Airbox size & openings - how much affect on performance?

[mugget]

Hi folks,

I am currently doing a KTM 380 supermoto build with custom bodywork. And that's where I need some 2T knowledge and advice...

The rear fender part has a small section which basically adds a little bit of volume to the airbox. My question is whether I can remove this section without massively affecting performance? (This will just make it a lot easier to make the bodywork.)

I think most of these bikes use this area to fit a battery (which would also reduce volume in the airbox), but I am planning to run everything direct off the stator, no battery. So maybe it will be fine? The area I would shorten the airbox by is definitely less than the space a battery would take up.

I'm also just wondering if there's a site out there that explains the basics of 2T airbox/carb operation? I've been doing a bit of searching (on other forums/Google) but found conflicting information - some say that the airbox volume affects the "resonant frequency". But there's others who drill holes in the airbox for more airflow (going by the old "more fuel + more air = more power" theory) and apparently some even remove the airbox completely and use an air filter with external cover to keep out large debris?

Maintaining the longevity/durability and ease of maintenance (so basically close to stock) is more important to me than outright performance (it has enough power as is!), but I'm still interested to know the options and learn a bit for my own knowledge.

Big thanks for any tips/links/info!

Cheers

[Jeram]

Hey Mugget you forum whore

airbox information I read from Jan Thiels posts on a french forum. Jan Thiel was the leading man behind the dominating Aprilia two stroke grand prix bikes.

As the bikes is a single cylinder and not an inline four the volume is especially critical because there isnt a constant flow of air entering the engine it is a suddent burst, one after the other which sends an enormous pressure wave through the airbox.

Anyhow, the rule of thumb is:

10x engine capacity is good
20x engine capacity is great
bigger is generally better
less than 10x engine capacity will result in some form of performance loss depending on how severe the volume restriction is


As for the no airbox question, it may potentially make more power on the dyno but at speed on the track or street you fill find that it significantly lowers your performance. The was I like to describe this is to liken it to a car traveling down the highway. If you crack the window slightly (like a std airbox) you get a smooth flow of air entering the carburetors. If you open one window fully then depending on the cars speed and the window size you will potentially get high and low pressure zones and eddies in the car which varry in size and location depending on on the cars speed. This leads to the 'air hammer' which you experience sometimes.

So removing the airbox is a no no!


my advise would be to either leave the airbox stock and accept that you might loose a hp or two. Or build a custom airbox which might be tedious or simple depending on how handy you are with either plastic welding or fiberglassing.

Personally I dont think there is much to gain as Jochen Jasinski on the husaberg 380 SM racebike looks like he is using a stock airbox which Im assuming is similar in size to the 380 unit
Just keep the airbox stock and if you want more power then there are many simpler ways to do this !

[twinrock]

well in my experience..which is limited to road going two strokes for almost 20yrs and the last 10 years being substantially modified bikes ..i have almost always run with no air box just pod filters..a big reason for this was simply ease of tunning and access to carbs
i have heard over the years arguments for both options...
for me i have always had gains with no air box..! altho jetting is required to get it great through out the rev range
even in pouring rain i have never suffered any probs..
how ever..i can only assume that alot of air box design is designed around noise reduction and fuel consumption
also on a motorcycle rarely is there the room to fit or create the best possible box, but instead have to cater and customize to fitment with the rest of the bike and chassis, also on a dirt orientated bike it would be crazy not to have a air box in regards to dirt mud and water.
also one has to take into account that the carbs and chambers have all been matched and tuned in accordance with this box,,what ever the configuration..
on a gp bike there is far more room no batt, oil res, that sort of stuff so it could afford to accommodate the box required
so for me..and 80,90 road going two strokes.. no air box has always proved a big improvement ...but on a ktm380.. which i can only guess is a very refined work of art .. probably not much in it..again i have no mathematical theory for this ..just my seat of the pants experience..
i have been currently working on and ridding an old dt200l... first thing i did was loose the air box and standard muffler...the difference was substantial..!!

