Airboxes R vs SP

[Crank]

Mbsteve - while I appreciate you pointing out the highly technical theory behind airboxes being developed today, in your opinion, how much of that was known and incorporated into the design of lets say a 3XV1 22 years ago?

[Need4speed]

Hi Crank - I was there 22 years ago and this was normal thinking in racing and factories - I normally explains this way:
Take a garden hose open the wather pipe to your decided pressaure - find the were the water comes out - squice with your finger so that the opening are reduced and the pressaure increase - if you squice to mutch the length of the water are shorter, if you open too much - the length af the water are shorter - the maximum performance are were the opening produce the most presaure without limiting the quantity of water passing... think this metaphor - when you are looking at the intake "snorkels" in any form and shape that they appear - then think "green garden water hose"  ..consider this in all aspects in you engine - not all tuning are done by making things bigger - it appears that smaller venturies incrase performance too ..

[EEKNOWS]

Hi Crank - I was there 22 years ago and this was normal thinking in racing and factories - I normally explains this way:
Take a garden hose open the wather pipe to your decided pressaure - find the were the water comes out - squice with your finger so that the opening are reduced and the pressaure increase - if you squice to mutch the length of the water are shorter, if you open too much - the length af the water are shorter - the maximum performance are were the opening produce the most presaure without limiting the quantity of water passing... think this metaphor - when you are looking at the intake "snorkels" in any form and shape that they appear - then think "green garden water hose"  ..consider this in all aspects in you engine - not all tuning are done by making things bigger - it appears that smaller venturies incrase performance too ..

That's not only with airboxes.

[zoomzoom]

Hi Crank - I was there 22 years ago and this was normal thinking in racing and factories - I normally explains this way:
Take a garden hose open the wather pipe to your decided pressaure - find the were the water comes out - squice with your finger so that the opening are reduced and the pressaure increase - if you squice to mutch the length of the water are shorter, if you open too much - the length af the water are shorter - the maximum performance are were the opening produce the most presaure without limiting the quantity of water passing... think this metaphor - when you are looking at the intake "snorkels" in any form and shape that they appear - then think "green garden water hose"  ..consider this in all aspects in you engine - not all tuning are done by making things bigger - it appears that smaller venturies incrase performance too ..

Hey Need4speed when you say open the tap to a desired pressure, my question to you is...
1.where is the pressure measured at in the pipe?
2.when you close the hose the pressure before your finger will increase and the " velocity" of the water exiting after you finger will increase.
3. the theory behind this is all to do with "air speed" and cylinder filling, thats why a smaller venturi will increase "torque" and rob HP high up in the rev range to a given point where you will need to "increase" the venturi to gain HP. lost by a small venturi.

So thus the theory of"go big or go home" doesnt always work in all applications.
If you gain both torque and hp throughout your rev range it means the venturi was to small in the first place.

[Neal]

Have you guys seen the "trumpets" that fit onto open carbs ? They are supposed to help with improving the velocity of the incoming air . The same principle if you have a 2L bottle of water - when you swirl it around and flip it upside down the water exits much faster .

[yorkee]

Eagle-eyed viewers will note that the lid on the far left is an SP lid. Useful to be able to see the SP intake trumpet like that.

Also of possible interest for those who like to ponder such things is the little 'boost bottle' that is oe fitment on the SP models' square lower cyl airbox intake pipe... Some kind of aid to bottom-end carburation I imagine?

  you know more about this airbox than I do.

This is basically a SPR airbox... Anyone have a SP airbox for comparision?

[Need4speed]

EEKNOWS: You are right, it's all the way from the snorkles to the end of the exhaust pipe.. 

zoomzoom: as I see it - our target are to increase the speed of air in the complete engine...one example are the chrankshaft of the 3MA1/2/3/5 versus the 3MA4 SP. The 3MA4 has, apart from a better balance, a bigger volume in the same chrankcase = more pressaure of air = more speed what is needed to accept the SP porttiming,(cylinder 20) and the SP exhaust venturi in the cylinder..
for sure the angels of the tranferport in the cylinder are important, as well as many other points - in my oppenion..

your point 1 : equal to what vaccum you decide to have to fill the chranchase
your point 2: that is my point - in the complete engine
your point 3: my experiance are not like you describe - but if you have another oppenion it's ok with me.

