Helmhotz Resonator

[Koncretekid]

I had decided to add "ram air" cool air induction to my single cylinder BSA powered partial streamliner and had already started on an airbox when I remembered reading about a Helmholtz resonator.  After searching the web, the best article I found was by Kevin Cameron who stated as follows:
 "a forward-facing intake, operating at an unlikely 100% pressure recovery, is only worth about a 3% pressure gain at 150 miles per hour.  Yet these “ram airboxes” were regularly showing much larger gains of 10 or even 15% more torque.  Where was the extra power coming from?"(http://www.thunderproducts.com/airboxesdynotech.htm)

He went on to state that some of the improvement undoubtedly came from the cool air, but that more of the gains came from the Helmholtz resonator effect. The formula for determining the frequency  expected was: R(cps) = 5280(a number that even I can remember) x square root of A/(VL), where A is the area of the inlet(s) in square cm, V is the volume of the airbox in cubic cm,and L is the length of the inlet pipes in cm (only the length of one pipe, if more than one is used).

In my case, "V" will be 15.24x15.24x15.24cm,  "A" will be  two 2" diameter tubes, or about 40.5 square cms, and L will be the distance from the carburetor to the front of the box, which is about 76cm.  Doing the math results in a frequency of 64.77 cps, or 3886 cpm.  Because I have a 4 stroke motor, I'm assuming that this will be equivalent to 7772 rpm (an intake pulse every other revolution).  Not too far off, but I would like to move this up to 8400 rpm.  I can do this by reducing the area of my inlets, but I won't until I can put it on a dyno.

So far, so good, but the air filter is a problem.   I built my box around the filter, which will probably kill the resonance.  I've read that the filter should be upstream from the resonator box, which is a little difficult unless I can find some that will fit in a 2" diameter tube.  I'm also wondering about my inlets, which are actually only 1-1/2" at present.

I'm looking for confirmation of my idea, and suggestions for air filters.  Also, since my airbox is .030" aluminum, I'm wondering if I'll get any resonance at all. I could wrap it in fiberglass and resin.
Tom

[manta22]

Tom;

Yes, an air filter in the box will damp the resonance. Flexible sidewalls will also kill the "Q" of the resonator. You can stiffen the walls by adding some exterior angle, marine plywood, etc. Think "subwoofer"..........

Regards, Neil  Tucson, AZ

[Dean Los Angeles]

Yet these “ram airboxes” were regularly showing much larger gains of 10 or even 15% more torque.

Yeah, and I've got a bridge in Brooklyn for sale too.
My PT Cruiser exhibits the Helmholtz resonance when I open the moon roof. Big time. At 54mph it will batter your eardrums. Cracking a window kills it.

Which brings up two points. You don't have a single opening, you have two. The air coming in has to go out to the engine. The effect is from the compressive factor in the air. You're going to be sucking out as fast as you are sucking it in, so where is the compression?

Ok, so maybe it does work. The fact that NO ONE is using it in racing might tell you something. And if it did work, it could be tuned to work at 8,400 rpm. Not 8,410 or 8,390. One frequency.

This is an area where guess work and calculations need to be backed up with dyno runs. You certainly need a pressure sensor in the air box to validate tuning.

[Rex Schimmer]

Thanks for posting the link to the Kevin Cameron article, great reading and by one of my very favorite tech writers. After reading the article it probably explains why some of the fastest cars that use motorcycle engines retain the original stock air box. Look at the Costella/Cunha streamliner or the the three engines in Jim Knapp's Barnyard Bearcat streamliner, all still fitted with the stock Suzuki air box. Horsepower is where you find it.

Rex

[dw230]

How will you duplicate the 150 MPH intake air on the dyno?

DW

[Tman]

How will you duplicate the 150 MPH intake air on the dyno?

DW

move the dyno REALLY FAST

[hotrod]

Yet these “ram airboxes” were regularly showing much larger gains of 10 or even 15% more torque.  Where was the extra power coming from?"

Increased volumetric efficiency. It is just a variation on conventional tuned intake runners. If properly designed you get pressure waves arriving at the intake valve at critical times at the tuned rpm. If the positive pressure wave arrives just as the intake valve opens, or just at the moment the intake airflow starts to slow down at the top of the intake stroke, it improves total filing of the cylinder. Such tuned systems often reach 105-115% volumetric efficiency at their ideal rpm.

See:

Scientific Design of Exhaust and Intake Systems (Engineering and Performance) [Paperback]
Philip H Smith (Author), John C Morrison (Author)

The charts and graphs in that book taken from real running engines is an eye opener about the complexity of the cylinder filling process and all the factors that impact how much fuel air mixture actually ends up trapped in the cylinder when the valves close.

