Bonneville America Forums Forums Technical Forums Performance Mods Airbox Theory

This article sheds some light on the function of airboxes. I found the information useful, as well the rest of the site.

Wow, ain't science grand! Is my understanding correct that we should not be touching our airboxs and snorkles because there will be a loss of mid range torque? If there was one thing everyone agreed upon in this site was the removal of the snorkles &/or A.I. I look forward to hearing from the teckies on the advantages or disadvantages in light of this information.

Sorry to be so uninformed, but what exactly is the snorkel? I removed the AI - does that have anything to do with it?

Snorkel is under your seat, attached to your airbox.

Disclaimer:
I don't know what the crap I'm talking about, but am gonna ramble on anyway. What is said is in NO WAY backed up by and scientific experimentation with double blind sampling or taste testing. Just thinking out loud and talking out my rear end. Feel free to jump in my poop and chastise me. I won't care anyway.

I don't know how relevent this all really is. Math can be manipulated to get pretty much any result they desire to prove they are right. So here's my math:

More air in + More gas in + good exhaust pipes = faster bike.
Granted our beloved(?) CVK carbs work via vacuum pressure and all that scientific nonsense so impeeding the airflow will cause a loss of power.
I know for a fact though, dyno prints to prove it, that my torque line flattened out and increased by removing airbox, upping main jets and adding aftermarket pipes. This is comparing my previous setup with Mikuni carbs and K&N filters, no airbox. I know have CVKs back on, and still have K&Ns, no airbox. As I tweak more and more, the torque curve keeps rising and getting flatter with each minor tweak and the HP keeps rising as well proportionally.
My assumptions are based soley by the seat of my pants, my gas consumption, my grin factor and the pesky little machine called a dynomometer. Actually the same results on 2 totally different dynos in case anyone cares.

Fire away, I can take it.

...makes sense to me. Maybe the mechanics I talked to know what they're talking about. After much grief dealing w/ the Freak/K&N setup I finally went to the boys @ RPM Cyle in Dallas. First thing they told me was that removing the airbox was not a good idea, basically because the CVK carbs worked better with it. After a few jetting changes and re-synchronizing, all is well. I may have dropped a few HP, but nothing noticeable. If I was really looking for blazing speed I'd probably get a Daytona. The problem is, @ 54 I can't figure out a way to decrease my reaction time.

First off, while I haven't done any calculations to verify it, I have an intuitive feeling that an airbox designed to specifically reduce noise, may not resonate at the right frequency for optimum engine operation.
Also, if the engine is designed as a complete system, there is no reason that this "midrange dip" he talks about can't be exactly at midrange. If all this is correct, the end result would be that redline or peak power, whichever they feel is best, should occure on the harmonic of the airbox, so you would get another boost running WFO. Even better, would be to design the airbox for top speed and tune for this "midrange dip" to be on the harmonic.
In the days when air filters were just large air strainers that were often not even used, there was no "midrange dip" in power and torque curves, which makes me think that airbox design is mostly sound damping and the resonance is selected to minimise their detrimental effect on engine tuning where the designer feels it is most needed.
The dyno charts I have seen for Freak equipped bikes do indeed show a bit of a negative spike in the power curves around 3000 - 3800 rpm's, but those seem to be cleared up by carefull carburettor tuning.

Quote:

---

Modern engines have throttle bodies or carburetors with throats that are typically about 45mm in diameter, about 16 sq.cm in area. The inlet snorkel to a modern air box will be roughly 300 to 800 sq.cm - much larger than the throttle body or carburetor throat. The idea that the snorkel makes for a significant impediment to air flow into the engine is questionable at best.

---

Our snorkle inlet is smaller than our two 36mm carbs...so yes it is an impediment.

All I know for sure in my two cents worth is that by removing
the snorkels and changing the jets,(I already had TOR pipes and
the AI removed) I have never and let me repeat that NEVER in
25 years of turning a wrench gotten SO much improvement by
doing so little to a gasoline burning engine. My snorkels
are gone never to be seen ever again. So I don't care what
the tech guys say because the seat o' my pants knows better.

