Helmhotz Resonator

[SPARKY]

The Original GM Vortec V-6 engs actually had a "gate" that swung open during the flat spot to take out a torque dip between ? and ? really took out the dip.

[Koncretekid]

As I stated, my original intention was just to take advantage of the ram air effect.  If I can get resonance at the correct frequency, that will be a bonus.

But one of my basic questions is, at what frequency for a 4 stroke engine? At 8400 cycles per minute (my peak hp RPM), or at 4200 cpm, because I only have 4200 intake valve openings at 8400 RPM?

I'm concerned that in the formula given for Helmholtz, that the "L" in the formula, for the intake runner length, will not work for long intake tracks that I need to get to the high pressure area at the front of the fairing.  However, Dyno work is scheduled in 2 weeks at NRHS Performance (Dan Dunn), so we will see if there is any here to be gained (or lost). We won't be able to test the 140 mph wind effect, unless I rent a couple of high speed leaf blowers.

These photos show the box and the runners.  I reinforced it with fiberglass mat and resin, and the Bondo is just to smooth out some of the irregularities.   My friend Doug Edwards whipped up a short airhorn to replace the air filter.

Tom

[hotrod]

The L should properly be referred to as the "effective length" of the intake pipe. Reflection and reversal of pressure occurs at the first major change in cross sectional area of the intake pipe. If that change is large enough to get full reflection, the engine does not know that there is more larger intake pipe beyond the reflection point. If the L you calculate is shorter than your total intake run, put a reflection point in the intake at the proper distance from the intake box. If it is longer tune it for an odd harmonic (ie 3rd harmonic).

The engine has maximum volumetric efficiency at your torque peak rpm that is why the torque peak occurs at that rpm.
I would tune it for the high rpm range where volumetric efficiency starts to fall off rapidly, probably for a point near the rpm you will drop to at your last gear shift as you enter the traps to be sure the engine can pull the gear change.

Best way to test would be to build a test intake that you could vary the volume  and intake length on, for testing then once you find the ideal dimensions on a dyno figure out how to fit all that into the bike.

By the way a resonator needs to be "high Q", that means low loss, so you want the walls of the intake box to be really stiff so they don't absorb and kill the pressure waves by flexing.

Larry

[Queeziryder]

Tom,
Fundamental design flaw !!!!

If you only pressurise the air inlet then you will get no fuel flow, as the differential pressure across the jet is what draws the fuel up.
You need to have the whole carb inside the airbox, have a look at the NHRA pro-stock Suzi's that will give you a better idea.

HTH
Neil

[Koncretekid]

The L should properly be referred to as the "effective length" of the intake pipe. Reflection and reversal of pressure occurs at the first major change in cross sectional area of the intake pipe. If that change is large enough to get full reflection, the engine does not know that there is more larger intake pipe beyond the reflection point. If the L you calculate is shorter than your total intake run, put a reflection point in the intake at the proper distance from the intake box. If it is longer tune it for an odd harmonic (ie 3rd harmonic).

The engine has maximum volumetric efficiency at your torque peak rpm that is why the torque peak occurs at that rpm.
I would tune it for the high rpm range where volumetric efficiency starts to fall off rapidly, probably for a point near the rpm you will drop to at your last gear shift as you enter the traps to be sure the engine can pull the gear change.

Best way to test would be to build a test intake that you could vary the volume  and intake length on, for testing then once you find the ideal dimensions on a dyno figure out how to fit all that into the bike.

By the way a resonator needs to be "high Q", that means low loss, so you want the walls of the intake box to be really stiff so they don't absorb and kill the pressure waves by flexing.

Larry

This makes sense.  I currently have a 2" hole in each side of the box and a 2" ID elbow connected to an 1-1/2" ABS pipe to the front of the bike.  I intended to increase these runners up to 2" continuously to the front for best ram effect.  However, I can try various lengths when I get it on the dyno to see if there is some length that gives me an increase at higher RPM's.  I have a broad torque curve now, with no trouble accelerating to 8000 RPM's. So I think I want the resonance to occur near 8300 RPM's (peak hp) for the extra boost.  I might be able to add a section of 3" or 4" pipe at the length that works best, and use that enlargement to house filters.

Tom,
Fundamental design flaw !!!!

If you only pressurise the air inlet then you will get no fuel flow, as the differential pressure across the jet is what draws the fuel up.
You need to have the whole carb inside the airbox, have a look at the NHRA pro-stock Suzi's that will give you a better idea.

HTH
Neil

Neil,
Thanks for the heads-up.  I intended to route my float bowl vents into the airbox, and can do the same with the tank vent, which I think will have the same effect as enclosing the carburetor, but much simpler to do.  The bowl will then see the same pressure increase (if any) that the air inlet sees.

Tom

[John Burk]

 I calculate intake runner length this way .

 deg pressure wave duration / 360 / rev per sec x sound f/s = pressure wave travel in ft

 ft x 12 / 6 (3 trips up & 3 down) = inches from plenum to valve

 Using 260 deg wave duration and 117 rev/sec (7000 rpm) and 1090 f/s (Speedweek mach 1)
 260 deg / 360 deg / 117 r/s x 1090 f/s = 6.728 ft x 12 / 6 = 13.46" runner length

 My 260 deg from impulse start to the ideal final return is a guess . Be glad to hear what deg others use .

 

[Jack Gifford]

...  = 13.46" runner length...

As I alluded to- this is a whole lot shorter than 76 cm. And it's for only 7,000 RPM, not 8,400.

