Bonneville America Forums Forums Technical Forums Performance Mods Airbox mod VS airbox removal?

Hi there!

Just a technical question.

Due to many experience in past I've seen that on many bikes remove completely the airbox is not better than study a good airbox modification.

I've seen lot of dyno charts showing same or better performance modifying the airbox instead of removing it and using pods in front of the carbs.

Just let me explain...
The airbox is an important invention, after '60s many cars and bikes started to use large aiboxes instead of an open filter in front of the carbs.
Before '80s noise was still not a big problem for EPA and pollution agencies, so the use of airbox was not only a noise matter.

So the reasons are technicals. The airbox has a breather function like a lung. The so called lung effect.

Into the airbox the intake phase find sufficient and right air quantity for the cylinder.
After the airbox is empty by the intake phase, the depression causes a fast air suction from the snorkel for the next intake phase.
The snorkel dimension is studied for speed and right air delivery for every intake phase.

Modifying the bike, using free flow exhaust we don't need much more air but a faster breathing for the engine, so we don't need a bigger airbox if we don't change displacement and valves diameter, or combustion chamber volume.

But the lung effect is interesting because it permits the better and smoothest power delivery.

How you can see with pods you'll need 150 or bigger jets, with an airbox mod 130-135 is sufficient.

I did lot of bike mods in my professional life when I've been motorcycle mechanic, on Suzuki Bandit 400, 600, 1200, Suzuki Marauder, Suzi DRZ400S, Harley Sportster, bigtwin and TC88, Kawasaki Vulcan and ZRX1200, some Ducatis.

On custom bikes with V-twin engine it's difficult to have an effcient airbox due to the intake position and small space, Harley V-Rod and Yamaha V-Max have a fake gas tank used like airbox and the gas tank is a plastic box under the rider seat.
So for v-twins the direct intake is the only way...

Pods or direct intake have also lot of intake noise, a "wooom" noise that not everybody like.
More limited with a modified airbox.

Some examples:
http://www.mechanicamente.it/component/option,com_ponygallery/Itemid,13/func,viewcategory/catid,3/
http://www.mechanicamente.it/component/option,com_ponygallery/Itemid,99999999/func,viewcategory/catid,2/

In my airbox mods of '04 Harley Sportster and Kawasaki Vulcan 1500 Classic, the airbox removal is a must.

But on Kawasaki ZRX1200
http://www.mechanicamente.it/component/option,com_ponygallery/Itemid,99999999/func,viewcategory/catid,5/

the best performance come from an airbox mod insted of direct intake using pod filters.
The same was with Suzuki DRZ400 dirt bike, and with the Bandit family (400, 600, 1200).
It's famous the 1" hole mod for the Suzuki Bandit 1200 SACS engine. A mod where a 1" hole is added to the snorkel removal.

I don't like big holes mod on airbox, I've found that lot of small holes is better than a bigger hole, due to a physic law that describe a lower pressure drop in smaller holes than a bigger one.

To explain how good is this theory I can say that on my Kawasaki ZRX1200 I've obtained 132bhp from the modified airbox against the 128bhp of the carb pods.
Also main jets are smaller, with 114 or 115 (from 96) with the airbox mod against the size of 132 if the pods were used.

How the power is delivered is another question.
With pods and direct intake, there's plenty of irregular power delivery in low and mid range.
The dyno chart shows a sawteeth shape curve in both power and torque delivery.
But with an airbox mod the curve is more smooth.

Sorry me if this thread is going so long... But there's another question about the Bonneville (America and Speedaster too) airbox shape.

I saw that this Triumph bikes use an inverted air filter shape, where the air filter is shaped like a cylinder and the air is sucked from inside the cylinder. This doesn't permit a big hole just like the cyilinder mouth, and an airbox drilling to permit more air on intake is impossible or dirt air will be sucked.

So I want to ask if someone of you has passed from an airbox mod (snorkel removal or other mods) to a Freak kit with airbox removal and has noticed better performance.

I saw also the air filter is mounted vertically (on America/Speedmaster) and the snorkel is under the seat.
If this mod doesn't permit good breathing when I'll buy my Triumph I'll study how to open an hole in the bottom side of the filter's rubber body and of the airbox, but avoiding an unfiltered air suction.

I fact I'm waiting to sell one of my bikes to buy a Speedmaster, it's not possible for my finances to own 3 bikes now...

We've discussed this before, and my personal theory is that if our airbox was actually designed as an airbox you might have something (like a Vrod) but ours are designed to fit in a space, no breathe well.

