I worked a bit with helmholtz resonance at my summer ME internship. cool stuff.

Looks like BMW has already invented/proven the theory with something called "DIVA"

I feel like I should design some sorta system like this for our fsae car... keep us updated.

Anyone have any technical information regarding best dimensions for the intake runners before I do the math and "over engineer" (i use the term lightly, because the end result will likely be crude at best :D ) the range of runner lengths?

If I remember correctly, the Mazda 787B had adjustiable length velocity stacks too, and they used the headlight motors from the same year RX-7 to move them. Aparently they were tough enough to survive 24 hrs and win
Le Mans in 1991 on an engine that redlined a bit higher then ours

Im sure they were tired into RPM somehow, not sure exactly how they worked though, if different pulsewidths would cause them to extend only so far or what ever, and im sure that they were geared so they would go from max to min position quickly... the headlights on mine move really quick, so i can see them extending and retracting the intake stacks faster then you reving it from idle to readline in nuetral

Theirs seems to be made of a FRP or some other composite material... probobly good so it does not heat soak i guess

Could probobly get more info from rx7club about the specifics

http://www.rx7.net.nz/r26b-2.jpg

That is very cool!

Just saw a neat article about this.

Porsche is using some form of variable length intake on one of their turbo cars. The intent is a little different though. Typically, like on the 787B, the intent is to use the resonance to compress the air into the cylinder.

Well, this creates excess heat.

So what Porsche is doing, is while not in boost (or in low boost, not sure), their manifold acts like normal and helps compress the air into the cylinders. Then as boost pressure builds, the harmonic tuning changes such that the manifold causes the air to expand. They're making up for the lost charge pressure with additional boost and allowing the intercooler to do its job. Just avoiding the addition heat generated by harmonic resonance compression.

Neat stuff....
--Alex

Dlaitini, thats fantastic! I just dont know how well it would fit into our engine bays :P

They are compensating for heat caused by harmonic tuning? It sounds like these guys are running on the ragged edge! I'm not sure it would be a good idea for porsche to put this into production cars, but good on them for engineering to such an exacting requirement!

Yeah that is what I was thinking. That has to be well into diminishing returns territory and not that important on a street car. That is the kind of design detail that makes high-end racing engines so interesting.

for some reason, the only way packing wise on a miata, i see an intake shaped like a trombone.

The runners will increase and decrease their lenght, and if also tied into an Plenum, can also use forced induction if the tolerances are close enough on the sliding part... or if enough air is pushed in to overpower what leaks, lol

I'm currently considering an idea which involves a rotating cylinder to increase/decrease runner length. An rotating inner cylinder would be completely encased in a non-moving outer cylinder with the exception of a seal where the servo connects to the inner cylinder assembly. The seal would look similar to the current VICS actuator seal.

I just need a better way to prototype it than what I'm currently doing.