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here i go again!!

  • Thread starter Thread starter nert
  • Start date Start date
N

nert

Guest
Somebody PLEASE explain to me how the ?tightness? of the air box seals affect anything. The amount of air that enters a cylinder is controlled by either a throttle plate or slide. We are talking about atmospheric pressure which is constant regardless of seals providing the sir box has an air inlet and a negative pressure caused by the piston on the intake stroke. The air flowing through the carburetor body (regulated by throttle valve or slide) creates low pressure causing fuel to flow through the jets. (ventury affect). A performance limiting factor in naturally aspirated engines has always been the inability to get enough air into a cylinder. (you can always get plenty of fuel) Enter, port and polishing, high lift cams, creative valve overlap, tuned exhaust, and eventually turbochargers and superchargers. Air box tightness does not affect other 4 cylinder engines, cars, boats, lawnmowers, etc. No matter what,?. ample air being present, the throttle valve and slide control the amount entering the cylinder. An air leak in the air box is not going to cause more air to enter the cylinder. And since there is only one route for air into the cylinder, via the carburetor body, I have no idea what all this tight air box talk is about. A restriction (clogged air filter) can limit the amount of air available and certainly affect performance. I can?t believe that a great amount of turbulence is caused by the ?air leaks? the starve the carburetors of air. I suspect many of these bike engines may have other problems instead. OR someone please explain the science behind the madness.
 
Dude, I think your totally confused as to what is important to be sealed. If there are air leaks between the carb body and the cylinders, the mixture will be all 'effed up, and your bike will run like crap. If there are leaks prior to the mixture, dirty air will be entering the carb and engine, possibly causing clogged or scored parts and leaving unwanted deposits.

The airbox should ideally be sealed entirely except for the cutouts by the factory. If your using the stock airbox, it is not the throttle bodies that dictate the max airflow, but the airbox vent holes itself. You'll notice that the if you measure the surface area of the carb intakes (on the plane) and the surface area of the airbox vents, the airbox vents have a smaller effective area, meaning that the max volume of air that can enter the motor is dictated by the airbox, not the throttle bodies.

Air leaks are so bad it's not even funny. I smoked a head due to the exhaust cam seats burning because my airbox was leaking and the bike was running real lean. Lean = hot. Too much air = lean = bad.
 
Many of the assumtions you've made are true for VM carbs only, where the throttle cable raises the slide.
If you're carbs are CV carbs (the throttle cable is NOT linked to the slide, but to a butterfly plate in the mouth leading to the cylinder) then the only thing that's lifting the slide is the pressure differential between the two mouths of the carb (in and out). If there are leaks in the air box then the pressure of the air entering the carb will be affected. A restricted filter (overly dirty) will also affect the pressure, but in the opposite way.
Even for VM carbs, some of them are manufactured with the assumption that the air entering the carb will be restricted somewhat by an air filter.
My Honda with a VM carb will run without the air filter but noticeably less smoothly and with less power.
My GS450 with CV carbs will stall as soon as I twist the thottle if there's no air filter.
 
I agree with your first paragraph. I think the second and third paragraphs are incorrect. If your telling me the air entering the cylinders is controlled by the air box inlets thats just WRONG. Thats the action the throttle valves and/or slides control. If you restrict the air to your carburetors you will hurt performance. The reason the engine will tend to get rich with a clogged air filter, is because the low pressure is still pressent and draw the air through the filter, causing a greater negative pressure that causes raw fuel to enter the carburetor throat. Your fuel mix, be it set, lean or rich is controlled by your mixture jets. The amount of fuel pulled into the engine is in proportion to the volume of air that goes into the engine through the carburetr throat "drawing" fuel along the way. If your jets are sized correctly, the proportions stay relatibly constant. The more air, (greater throttle opening) the more fuel flows. Less air (less throttle opening) less fuel flows. You still desire the slightly fatter than 14.7 to 1 fuel mixture. Its the volume of air and fuel with that same mixture that gives you more or less bang. You may have have cooked a head, but probably due to lean jets!
 
