the pressure increase would be less. These changes are relative to the corresponding values in stationary air such as on a dyno or in slower-moving events such as tractor pulling.
A head- 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. Conversely, a vehicle traveling 200 mph but with a 30 mph tail-wind can now be considered to be going 170 mph for the purposes of ram air adjustments. In both cases, the fuel curve adjustments can be dramatically different.
Effect Of High-Speed Bypass
Naturally-aspirated racing engines often use a high-speed bypass to lean out the fuel mixture at high engine RPM. If the racing vehicle is equipped with a forward-facing air scoop or fuel injection hat, less reduction of high speed bypass fuel flow is needed with speed increases. For stack-type fuel injection setups, more reduction of high-speed bypass fuel flow is needed. With speed increases, the air velocity over open stacks may reduce the actual air intake.
The high-speed bypass is often used to lean out the top end in supercharged engines with high blower overdrives. Blower efficiency reductions occur with high blower revolutions. In many racecars and race boats, ram air can recover some of that air loss from blower efficiency reductions. In many cases, no high speed lean-out is needed.
Examples of Special Cases With Ram Air
The tall blower hat air scoop in the below photo is a vivid example of a purposeful ram air scoop. Prior instrumentation by Don Jackson Engineering revealed intermittent pressure drops in blower hat scoops that were lower. Pressure drops were a result of vortex air turbulences from the racecar roll cage immediately in front of the air scoop inlet. To fix this, the air scoop inlet was redesigned to be well above the roll cage to capture clean air. A front opening was made as large as governing rules would allow. For optimum air feed into the blower, the large width of the air scoop narrows down considerably to a smaller inlet opening.
For our drag racing Altered, we used an old Crower eight-port fuel-injection and air scoop hat. This configuration used a vertical throttle assembly that was fed by a large air scoop casting facing the air stream. The scoop had an air space that extended around the throttle inlets and also behind the rear-most throttles.
At 200 mph speeds, we observed only a small boost increase. That increase was considerably lower than what the calculations indicated. We found it was a result of turbulence inside the air scoop as the air transitioned from the horizontal inlet into eight smaller throttle ports.
Additionally, we ran large amounts of fuel through the blower hat into the blower, instead of below the blower directly into the inlet ports. Our rich mixture in the hat reduced the boost reading from excessive manifold cooling.
In one of our blown alcohol combinations, we determined the following:
- Air/fuel ratio stationary for best power: 3.4 to 1
- Air/fuel ratio at 200 mph with ram air and no fuel enrichment: 3.6 to 1
- Air/fuel ratio at 200 mph with ram air and appropriate fuel enrichment: 3.4 to 1
- Power stationary: 1,800 horsepower
- Power with ram air: 2,000 horsepower
The effects of ram air at high speeds can cause considerable boosts in power or unexpected complications. Taking into effect engine performance at higher speeds becomes complicated at times but can mean the difference between a screaming run and a surprise slump.