[casal-fan]

Have you guys seen the "trumpets" that fit onto open carbs ? They are supposed to help with improving the velocity of the incoming air . The same principle if you have a 2L bottle of water - when you swirl it around and flip it upside down the water exits much faster .

There were some experiments by the legendary prof. Blair on the subject.
Wouldn´t belive it blindly though, there are a lot of clever people stating that intake flow never reaches sonic speeds, and in this document I can see numbers higher then mach2.
http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

mbsteve, that is a nice article, please don´t get me wrong, not trying to shoot anything down here, but...
That formula, seems to have it´s roots in helmholtz resonance formula?
If so, there is a problem imo. One hears a lot about helmholtz resonance on a 2stroke engine, but the basic helmoltz resonace does not apply on a 2stroke engine.
Specific helmholz resonance calcultions, apply to a set of parameter that are not all present in a 2stroke engine, neither in the airbox of a 2 stroke engine.
It has to do with superfast pressure changes in an enclosed system (or better with only one opening, like a beer bottle f. exe.)
Now, a 2 stroke airbox is not an enclosed system. Sometimes there is a way to the crankcase, sometimes there is a way to the cylinder... and exhaust... and the atmosphere... and the outer space
So, helmholtz resonance (or derived) formulas maybe do not apply?
But maybe that formula accounts for that, I don´t know. I have a set of R airboxes laying around, it would be interesting to mesure them, and see what the result is when the numbers are dialed in that formula.

And I actually got an idea that maybe could work, and maybe not so hard to test.
Here is the background:
http://www.youtube.com/watch?v=K4Acofm09EA
From 0.35 to 0.42 there is a sequence of 4 accelerations where it is clear that back flow is happening.
To the best of my understanding this happens because of a pressure diferential when crankcase is pressurised, and the reeds are still open - backflow occours.

Relating this to the airbox. Well, if "back flow" occours at the airbox intake at an rpm regime where the motor is intend to work good, f.exe. the powerband, then probably there is something wrong.
This could maybe be tested by reving the engine under load, with a thin piece of paper hanging close to the box intake. That would maybe tell what is happening with airflow at the box intake.

[Crank]

Relating this to the airbox. Well, if "back flow" occours at the airbox intake at an rpm regime where the motor is intend to work good, f.exe. the powerband, then probably there is something wrong.
This could maybe be tested by reving the engine under load, with a thin piece of paper hanging close to the box intake. That would maybe tell what is happening with airflow at the box intake.

Now there is a realtime basic test that might show what the airbox is doing at different points in the rev range.
Easy to do on a dyno, although not super accurate, it will indicate at what rpm the box flows best.

[Crank]

I would still like to know if any of the theory in this thread was known and incorporated into the design of our machines produced 20 odd years ago.....

[zoomzoom]

EEKNOWS: You are right, it's all the way from the snorkles to the end of the exhaust pipe.. 

zoomzoom: as I see it - our target are to increase the speed of air in the complete engine...one example are the chrankshaft of the 3MA1/2/3/5 versus the 3MA4 SP. The 3MA4 has, apart from a better balance, a bigger volume in the same chrankcase = more pressaure of air = more speed what is needed to accept the SP porttiming,(cylinder 20) and the SP exhaust venturi in the cylinder..
for sure the angels of the tranferport in the cylinder are important, as well as many other points - in my oppenion..

your point 1 : equal to what vaccum you decide to have to fill the chranchase
your point 2: that is my point - in the complete engine
your point 3: my experiance are not like you describe - but if you have another oppenion it's ok with me.

yeah neal i agree in some respects, but hey its all good if she share opinions and info..

[zoomzoom]

Relating this to the airbox. Well, if "back flow" occours at the airbox intake at an rpm regime where the motor is intend to work good, f.exe. the powerband, then probably there is something wrong.
This could maybe be tested by reving the engine under load, with a thin piece of paper hanging close to the box intake. That would maybe tell what is happening with airflow at the box intake.

Now there is a realtime basic test that might show what the airbox is doing at different points in the rev range.
Easy to do on a dyno, although not super accurate, it will indicate at what rpm the box flows best.

Crank, im picking up my dyno tommorrow mornig and once sorted we can start doing doing some testing.....

