Removing Backfire Screen

55 replies to this topic
  • MXKyle

Posted October 19, 2004 - 01:11 PM


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.

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.

Did I hear you mention Boost Bottle?

Let’s see if I really understand this properly. The 450's max intake velocity through the carb would be roughly 95 ft/sec (if there were no losses) which is well below the subsonic. The intake system using valves has an intake flow that is essentially a square wave. There is no back flow only forward pulsing. This is compressible flow so the mass of the air maintains some forward momentum even when the intake valve is closed. Since the intake flow is not at supersonic speeds you are not getting shockwave pulses back through the system. So to simplify the system for the 4-stroke sicle the overall system flow is for air to move from the outside of the airbox to the inside of the cylinder at varying forward velocity. Ok, it accelerates and decelerates in pulses (square wave). Still the overall movement is in a constant direction….In. I would hate to have to calculate the Reynolds Numberes for this particular system because as we know that is really a number relating to the mass flow rate. So I can't just give you a number regarding the loss over the screen. At any rate, regardless of the change in velocity, there is a forward mass flow even though in this case the density of the air is variable. Like I said, since the intake velocity is variable the mass flow rate is also leaving us with some difficult calculations to determine the actual head loss for any given obstruction. But..., here is the critical point… any obstruction can be construed as a head loss to the system. The screen causes a head loss that can be calculated given a determined (read ‘hard headed’) person. Even the plenum has an inlet and outlet and in this case curves. These too create head loss. Head loss means negative delta V and loss of horsepower. There is no getting around it.

You can claim it’s inconsequential but people go a long way (and pay lots of money) for ANY power gain. And as you may know many tout the KTM’s intake tract because it straighter due to not having to route around a shock/linkage system. The screen should have just as much effect.


There, I knew all that hot air would come in handy.

  • Ga426owner

Posted October 19, 2004 - 04:59 PM


Enough already! Just cut the friggen screen out!!!!!!!!!!! :cry:

  • ncmountainman

Posted October 19, 2004 - 05:46 PM


really,i think my brain melted tryin to follow that! twinair has a filter that is backfire resistant and doesn't hinder flow that you can get with a screenless aluminum cage that replaces the stock one(($80),or you can buy the filter seperate($24)and just cut your screen out,the fact that there is a demand for such a gizmo must justify sumthin. :cry: :cry:

  • grayracer513

Posted October 19, 2004 - 07:10 PM


No, you didn't hear me say that.

As for the rest, I think I'll handle as would FDR:

Kyle, you're absolutely right.

  • Blue4Ever

Posted October 19, 2004 - 07:32 PM


Just curious? since you guys appear to have some knowledge of airflow this may be too easy! If you remove the backfire screen & replace the filter w/twin air's backfire resistant filter that as they advertise have added a layer of fire retartand foam inside, wouldn't this offset. Without being educated in this field it would be my guess that the extra layer of foam would be a greater restriction than the screen. Also, what about the cage frame? The twin air one has less spars from the center to the perimeter than the orig. one, should we consider cutting a few of those out also. Enjoyed the debate you guys had! It was more interesting than any I have heard latly, if you know what I mean!

  • grayracer513

Posted October 19, 2004 - 08:47 PM


Nothing like the free exchange of ideas, eh?

Several of the premium filter elements currently available are constructed with two layers of foam of varying densities. There different reasons for doing this. Most of the time, it has to do with trying to improve filtering. In the case of the Twin Air, the interior layer is made of foam which is more resistant to flame exposure, with the idea being that this will allow the removal of the backfire screen without the usual risks.

I don't know whether this element is any more or less restrictive because of this, but I wouldn't reflexively think so. The flame retardant nature of the foam is more probably due to a different chemical formula used in its manufacture, rather than something about its construction. Anyway, The filter element in the YZ airbox has a generous surface area, (in fact, the Twin Air looks like it has more than many do) and should not offer much significant restriction.

I'm glad you liked the debate. One thing I can tell you about me is that there will be others. :cry:

  • Blue4Ever

Posted October 19, 2004 - 09:13 PM


Correct me if I am wrong but I thought the Twin Air site stated they added a layer, making that a total of three layers, hence the check & balance of the screen removal?! :cry: :cry:

  • YZ250F_Rider

Posted October 20, 2004 - 02:45 AM


well since I run the twinair backfire filter on all 3 bikes, I can tell you what I have seen with them.

The backfire layer is coated with some kind of fire retardent chemical and is really coarse. The holes are huge compared to the ones in the screen. It does not filter in any stretch of the word.

As for the cage, I would use the 125/250 cage. I have even gone a bit further with that and reshaped the spars so that they are less thick and more streamlined. You can actually make them wafer thin and they will still work fine.

One other thing you can do as well is trim back the inside lip of the filter itself. It has over a huge lip that hangs out into prime airflow path. The cage rests on the inside corner of the filter so you can trim the lip back considerably and get it out of the airflow as well. That might be worth 1/64 of a hp. :cry:

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  • Satch0922

Posted October 20, 2004 - 03:27 AM



Here is an idea and it's a free mod.......try holding the throttle wide open all the way around the track! Bet you can't do that! And if you can't , why do you need MORE power? LOL :cry: :cry:

  • YZ250F_Rider

Posted October 20, 2004 - 03:41 AM


Make it stop? why dont you stop clicking this thread and bashing in it instead?

  • Satch0922

Posted October 20, 2004 - 04:20 AM


Make it stop? why dont you stop clicking this thread and bashing in it instead?

:cry: Honestly......I just have to see what you guys are saying that could possibly keep this thread going. I am not bashing the thread....I am bashing you ! :cry:

I promise I won't click on it again :cry:

  • YZ250F_Rider

Posted October 20, 2004 - 04:49 AM


Bash away. Far away. :cry:

  • grayracer513

Posted October 20, 2004 - 08:02 AM


I think he's trying to harass us.

But nevertheless: :cry: :cry: :cry:

  • picacosa

Posted November 20, 2005 - 11:42 AM


I was the second incident.
Not cool at all when you watch your bike melt.

  • Ga426owner

Posted November 20, 2005 - 01:47 PM


I was the second incident.
Not cool at all when you watch your bike melt.

it wouldnot have melted if you had the twin air setup = I use it, it works and it adds power.

  • Satch0922

Posted November 20, 2005 - 05:39 PM


I think if you NEED to do this mod you should spend more time practicing cornerning....


I did the mod...I would not do it again....and my bike did not burn.

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