Found this nice site:

http://www.frontiernet.net/~dmclec/airbox.html

I believe this mod is far closed to a Freak kit, with a smooth power delivery at a fraction of the cost.

Sure using a dremel or drill tool the hole cut is cleaner than using a saw.

Bonjerno Roberto,

I've been lucky enought to travel to your beautiful country a few times in the recent past. Brindisi was very nice. Good people and some of the best food and wine in the world!

I first modified my airbox using a hole saw as outlined here: Hole-Saw (my post is about 2/3 the way down).

Not too long after posting that I opened the bottom hole up even larger with a Dremmel tool being careful not to destroy the small protrusions in the bottom of the box that center the filter. I had to rejet the mains to 132.

Since then I performed major surgery on the airbox with the Ghetto Freak airbox modification and had to rejet to 150's and I'm acutally considering a little larger (may be 155's).

My Ghetto Freak talking paper in pdf format is also located in the Tech Vault.

One of these days I'll have to break down and pay for a dyno to compare my current configuration to stock numbers and enable fine tuning. The difference between the amount of air the holed box passed and the pod filters pass is apparent due to the necessity to increase jet size (132 to 150s) to mitigate popping on decel and ensure proper plug coloring.

Just my input ......

Regards,

Tom

Hi Robert, great to see someone from another country giving his two bobs (50cents) worth. I found what you said interesting regarding how some bikes actually work better with airbox mods rather than the pods. From what I can gather though, most riders here that want to experiment with more horsies start obviously with less restrictive pipes and then airbox mods as explained. Once they have the bug, a lot then intend or do the freak kit with even bigger jets. Again the bug for more horsies has struck and we have our resident mod guru, Pat experimenting with Mikuni carbs as we speak. So if you do buy a speedmaster, it will be interesting if you did try different airbox mods as opposed to pods and we certainly will be interested in results. The best thing about this site is that everyone shares info freely to try and help each other so welcome and hope to hear more from you.

Very interesting read indeed. It really makes me think. I know some people swear by the Freak, but I also know some others that have tried the Freak, played with all kinds of jetting and adjustments, but the bike ran like garbage no matter what and ended up going back to the stock airbox.

Soren

If any of this were true, bike's used in racing application's would still have airboxes as opposed to pods.

Having to run larger jet's is the first indication that more air is getting in the motor...not less.

In fact racing bikes don't use pods from 15 years!!!

Every engine has a fluid-dinamically engineered intake (and airbox) volume studied for better performance.

The first thing is to calculate an air volume during engineering and dyno tests.
With this info the racing team builds an airbox that match the bike style and tank/frame shape. They dyno again the bike to find the better shape to match the flow and volume values given by the engine engineers.

Only into AMA races someone still uses pods.

I've obtained 132bhp and smooth power delivery with airbox mods, against 128bhp and nervous not constant power delivery using pods, on my Kawasaki ZRX1200.

The needing of bigger main jet doesn't mean nothing, it's only to compensate a bigger amount of air, but the combustion chamber and valve sizes don't permit better performance with bigger amount of air if they have reached the better performance at a lower air/fuel level.

It's not difficult to understand that adding holes to the airbox (and correcting carburetion at the same time) will rise power to an upper limit, raising it you can't obtain more power adding more holes or removing everything and using pods.

The job of a tuner is to find that upper level of performance possible with your airbox matched to your engine setup.

The right balance is where if you go lower or upper (with air/fuel mixture) doesn't give you any better gain.

All that is true, but only for airboxes that are actually designed for performance... Ours were designed to fit under the seat. Period.

I wonder, what is the efficiency our the stock intake/exhaust systems on our bikes. In other words, for a 790cc engine, with the stock cam/valve setup, are we actually pulling in 790cc of air at ambient pressure with every intake stoke? Probably not. I'm guessing the efficiency is around 70 or 80 percent, but I may be way off. I suppose it wouldn't be too difficult to hook up an air flow meter to the opening on top of the airbox. It would take a little more ingenuity with the airbox removed. You could measure air volume against rpms and get an idea of the cylinder efficiency. I wonder if you could get over 100 percent with a turbo system.

A lot of things affect the intake, like air friction, air turbulence, and pressure drops throughout the system. I have heard of a thing called scavenging, which is where the exhaust pulse bounces back from the end of the pipe. It hits the exhaust valve as it opens causing a slight vacuum to help evacuate more exhaust from the cylinder. That allows a bit more air mix to enter on the intake stroke, and thus more power. Scavenging depends on the shape and the length of the exhaust pipe, and then it only works in a narrow rpm band. It would be like tuning a trombone. You have to get the length just right for the travel and timing of the pulses to work right.

