here i go again!!

[QuaiChangKane]

Re: here i go again!!

The amount of air that enters a cylinder is controlled by either a throttle plate or slide.

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.

[nert]

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.

[nert]

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.

[QuaiChangKane]

However, the engine does not suck in air. The air pressure outside of the displacing cylinder filling a void that has less (negative) pressure.

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.

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.

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.

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.

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!

[QuaiChangKane]

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.

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!

[nert]

Not so! From Motorcycle Carburetor Theory 101 www.motocross.com
?The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor. By placing tubes inside the venturi, we can cause this lower pressure to put fuel into the air stream.?
So your theory is based on incorrect assumptions.

[nert]

"quiet bone-stock Harleys" WHAT a SHAME!!! (where they running?)Thats one of their few endering qualities. hahaha. I love that sound. Often imitated, never duplicated. Really, I do like Harleys. Just too many of them.

The fuel is NOT pulled into the carburetor. It is pushed. The higher pressure in the fuel bowl pushes the fuel in to the lower pressure of the carburetor throat. FACT!

The reason fluid goes up a straw when drinking, is because the pressure in the bottle is greater than the pressure in the straw. basic science. Just like water seeks its own level, pressure seeks to equalize.

[QuaiChangKane]

The reason fluid goes up a straw is because a force is acting upon it to move it. There is greater pressure inside the bottle of fluid than there is in the atmosphere, but a soda pop won't start spraying into the air the minute you put a straw in the bottle. Someone has to suck on the straw to create the negative pressure - or "pull" the fluid up against gravity.


Pushing a fluid involves pressurizing the source - like a municiple water tank forcing the water through the end of your water hose.

Fuel in the bottom of your float bowl does nothing on its own but sit there.

[nert]

I am sorry, science says you are incorrect. There is not greater pressure in the bottle than there is in the atmosphere either as you stat!!!

yes, you create a negative pressure by sucking on the straw, BUT it is the positive/greater atmospheric pressure that pushes the fluid up the straw.

If you blow gently into the straw (not enough to make bubbles) but to increase the air pressure in the straw side you raise the level of fluid in the bottle. Are you going to claim the atmoshere is sucking the fluid up???

If you sucked on the bottle completly enclosed in your mouth, You can't pull the fluid level up. You must have a greater pressure on the opposite side of the low pressure to push the fluid!! Ever shotgun a can of beer???

Thats the way it is!! ITs not my theory. ITs science, Its a fact. Believe it!!

[earlfor]

As velocity increases, pressure decreases. However, intake volume required is controlled by displacement and rpm. Air volume is a variable. Velocity/vacuum is not. It is controlled. That is the point of a CV carb. At low rpm the engine is circulating a small volume of air. Consequently, the slides are in the lowered position and the cross sectional/throughflow area is restricted, hence velocityvacuum will be at value "X". As rpm increases, required air volume increases, and the slides are raised which provides a larger throughflow area, hence vacuum remains the same at "X". Fuel is drawn through the jets due to a pressure differential. It rises into the carb throat. Therefore, there must be a vacuum in the carb. The float bowls are at atomspheric pressure. Any fluid will follow the path of least resistance. In this case, it flows towards the negative pressure area.
Fuel is not being pushed. It is being siphoned.

Earl

Not so! From Motorcycle Carburetor Theory 101 www.motocross.com
?The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor. By placing tubes inside the venturi, we can cause this lower pressure to put fuel into the air stream.?
So your theory is based on incorrect assumptions.

[nert]

which is all of what i have been saying. the velocity through the carburetor creates a negative pressure. call it a siphon if you like, but as you said "In this case, it flows towards the negative pressure area."

actually siphon has to do with moving liquid from a container through a "n" shaped tube. moving the liquid up first than down to a lower second container. Different principles apply.

The greater atmospheric pressure IS pushing the fuel. Just like a U tube manometer. If you apply a negative pressure to one leg of the tube, (not enough to siphon) the fluid will rise to the low pressure side due to the greater atmospheric pressure on the other leg of the tube.

anyway, so much for air box leaks.

[earlfor]

No, it is not a different principle. Fluid moves due to a pressure differential.
You may create a flow in any number of ways, but the principle by which a fluid flows is a constant. there MUST be a pressure differential.

Earl


[quote="nert"]
actually siphon has to do with moving liquid from a container through a "n" shaped tube and moving it down to a lower second container. Different principles apply.

[nert]

differant

Britannica: "The action [of a siphon] depends upon the influence of gravity (not, as sometimes thought, on the difference in atmospheric pressure--a siphon will work in a vacuum) and upon the cohesive forces that prevent the columns of liquid in the legs of the siphon from breaking under their own weight."

[earlfor]

I have to give you a "NOPE" on that one too. The airbox is restricted so as to match engine intake air volume with jet size to yield an efficient fuel air mixture. Engine efficiency curves reach a point where fuel consumption outweighs HP gains. It is sound engineering to calibrate the induction, exhaust and fuel metering system so as to provide the best fuel economy and the greatest HP. These two are in opposition. The result is a compromise. Noise level and exhaust emissions are also a consideration.
Not sealing the airbox will/can result in the engine running lean if jetting is not changed. Some motorcycles wil run (although not well) with the airbox removed and no jetting changes. The GS's will barely run at all if you remove the airbox and do not rejet. Removing the airbox and rejetting will not noticably increase performance, but it will decrease gas milage. To increase performance, intake, exhaust and fuel delivery must be taken in to account. The whole system must be balanced. For example, discarding the airbox, installing Pods, rejetting and adding a less restricted 4 into 1 header will increase HP. It will also ALWAYS decrease fuel efficiency. In short, you are just reversing the engineering compromise of the manufacturer. Pods, jets and a 4 into 1 may add 7-10 hp to a 750cc size machine, but they will decrease gas milage by 10 mpg. 10 hp on a 750 lb gross weight machine will yield a minimal increase in acceleration for a loss of 25% in gas milage. That is not a good trade off for the normal cruising recreational rider. Keep the airbox and seal it as it should be.

Earl

anyway, so much for air box leaks.

[earlfor]

:-) What is it you think gravity causes? IT IS A DIFFERENTIAL. The weight of the receeding water in the tube causes the differential. How do you initiate the siphoning process? Maybe by sucking on the lower end of the tube? Since air and water are both fluids, there is no difference in principle between the mechanics of sucking on the tube with your mouth and the following weight of water creating the effect. Agreed, gravity results in weight. Agreed, you can have gravity/weight and a vacuum at the same time.

You cannot have a bucket of water filled to the same level and placed on each side of a 3 foot high wall and expect to siphon fluid from one, over the wall and into the other. Gravity is the same on both sides of the wall. Siphoning will only take place when the fluids are not in equilibrium. Therefore, whether it is gravity induced or mechanically induced, there must be an imbalance to start, and continue the flow. As I said, there MUST be a differential. Britannica is correct............as far as they go.
The principle their observation works by is called differential.

I think you are confusing atomspheric pressure with pressure differential. You may have a pressure differential in the absence of an atomsphere.

Earl

differant

Britannica: "The action [of a siphon] depends upon the influence of gravity (not, as sometimes thought, on the difference in atmospheric pressure--a siphon will work in a vacuum) and upon the cohesive forces that prevent the columns of liquid in the legs of the siphon from breaking under their own weight."