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About
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The intake system of an engine generally includes a filter system, a throttle body, an intake manifold,
and intake valves (and intake ports with in the head). Each piece is important in making power.
FILTER SYSTEM (Intakes) – Basic Overview
The filter system of most cars is not designed for maximum performance but for sound reduction. Upgrading
the filter system on your car can often be the single most cost effective performance improvement.
There are many types of air filter upgrades. We will cover a few.
1. Drop in filter
A drop in filter will give you minimal results because it does not replace the factory sound dampening
apparatus. Any improvement in power is limited to the differences of airflow between the stock and
replacement filter (within the stock filter system). Expect a 1-2% hp increase at most.
2. Short Ram Intake (SRI)
A SRI system replaces the factory air box and filter with a cone filter and a short air tube. These
systems will in theory provide a high peak horsepower result, but are handicapped by breathing in hot
air from underneath the car hood. A SRI provides better throttle response that a CAI or most hybrid
intakes. These intakes may dyno higher than CAI and hybrid Intakes (depending on the testing methods),
but RARELY out perform in real world situations. Expect a 2-3% hp increase.
3. Cold Air Intake (CAI)
A CAI system replaces the factory air box and filter with a cone filter and a long tube that extends
the filter to an area of the car that has access to fresh air. Fresh air is cooler and denser than under
hood air and results in greater HP results. The long tubes often take advantage of acoustic resonance that
provides increased efficiency in a certain power band (ie: power). This point is sometimes referred to as
the AEM hump. Expect results between 4-8% hp increase.
4. Ram Air Intake
A ram air intake system is set up so that outside air has direct access to the air filter. In theory this
creates a ramming effect of the air into the engine. In practice the ramming effect is small. IE: At 320 kph
(200mph) the ramming effect is ~ 1psi minus plumbing loses (1 ATM = ~14psi). In a streetcar this translates
into very little gain in HP (even in the ¼ mile). The biggest factor in these setups is the cold air. Expect
results similar to CAI with possible small gains (1%-2%) in real world gains near 100mph. This type of setup
would be more effective in a land speed setup, ie: Bonneville.
5. Hybrid Systems
Hybrid systems are intakes that cannot be categorized into any of the above setups. The Comptech ICE BOX is a
good example. It includes a SRI and an air box with an air snorkel (tube to get cold air). Performance is similar
to a CAI, but without the midrange resonance effect. Peak SRI gains are effectively negated by the air snorkel.
The ICE BOX is often used in wet climates where a CAI risks hydro lock.
For a more advanced/detailer explanation of the physical effect occurring in intake filter systems see The Physics
of Intake Systems below.
Throttle Body
The size of throttle body effects engine response and peak hp. Here are some simple rules.
Large cross-sectional area increases throttle response.
Large cross-sectional area increased high rpm torque.
Small cross-sectional area decreased throttle response.
Large cross-sectional area increased low rpm torque.
Note: that “required” cross-sectional area is dependant on HP.
The following equation is a good rule of thumb for throttle body diameter (~ +/- 5%)
D (“) = sqrt[(WHP x 4)/(AP x CR)]
sqrt = square root
D = diameter of throttle body (single)
WHP = wheel horse power
AP = Atmospheric Pressure (14psi)
CR = Engine compression ratio
Cross Sectional Area = 3.14 x (D/2)
Note: Some engines have dual throttle bodies. The cross-section area of the dual TB should equal the area of the single
in the equation above.
Intake Manifolds
Choosing the right intake manifold is key to the type of characteristic a motor will have. Things to consider are runner
cross-section area (intake velocities), runner length (Helmholtz resonance), and plenum volume.
Some simple rules:
Short Intake Runners help high RPM torque.
Long Intake Runners help low RPM torque.
Large area intake runners help high RPM torque.
Small area intake runners help low RPM torque.
Plenum volume should be kept between ~40-50% of engine displacement. Small plenum volumes lower the rpm of peak torque.
Larger plenum volumes increased the rpm of peak torque. A plenum that is too small for the motor will starve the engine
of air, a plenum to large will bog it.
For detailed information on these effects check out the physics of intakes below.
Physics of Intake Systems
Cold Air:
Cold air is denser air. Engines need two things to make power, air and fuel. For every 7 degrees (F) of temperature
difference air density changes by ~ 1 %. Having access to cooler air can have a huge effect on hp.
Acoustic resonance:
Sounds are waves in the air, specifically longitudinal waves. These type of waves cause pressure waves (90 degrees
out of phase with wave itself) that are directly proportional in strength to the amplitude of the sound wave. It is
possible to utilize this effect to increase intake air pressures within the intake tube. In most instances the
amplitude of a sound wave is not that large. During resonance however the sound wave amplitude becomes much larger
(and so does the pressure wave following it). During this resonance it is possible to gain significant amounts of
torque and actually create positive manifold pressure (above 1 atm). This effect is sometimes called the AEM HUMP,
where power spikes ~10hp for a couple hundred RPM around 4500 RPM in their cold air intakes. The resonance frequency
of a pipe depends on the length and the diameter of the tube.
Fn= n (v/4L)
n = wave number, v = velocity of sound (~340m/s), L length of pipe
Note: v the speed of sound changes with temperature.
