Dirt Rider . Net Text Version Home
Pages: 1
Carb restriction work-arounds
(Click here to view the original thread with full colors/images)
Posted by: David Trustrum
Ok so this isn’t hugely dirtbike oriented but there is some good knowledge here so it is worth asking.
A web-search has sadly only unearthed myself bleating my limited knowledge on it. Tragic.
I am building an engine for a class with a 125cc 2 stroke but limited to a 24mm carb.
Now this attracted me as I have read how back in the old days of F750 racing in the states, the powers that be were trying to curb speed at Daytona etc. So they introduced a carb restriction.
But the fiendish Japanese engineers foiled this & the bikes were just as fast or faster next year.
The idea is you gently taper the entry to the carb before continuing to the usual bellmouth shape. This increases the velocity & the carb flows pretty well reducing the restriction of the small carb.
So I am well keen to experiment & see how well I can work this.
I was intending to add a close fitted extension to the bellmouth to essentially add a gentle tapered bit to the slide & replicate the bellmouth before that.
& also an extension to the other side to taper it back before it hits the reedblock.
Obviously I don’t want to add huge length to the inlet, although the reedblock goes a way to alleviate concerns here.
The question is: (Assuming someone here has ½ an idea)
Assuming I want to replicate a 28mm carb (bigger would be ideal but beggars/choosing etc) how far should I extend this taper?
I am going to start the taper in the carb body itself on the exit side.
To add to my woes the damn thing has to be air-cooled so any tips from old timers here appreciated.
Posted by: steve.emma
just my 2 cents worth here David, BUT if your main power restriction is the size of the carb bore i.e. 24mm. then if you add a taper from the carb to the reedblock this will then have to be smaller than 24mm (or it wouldnt be a taper). so im thinking that any benefit that this taper would give in terms of flow velocity it would be more than cancelled out by then added restriction. not only that but for top end power the carb would normally want to be a close as possible to the cylinder not spaced away from it.
steve.
Posted by: Rich Rohrich
David - 7-8 degrees of taper is considered the most efficient diffuser angle and will tend to minimize flow seperation right after the restriction. If the air seperates too quickly before or after the restriction it tends to tumble around and make the restriction seem greater to the engine. Some simple math should give you a pretty good idea of the length of the taper from the main restrictor point to acheive this angle. If there isn't enough space to accomplish this flat of a diffuser angle I think you'll find that the closer you get to this angle, the more efficient the flow path will be. We face a similar problem when working on the flow bench with valve and seat angles on four-strokes. That interface is just a small convergent cone (the port throat, valve head and backside angles ) leading to a diffuser ( the exit angles past the seat and the shape leading into the chamber).
Posted by: Flummo
The choice of the carburettor might be just as important as the shape and size of the airpassages before and after the carb. Might I suggest a Mikuni TM24 flatslide?
Rich: you talk about the angle after the carb. What about altering the shape and/or length on the carburettor inlet?
Posted by: SFO
find out who is building the fastest ones in your class.
Have them build your motor.
Then dissasemble it and build on that.
There is always a way to push the rules, see how they are written eactly in regards to carb choice.
Posted by: David Trustrum
Steve: I meant ‘taper out’, not ‘taper in’ [thinks for a bit, grabs Oxford Concise dictionary] Oops! 
How about that?
ok so I meant a taper from the reed block to the carb.
Flummo, that’s what I’m running. Or rather will, when it’s all together.
SFO, there aren’t any fast ones, most are running 150 4-strokes also allowed. But on short tracks I figure the 125 2smoke should be easy to race even with the restriction. And I had this carb already. . .
Rich. Yes, thanks, that makes sense. Space isn’t a problem (well it is, it's being squeezed into an RS125 chassis, but space back from the carb isn’t a problem). So I’ll machine some adaptors at that angle & see what the engine likes in terms of final diameter & hence length.
So at 7deg (or 14 included) from 24 to 28 is only 16mm. Does seem a little steep. Did you mean included?
