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