[mugget]

Hey Jeram. Yes I'll join any forum, no shame.
Haha.

Cheers for that info. I will leave the airbox as-is, just removing this little section on the rear fender. I'll bet it's going to have minimal effect anyway... I can always put it back to stock shape if needed.

I actually thought about making a replacement airbox, but I'm in deep as it is. Starting to just wish that the bike was on the road already!

I don't want to make it guzzle a bunch more fuel than necessary either, so keeping it stock seems like the good idea there.

Cheers fellas.

[OzzyElsie]

The general rule is; for a 4t concentrate on the intake, on a 2t concentrate on the exhaust. That holds true but it is not to say HP can't be found in the airbox of a 2t. And more to the point, for both 2tg's and 4t's the best result is in the overall package.

We call it an air box and they generally hold the air filter but I think of it as a 'still air box'. Twin Rock talks of a performance increase when removing the OEM airbox. To me that points to a restrictive air filter rather than a restrictive air flow due to the air box.

Many years ago I accidentally found out the benefits of a big still air box. I was enduroising a DT360 and in the process of losing weight I removed the OEM air box and made a fibreglass replacement. As an expedient I made a large still box box immediately in front of the carb intake with a pod up high leading into the still air box. What a difference, you would believe it. More HP and responsiveness everywhere. (it also had an EBC pipe but little else).

It would appear that the manufacturers have also realised the benefits of a large still air box. It is a feature of modern bikes. Over the past 30 years airbox have grown bigger and bigger with a distinct still air box immediately in front of the carbs. Even the factory superbike kits include an airbox.

To make the air box it wasn't hard. I just used poster cardboard, cut to shape using ya mum's scissors, glue and tape together, and then brush with fibreglass resin. Once set put a layer or two of fibreglass mat and resin on the outside.

[Jeram]

Nice post!

[mugget]

OzzyElsie - that sounds interesting, do you have any photos or drawings of your fibreglass replacement? I'm not familiar with the DT360 - did you add a larger airbox, or didn't it have one as stock?

Not that I'm going to do anything like that, but I'm still interested in learning about 2T.

[OzzyElsie]

OzzyElsie - that sounds interesting, do you have any photos or drawings of your fibreglass replacement? I'm not familiar with the DT360 - did you add a larger airbox, or didn't it have one as stock?

Not that I'm going to do anything like that, but I'm still interested in learning about 2T.

It was all pretty unscientific and ad hoc although, to give myself full credit, I did have some vague ideas to save weight and to get an unrestricted filtered air flow. The exact design come about by expediency and trial and error - which is why I used the cardboard. But the trick of making it in cardboard and then 'painting' the cardboard in resin worked well (the resin soaked into the cardboard). By no means did it look trick although painted black it hid unobtrusively under the seat and outa sight.

I'm sorry it was done in the age of the box brownie and throw away camera and recording the mundane on film wasn't the 'done thing'. So no photos or drawings - but it was pretty simple to do.



Here is the parts diagram of the air box. Looking at the design you can see there is virtually no still air box. The 'air box' is really just a container for the filter with a hose inlet and out let. Both had a pretty torturous path.

I replaced the 'container' with a still air box with a hose coming directly off the carb. The whole area back to the mudguard was air box. Where part 7 is, a heavy rubber snorkel, I placed the 50x40mm air pod feeding into the still air box. The pod was shielded by fibreglass that reached up nearly to the seat. It gave good pretect, deep fording and good breathing (air flow).

There ya go - the original in all it's heavyweight glory.......

[2TInstitute]

Engine will be easier to jet and make more hp with a airbox. An example is a gixxer 750 used for P6 racing here was fitted with a heat shield and no airbox as ran back in the day. A stock airbox was fitted and it made 12HP more at peak. Incredible value considering you get one for free with the bike.
On Aprilia RS 125 we removed the 'restrictive' snorkel and dropped 1.5hp and about 3 on over rev.
The things to consider if you make an airbox is first try to enclose the carb inside, if you can't do that take some time to consider how to install and remove the carband particularly the carb to airbox manifold.