OK, this ain't brain surgbery, it is simple airflow velocity at work.
Simply put, taking the length of the air duct off a two stroke, even a piston ported one, increases the rpm band, and sets the tune rpms past the level the engine can survive at. Stopping down the velocity with NO length past the inlet of the carb before air is available, sets the jetting at so low a velocity, it has to be radically changed for the reduced signal of pull at the jet orifii.
Adding the tuned length of the velocity "stack" back, makes for a stronger signal at the jet orifii, and the float levels and jetting don't hqave to be radically altered to get the fuel into the engine.
I've been doing this kind of work for 35 years, and I know what works and doesn't, so please, don't give me all that pod filters crud about how good they work over a properly designed airbox and ducting, it just won't fly, and never has.
The airboxes on the older GP 500/250/125 2 stroke engines are just that, air boxes with fairly long inlet velocity tracts, same as MotoGP 4 striker engines are, NONE of them use open stacks, pods or anything else.
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Moo wrote:With a few thousand dollar puter program, or some flow dynamic phd level college classes......rngdng wrote:How do you determine the proper length of the inlets, and volume of the box? Lane
Maybe we should get somebody with a flowbench to figure it out for us. There's this guy in Texas...............
Lane
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Look to the current four stroke street bikes for direction. They are pretty much state of the art for design and it does work on our older two strokes.
Dave Crussell's H1R airbox is huge, is also the rear fender, so that gives you an idea as to how large it is. His carbs are also very large, and he uses air ducts form a four stroke street bike as well. What can also be used is radiator hose, other hoses to lengthen the "pods" out, but a still air box is the way to go, and experimentation is the order of the day, not fancy computer programs, simply because they aren't "active". They don't change the parameters they way they should be changed, as the inlet velocity changes.
I remember reading recvently that Ralph Spencer built an airbox setup for his T500 racer,. with less than workable results. When I saw the pics, first thing I figured was that the air box itself was way too small in volume. Testing showed that to be true. That particular box needed to be about 4 times the volume it was, but at least, Ralph gave it a try, and learned a lot more than not trying at all.
Dave Crussell's H1R airbox is huge, is also the rear fender, so that gives you an idea as to how large it is. His carbs are also very large, and he uses air ducts form a four stroke street bike as well. What can also be used is radiator hose, other hoses to lengthen the "pods" out, but a still air box is the way to go, and experimentation is the order of the day, not fancy computer programs, simply because they aren't "active". They don't change the parameters they way they should be changed, as the inlet velocity changes.
I remember reading recvently that Ralph Spencer built an airbox setup for his T500 racer,. with less than workable results. When I saw the pics, first thing I figured was that the air box itself was way too small in volume. Testing showed that to be true. That particular box needed to be about 4 times the volume it was, but at least, Ralph gave it a try, and learned a lot more than not trying at all.
Last edited by Admin on Mon Aug 21, 2006 11:12 am, edited 1 time in total.
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As H2R Tuner states my cool "liitle" airbox didn't work on my race bike, actually restricted airflow. Think I'll try this airbox on of of the street bikes I'm building with milder engine porting.
This this off season I'll make another stab at designing a larger airbox. The frame geometry of my bike with the added gusset tubes gets in the way of making a simple airbox design, along with the angled carbs. I can't just have runners extend without interfering with the frame gussets.
At Mid-Ohio I finally ditched the pod filters and went with open carbs...with the ported engine I have intake reversion was fogging/fouling the foam pod filters with fuel-oil mist and the bike ran poorly. Open carbs allowed the engine to clear out better an pull upper revs better.
Next step for my race bike thing I'll try some screened velocity stacks I've had around for years, I'll see how they work at next race at Sandia. Of course at Sandia I'm sure jetting and gearing selection will be way different from sea level tracks.
This this off season I'll make another stab at designing a larger airbox. The frame geometry of my bike with the added gusset tubes gets in the way of making a simple airbox design, along with the angled carbs. I can't just have runners extend without interfering with the frame gussets.
At Mid-Ohio I finally ditched the pod filters and went with open carbs...with the ported engine I have intake reversion was fogging/fouling the foam pod filters with fuel-oil mist and the bike ran poorly. Open carbs allowed the engine to clear out better an pull upper revs better.
Next step for my race bike thing I'll try some screened velocity stacks I've had around for years, I'll see how they work at next race at Sandia. Of course at Sandia I'm sure jetting and gearing selection will be way different from sea level tracks.
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Exactly my experience in 1979....desmocat wrote: At Mid-Ohio I finally ditched the pod filters and went with open carbs...with the ported engine I have intake reversion was fogging/fouling the foam pod filters with fuel-oil mist and the bike ran poorly. Open carbs allowed the engine to clear out better an pull upper revs better.
Save your time and possibly a seizure. They were tried long ago when the TZ guys thought they were cool when they saw them on the superbikes. They create suprising lean conditions.desmocat wrote: Next step for my race bike thing I'll try some screened velocity stacks I've had around for years,....
