Cross wind power loss.

[gsrick]

After reading AJs thread about pods and the pressure equalizing tubes it got me to thinking about something else I've noticed with running pods.

http://www.thegsresources.com/_forum/showthread.php?t=208090

I've noticed while riding both of my 1000s(Suzy '78 1000C emgo pods, SuzyII '80 1000G K&N pods), if I get hit by a cross wind there is a significant power loss. We don't get too many strong gusts in central Fl. unless a storm is coming or you're on the coast.

I really noticed it last month riding in the Tennesee hills on SuzyII. We were riding on some back roads and came out of a small cut through road(canyon) and got hit by a gust that was wraping around the hill. It immediatly acted like it was running out of gas. I was down to a 3/8 of a tank, well above reserve time. So we pulled over and investigated, just to make sure nothing was wrong. Everything seemed fine, it reved up with a twist of the throttle and there was plenty of gas in the tank after a vivual check.

Off we went with not an issue until coming around another hill and another gust struck us from the side. Same thing happened as before, so I just went along untill the wind stopped and everthing was honky dory once more.

Now back to AJs thread, here is the deal or question. I've heard of the "Vortex effect" before. About crosswind reaking havoc on the pressure around the pods. Some say it's true and some say it's BS. Now if these tubes or hoses, which I believe are commonly called vent tubes are the culprit, then putting them in a place that can't recieve a change should be the best place.

The experts at K&N call to remove the hoses completely, but to leave them unblocked. Now what exactlly do these tubes do, because there is an anomally occuring here. Here is some visuals.

First photo(SuzyII) is the vent tubes sticking down just below the filters and this is the set up I was running in Tennesee.


Next visual is the carbs off of Suzy and type of pods that I run. As you can see the tubes are completely blocked off. This is the way I got it and have been riding that way for three years now.

My old exmechanic said these tubes should not be blocked off and shouldn't run like that and asked why I blocked them. I told him thats the way it's been since I got it and when he cleaned the carbs a year earlier. If I open them up it runs like crap.

Please some or anyone explain whats up. I'm getting ready to reclean them and put in new orings and intake orings, so I'm going to have to rethink what I should do with these tubes and the tubes on SuzyII.

Just for info, I'm at near zero feet above sealevel.

[AJ]

I called them "pressure equalization tubes" because that's what they are.

Here's the short version of how it works.
(1) Regular atmospheric pressure is pressing down on the gasoline in the float bowl. This is determined by the "equalization tube" or "vent tube" that lets that air pressure from the atmosphere reach the top of the gas in the float bowl. Lets call that pressure at the BOTTOM of the jet Pbottom.

(2) Air is drawn by the engine through the filter and into the throat of the carburetor. The air moves over the carburetor jets, dropping the air pressure via the venturi effect at the top of the jet. Let's call that pressure at the TOP of the jet Ptop.

(3) The jet is like a little straw, with a precision hole in it. By items 1 and 2 above, the jet has Pbottom below and Ptop above. Because of the venturi effect, Pbottom is greater Ptop (Pbottom > Ptop) so the gasoline is forced up through the jet and enters the air stream in the throat of the carb.

Ideally, Pbottom should equal the pressure of the air right at the intakes to the carbs (before the low pressure Ptop is created in the throat of the carb). This is why that vent tube needs to be located near the air filters, so it can equalize the pressure between what's pressing on the gasoline and the pressure that is moving into the throat of the carbs.

If you block the vent tube, the venturi effect will suck gas out of the float bowl, but since no new air can get in as the gasoline is pulled out of the bowl, the pressure inside the bowl drops so Pbottom decreases to the point where Pbottom = Ptop and no more gas is pushed through the jet.

If you have your vent tube out in the air stream (or a strong crosswind), air can move across the opening of the vent tube, creating a venturi effect there, effectively decreasing Pbottom, again, keeping gasoline from being pushed up through the jet.