[WhizzbangK.C.]

Hey Tom, you're sure getting back onto it faster than I am!

One thing to consider when setting up for ram air is to make the pressure line to the fuel tank as as large as you possibly can. The smaller the line the longer it will take the air pressure in the tank to equalize with the pressure in the intake, and it needs to equalize instantly to keep the fuel flowing to the carb. I learned that one the hard way years ago, couldn't figure out why it wouldn't pull through that killer dead spot right where it should have been pulling the hardest.

On the plus side, the airbox should help keep the carb from falling completely off when the bolts back out. 

The L should properly be referred to as the "effective length" of the intake pipe. Reflection and reversal of pressure occurs at the first major change in cross sectional area of the intake pipe. If that change is large enough to get full reflection, the engine does not know that there is more larger intake pipe beyond the reflection point. If the L you calculate is shorter than your total intake run, put a reflection point in the intake at the proper distance from the intake box. If it is longer tune it for an odd harmonic (ie 3rd harmonic).

The engine has maximum volumetric efficiency at your torque peak rpm that is why the torque peak occurs at that rpm.
I would tune it for the high rpm range where volumetric efficiency starts to fall off rapidly, probably for a point near the rpm you will drop to at your last gear shift as you enter the traps to be sure the engine can pull the gear change.

Best way to test would be to build a test intake that you could vary the volume  and intake length on, for testing then once you find the ideal dimensions on a dyno figure out how to fit all that into the bike.

By the way a resonator needs to be "high Q", that means low loss, so you want the walls of the intake box to be really stiff so they don't absorb and kill the pressure waves by flexing.

Larry

This makes sense.  I currently have a 2" hole in each side of the box and a 2" ID elbow connected to an 1-1/2" ABS pipe to the front of the bike.  I intended to increase these runners up to 2" continuously to the front for best ram effect.  However, I can try various lengths when I get it on the dyno to see if there is some length that gives me an increase at higher RPM's.  I have a broad torque curve now, with no trouble accelerating to 8000 RPM's. So I think I want the resonance to occur near 8300 RPM's (peak hp) for the extra boost.  I might be able to add a section of 3" or 4" pipe at the length that works best, and use that enlargement to house filters.

Tom,
Fundamental design flaw !!!!

If you only pressurise the air inlet then you will get no fuel flow, as the differential pressure across the jet is what draws the fuel up.
You need to have the whole carb inside the airbox, have a look at the NHRA pro-stock Suzi's that will give you a better idea.

HTH
Neil

Neil,
Thanks for the heads-up.  I intended to route my float bowl vents into the airbox, and can do the same with the tank vent, which I think will have the same effect as enclosing the carburetor, but much simpler to do.  The bowl will then see the same pressure increase (if any) that the air inlet sees.

Tom

[wobblywalrus]

Tom, you are getting some good advice in these recent posts.  PipeMax will give you some intake runner lengths for the various harmonics.  Expensive experience has taught me to locate the carb intakes in an area with cool and calm air that does not change pressure as the bike speeds vary.    

[John Burk]

With a forward facing air scoop you're trading the air's inertia for pressure . There are two ways to do it .

(1) Size it just right so in high gear the air maintains its speed and decelerates inside the duct . The math needs to be correct and in all but high gear there is a slight strangling effect .

(2) If there is a rounded , unobstructed area on the front of the body / fairing there is a small area in the center sees full impact pressure at all speeds . If the inlet is there it doesn't need to be accurately sized except to stay were the impact pressure is high . That's where my engine inhales .
 
http://www.motorsportsinnovations.com/Bvile-pics/bs%20nose%203-07%20(1).jpg

[Koncretekid]

As I floundered around on the internet trying to find out more information, I came across a 5 page posting on the Speedtalk forum which had more information than I got from all the other sites put together.  Plus some interesting heated discussion between some very knowledgeable people.  References were made to Phillip Smith's book, mentioned here already, Gordon Blair's, and included comments by Billy Shope (one of the original designers of the Chrysler ram tunnel intake manifolds), and our own Hotrod.  You can follow it here: http://speedtalk.com/forum/viewtopic.php?f=1&t=1163&start=60.  I had to register to view it, but it was well worth it.

Most of what is known deals with the length of the intake runner between the carburetor and the intake valve,  and I find that Wallace Racing (wallaceracing.com) calculator gives the best information on this, because it gives the answer according to Gordon Blair's formula as well as the Chrysler formulae and others, so you can take your pick. Only Gordon Blair's formula seems to take into consideration valve timing events.

The Helmholtz resonator concept is also explained by Hotrod on that site (as well as here), but no formula is given.  I'm of the opinion that there is no set-in-stone formula to deal with my situation with the long runners leading into the box. I know that some of the modern bikes are using such a principle, I just need to figure out how to make it work with my "almost antique" BSA.

As for placing a forward air scoop, that is what I am trying to do, but cannot use the immediate front location because it would direct the air smack into the lower triple clamp as well as my body.  Hence I'm using holes, maybe converted to pitot tubes by next year, just to each side of the front fender (see photo above.)

Thanks for the comments. 
Tom

[WOODY@DDLLC]

http://www.circletrack.com/drivetraintech/ctrp_0905_petes_torque_converter/viewall.html

It's titled exhaust but has the whole enchilada. Peter Vorum actually worked with the guys who "figured" it all out! Hemi's, too!

http://etd.ohiolink.edu/send-pdf.cgi/Vorum%20Peter%20Carl.pdf?osu1298649174

Don't hurt your brain!