Still air box on a performance 2 stroke

[john durrill]

I posted this on Vinduro but hope some one on here may have some ideas or experience in this area.


Ok folks,
  Here is another thing that has been kicking around in my head  for  several years. What are the advantages if any of a still air box on the
 intake system of performance 2 stroke dirt bikes.
 I know KTM designed them on the 72-76 Pentons and one box and air filter intake setup worked for all the engines 100-400cc , 27 - 38 mm carbs.
 I remember a tec sheet from Dick Obrian for my 165 Harley that stated for ever inch we moved the air cleaner back from the carb we needed to double the diameter of the air boot.
 We installed a Harley 74 air cleaner to replace the stock Brillo pad one. That had to be moved up and back on the frame for water crossings.

 Can anyone shed some light or experience they have had with this?
 We are playing with a bike build and wonder if it would be worth the effort to make one up. The bike has a long snorkel type boot now from the carb to the air cleaner.
 Thanks,
 john D.

[Merlin]

Is that you Mr.Chuckle Chuckle, you should know already.

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[john durrill]

Merlin ,
 I'm afraid i don't understand half the things i " know " chuckle chuckle!
 I see what engineers have done on other bikes. I see that still air tec is used on new cars and trucks. The 2 GM vehicles i drive  a lot use it.
 For me to use a thing properly i have to understand how it works i'm afraid , or at least the theory behind it. Do you have an SAE paper or an engineering Doc. you could Point me too?
 Thanks for the reply. So far i'm not getting a lot of input LOL!!!!
 John D.

[Lew Mayer]

I understand your question ,John, and wish I knew the answer. The effect of air box volume size was shown to me clearly on my '93 ST1100. With the top of the air cleaner/box off, the bike would barely or ,at least, poorly rev and backfire. Reinstall it and it revved fine. Why didn't I go to college?[?]




Lew Mayer

[OUCWBOY]

I went to collage, but all I know how to analyze it people, not machines. LOL

Donny Smith
Paragould, AR

[john durrill]

Donny ,
 I must have missed the class on it. It might have been the day i had 2 scheduled at the same time , one was how to hit a trash can at 11 kilometers with a 37 lbs of HE. Thats the one i went to. I should have skipped that one and gone for the still air box theory class!!!!
 ROFL!!!!!
 John D

[john durrill]

Lew , Donny ,
 David Haines sent me this reply on the Vinduro board.
 David is a mining engineer and a POG member.
 
 
Hi John
As I recall from our bits and pieces of technical discussions, the
taper of the airboot does have an effect on the air flow. On a
practical level, just adding the airboot to my Kawasaki 500
(triple), caused a reduction in power compared to open carbs.

I'd say that the most effecient form of inlet boot would have a
varying taper like a velocity stack, which brings me to the case of
the Zundapp inlet pipe/stack which sits inside the air box and makes
a big difference in performance (correct Andre?).

The general aim at the tapered air inlet is to prevent the energy
loss at sudden contraction of air flow through an orifice. I'd say
that to double tha area, for every increase of pipe length equal to
the diameter of the smaller end of the taper.

I carried out some experimentation with my Penton 400 and a 38mm
Mikuni using an alloy velocity stack which poked into the still
airbox. The need to use the velocity stack was driven mainly by the
need to create a clear passage to the (larger) Mikuni bellmouth. The
results were good in that the Penton pulled like a trials bike in
the rocks and generally went well throughout the rev range. I was
able to take the tuning process to 80-90% complete but as I did not
have a fresh Bing carb to compare the results to, I cant offer up
conclusive evidence if the tapered stack offered an advantage.
Nontheless, it's an experiment that can continue in the future.

I went through my technical books and got the following rough
guideline from " Motorcycle tuning: Two Stroke by John Robinson.
(Butterworth Heinemann 1986 (re printed 1992) ISBN 0 7506 0682 7)

The volume of the still airbox should be in the region of 10 times
engine capacity or roughly 1 litre for each 15 BHP.

Regards
David
 It will give me a starting point if i do this on the bike we aer playing with now.
John D.

[sachsmx]

I am an engineer for a large turbocharger manufacturer and can tell you that there is still a lot of cut and test when it comes to airflow analysis.
A rule of thumb is that you would like a convergence angle of up to 20 degrees/side when changing diameters in the inlet tract. Another is that a bellows type airboot with the same size ID as a smooth airboot will flow substantially less air. The reason is that airflow along the wall of the bellows forms a boundary layer of turbulent air, effectively decreasing the ID. I believe Maico had a good bellows style airboot design as it is much larger in diameter throughout its length than the throat of the carb.
I could get our aerodynamacist to chime in here, but he would very quickly get way over most of our heads

[Lew Mayer]

I believe Canned Hams have some type of rubber velocity stack on their carbs sticking into the airbox and wondered if a setup like that may help on my JP. I'll let you know.

No,Ron,I can't leave well enough alone.

Lew Mayer

[brian kirby]

That is correct, on the original Canned Ham Bing airbox boot there is an integrated velocity stack shaped peice. I dont use those boots as they are only for the Bing and I run Mikunis on my Canned Hams. The larger bell on the Mikuni prevents its boot from having a similar molded in velocity stack.

Brian

[Lew Mayer]

Brian, do you have any leftovers you're not using that would fit a 30MM Bing for a little experimentation. For sale, of course.

Lew Mayer

[john durrill]

Lew,
 Check Sudco's page 55 of the online catalog.
 Measure the bell on the 30 mm first. they sell several different size and length Mikuni V stacks.
 John D.

[john durrill]

Thanks SachsMX,
 If I understand you correctly then ,rule of thumb on the air box volume          from Davids post should be 1 to 1.5 liters with a transfer from that volume to the carb bell of not more than a 20 deg angle on the sides of the taper? Smooth as possible transition on the boot walls.
 That would be a ball park starting place for a 125 making 15 to 22 or so HP?
 John D ( working on what to use for a quick throw together test box that could be redesigned out of good materials once the size and shape was nailed down )

[sachsmx]

Yes, no more than 20 degrees per side on the taper and the walls as smooth as possible. Also, no bends upstream of the carb is ideal. If you have to make a bend upstream make it as smooth of a transition as possible. Tight bends = turbulence and turbulence = less flow. If you must make a tight bend then a "D" shaped cross section through the bend is best as it keeps the airflow straighter thru the bend than does a round cross section. Better yet you could put a "turning vane" in the bend, which is a straight wall that spilts the airway into two "D" shaped sections. We do this in a lot of applications were space is at a premium.

[hrbay]

Do reflected sound waves in the intake tract effect anything... a scavenging type thing going on in the slide side.

GC