(Home) Hiace air intake Helmholtz resonator

well, i don't think i've a problem as such but just curious. this is a repost to one many months ago....looked at the web info on resonators but i still don't understand.
at around 2900 rpm (stationary)the intake air is halved or less (the intake by my right ear goes very quite...and my hand over the intake demonstrates the reduction of air intake). any more or less rpm and the intake doubles (or more).
whats going on here? is this its most efficient or least efficient running speed?
vehicle is hiace supercustom 2.8 diesel,
as i said, not a problem for me....but i would like to understand why.......

- (#14416) chris turner, 5 Dec 04 15:34

In the case of a TD it isn't really about efficiency. In a normally aspirated application, the resonator can have some effect on the "tuned length" of the intake, which can affect torque. Simply, as each inlet valve opens and closes it created pulses in the inlet tract. Likewise exhaust valves and pulses. At certain combinations of revs and load these will positively or negatively interact to produce pressure differences.
Negative pressure in the exhaust helps remove spent gasses, positive pressure in the inlet forces fresh air in. See, not so relevant if you've got a turbo in the way of things...

Now, any column of air in a tube will resonate in this manner. Just like an organpipe, in fact. So whilst the tubes are tuned to make the engine efficient, that must also be tuned to make a nice noise. Your Helmholtz resonator creates a body of air "attached to" the tube in question and the air moving in the tube moves the air in the resonator box too. The effect is of reducing the intensity of a particular frequency of "drone" produced by the tube.

- (#14416) david miller, 6 Dec 04 02:43

Thanks David.....still a bit perplexed why all goes quiet (low intake) at 2900rpm only. Guess in theory its all out the pipe and cool running at this narrow band.
Comparatively speaking at 2900 its silent...anywhere else there is a hell of a drone through the intake.
If I bypassed the resonator altogether and fitted a remote filter instead...would this be a bad move?
Its a naturally asthmatic...no turbo.
I had got used to the racket, but its been resting in the garden a couple of months now. However its MOT time....and I'm rediscovering the quirks I had previously forgotten about...!

- (#14416) chris turner, 6 Dec 04 04:29

Well, you could try removing the resonator and blocking the hole left up. It'll do no harm, but might give you an even more unpleasant drone at some other revs...

- (#14416) david miller, 6 Dec 04 07:47