Posted by: JFN
Hello David;
There has been some good research on restrictive intake designs. The research was done for the Formula SAE racing cars using a 600cc four stroke engine. One SAE paper is # 2001-01-1211 titled, Evolution of Intake Design for a Small Engine Formula Vehicle, from Lawrence Technological University. The paper deals with using Helmholtz Resonate Frequency Tuning. Also there has just lately been a updated paper on this subject but I don't know the number and I haven't read it yet.
Another good paper is titled: Improvement of the Torque Characteristics of a Small, High-Speed Engine Through the Design of Helmholtz-Tuned Manifolding from Cornell University, SAE paper # 900680. Excellent info. Although this research has been on 4-stroke engines. They also work for 2-strokes
Of course the ultimate would to be able to use Virtual 2-stroke simulation program from OPT.
One of the main things they illustrate in these papers is the use of a very long tapered inlet pipe and the use of plenum's. Also there are lots & lots of formula's. One of the main items they tell you is to use a screen mesh just before the restrictor to help the transition from turbulent to laminar flow into the restrictor. In your case the 24mm carb. Check them out.
Posted by: David Trustrum
Presumably the mesh idea is used when there is a nasty flat restrictor plate with a hole cut in it, but by having the freedom to taper down to size on both sides of the restriction this shouldn’t be necessary as the transition shouldn’t be turbulent.
Interesting idea though, I would have never thought of it.
The other quick & dirty idea people have used in this instance (big V8s with flat restrictor plates) is to jet rich & make sure the engine leaks. This strictly speaking is cheating & I think anyone who is introducing a leak into the engine of a 2 stroke is likely to reap the punishment at any rate.
with 24mm the Carb velocity will be pretty high though, this will limit the revs we can pull.
Posted by: JFN
In the paper, SAE# 2001-01-1211, there are no flat restrictor plates. There is a taper on both ends and the restriction is in the form of a venturi. The restrictor shown uses a 32mm inlet (front of carb) tapered to a 20mm venturi then they use a long taper of 7 degs to a 50.4mm opening. Also do the rules say you must use a 24mm carb or a 24mm restriction or restrictor? I have been using mesh screen on carbs for a long time now and you would not believe the difference they make in airflow.
Posted by: David Trustrum
Rules say carb to the equivalent size of one 24mm.
So what are you using for mesh? Door flyscreen? Where are you mounting it? At the bellmouth?
I would have thought on a 2 stroke the blowback may have caused an issue, but I have an open mind.
Again, by 7deg you mean 14deg included angle?
That would make the last section 124mm long in your example or 248 if 7deg included.
thanks for the reply.
Posted by: JFN
The paper reads 7degs...incline...taper. I read this as 14 degs included angle...but...here I make an assumption, since I have done this in the past and it worked fairly well, I measure the picture in the SAE paper as to the size of the restriction and work the math to compare to the pictures' measured length ( I hope I'm saying this correctly) and I come up with the 7degs as a included angle and NOT 14degs. I can e-mail you the picture if you want it. But the paper has all the formula's needed and the example used. I'll quote this from the paper..."Calculations showed that considerable improvements in airflow could be obtained by increasing diffuser cone lengths up to ten inches.It was also observed that a longer diffuser with a smaller angle could produce more airflow at lower pressure drops."
As to where to mount the screen mesh, since.... "Even bellmouth entrance/exit configurations allow separation of the fluid boundary layer which creates kinetic energy losses."..., mount the mesh between the bellmouth and the venturi.
As to what to use...I have seen doorscreen used but I feel that to create laminar flow you need some length to the screen, ie.thick doorscreen? I have a source that charges me 'x' amount. E-mail me if your interested.
Posted by: David Trustrum
Hmm, interesting. Although increased airflow sounds promising in a dynamic engine I suspect throttle response may be a bit ugly with fuel stand off in a reeal long tube. I am expecting to make compromises along the line.
I’m not too cheap to buy the paper, but to be a Luddite, I hate/won’t do internet CC purchases.
Text Version Home
vBulletin Copyright ©2000 - 2008,
Jelsoft Enterprises Limited.
vB Easy Archive Final ©2000 - 2008
- Created by Stefan "Xenon" Kaeser
