Removing Backfire Screen

I agree with Motobark. I just rode my 01' YZ426f after stepping from a 125 2 bang, I learn that having an underpowered bike is kinda like a trainer. Power when your CCs are in the 4xx is personal power, well if you are Chad Reed trying to catch the pesky #4 then I'd say go for it.I am not gonna cut it out because I just dont want to have a Fireball between my legs, there is something special there I dont want to melt.

Good post Motobark. :cry:

I just used a heat gun and got the plastic soft enough and the screen just lifted off.

Good post Motobark.

Thanks!

John

Here's the pics I put of mine in another thread. If you look in the second one you can see the hard line where the wall was on the box. That wall blocks off all of the path for the air except for the small area below the seat to about the bottom of the subframe tubes. Less than an inch in height.

It should be obvious that with out the wall the air has much less obstruction in flowing to the filter. And it doesnt suffer from having the air peel off from the wall shaving it as it flows up and over it. That peeling off causes turbulence and impedes the airflow on the fender side of the box. And of course without the airscreen it has no obstruction other than the filter itself in getting to the carb.

What might not be so obvious it that these two changes lets the air pick up more speed as it gets pulled in by the piston. It's all about the velocity of the air.

The speed of the mix at the bottom of the stroke determines just how much it continues to fill the cylinder as the piston travels up for compression. The more obstructions that are placed in the path, the less velocity it will have to continue filling the cylinder. It's the same reason they reshaped the boot leading to the carb from the box this year too, to gain velocity. Velocity equates to more potential hp, albeit only 1/32 of a hp for some. :cry:

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I removed the screen in my '99 YZ400 after reading that article about it in MXA and I can tell you that the slight power increase I gained at full throttle isn't close to being a good trade off for all the puncture wounds I get from those damn residual wires poking off the cage every time I clean my filter!

Give me a break! If you guys can tell a noticeable difference from taking out a backfire screen, why does anyone bother using a dyno, why don't they just have guys like you ride the bikes. I've got $100 that says I can switch air cleaners all day long in your bikes and you won't be able to tell me if the screen is in or not.

John

John, I tested with a YZ250 screen. I bike definetly reved quicker and also needed a larger main jet. That's not to say it made for faster lap times. I went back to the screen for slightly smoother delivery.

It does make a difference. Anyone should be able to feel the difference but few would be able to say it's a positive difference. I suspect anyone that has a years worth of riding time on a bike can tell the difference between a dirty filter and a clean one. That's about the level of difference we are talking about.

BTW - there are a lot of good riders out there with enough seat time that they will be able to tell you that the air pressure in their tires is 1 PSI off. I don't think that you can make any change to their toys without them feeling it.

--KT--

I agree - it definitely was a plus to remove it on my 03 450 - I went with the Twin air kit and added a powernow for even better intake breathing - I would caution if you cut it out buy a flame retardent air filter from twin air. They can catch fire..... :cry:

The area available between the seat/airbox/subframe at the rear is just about exactly 1x7 inches on my '01 YZF. If this were the only airpath to the filter, it would 45.1 square centimeters in area. The bore of the carb on a YZ450 is 39mm, so it has a cross-sectional area of just less than 12 square cm. The opening over the seat is in excess of three time the size of the carb.

This means that the air flowing through that path will be traveling less than one third the speed of air passing through the carb.

Any "conditioning" one might accomplish to the airflow at this point will be completely revised when the air flows through the filter element. The inside diameter of the filter flange is an odd shape, but for now let's call it a 4.5 inch circle. That gives it a cross section of over 102 sq. cm. The area inside of the filter is in gas dynamics terms, a plenum. It acts a great deal like a false atmosphere because air velocities are so low here that the engine sees the end of the air boot as being "out in the air". The airboot is where the airflow is directed and accelerated into what amounts to a built in "velocity stack" designed into the boot itself.

Because of its size, airflow across the backfire screen in a 450 will be moving at less than 8% of the speed that the same air will flow through the carb. In a 450 operating at 80% volumetric efficiency at 10,000 rpm this would be about 6.5 feet per second across the screen, or about 4.6 miles per hour. It would be about half that in a 250F.

I don't see how it will help.

you are leaving out the second most important part.

It not only has to have the area, it also needs a clear path. Dont straight pipes flow faster than ones with bends in them?

The way it's set up it it's like an obstacle course in there. Every time you make the aif flow change direction you slow it down.

