The great Air Box Debate!

[guzzi jon]

Don,

My take on your less than expected horsepower was due to one or two factors, first, it was 107 degrees when your bike was dynoed, this cuts horsepower. Second, your newly jethotted bubs are smaller and may be a bit clogged up decreasing ultimate horsepower. I dont think there is that much difference in H-pipes, but you never know. If you want, I have an identical pair of bubs that are not jet hotted as well as a full NOS BUB system with the reverse black cones, we could head to Louies and do a number of runs with different pipes etc to extract the most. It would be an interesting documentary...

[Guest geno929]

Hey there one and all. Thank's for all the well written replys.

I've got a 2000 V11 with FBF carbon pipes and the turn down snorkles removed, K&N installed. Stock crossover and no PC. Where should I spend my limited money? Crossover? PC or what? What mod gives the best bang for the buck? Do I REALLY need a PC? Help! And Thank's, Geno.

[guzzi jon]

I would sure do the X-over and the PCIII, get a used serial port PCIII, see em pretty cheap.

[Skeeve]

If you cut a 4x4-inch hole in just the flat top surface of the airbox, you get the flow, and the vertical sides of the box lid keeps the honk form getting to your ears.

 

And keep the paper filter, unless you need every tenth of a horsepower . . . for a while, anyway. Afer 30,000 miles of the K$N letting dirt in, your ring seal will slip to the point where the power drops below what a bike with the stock dirt grabber would make anyway . . .

 

The horns on the airbox clearly aren't as restrictive as people seem to think; Gordon Jennings in his Sportbike Performance book talks about how back in the 70s, there was some race series that required an intake restriction of xx dimension per cylinder (let's call it an inch - 25mm to make it easy to visualize, and suitably "restrictive.") He pointed out that Yamaha found that their TZ750 wound up making only some piffling amount less [1? 2 ponies?], after the engineers got finished fiddling around. How did they manage this magic? By putting the restriction about a foot (call it .4m for you metrical types) behind the carbs, waaaay a the back of the airbox, with a looong, shallow angle horn leading up to & away from the point of restriction.

 

At any rate, back to the subject at hand: swissing (as in cheese w/ holes in it! ) the airbox:

 

If the redline is 8k rpm, and the engine is a 4-stroke so there's one intake cycle per 2 rotations and 1 minute/60 secs, that equals:

 

8000 rotations/min * 1 cycles/ 2 rotations * 1 min/60secs = 800/12 cycles per sec or 66.6667 hz natural intake frequency , which is a nice basso profundo note, just above the 60hz "bottom" of the normal range of human hearing. This is the "honk" that we love so much from our geese!

 

Now, 1080 fps / 67 hz = 16.1 ft Obviously, we can't fit a hole 16.12 ft long in our airbox! So we divide by 4 (I won't go into it, but it has to do w/ harmonic reinforcement, & why you can have a stubby little rubber coated antenna on your car that pulls in as well as the long whippy steel OEM one...), divide by 4 again, and again, to end up with a hole size of about 3 in for "best sound." Naturally, you'll be getting all sorts of other harmonics being reinforced as well, since we had to do so many of the 1/4 wave reductions. Now, mix in a few 1.87 in holes (our 3in hole divided by phi, the golden ratio) and 1.15 in holes (again with the phi), & you should get a nice pleasing chord of intake music, instead of just that nasty roar of high-frequency random sh!t...

[Guest ratchethack]

Say, Skeeve -- I'll bet you can tune a bass reflex speaker to perfection. Y'er calculations look amazingly similar to the physics I used to calculate a bass reflex port diameter and internal tube length for a set of speakers in college. Still got the speakers and still use 'em in my office.

 

I guess you know that y'er calculations WRT most of this forum may as well be for tuning a contra-bassoon. . . . . . It IS kinda impressive, nonetheless.

 

My hat's off to ya - and y'er ref. to Jennings' article on the TZ750 was spot-on. Some aspects of intake harmonic wave physics seem a little counter-intuitive sometimes. Ya gotta admire engine tuners who're smart enough to tune their way entirely around and beyond the restrictions.

 

You've encouraged me to try a less audio-antagonistic variation on an open airbox top. Thanks.

 

Why, you may ask? Entirely for the sheer aural joy of the symphony.

[Ballacraine]

 

You've encouraged me to try a less audio-antagonistic variation on an open airbox top. Thanks.

 

Y' mean you would consider using it with a louder HONK?

 

Nige.

[Guest ratchethack]

Can't take much louder, and the wide-open, raw honk is definitely OUT. More melodically pleasing, to more harmonically accompany the FBF oval carbons singing through the Stucchi crossover -- that's the ticket.

[guzzi jon]

Next time I have my tank off, I'll post a pic of my modded air box, still has the snorkels as they reach down in cooler air, I have a combo of raised flaps to catch the air as well as open holes. I could not tell any difference from the stock airbox in intak. I do have a FBF x-over and mistrals, but no difference in sound, just performance

 

Jon