The diameter of a pipe has a small effect on the resonance frequency as well. The sound wave behavior remains consistent
with that of a pipe for ~.6 times the diameter (D). So
Fn= n (v)
___________
4(L+.6(D))
For a 1-meter intake pipe this translates into a resonance frequency of 85 hz or 5100 RPM. The intake pipe reaches
resonance frequency when the driving force of the system (the engine) is also at that frequency. This is RPM/60;
rotations per second.
Ram air effect:
Ram air is the process of using the relative airspeed of the outside air to ram itself into the motor. This effect
is negligible at low speeds, but at high speeds it can be utilized to a small effect. Since air is non-compressible
at speeds less Mach 1 the pressure created by velocity can be calculated using Bernoulli’s equation for dynamic pressure.
Static Pressure + Dynamic Pressure = Total Pressure
Dynamic pressure = ½ (p) V2
P = air density
V = Velocity.
This translates into:
P (psi – above atmospheric) = (.0000176) V2
V = Velocity (MPH)
Venturi effect:
The venturi effect makes uses the laws of fluid dynamics, specifically the “equations of continuity”. Most people think
of pressure with reference to Boyle’s Laws for pressures in closed containers; P1V1=P2V2 (P= Pressure, V= Volume).
This equation is for static fluids and gases. When gases are moving the rules change a little. For gases in motion the
amount of flow is constant at the inlet and outlet. This means the area of the inlet/outlet determines flow velocity.
This makes since and is often utilized by someone with a garden hose. Placing your finger in front of the hose creates
a smaller stream of water that is moving much faster.
Equations of Continuity Q = V1A1=V2A2 (non compression gas fluid)
Q = Flow Volume
V = Flow Velocity
A = Area
This is where things get “weird”. Bernoulli’s equation tells us that the total pressure must remain the same at the inlet
and outlet. However we also know that the equations of continuity tell us that the dynamic pressure increases with air velocity.
Ps1 + Pd1= Pt = Ps1 + Pd1
Ps1 = Static pressure
Pd1= Dynamic pressure
However we also know that the equations of continuity tell us that the dynamic pressure increases with air velocity.
Dynamic pressure = ½ (p) V2
P = air density (constant)
V = Velocity.
This means that static pressure must go up within the intake tube when flow velocities drop and cross sectional areas get larger
(and vise versa). This seems counter intuitive, but here lies the fun of fluid dynamics.
So what does this mean and how can I use this to advantage…
Two ways:
1. Increase velocities to tune for a specific RPM. The Spoon Venturi intake manifold gasket (rubber chicken) has a reduced
intake manifold gasket size to increase velocities into the motor. Peak torque occurs when intake flow rates are ~240-260
ft/s. With a smaller runner area, peak torque is lowered. This helps low end torque on smaller displacement motors at the
expense of high-end power.
2. Reduce velocities to create greater pressure to help push air into a new camber. This technique can be utilized directly in
front of the throttle body to help flow air into the intake manifold. By increasing the intake diameter in front of the TB
(ie: intake diameter is larger than the TB) the pressure is increased before the throttle body opening. This increased
pressure can help move air through the TB and into the intake manifold. Larry Wilmer (of Endyn) suggests a 25% difference
between intake and throttle body cross-sectional area directly before the throttle body.
Helmholtz resonance:
Applies to intake manifolds. This resonance effect on the intake filter system is negligible. Helmholtz resonance applies to IM and
is due to the intake valves opening and closing. This results in pressure build-ups and releases and air traveling back and forth
with into the intake manifold. The effect is comparable to a mass on a spring, oscillating up and down. Runner length (and area)
and plenum volume on an intake manifold are sometimes designed to effectively use this pulsating effect to increase efficiency in
certain rpm ranges. In the 1960’s Chrysler did a lot of research in the area of Helmholtz resonators. A quick little formula was
found to find the approximate area for this effect.
RPM ~ 88400 / L
It is found from the following.
Speed of Sound (V)
V=sqrt(Y * R * T)
Y= 1.4 (specific heat of air)
R= 286m^2/s^2/k (gas constant of air)
T=273.15 k (0 c)
V=340m/s=13400”/s (~15 c)
V=355m/s=14000”/s (~40 c)
D = Distance
L = Intake Length
V = Velocity
T = Time
F = frequency
N = Round trips
RPM = RPM of Peak Efficiency
D = Vt
T = D/V
F = 1/T
D = 2L * N
RPM = (V/D)*60
RPM ~ 84000 / L (5 return trips).
The shock wave or wave front from the valve events oscilates/bounces back and forth several times (5 times) between valve events.
Ideally at "resonance" the wave front hits the valve as it opens.
That is the theory behind the N = 84000 / L ~ (V/2L *5)* 60 (depending on V - seed of sound)
If your intake manifold runner length (to the valve) it is possible to hit multiple “resonance points”. With a runner length of 16”
resonance will occur at N-5,4,3 with peak efficiencies at ~ 5250, 6550, 8700 RPM. A change to 14” will produce peak efficiencies
at ~ 6000, 7500, & 10,000 RPM.
N is the ~ engine RPM of peak torque
L (“) is runner length from the IM plenum to the valve.
Short Intake Runners help high RPM torque.
Long Intake Runners help low RPM torque.
Intake velocities:
It is generally accepted that peak torque occurs when flow velocities are ~ 240-260 ft/s into the motor. These velocities depend on
engine displacement, flow area, and RPM. The following equation can be used to determine peak torque due to intake runner area.
Peak Torque (RPM) = 88200 * Area / Cylinder Displacement.
Veris
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