easy way to look at it

easy way to look at it

An easy wat to look at it is these bikes are built with a restrictive air box and as such are jetted down to accomodate this restriction, if you open up the airbox then you need to jet up for the extra air, THE REASON FOR THE RESTRICTIVE AIRBOX IS NOISE REDUCTION witch was a mandate of manufacters laws, along with exhaust noise and emmissions
 
nabrams
i agree, a resticted filter will affect the way the slide adjusts accrding to pressure. But i can't see that an air leak in the air box is going to make it run lean, not if the jets are are correct. And to "adjust" fuel mixture to the engine by the size of the air box inlet is crazy. why not install a smaller throat carburetor instead and have better control over the mixture.
 
and if all that is true, an EASY and CHEAP upgrade to horsepower is no air box and fatter jets. AND why do some people claim their engines won't even idle with an air box or air filter on. Something very wrong there if the air box restriction is throttling the engine at idle. That much restriction the damn engine wouldn't have enough air to rev. with the box in place.
 
I think the key thing you're missing is that you assume that CV carbs control mixture with jet size primarily and pressure secondarily. Once again that may be true for VM carbs - NOT true for CV carbs. The pressure differential contols that slide (via the diaphragm), which of course will change the volume of air flowing through - but the pressure that volume of air is at can greatly affect how much fuel gets sucked up through the jet.

I think this is the "technical" explanantion:
CV stands for "Constant Velocity" - the velocity of air in a restricted space (like the throat of a carb) directly affects the pressure. If that target velocity that the engineers were looking for (which is the same for all slide positions - thus "Constant Velocity") is not met then your pressure will be off, affecting the mixture. For a larger air volume at the right target velocity the diaphragm will rise more, and so will the needle (widening the "fuel hole"). The velocity of the volume should remain constant while the pressure and size of that volume changes for various throttle settings. Once again, the pressure will greatly affect how much fuel gets sucked out of that widened "fuel hole".

And yes, you are right about the "cheap" way to increase horsepower - that's exactly what some people do. They put in fatter jets, increase air flow to the carbs and put more freely flowing exhaust pipes (essential for correct back-pressure). Of course all this is extremely difficult to get right (much trial-and-error) and it puts higher stresses on the engine and will wear it out more quickly.
 
I like your technicle explanation. However I don't see how it would be possible to have "un equal" pressure for measurable length of time. If the air is available to allow a velocity increase, than that same air is available to float the slide. If you restrict available air to the carburetors, than you can't have increased velocity and you won't float the slide any higher. IT seems to me that a CV carb does a very good job of providing a "constant" fuel mixture through differant operating ranges and throttle positions. More like a engine governor in response to throttle plate position. Much less chance of engine bog, flat spots, etc.

My second thought on my long drive home is to the comment made by Jethro. "You'll notice that the if you measure the surface area of the carb intakes (on the plane) and the surface area of the airbox vents, the airbox vents have a smaller effective area, meaning that the max volume of air that can enter the motor is dictated by the airbox, not the throttle bodies." Maybe the window for air in, is smaller than the sum of the 4 carburetor inlets. BUT!!! maybe!! 4 stroke engine, 4 cylinders. (1) cylinder on compression stroke, (1) cylinder on power stroke, (1) cylinder on exhaust stroke, leaves only (1) cylinder on intake. Yes there is some crank degrees of overlap, cam advance, etc. Its not a vacuum cleaner, there are not (4) carbs sucking air all at the same time, continuously. So unless the air inlet into the air box is smaller than a single carb, there should be no problem with air supply. Atmospheric pressure takes care of keeping the air box filled between carb gulps.
 
Maybe the window for air in, is smaller than the sum of the 4 carburetor inlets. BUT!!! maybe!! 4 stroke engine, 4 cylinders. (1) cylinder on compression stroke, (1) cylinder on power stroke, (1) cylinder on exhaust stroke, leaves only (1) cylinder on intake. Yes there is some crank degrees of overlap, cam advance, etc. Its not a vacuum cleaner, there are not (4) carbs sucking air all at the same time, continuously. So unless the air inlet into the air box is smaller than a single carb, there should be no problem with air supply. Atmospheric pressure takes care of keeping the air box filled between carb gulps.
Click to expand...

