JOE LEEN

The Helmholtz resonator analogy has led to the term

twohoos

Where does R appear in the equation?

JOE LEEN

a is the speed of sound, the equation is given by

a = ( (gamma * R * T) ^ 0.5 )

a ---- Speed of Sound ---- Meters/Sec

gamma ---- Ratio of Specific Heats

R ---- Universal Gas Constant ---- J/Kg K

T ---- Gas Temperature ---- Kelvin (K)

Gamma is approximately 1.4 for air and 1.35 for exhaust fumes

R is equal to 287 J/Kg K

Road Rage

Thanks for the great detail - so was I on track that Helmholtz factors figure into the intake/exhaust tract tuning?

jzr

Absolutely. F1 cars utilize motorized intake runners that constantly change length throughout the RPM range to keep the engine in its "supercharged" state. A number of production cars use a more simplistic approach of having 2 or more possible intake paths, with a valve to switch between them at a set rpm.

In practice for us mortals, evidence of the Helmholtz resonance is most apparent when dyno-tuning cars with aftermarket or homemade intakes. Changing the length (or diameter) of the tubing from throttle body to filter moves the location of the torque spikes in the RPM band. Where possible, the final intake length can be tuned to produce the spike where it's most beneficial for the given application. That's how AEM, Injen, and K&N make their money on CAI's...

MacGyver

j,

Exactly how do you lengthen/shorten a solid pipe with motors? I understand the principle behind the reason, but I can't get a pic in my head how a motor can lengthen a pipe.

jzr

Thruthfully, I don't know the mechanism by which this is accomplished. I've often thought of building something like this for the race car, and I keep coming back to some sort of imaginary telescoping tube thing.

Here's a paper I came across some time ago on the subject, for further reading:

http://pdmec4.mecc.unipd.it/~cos/DINAMOTO/...isuonatore.html

Gernby

I picture something like the slider on a trombone.

MacGyver

That was my only thought...the moving piece HAS to be on a bend. It can't be on a straight piece. I'm trying to imagine finding enough space in an engine bay to move four 2.5-3" pipe bends back and forth!

Gernby

I wonder if an adjustable length Helmholtz resonator would work if it was designed similarly to the one on the OEM exhaust. The air flow could follow a fixed length passage from the air filter to the TB, but with an adjustable Helmholtz resonator (closed ended) hanging off somewhere in the middle. It seems that this could be done with a straight pipe and a piston connected to a hydraulic or mechanical actuator.