Besides h2rtuner calling my observations 'crud' while misquoting me, he is incredably nieve about how a computer program would handle determing a very good starting place. Sure a program that simply took Gordon Jennings equations and calculated something would be difficult to get something meaningful out of without a lot of manual intervention. But Jennings equations were the result of making assumptions in differential equations that are 'active'. This kind of computation has been going on since the 60s, and has gotten incredably accurate due to the increased speed of the processors. We used to run simulations over night that take a few minutes on a current pc. Actually, using an analog computer you could change the system dynamically by turn a potentiometer(s) and watch the output in real time.
No it isn't brain surgery - it's fluid dynamics - much more difficult. There is no 'intuitive' in fluid dynamics. Ask the people who designed intakes and exhausts before around 1970. They guessed, built, tested, and trashed until something works. Recent engines are probably close to 98% right after the computer simulations, and they are considerably more complex.
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Pods
DTKid,
Heres the breakdown.
Your bike averages 3-4K rev. not actually high enough to worry about the inlet speed, air filter tuning. ( thats for the new R6 that rev at ave 12K)
Deal with the fact that you will have to re-jet the carbs. Just do it and get it over with. Its really easy. just go 1 size at a time.
New pod filters are... well.. new , cheap and avail at the local store.
just do it, relax and enjoy !
seriously made a HUGE difference on my bike.
-schmooooooooke
Heres the breakdown.
Your bike averages 3-4K rev. not actually high enough to worry about the inlet speed, air filter tuning. ( thats for the new R6 that rev at ave 12K)
Deal with the fact that you will have to re-jet the carbs. Just do it and get it over with. Its really easy. just go 1 size at a time.
New pod filters are... well.. new , cheap and avail at the local store.
just do it, relax and enjoy !
seriously made a HUGE difference on my bike.
-schmooooooooke
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Well, sorry, sonny, but I am not anywhere nieve. I DO know what I am speaking of.
You worked for who?
Ralph Spencer's airbox should hold 5 to 6 quarts of air minimum, Dave Crussell's looks to hold over 12. A certian number of Yamaha MotoGP bikes I know of carry 14 quarts of air volume.
And, how many air inlet systems have you designed?
And...yes...I use a very advanced computer program to design what I do, a really nice one from the factory. No...I can't share it with anyone, and I wouldn't give you any info from it, after all, your posts above tell all of us you know it all already, and that you already use a computer program to design your airboxes and inlet systems.
You worked for who?
You worked for who?
Ralph Spencer's airbox should hold 5 to 6 quarts of air minimum, Dave Crussell's looks to hold over 12. A certian number of Yamaha MotoGP bikes I know of carry 14 quarts of air volume.
And, how many air inlet systems have you designed?
And...yes...I use a very advanced computer program to design what I do, a really nice one from the factory. No...I can't share it with anyone, and I wouldn't give you any info from it, after all, your posts above tell all of us you know it all already, and that you already use a computer program to design your airboxes and inlet systems.
You worked for who?
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Hmmm, now I have to make a CFD model of an airbox just to see what happens. A problem would be to set the boundary condition at the outlet, best thing would be to run the model from the piston and all the way out too the free air, but a boudary condition where tou have changing temp, pressure and shape sounds like a nightmare... I'll guess I'll just start with running a few preassure pulses through a pipe.
I will run the simulation on Ansys CFX 10 and make the grid with Solidworks. Hope I'll can get some results to post here.
I will run the simulation on Ansys CFX 10 and make the grid with Solidworks. Hope I'll can get some results to post here.
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I'd be interested in your results. I work with a very nice factory Yamaha program for the MotoGP engines (did the same when we did the Honda stuff, both two and four stroke), that I canot share, but I can make suggestions from what you get from your efforts.
I'd like to see a full closed airbox, with the inlet someplace in the front of the fairing, or a non-pressure flow area on the frame, then, a blow off valving to not overpressure the box.
Crussell's box inlets just past his legs on both sides, not the best place, but it works, and doesn't disturb the air inside the box.
On the poster above with fillling the pods with oil/fuel mix from reversion/standoff, too rich. Whenever I see a newer roadrace two stroke with a very high standoff in the airboxe(s), it is from the carburetion being just plain too rich, no other factor. Properly jetted, there shouldn't be a lot of standoff, should be able to wipe it out of the box with a paper towel, not have to give the box a solvent bath.
Since an airbox will allow for a better setup as far as outside influences of air at the carb inlet, the jetting can be very nicely corrected, not as with an open or "podded" inlet.
I'd like to see a full closed airbox, with the inlet someplace in the front of the fairing, or a non-pressure flow area on the frame, then, a blow off valving to not overpressure the box.
Crussell's box inlets just past his legs on both sides, not the best place, but it works, and doesn't disturb the air inside the box.
On the poster above with fillling the pods with oil/fuel mix from reversion/standoff, too rich. Whenever I see a newer roadrace two stroke with a very high standoff in the airboxe(s), it is from the carburetion being just plain too rich, no other factor. Properly jetted, there shouldn't be a lot of standoff, should be able to wipe it out of the box with a paper towel, not have to give the box a solvent bath.
Since an airbox will allow for a better setup as far as outside influences of air at the carb inlet, the jetting can be very nicely corrected, not as with an open or "podded" inlet.