Yes they do, but what you have here is like a pipe that changes cross section several times. The "fat" parts will have a lower velocity than the narrow sections. With air boxes, this often done by design to force the air to turn suddenly and "throw off" the dirt solids as it goes around the corner, or simply let it fall to the bottom as the air slows to a near stop.

Bear in mind that at no point in the air path of a YZ does the air velocity exceed 1/3 that which occurs inside the carb bore, even assuming that all of the air must come over the back edge of the airbox.

Also remember that the single greatest increase in section, and associated velocity drop is at the mouth of the air boot where it opens into the filter. This is designed into the intake tract to make the engine behave as if it has nothing between it and the outside air.

Because the engine's intake signal terminates in this relatively still area, the velocity and character of the air entering that larger section is not particularly significant.

You guys are still talking about this? LOL You really need to get out and ride and practice your cornering, whoops and jumping skills....THAT will make you (and me) faster! :cry:

Yes they do, but what you have here is like a pipe that changes cross section several times. The "fat" parts will have a lower velocity than the narrow sections. With air boxes, this often done by design to force the air to turn suddenly and "throw off" the dirt solids as it goes around the corner, or simply let it fall to the bottom as the air slows to a near stop.

yes in stock form. All I have done is tried to keep the path consistent so that the resistance to change is minimized. The air starts from a dead stop and reaches a maximum velocity every stroke.

Bear in mind that at no point in the air path of a YZ does the air velocity exceed 1/3 that which occurs inside the carb bore, even assuming that all of the air must come over the back edge of the airbox.

well the maximum velocity anywhere in the path depends on the ratio of the cross sectional area of the two. But one thing is for certain, it can absolutely go slower than 1/3 of the carb's venturi velocity when there is obstructions to it's flow. Take them out and it will approach the maximum.

Also remember that the single greatest increase in section, and associated velocity drop is at the mouth of the air boot where it opens into the filter. This is designed into the intake tract to make the engine behave as if it has nothing between it and the outside air.

yamaha has two radically different transitions now. Last years is the abrupt transition, and this year is a conical form of transition. the conical form increases power at higher rpms. There is no such thing as nothingness between the carb and the air.

Because the engine's intake signal terminates in this relatively still area, the velocity and character of the air entering that larger section is not particularly significant.

But that assumes there is nothing beyond the mouth of the carb or the mouth of the boot. The screen is there and the wall is there. Take them out and the velocity of the flow goes up, along with the hp. Just as reshaping the boot to a conical form increases velocity so does removing restrictions to the airflow.

--------

My turn for philosphy (or just a short course in statics and dynamics :cry: )

It's a system. To disregard a component of that complete system is to disregard the system. Everything contributes. And with engines it's maximizing airflow at maximum velocity is what makes maximum power. In our case all we have is a single cylinder which does not have continuous air flow but pulsates the pull of air.

Anything obstruction in that path, although it may statically allow enough air flow (flow bench's are static), will in fact create a dynamic resistance to a pulsating air flow.

That dynamic resistance can be higher than the static resistance. (It's called impedence for dynamic situations and resistance for static ones).

thats why removing the wall and screen make a difference, Statics and dynamics. They present an impedence to a dynamic system.

Dont make me pull out the Z transforms for this system on you. :cry:

Satch, debates are great. They empower everyone that can take something from them. Of course they are always better when powered by beer. :LOL

Satch, debates are great. They empower everyone that can take something from them. Of course they are always better when powered by beer. :LOL

True Dat!!!! (but I dont drink beer! LOL) :cry:

Give me a break, Satch, it's a weekday and it's raining and stuff. Besides, a)I've got him where I want him. :cry: At my age, I bang heads a whole lot better than I ride. :cry:

All I have done is tried to keep the path consistent so that the resistance to change is minimized. The air starts from a dead stop and reaches a maximum velocity every stroke.

...The maximum velocity anywhere in the path depends on the ratio of the cross sectional area of the two. But one thing is for certain, it can absolutely go slower than 1/3 of the carb's venturi velocity when there is obstructions to it's flow. Take them out and it will approach the maximum.

OK, let's look at this. You are suggesting that it would be advantageous to have the air mass entering the engine already up to speed as it enters the venturi. With out a supercharger, that isn't possible, given the number of section changes in the air path.

The total length of the air path in this case is what, about 18"? Since the rate air will flow is ultimately determined by the smallest section (the venturi) and atmosheric pressure, it will not go faster at any point than in the venturi. Allowing any increase in section will slow it down, but not restrict it. If you wanted to have air flow at a steady rate over the entire length you would have to run an 18" tube from the carb mouth all the way back. You'll have to figure out for yourself how to filter that one. In a world where the air filter was not required, would you create an 18" air stack for an engine like the YZF? I doubt it, and it wouldn't work as well as the stock setup if you did.