[casal-fan]

Relating this to the airbox. Well, if "back flow" occours at the airbox intake at an rpm regime where the motor is intend to work good, f.exe. the powerband, then probably there is something wrong.
This could maybe be tested by reving the engine under load, with a thin piece of paper hanging close to the box intake. That would maybe tell what is happening with airflow at the box intake.

Now there is a realtime basic test that might show what the airbox is doing at different points in the rev range.
Easy to do on a dyno, although not super accurate, it will indicate at what rpm the box flows best.

Hummm, sometimes, finger can be fast a typing...
While this test could be interesting to do, if the findings are not very good, and a lot of inconsistency shows at the airbox intake, it could be that the undesired effects have they roots somewhere else either then the airbox.
And now, I´m probably gonna walk into deep water here, without knowing a heck of a lot about swimming... but anyway...

Resonances in an engine are supposed to play a major role. While (at least to my knowledge) it would be very difficult to quantify them in an empirical manner, the theorys still apply.
F.exe. if looking at the crankcase filling, the intake period.
A depression is caused in the crankcase a column of  mixture rushes into it. At some time, the low pressure area in the crankcase becomes a high pressure area. Since mixture (as well as other matter) flows from high to low pressure areas, then the direction of the flow would be reversed.
Now, the time this event (in and out of the crankcase) takes, is a frequency.
This frequency can be matched, by adjusting a handfull of variables, all playing a role upon eachother.
Some of the variables I can think of would be, crankcase pressure, inlet length (this is a trycky one... what is inlet length?, with an airbox, is the airbox part of the inlet? Is it not? Have no idea), carburetor diameter.

Assuming f.exe. an engine is rotating steady at 6000rpm. If intake frequency is matched exactly for 6000 rpm, that would mean that the crankcase would have optimal filling in every rotation. The stream thrugh the carb would be constant, and no intake adverse happenings would occour.
Now, to quantify this? Well, helmholtz resonance would be the one, IF the crankcase was sealed, and had a steady volume. That of course does not happen, crankcase opens to transfers, and the piston running up and down makes sure that the crankcase volume is constantly changing. Quantifying, like in putting all this into a formula? I don´t think so... but that doesn´t mean that a good result can be obtained by playing with helmholtz resonance principles.

So, hurra, now one can test to death, and have an engine that is perfectly tuned for lets say 6000 RPM.
Now, all that is needed is to not go out of 6000rpm, because as soon as that changes, the engine frequency changes, and the well tuned intake frequency for 6000RPM will no longer work at its best.
So, if the TZR engine (as well as any roadgoing engine) must operate in a vast rpm range, compromises have to be made, and mesures to atenuate the bad performance in rpm regimes where nothing sings along have to be taken.

Coming back to the airbox.
If a manufactor designs an engine that is supposed to be a sports engine (but still for the road, and it has to go around a city at 4-5k rpm otherwise reviews will shoot it down), after parts have been chosen (carb, etc) intake testing has been done to satisfy the wish of a sports, revy engine, then he will probably meet up with an utter shity low rpm range. But lucky, he has the airbox. He has the airbox, and the oportunity to tune the airbox to a frequency that will somehow interact with the hole engine and intake (without airbox) frequency, and improve engine performance at the ranges where it was not happy.
And while this can be tottaly wrong, (its just an opinion in a forum, and not a cientific document), many repports around that when taking the airbox out, engine will loose at lower rpm regimes.

And to finish, just a practic experiment to see how this all thing MAYBE works.
A bottle, blow air into it by its neck, notice the sound.
Half filled with water, or beer - higher frequency
Insert a tube in the bottles neck so that it works as if the neck was made longer - lower frequency
Inset a tube trugh the neck that has a small inside diameter - lower frequency

[EEKNOWS]

Back flow out the carb is more to do with the intake tract resonances,

[casal-fan]

Back flow out the carb is more to do with the intake tract resonances,

Agree.
I just wonder if having the airbox as some kind of independant resonation element, would somehow "counteract" rpm areas where frequencys are inadequate.
Did some "homework" during the weekend, on this, and ended up with more questions then answers. (It usually goes like this).

Eeknows, what is it you refer to as intake tract? Is it the combo crankcase-carb bell mouth-and everything in between?
And another one, the aprilia 125 you tested with and without snorkels, did it have the 28mm carbs?