Rear Facing Airbox Ideas

[hwr]

Looking to run an airbox for a mina with a rear facing intake.

Has anyone ever seen someone run a junior motorcross airbox from the 65cc class (running a 70cc BBK) for example, kx, sx, rm, cr, yz. Can find cheapies on ebay and local classifieds for ~$30 AUD.
Looking at a few dyno pulls of 65cc class has them making peak power around 9-12k rpm which is similar to what my port timings align with. Potential for some beneficial yamaha developed intake resonance!!.

Also considering airboxes commonly used for race go-karts, however these seem too large considering typically higher displacements of the karts. They're also quite expensive.

Final idea is to get something 3D printed, im relatively handy with SolidWorks so I could whip something together which could be cool, but i imagine oil and fuel will not be compatible with most plastics.

Any cool ideas? things youve seen?

Let me know!

[benji]

The vento zip has a rear facing carb w airbox over the cvt cover next to the shock and wheel

[ThomasTPFL]

Stock Buddy 50 is sort of sideways rearfacing. Similar with TGB.

139qmb too.

[hwr]

the scoot is a keeway hurricane basically the same as a vento zip i think. Its properly rear facing to straighten the intake path. Didnt even need to modify the hanger.

[ThaiGyro]

Hey hwr...
I have had some contacts with a professor in a European University regarding air box designs. Not sure why you are looking at "rear" facing...why do you ask?

My research highly involves using an airbox, versus not. I always use a box. That said, understanding the engineering is very complicated. My meaning is...there are single stage air-box designs, that effectively do nothing but attenuate sound. They take air into a chamber, however that ends up with dynamic pressure and "allows" it to be vaccumed through a venturi and beyond.

There are others...usually 2 stage designs, that add performance value. A 2 stage box, generally, takes in air through an inlet tube to the primary chamber, creating as much high pressure as possible across a filter membrane. This membrane has a very large surface area...better filtering/more air.

I have noted variations of forward facing inlets versus rear facing. It is interesting, really! In a single stage box, no matter where you face the opening...you get the best power (If you can actually measure it) when the inlet tube has the best flow dynamic. Lots of words to say...the direction can influence the pressure, but depending on all other ambient air influences. Side winds? Frontal? Rear? Low to high/high to low?

It is all a matter of nano data, which may, or may not be valuable. In a 65 cc engine? We have a KTM clone that does 9.5hp. It is now 4 years old. Test it? 9.5+/- ambient conditions. The pipe? POS..."Piece-of-shit". When I put a good pipe on it and carb and sound tuning? Blew out seals...Piece of shit.

I think you must find the right combo for your setup...and that very well could mean adjustment every time you test/race/practice. Our tests, when racing, were 15-20 minutes track time for every 2 hours of wrench time.

My suggestion is to find a box that can handle just a tad more than the volume you engine needs at any desired point. I like high end street stuff. I tune for an agressive torque curve, as flat as possible to the rpm max. Soft curves are ok for an average rider.

Not enough air is a top end limiter, but too disturbed air can be worse in mid range. Depends on the racing type...offroad, street, motard, MX, drag...not the same at all. The issue you have is time old...it is a box. It's dynamics are not controlled...think about that! They can be, but the returns are not measurable.

[ThomasTPFL]

Here's some more info. www.mopedarmy.com/wiki/Airbox_Calculations

[ThaiGyro]

Haha...ThomasTPFL, I have seen that before...tons of math. Nice page!

I am an engineer, so I like that stuff. The however is, here, on this site, most don't have a wind tunnel, nor a flow bench, nor a dynamometer. If you break down the principal elements of the math...you still have a single or two-stage box, with an inlet pipe, (or not) and a filter, (or not) and a velocity stack. (or not)

With all kind of algorithms in my drain hole...I still simply play with air space, generally using the OEM box. When I can dyno test, I bring all of my crazy designs! My latest is a high velocity stack inlet, into a conical filter inside a housing, (to avoid moisture) down to a shorter than stock corrugated tube, into the stock box with OEM filter removed. The carb interfaces with the box using a smaller velocity stack, though the box wall, into the carb inlet. The stack (bell mouth) is sealed at the outer wall...

Mt next testing design is the same inlet/box setup, but with n anti-chamber at the carb. My thought is to provide a tad more available pressure, as close to the reed block as I can. I am also connecting a MAP sensor in the anti-chamber, so I can "see" WTF I am getting...(or not)

My engineer brain trust colleagues like the idea, but think the usable returns are too low on a two-stroke. I am testing it with an open passage from the resonant box and with a diaphragm. Fun!

[hwr]

There is a relatively simple formula in Jennings two-stroke tuners handbook pg92

F=(V_s)/(2*pi) * sqrt[A/(V_c(L + 0.5*sqrt(pi * A)]
F = frequency
V_s = sonic speed (1100 ft/s)
A = inlet area
L = inlet length
V_c = crankcase volume

My intention is to use this formula empirically and alter one of the parameters.
Using a 3d printer I could include three components carb side, middle section, inlet side
Using the 3d printer I could alter the inlet area or length relatively easy with threaded or press connections to keep it rigid.

Frequency ~proportional~ sqrt(Inlet Area)
Frequency ~proportional~ 1/sqrt(Inlet Length)

By altering either I could test fit different inlets until a beneficial balance was found

[ThaiGyro]

I have seen Gordon Jenning's stuff. Still have some files from his early 70's writings. Nothing wrong with the formula. The only issue we have had, repeatedly, is that it doesn't take into account form and shape or many other little inducements. I can see why now, that you want a rear facing inlet.
For me, the original calcs are a good place to start, but my play time has not yielded much that is measurable.. I do like your approach. One alteration at a time. I wish I had the funds for an engine dyno.

[jackrides]

Gordon Jennings still Rocks!

[ThaiGyro]

In somewhat related engineering...Rotax has it's own design or maybe more than one for a race-type airbox. Though I cannot find details, yet, you can see pictures on various websites. The pictures here are for the Rotax Max 125cc, with the box behind the engine and dual inlet horns facing forward, but on the bottom. (In a kart, there is open air from under the chassis...also note that these 125's are capable of speeds above ram effect)

What I see, is a low to high air flow, which makes some sense, as air will rise as it heats. The filter is really massive, matching the box dimensions, though it is not tuned for pressure on the low side. These engines are capable of 34 hp, with a funky looking pipe. It looks as if they are slightly reducing box dimension as the air is forced up, through the filter and again forced toward the carb bell via a reduced , but gently curve.







My Honda is also low to high flow, in similar fashion, but has a high "snorkel" to eliminate water.

[ThomasTPFL]

My next airbox plan will actually look very similar to that, but be made out of a pair of snap lid food containers.

[ThaiGyro]

Deranged minds think alike Thomas..my latest "experiment" is an anti chamber at the carb...narrowing diameter to get a similar pressure effect as the Rotax above. It too is a clear plastic food container, with inlet horn from the OEM box, sealed with my home-made clear silicon gaskets. Looks quite dumb, but needed for testing. (I am mounting my map sensor there to measure effects of all other intake track changes.)

[ThomasTPFL]

For me it's less about a real performance improvement and more about maintaining consistency. My bikes so hacked I have no chance of a stock box, and when I had a pod on there it took it no time to clog enough to hurt performance even with a rear fender.

I'm thinking I'll just use a simplistic interpretation of the math and make sure I have about 1000cc inside the box on both sides of the filter.