There is no such thing as nothingness between the carb and the air.

There is, however such a thing as a closed area that has the effect of being open. That is the function of a plenum. As you point out, an engine is a dynamic series of overlapping events. Flow benches that lack any means of recreating the pressure waves that occur in the intake tract are useful to identify areas that need improvement, but don't tell the entire story.

When the intake valves open, the remaining pressure caused by the momentum of the previous intake stroke is dispersed into the cylinder and a vacuum begins to form as the piston travels downward (there should also be negative pressure in the exhaust at this point, but that heads off in another discussion). Since air is a compressible medium, this vacuum, for the moment exists only in the immediate area of the combustion chamber. It then travels outward toward the open end of the intake tract as a wave of negative pressure, as though someone had yanked on the end of a long, weak spring, traveling at about the speed of sound. When it reaches a major increase in section, in this case the air boot (either design. the newer one has the effect of being shorter to the valves), it bursts out into the larger space and is absorbed.

At this moment, air begins rushing in to fill the depression in the intake tract from the plenum at the end of the boot, reversing the negative wave with a positive one. If this second, positive wave arrives at the intake valves as they begin to close, it can significantly increase the amount of air/fuel mix drawn into the combustion chamber.

Since we cannot really alter the speed of the pressure waves from sonic velocities, the critical thing in timing all of this so that the positive wave gets to the engine at the right time is the length of the the air tract from the valves to the first major increase in section. This distance can't be very long, because at 10,000 rpm, the intake event only lasts about 5 milliseconds. Any section increase, if large enough to accomplish the absorbtion of the negative wave and provide enough "still" air to create the positive wave, is for all practical purposes, "open" air, at least in the "eyes" of the engine.

If you were to eliminate the air box altogether, remove the number plates, and leave the filter where it is, the pressure wave behavior would not change, because it is independent of everthing after the divergent section of the air boot. This is because the volume of near still air within the box is large enough to serve as a buffer and allow the pressure reversion to take place. It would behave the same way if we put the stock air filter into a closed 2 gallon can and allowed air into the opposite end through a hole no more than twice as large as the carburetor bore. The can, acting as a buffer, absorbs and eliminates the intake pulses, and air flowing into it becomes much more "static". As long as the most restrictive obstruction in the path is not great enough to lower manifold air pressure, and the buffer is maintained, the restriction is insignificant.

Having said all of that, I will grant you these two things:

Yamaha lowered the back edge of the airbox by about 1/2 inch on the '03 model compared with earlier versions.

I can imagine the possiblity that the backfire screen might interfere with the sonic shock waves that are associated with the negative pressure waves in some way, but the effect of that should be minimal.

And, of course in Supermoto, where speeds can run over 100 mph, there could be turbulence issues with getting the air around the rider and under the seat to begin with. So that's 3.

The key is really just this: if there is no pressure drop across the backfire screen, there is no reason to remove it. As I was writing this, I thought of a practical way to test for that. I'll let you know if I find anything out.

So, this was fun. The cool thing is that until now, I didn't think there was anyone else as stubborn as me. :cry:

And to think, we've had this whole "debate" with no name calling.

Dont make me pull out the Z transforms for this system on you. :cry:

Hey, as long as it doesn't turn my bikes red, I can take it.

The overwhelming majority of people that try it have responded favorably to the mod, the bike likes it. The fact that you can increase the main afterwards says it's simply allowing more air to flow. It really does make a difference for the better.

The key is really just this: if there is no pressure drop across the backfire screen, there is no reason to remove it. As I was writing this, I thought of a practical way to test for that. I'll let you know if I find anything out

just take a piece of nylon screen, hold it up and blow on it. How slow do you have to blow against it before it doesnt move. How far does it move when you blow on it as hard as you can.

The fact that it moves at all says it presents an obstruction to airflow, and that it prevents the air from reaching the maximum speed that it could.

:cry:

The ability to increase the jet size suggests your argument is sound. Still, until I test it, I will remain a sceptic.

As far as people's opinions, I once got 70% of a small test group to say that the gas they poured into their bike out of my U4 can gave them a noticable performance boost. The only problem with that was that it was plain old Arco premium.

Anyway, we'll see. :cry: :cry:

The ability to increase the jet size suggests your argument is sound. Still, until I test it, I will remain a sceptic.

Back in the beginning when I posted as sceptor the hawaiians used to call me sceptic. But for a very different reason. :cry:

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