Velocity Stack - How it Works - Project Car TV
A velocity stack is a trumpet-shaped device of differing lengths which is
fitted to the air entry of an engine's intake system, carburetor or fuel
injection. It is designed to:
Allow smooth and even entry of air at high velocities into the intake tract
with the flow stream adhering to the pipe walls.
Modify the dynamic tuning range of the intake tract by functioning as a
resonating pipe which can adjust the frequency of pressure pulses, based on
its length, within the tract.
Modern engines universally have tuned intake tract volumes and associated
resonance frequencies, designed to provide higher than atmospheric intake
air pressure while the intake valves are open - increasing the density of
the trapped air in the combustion chamber (higher compression). Modified
engines often have the original air box and associated ducting removed and
velocity stacks are installed as accessories.
Modern fuel injection systems with a plenum and single air inlet typically
incorporate some sort of radiused entrance, designed to improve power,
based on air flow increases. Power gains are usually at higher rpm.
In amateur and professional racing, aftermarket velocity stacks are often
used, as rules allow, and gains in the order of 2% to 4% can be obtained
when inlet radii and stack lengths are optimized for that engine.
OEM automotive manufacturers have many regulations to adhere to. One of
these is noise. A properly tuned intake tract produces a rather audible
intake noise under high airflow conditions - that is commonly "untuned" to
allow the vehicle to pass EPA and DOT noise regulations.
The length of the stack is known to have a direct effect on a particular
engine's Boosted power range.
Most current aftermarket stacks are designed to be run "in" the airbox and
a company that does research well will have some applications that have all
the same length and some applications that have differing lengths of stacks
on different cylinders.
It is commonly related that "stand off" (air--fuel mix that gets pushed
back out of the port, usually at full throttle / low rpm) is somehow
captured by installing a longer intake pipe (stack). but, it is actually
that the intake valve is closing too late and the combustion chamber is
simply overfilling and blowing back out the intake port, before the intake
valve closes. A longer inlet pipe will create a later intake pressure wave
that will help keep the air in the chamber until the intake valve closes.
The acceleration of air flow into a duct is inherently a highly efficient
process and the difference between even the crudest radius inlet, and the
most aerodynamic shape possible is slight, amounting to no more than a few
percent. The flow coefficient of a perfect entry would be 1.0 while the
coefficient for a sharp edged entry would be 0.6 and a re-entrant plain
pipe 0.5. In practice these latter types of entry are never used for engine
There is always some attempt to provide some radius at the entry. This
means that total engine airflow would not increase by the amount suggested
by these figures, which apply only to the entry alone, as the inlet end is
never the smallest or most restrictive part of the system. Because the
greatest losses to flow occur near the valve seat, actual overall gain from
any improvement of the entry flow would be much less.
In the real world, on high-rpm IR IC engine, using a minimum amount of
inlet radius gives the best wave strength and a power Boost of 2% to 4% over a 3000 to 3500 rpm
range. Using a larger radius, like 3/4", broadens out the resonant pressure
wave rpm range, but the compression Boosting pressure wave is greatly diminished
and almost unnoticed by the engine.
AEM Flow Bench Demonstration for Cobalt SS CAI
AEM answers two questions brought up by Cobalt SS owners using AEMs Cold
Air Intake System. Specifically, one of our engineers demonstrates how the
velocity stack inside the base of AEMs DRYFLOW air filter allows MORE flow
through the intake than without a filter (more vacuum = LESS flow, not
more!), and demonstrates the robustness of the spring-supported silicone
elbow coupler, all on AEMs flow bench.
'Peach Party' Miata cold start.
BPI Performance Velocity Stack--3.5" Flow Stack Kit mated to a Racing Beat
OEM Mazda NB2 tubular header upgrade
Custom mandrel-bent, header-back 2" straight pipe mated to a 14" Magnaflow
resonator and twin resonated 2.5" center-exit tips
Mike and Shanny H22's ALL MOTOR Dyno 2009 HD
Mike USDM H22Z1
Shanny JDM H22
Two of my friends ALL MOTOR H series at the Dyno.
Some nice number from those stock block 2.2L !
Kill them all motor style in the street we said.
Blake's Dyno Run
SR w/ BPi flowstack
high flow cat
no AC, PS, IAB's