It would be fun to have a well stocked garage with a dyno and an exhaust gas meter and air flow meters to experiment and tune our bikes to the max. Unfortunately, most of us don't have those things. All we can do is make informed guesses and hope we don't screw things up too badly.

Cody

There's no such thing as 100% efficiency But then again it depends what you are really measuring. So it really wouldn't be energy efficiency you are talking, more just mass air flow right? Good question. I would be interested to know how people measure airbox performance when designing them.

This was my original post in 03 or so, I was going back & forth w/a brit racer who was the beta tester & first 2 make the Mod.
After he sent me his Dyno test I was the next 2 make the Mod , test it & post it.
Spade drill three 1 1/4 holes in the filter bottom (in a triangle pattern) then place back in air box, mark holes. (with grease pencil) take out filter & spade drill out the same holes in airbox bottom. Y can stager the holes if U want (so they don't line up but U WILL get MAD AIR!
This works because our air boxes have no breather valves (or vents) & require no airbox for engine to run correctly. More air & fuel combined w/ sync of carbs = POWER
My bike is TOTALLY different since & I highly recommend the Mod.
Another Mod: I also noticed that the Pingle petcok made a diffrence as well, stock filter is restrictive & made like crap! Bike was looking for more fuel & breaking up @ high speeds
I have made the Mod on a few TBA/Speedmasters w/nothing but + results & big smiles! O-yea,
Also: Valvoline 4 stroke wet clutch motor oil ($4 a qt. NAPA & other stores) w/half quart of lucas oil treatment has been my secret weapon mixture since 02.
No dry starts in summer or after sitting. Take off your valve cover & look @ how the Lucas coats the top end!
It's in all my vehicles & my compression tests prove how great the product works, crazy cool!

Sincerely
G@ry J@mes

i def do not have any kind of roughish power delivery, bike never felt better

Frank

The air box on cars and some of the bikes he is talking about perform two functions. 10, to seal out the hot air in the engine bay and 2) draw/duct cold air from outside the vehicle. Cooler air is more dense than hot air. The air box on the Bonnie is not a form-follows-function approach. As it was said- the stock box is there because that is where they had space to put it. Functionally, it's junk- too, ugly to even be a planter pot.

You can get a good idea of the efficiency of the airbox by connecting a vacuum gauge to the port where the air injector gets its air. This will give you an accurate picture of the average pressure drop across the filter and inlet port. Next, move the guage to one of the vac ports on the manifolds (or use second gauge), run the bike WFO under load and you will see the pressure drop across the carbs and the outlet paths of the airbox.

Interesting post. If you are looking for maximum HP, the airbox has to go. Check any of the few Bonnie's built to race, I guarantee they aren't using the stock airbox.

short trips huh, guess my 1000 mile weekends and 300 mile days with the lady should not be as easy as they are. there have been many who have put big bore kits and other mods on these bike and not once have i heard it hurt overall rideability. try removing the airbox and see how it runs. a well tuned airbox may very well be better but we dont have the luxury of that as an option.

Frank

Like the man said, short trips. You need something like a Tiger to cross Africa or circle the globe.

Mechano:
I agree with your statements completely. All of us here don't race and never will. It's just that your opening statement didn't reflect that:

"Just a technical question.

Due to many experience in past I've seen that on many bikes remove completely the airbox is not better than study a good airbox modification.

I've seen lot of dyno charts showing same or better performance modifying the airbox instead of removing it and using pods in front of the carbs."

No airbox will always make more peak HP than a stock airbox, there is no denying that fact. Of course, none of us here need peak HP, we need ridability. But we dont seem to let that stop us from trying to attain a few more precious ponies.

Good deal Mechano, keep us posted. We're pretty good here at tinkering with things, but not so good at doing, good, hard, dyno-based research (we're busy riding!).

I actually started doing the Ghetto airbox, but abandoned the idea when I couldn't come up with brackets that suit me and allowed me to keep the side cover. Still have the pods and the stock airbox I cut up. To the best of my memory, no one has dyno'd one of these Ghetto airboxes. I'm curious to see what kind of numbers they generate and what type of tuning is required to dial it in.

I know people able to put 300mi on a rigid bobber in one day. But this is a different question.

If you do so much miles with your bike, I think you'll not like to rest somewhere with a broken engine because of your mods.
That's another reason because of the mods have to be the most reliable, not only the most powerful.

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so far all of the mods for our bike are as reliable as stock. unless your talking forced induction or NOS. there are not options currently available that will compramise (spelling) reliability. only think with the bog bores is they may need to be honed and new rings every 50k wich i find acceptable for the amount of performance you gain.

Frank