On a stock setup with a stock airbox, the inlet to the airbox is carefully put up under the seat where the air is fairly stable. The vent tube is routed to be in exactly the same spot, just at the inlet to the airbox.

Removing all that and installing pod filters shortens the intake tract and remove resistance to airflow into the engine (allowing it to breathe more and make more power). BUT is also puts the filters in a place directly behind the engine of a vehicle moving forward. The engine blocks the air coming into the filters and the airflow is really turbulent right there (unlike the stock airbox location up under the seat). This is where there are problems with the filters. Modern bikes with big swoopy fairings use air ducts from the front of the bike to deliver a smooth, high-pressure air inlet to the engine.

[AJ]

Here's a really bad picture I drew.

[Agemax]

also you have to consider that when you are moving forward at speed, the air hits the front of the bike and is forced out around the engine, and your knees. the engine will be jetted to compensate for the amount of air that the carbs/filters can suck in this situation.

subject it to a massive side wind, almost has a turbo affect as it will force more air in from the side that the bike is set up for. resulting in a lean mixture, which would show as a loss of power at higher speeds.

[posplayr]

I think the jury is still out as to how much this effect is due to air flow around the pods or air flow around the equalization ports. My guess ( educated?) is it is not hard to imagine having local pressure variations between the equalization ports on the various carbs. And as AJ has described the fuel metering through the jets is proportional to the pressure differential between the intake Venturi and the pressure above the gas in the float bowl. So even a 1 psi variation in the turbulent flow around vent port would be like several step changes on the main. As a practical matter these variations must be happening fast enough to effectively average out (e.g. Flow flutter if you will at 1-2 Hz). However as age has described the pods/carbs are operating in a complex flow feild sitting behind the engine. Aerodynamically the carbs will be relatively sheltered from the flow as there isgoing to be kind of an air bubble at standard atmosphere behind the engine. outside the bubble will be a transition area (i.e boundary layer) that is turbulent that extends out into the freestream slip-lines of the air rushing past. It is not hard to see how this flow although relatively stable could be affected by a large change of lateral angle of attack. Almost the definition of lift (pressure differential between surfaces) that the outside pod on one side (exposed to the blast ) will be at higher pressure than the opposite side which will be at a relatively lower pressure. The question is how much affect does it have and is there anything you can do about it. More on that later I'm typing on an IPhone

[AJ]

There was a great tech note about this I read a few years ago, but cannot find it anymore. It was about turbulence on the pod filters - specifically on the GS1100E - increasing 1/4 mile times by tenths of a second. It was on one of the big names in drag racing websites, like Schnitz or Vance & Hines.

Anyway, I did find the following on K&N's website (http://www.knfilters.com/filter_facts.htm) about running open (unfiltered) velocity stacks on car engines:

Racing One might consider a paved road course or oval track as a clean air zone. After all, how much dirt and debris could be hovering above an asphalt track?


Subscribing to that theory, a road racer may elect to forgo an air filter in favor of large volumes of unrestricted air. However, testing the theory using an air filter enclosed in a vented housing should dispel the myth. The filter and housing will trap particles of loose trash kicked up by other race cars during the heat of battle. Dirt, small stones and pieces of shredded rubber expelled from soft compound racing tires can be found inside the housing after even a short race. Once a driver, car owner or engine builder realizes just how much trash is thrown around during a normal race, few would expose their expensive engines to unfiltered air in future events.


Whenever possible, performance enthusiasts should install a K&N 360 degree open-element filter. A correctly sized conical or round filter will deliver virtually unrestricted air flow. And, as we have learned, providing the engine with all of the air it needs promotes optimum performance. In a high speed application, a K&N filter will straighten the air which counteracts turbulence.


Straight cut velocity stacks, for example, pose a unique problem. Exposed to the outside air, velocity stacks experience a phenomena that actually hinders performance at high speed. We are referring to stacks and air horns that protrude through the hood and extend into the air stream so the direction of the air rushing over the car is at a perpendicular angle to the length of the tube.