I suppose that is a good point, I never really thought of that. Nabrams comment on the air box restricting sound may be right on too. I'm gonna have to think this over in the shop with the air box and carbs (they are currently out on the bench).



and if all that is true, an EASY and CHEAP upgrade to horsepower is no air box and fatter jets. AND why do some people claim their engines won't even idle with an air box or air filter on. Something very wrong there if the air box restriction is throttling the engine at idle. That much restriction the damn engine wouldn't have enough air to rev. with the box in place.
Click to expand...

That is a common upgrade to horsepower- pods and jets. And I agree with your last comment there.
 
Well, I measeured the openings to the air box on my 81 1100, wouldn't you know it, the air box openings are slightly larger than the intake manifolds! This would leave me to believe that the air box would not restrict air at all.
 
Jethro
thanks for measuring. my 82' 1100 is not so accessable right now.
 
Gentlemen,

I glossed over one piece of information in my technical explanation which I believe will complete the picture. I will attempt to explain it in more detail here.
Nert: You indicated that you understood how an overly restrictive air filter (like one that's too dirty) might reduce the pressure of the air coming into the carb. Of course, this would affect the pressure DIFFERNTIAL between the two mouths of the carbs as well as the pressure in the throat of the carb. This situation is intuitive and easily understood. By the way, the pressure at the mouth of the carb that leads to the cylinder is variable based upon the position of the butterfly plate.
Here's the key point: An air box that has leaks on the side of the filter that is closer to the carb inlet mouth has the effect of simulating a filter that is not restrictive ENOUGH. This will have the affect of INCREASING the pressure at the inlet mouth of the carb compared to what the target pressure should be. Once again this will greatly affect both the pressure DIFFERNTIAL and the pressure in the throat of the carb. The "constant" atmospheric pressure is OUTSIDE of the airbox. The pressure IN the airbox is varying based upon throttle position. For each butterfly plate position, the pressure at the exit mouth of the carb changes, which will result in a desired "design" pressure at the inlet mouth of the carb. If the actual pressure is significantly different than the desired "design" pressure (at the inlet mouth) due to a messed up airbox and/or air filter then your bike will run like crap.
 
Atmospheric pressure is a constant.


Airbox pressure is not. When a vacuum is introduced, the air being sucked into the carbs has a higher pressure than the air entering the airbox, as it is being squeezed into a tighter opening.


Large airbox volume, small airbox intake port, small carb intake port.


The air being sucked into the intake port of the airbox is momentarily pressurized as it enters the intake port. Once inside the box, which is far larger than the intake port, it expands again to match atmospheric pressure. From there, it is sucked into the carburetor, and its pressure is increased as it enters the carb. The air is moving fast enough by this time that it creates negative pressure inside the airbox (vacuum).

Introduce larger holes into your airbox, seal leaks, or simply do away with the box and go with velocity stacks, and you dramatically reduce the resistance to airflow. Your carburetors will suck more air, and will run lean since they were jetted to match a restrictive factory airbox.

Atmospheric air pressure has nothing to do with dictating the rate of flow of air into your carbs. All atmospheric pressure does is change the concentration of oxygen in the air going into your carbs (more oxygen on a cold day, less on a hot, less at high altitude, etc.).


A carburetor will continue to suck as much air as it can get - that's why you can gain power by using velocity stacks or pods along with richer jetting. But to reduce the resistance to air and allow your carbs to suck more air without enrichening your carbs to match is what causes a bike to run poorly.

If a bike were not running, the air pressure inside the airbox, the air pressure inside the carbs, and the air pressure inside the cylinders would all match the atmospheric air pressure - basic theory of osmosis. But since your engine is running, osmosis is no longer the controlling factor - vacuum is.




-Q!
 
I'm pretty sure evrything QuaiChangKane said agrees with what I've said.
 
Re: here i go again!!

nert said:
The amount of air that enters a cylinder is controlled by either a throttle plate or slide.
Click to expand...



This is almost true. The amount of available air that enters a cylinder is controlled by a throttle plate or slide.

By increasing air intake to the airbox, you're increasing the amount of available air. Just like replacing a clogged air filter, increasing the size of the airbox opening (drilling holes or having leaky seals) will decrease the resistance to air being sucked into the airbox, increasing the amount of air available to the engine.
 
