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Discussion Starter · #1 ·
ok starting this thread cos I need inspiration lol

seems aprilia ran out of money or just gave up on the air intake side of things

my own personal belief was they were at some point (late 90s) toying with the idea of a gp front mounted ram air pressurised box and just gave in

I currently run a open filter on my rs due to the port work larger intakes etc etc

now it runs like a sack of sh*t in the mid range and eats the reeds weekly I think this is due to turbulence and maybe the lack of a pressurized box

fact remains the bike pulls stronger and has more torqe everywere with an open filter although the noise is loud and can be annoying while at lights next to the local fuzz lol

so basically im looking into air box designs and priinciples trying to find the rite way to go

ive read that changing the lenth and width of the velocity stacks on the air box (rubber snorkels on a rs) can give mixed results apparently opening or replaceing these to the same size as the carb being used (34mm) does have its gains

taken from dyno testing a rgv with custom airbox

longer snorkels = gave less midrange more top end

shorter snorkels = short ones gave the same midrange but a little more top end and a little more on the over run

example of what I mean ..... this is a mc 21 nsr air box lid modified with custom stacks



and a rgv 250 airbox modified with custom stacks



what im thinking is takeing the rs air box or may be even something bigger (big enough to fill the void were the airbox sits ) and use a cold air feed off the 1996 ram air tubes ( or make a scoop ) in the front fairing to feed around the front of the trumpets of this box

from what i can make out its actually not about RAM AIR its about haveing still cold air around the trumpets and a SLOW steady flow in the box

forced induction of air into a pressureized box wont work (if my theory and a weekends worth of reading up on is correct lol)

the air inside the airbox will be turbulent and you're more likely to loose horsepower at high speed and wide open throttle with forced induction in to the box and will play ahvoc with the fuel pressure and temperature in the carbs with no pump

i beleive the way to go is to make an airduct from the from the front that gradually increases in cross sectional area, thereby SLOWING down the air as it get towards the airbox

as far as i can work out the bigger the size of the actual box the better

http://www.sportrider.com/tech/146_9910_ram/ nice read and results of ram air testing <<<

bought a few things to try and experiment with grabbed a cheap rotax max air box off flea bay dont even know if i can get it to fit in the void but for the price i just grabbed it lol



will defo be looking velocity stacks tuned lenth too

just posting all this for a discussion with the guys that have a bit of experiance with these matters

bounceing off each others ideas so to speak

suggestions comments welcome :)
 

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and while you're at it convert to rotary valve - shouldn't be to difficult.....

(I am not sure reed valves work with pressurized intakes)
(I am not sure reed valves work with pressurized intakes)

its been experimented with on rgv 250 forum ;)




there is little to o information on improveing the air intake / box for a 2 stroke all over the net lol

have been talking to stefano from italy on facebook about his design on his aprilia rs 125 gp project (JE RACEING)

seems carb inside and pressurized with a cold ram feed is the way to go but im not sure you would need a fuel pump or not for this to work

which is why i was trying to stay away fromt he pressurized box route

stefanos ......





http://en.wikipedia.org/wiki/Helmholtz_resonance <<< another interesting read
 

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I have thought about using a kart airbox on my 250
have a look there are CIK approved ones
as you said above I just need to have a measure of the space available and the dimensions of the airbox

the rotax max karts have a fuel pump (not a great one at that), but I think its just because the tank is so low down on them
 

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The max ones don't fit (I tried), the hole in the bottom of the box is too high so the bottom sits too low. I s

I've been toying with the idea testing different airboxes for quite some time and have seen a few I'd like to try but for a plastic box they're pretty expensive!







 

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I always thought the key to a good intake was to do with the air speed. The quicker the air speed is, the more air is gonna be drawn in through the carb. The thing to remember is the air doesn't flow into the motor, the carby sucks it in. But the volume of air still has to be readily available so trumpet stacks that lead off into a big capacity air box would be the way to go. If it's pressurized it's only going to be upping the rate of speed that the air is being sucked at. May have some ideas floating around...
 

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That's right monty there are different sized velocity stacks for the k&n. it looks quite a neat design, has a cone filter that fits internally.

I have seen one from a big Kart tuner (motiv) that looks like a Tupperware tub with a cone filter inside but loads of kart racers use it so can't be that bad ( e.g. http://item.mobileweb.ebay.co.uk/viewitem?itemId=181136161535 ). I am going to try to make one but haven't come across a container of the right size or shape yet.

I know the kart guys would rather run open filters but due to eu noise restrictions have to use an airbox in most if not all classes now.
 

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Discussion Starter · #12 ·
I have seen that one before as well parksie
need to get some measurements and give one a try
I do like the sound of an open carb or cone though
the other thing that interest me is why the trumpets on some of them kart boxes are faceing the rear away ffrom the flow of air
 

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Speaking of kart airboxes This was ours:



Little rotax 100cc with diaphragm carb. Airbox was filled with yellow "airfoam"

One thing i was told by other drivers was when pulling out of a corner, choke the airbox slightly and you'll get a quick burst of acceleration. I never had the opportunity to do this. Being we are on the subject of airboxes i thought id fire that question at you. is it true?
 

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the other thing that interest me is why the trumpets on some of them kart boxes are faceing the rear away ffrom the flow of air
Maybe that's to do with the fact that air is sucked into carbs not blown in as mentioned earlier?

Air boxes on karts aren't considered to be a tuning part, they are seen as more of a hinderence just there to make things quieter.

Luke you tend to cover the intake under braking at the end of a straight going into a bend to choke the motor which helps to cool the engine and gives a bit more acceleration on the way out on the 100cc karts (used to have a junior tkm back in the day)
 

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Very nice you've started looking into this, Pego. After having been looking for work for a whole year, I finally got a job two months ago. So I bought another 2 cheap RS125 from which I can make a nice commuter to bring me to work. Rebuild engine, skimmed head, maybe have a faff around with the Dremmel, VForce, a new 34 flatslide, maybe put my HPI CDI, Jolly Moto, and the usual stuff.

So my proper build still has to wait some time before I can get into the serious stuff. So little to share regarding air box design for the RS125. I'd go front feed. As large box as possible - lots of std stuff can be scrapped/relocated to make space for a larger/different design air box. Carb inside the air box. Air bleeders in the box, making sure to avoid any build up of pressure, as this will not be my goal. From this, I guess it is a matter of doing lots and lots of dyno and track testing to develop the box into something better than std. With different stacks, different layout inside the box, and so on. I am confident there are HP's to be gained. The big question is - how much work will it take?
 

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Hi

Not sure how much effect the length of the intake into the airbox could have. Direct onto the carb they should take advantage of pressure waves with the opening / closing of the "valves", but I would expect any decent volume airbox would provide a large enough still reservoir of air which would damp out these pressure waves.

Pressurising the airbox would probably require a fuel pump, if you got enough air pressure from the ram air to make any difference to power that is. And you would need to pressurise the float bowl as well by the same amount as the incoming air (hence the easy solution is to put the whole carb inside the airbox - Maserati did something similar with the carb on the early Biturbo).

The shape of the intake to the trumpet can make a massive difference to how well they flow. Enough that I strongly suspect removing the intake trumpet (proper one with both holes open) from a standard airbox will flow quite a bit less.

The RS might well gain from a larger airbox (although from memory as a rule of thumb an airbox should be 10 times the engine capacity to avoid being restrictive), suspect that Aprilia didn't bother as the gains would be small and it would make it a nightmare to get it in place on the production line.

All the best

Keith
 
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