I have to agree with greg, not only is the snorkle area smaller than the carbs, but it reduces the airflow down to the bottom one inch of the filter element. When I took mine out, that inch of filter was dirt, and the other 4 inches were clean as a whistle..
Not only that, the web page (which is real interesting by the way) is very general. All air boxes are different, case and point the bonnie airbox is so much different than ours! Side intake VS under seat, etc. So while some boxes, say on a daytona or whatever, may be specifically designed, I think ours is more just to fit than anything else.

i seem to recall someone even did some dyno runs on this and the setup w/out the snorkel and drilled airbox came out w/ more power than the stock setup-- so as in all general approaches to specific science, there's a little fat to chew.

like someone said, our snorkels are not huge air vortexes.

cat

The article is pretty good on the science but who knows if they right-sized our airbox. I can't see some designer saying 'make sure we have room on the BA for an eight liter airbox'. They probably ball-parked it and made it fit. The article doesn't consider air dynamics either. Air is not a 'slug'. It behaves much like a fluid. It swirls either clockwise or ccw as it moves - that's why engine stuff is round. I tend to agree with Dingua's dyno, more air=better.

Custus

Yes Castus, exactly. The bike was obviously put together for the style, the airbox is only as big as it could be to fit, maybe the only consolation to it being where the battery is placed to make room for hte airbox. Other than that, it just big enough for the filter and to help with noise.

And any dip in the mid range torque because of air box removal can be remedied with Thruxton or TBS needles....although I would like to see before and after dyno printouts of say, a freak install with nothing more than jet changes, and another dyno run with the same setup after TBS or Thruxton needles. Has anyone posted this kind of info?

I have a BMW page in my favorites that also goes through all the technical jargon about slugs and pulses and snorkel tube length and such things. Not being an engineer, I can't argue with the math either. I have to wonder just how relevant all this techno-babble actually is when dealing with a 790 cc engine.
When that piston goes down and that intake valve opens, the cylinder is only going to take in as much air and fuel as is available. It doesn't care if Poisson's stagnation is in effect, it is going to try to suck whatever it can get. (I know, it's really atmospheric pressure...) Same with the exhaust, you can blow a lot more air thru a 1 inch pipe than you can a soda straw.
My point is unless you are fine-tuning an engine for racing purposes, I don't think we need to worry about the slugs and pulses etc. Match your inflow to your outgo so that your plugs burn tan and when you twist the happy handle you grin.
The 650 Bonnevilles didn't have air boxes, and they ran quite well....

Well I know this probably belongs in other bikes, but it's a bleed over about airbox removal. On Wednesday I removed the airbox, installed K&N's, and rejetted my Thruxton. (THANKS Brent) I already had the off-road pipes.
On Thursday I tested the setup out on my favorite straight on-ramp and two things happened. First I slid back in my seat about 4". Then the front wheel just about came off the ground. These things have never happened with the stock airbox setup. I'm sold on removing the airbox.

You wrote "snorkles"... exactly how many did your bike come with? Last time I checked I only had one on my Speedy. It's out and the Uni filter in, TOR on and carb rejetted/sync'd. Today while riding "two up" from Fillmore to Malibu over the hills I could have used some more power. Am I missing something? To snorkle or not to snorkle? That is the question.

Bigsteve

bigsteve,
Supposedly, there are 2 rubber or plastic pieces in the airbox, 1 at the passage to each carb. Pull the plate/snorkel off and pull the filter out and you are supposed to be able to stick your finger in each one and yank it out, thus allowing more airflow into the carb.
I say supposed because my '03 TA never had any in it from the factory, so I never had to do this myself. Check yours and see....

I was just at the Delphi board for Kawa Vulcan 800's today (researching for a buddy of mine). One link about carb mods said not to bother doing an air box mod unless you freed up the stock exhaust! Sounds about right to me.....

As for the CV carbs working better with the restrictive intake box, the cure for that is the 7/64's inch drill-out of the slide hole.

I've been down this road before. There's only one snokle on a Speedmaster....the rubber one coming out of the top of the air filter. Speedmasters don't have a restrictor plate kind of snorkel between the airbox and the the intake rubbers.