However, the engine does not suck in air. The air pressure outside of the displacing cylinder filling a void that has less (negative) pressure. The pressure in the carb does not change as much as air SPEED changes as it goes through a narrower opening. Also gives a little ram affect (forcing more fuel/air into the cylinder) due to the increased speed and momentum. Witness a flowing river with narrowing banks, the water flow speeds up. I am sure the water speed is more obvious than an increase in pressure. The slight increase in pressure is due to restriction of the narrowing banks. With out restriction, there is no pressure. Thus the act of Velocity stacks. Increasings the air speed accross the jet syphoning a greater amount of fuel. Less air draws less fuel. And i can't believe this built in FACTORY restrictive flow is gonna do diddly at idle and low RPM. What is the point of the factory restricting the flow? Can't be noise abatement. Ever here a quiet Harley? If they fatten the fuel along with better air flow they would have more horespower. Horsepower and performance sells bikes.
 
My Correction: As the air speeds up in the carburetor venturi, pressure DECREASES. This is what allows the fuel to be pushed up through the fuel "pipes", affording they have atmospheric pressure in the fuel bowls. (fuel bowl vent). So again, the more air available>flow>air speed>fuel.
 
nert said:
However, the engine does not suck in air. The air pressure outside of the displacing cylinder filling a void that has less (negative) pressure.
Click to expand...

Called "sucking". With the exhaust valves closed, intake valves open, and piston being pulled down in the cyclinder (intake stroke), the engine acts as a syphon pump.

The pressure in the carb does not change as much as air SPEED changes as it goes through a narrower opening.
Click to expand...

It's not the pressure in the carb that changes - it's the difference in air pressure from a large chambered negatively pressurized air box to a small throated positively pressurized carburetor. The air is pressurized as it enters the carb. As long as the resistance to air entering the air box is greater than the amount of vacuum pulling through the carbs, the air box will be negatively pressurized because of its high cubic volume. Once that negatively pressurized air is sucked into the substantially smaller mouth of the carburetor, it becomes positively pressurized. A carb doesn't compress air, per se, but the vacuum action forcing the air into the carburetor through a smaller passage does increase the air pressure, along with air speed.


Also gives a little ram affect (forcing more fuel/air into the cylinder) due to the increased speed and momentum. Witness a flowing river with narrowing banks, the water flow speeds up. I am sure the water speed is more obvious than an increase in pressure. The slight increase in pressure is due to restriction of the narrowing banks. With out restriction, there is no pressure.
Click to expand...



Exactly my point.



Thus the act of Velocity stacks. Increasings the air speed accross the jet syphoning a greater amount of fuel. Less air draws less fuel. And i can't believe this built in FACTORY restrictive flow is gonna do diddly at idle and low RPM.
Click to expand...

Sure it will - tape off half of your air box opening and cruise around the block for awhile. Restricting air flow into your air box at idle speed is enrichening your mixture - alot like using the choke.

What is the point of the factory restricting the flow? Can't be noise abatement. Ever here a quiet Harley? If they fatten the fuel along with better air flow they would have more horespower. Horsepower and performance sells bikes.
Click to expand...


Actually, I've heard several quiet bone-stock Harleys. In fact, performance and horsepower are not the driving factor behind H-D sales, because they have neither. If a Hayabusa can make a claimed 175 hp, why do people still buy bikes that make less power???


Emissions. That is the driving factor behind mfr settings for carburetion and timing on motorcycles.



-Q!
 
nert said:
My Correction: As the air speeds up in the carburetor venturi, pressure DECREASES. This is what allows the fuel to be pushed up through the fuel "pipes", affording they have atmospheric pressure in the fuel bowls. (fuel bowl vent). So again, the more air available>flow>air speed>fuel.
Click to expand...


Pressure doesn't decrease in the carbs - the vacuum pressure drawing air through the carbs is constant based on the syphoning action of the piston. As the throttle is opened, the volume of air being sucked into the carb increases and sucks more fuel through the jets through an opening small enough to vaporize the fuel and ensure it's mixed with the air. It is the jets or fuel "pipes" that are negatively pressurized due to the speed of air flowing by them which is being sucked into the engine by the piston.

(not taking into account different jet circuits)


The fuel isn't "pushed" into the air mixture, it's "pulled".


-Q!
 
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