Older book but well worth reading.
It also discusses the historic evolution of intake and exhaust tuning and changes in the understanding of valve timing events and their impact on the cylinder filling process.

Larry

[manta22]

A Helmholz resonator is a variation on the familiar "ram tuning" that one gets from long stacks like on a Hilborn or Crower injection manifold. It isn't magic.

Regards, Neil  Tucson

[SaltRat]

"hanks for posting the link to the Kevin Cameron article, great reading and by one of my very favorite tech writers. After reading the article it probably explains why some of the fastest cars that use motorcycle engines retain the original stock air box. Look at the Costella/Cunha streamliner or the the three engines in Jim Knapp's Barnyard Bearcat streamliner, all still fitted with the stock Suzuki air box. Horsepower is where you find it."

It is very hard to outthink the R & D of a large corporation.  Even though thats what we all try to do. 

[wobblywalrus]

The Helmholtz resonators are most popular with the folks building fast Japanese in-line four cars.  Search for books by Vizard.  He uses this method.  This is a quote from Vizard in "How to Build Horsepower."  "To be of the greatest benefit, a Helmholz resonator needs to boost power just below the RPM at which the intake runner length comes into play."  This would be ideal for the Beezer during the shift from third to top.

Vizard goes onto say "Although this works well with four cylinders attached to the plenum, the best is to have three cylinders so attached."  This is a sticky problem.  Most of the formulae I have seen are empirical and based on four or more cylinder engines.  They might not apply to a single.

A suggestion is to discuss this with the people who develop the engine simulation programs.  It may be possible to virtually develop a resonator and to fine tune it on the dyno.  That is certainly what I would do.     

[Jack Gifford]

Intake tract length of over 30 inches "tuned" @ 8,400 RPM   I doubt that's correct.

[Koncretekid]

There are two different phenomena which will affect the performance of my airbox.  The first is to simply take advantage of the free supercharging effect of 137 (hopefully faster) mph air being forced into the box (ram air effect).  For this, I think I just have to ensure that my intake runners (not to be confused with the intake manifold and carb runner) will flow sufficient air not to starve the motor.  With two 2" diameter runners and only 30 cubic inches of motor, I think this will be sufficient (I did the math somewhere, but can't find it now).  No resonance is required, and I might get 1 psi boost, which is about an 8% gain in pressure.  Similar to running at sea level. And a subsequent 2-1/2% gain in speed (cube root of 1.08), which equates to 140mph.

The Helmholtz effect has to do with trapping and reflecting the waves that are exiting the carburetor mouth, and if done correctly, will reflect these waves at the correct time to give additional boost at a given RPM.  If this RPM is at my WFO throttle at the peak horsepower and at my optimum gearing, I just might get a little extra.

Only the Great White Dyno (or perhaps the Loring AFB) will prove if the two theories can work together.

Tom

[Jack Gifford]

Hello? Testing... 1.. 2.. 3..

[Jonny Hotnuts]

I remember a long time ago I had a conversation with an engineer from Suzuki and we spoke of Helmholtz and MC airboxes. My recall of the details are somewhat clouded....

but (*some things I can remember.....)

There is a reason the boxes are V shaped, with the intake going into the point of the V and the TB in the wide area. Something about the speed of air going into the box from ram air as it enters the box....because the volume was increasing would slow down the wave but it maintains even speed as the intake draw tries and speed up the pulse. It was about keeping the waves constant or wider without compression.

It is also necessary to have the intake tube taper as it goes into the box. The length, taper in and out were important.

A STOCK BUSA MOTOR WILL GAIN HP BY A SMALL INCREASE OF THE SIZE OF THE OEM AIRBOX!


The funny thing is that there is REAL horsepower gains from a properly designed Helmholtz airbox. MC guys know it, OEM car/truck manufactures know it, F1 knows it....
I am willing to bet that once someone has a good helmholtz airbox design for a BB or SB that can be used in LSR, some long standing records will go up about 15%.

~JH

[John Burk]

Jack Gifford asked
"Intake tract length of over 30 inches "tuned" @ 8,400 RPM  huh I doubt that's correct."

For space reasons intake runners are usually tuned for the 3rd cycle (3 round trips for the pressure wave) . Over 30" for 8400 rpm sounds like it's tuned for 1 round trip .

Every time a positive of negative pressure pulse returns from the open end of a tube it changes phase . On the 1st and 3rd round trip it arrives at the valve as a pressure wave . On the 2nd and 4th it's negative . That's one reason the dyno might show a hp dip at maybe 5000 rpm when the when the negative 4th cycle returns .