Air moving rapidly over these stacks create turbulence inside the opening. At high speed, the rushing air tends to create a partial vacuum inside the tube. The condition is counterproductive to air flow. The phenomena also effects open carburetors. The higher the ground speed, the greater the problem. Vacuum created by the engine is trying to coax air into the cylinders and the high speed air flowing over the open end of the stack is causing resistance.


Reversion creates other problems. In an automotive application, reversion refers to reversed air flow, or in simpler terms, it’s when air in the intake runner reverses direction for a split second. The condition is caused when a burst of pressure escapes into the intake runner from the cylinder during valve overlap.


Reversion creates resonance shock waves inside the tubes which exit the open end of the tube at various rates depending on engine speed. It has also been proven that these shock waves interfere with each other when the stacks are in close proximity.


Installing a free-flowing air filter on top of each stack or over the carburetor air horn eliminates these conditions. How? The solution is simply explained. The filter creates a plenum over the opening. Air entering the filter is slowed, smoothed and straightened. The filter then becomes an endless source of calm, clean air. Shock waves dissipate within the confines of the plenum without interfering with the shock waves emitted from an adjacent stack.

Given that, it seems to me that it would be good to add some kind of filter to the end of the float bowl vent tubes to reduce turbulence in the tube. Maybe one of those little filters people put on crankcase vents would work. (plus I have a spare one from my Harley!)

[81ZUKI75]

And I've experienced the effect even with an airbox. To wit...

On my stock 1981 GS750E, while riding at cruising speed on a four lane raised highway I experienced the sensation of low fuel when it surely wasn't. I discovered that whenever I moved my right foot forward on the peg (I normally ride on the balls of my feet) a stumble would occur. I noticed that the end of the vent line on the right side had slipped out from its normal position tucked in between the airbox. Now it was pointed out in the open and my boot location was somehow causing the stumble. Call me crazy but it was a cause-and-effect moment. I don't know/care whether the air was being directed into the vent line pressurizing the float bowls (probably not), or if the air was racing past the open end (lowering pressure in the float bowl (probably) but since it has been tucked away out of the wind, nor more stumbling

[chuck hahn]

I dont think it so much power loss as it is the fact that your hitting a wall of increased resistance at said speed. Soon as the pass thru is complete the bike is normal, so why over analyse the situation.

[gsrick]

Great stuff so far, I will be rereading it so it sinks in, but as my used to be mechanic said, "it shouldn't even run like that". Blocked off tubes that is. Eric has suggested that he runs the tubes up under the seat near the battery. I'm sure I'll hear from him if I stated that wrong and edit to suit, but why do I get the effect with blocked tubes and would shielding the pods from the offending crosswinds. This is not a problem I come across often, I'm just trying to learn more.

[posplayr]

And I've experienced the effect even with an airbox. To wit...

On my stock 1981 GS750E, while riding at cruising speed on a four lane raised highway I experienced the sensation of low fuel when it surely wasn't. I discovered that whenever I moved my right foot forward on the peg (I normally ride on the balls of my feet) a stumble would occur. I noticed that the end of the vent line on the right side had slipped out from its normal position tucked in between the airbox. Now it was pointed out in the open and my boot location was somehow causing the stumble. Call me crazy but it was a cause-and-effect moment. I don't know/care whether the air was being directed into the vent line pressurizing the float bowls (probably not), or if the air was racing past the open end (lowering pressure in the float bowl (probably) but since it has been tucked away out of the wind, nor more stumbling

Glad you point that out . While I was positing before but without conclusion I was starting to surmise that if there was anything that you could do it would be to avoid the differential power variation (between carbs/cylinders) due to the localized pressure variation at the vent port. My guess was that a local pressure variation for a pod was probably not as big of an effect as the vent port. The pods are kind of mini plenums with sufficient surface area to flow sufficient flow to meet the engine need. That big surface makes them less susceptible to local pressure changes. On the other hand the pressure vent is quite small in comparison and can have the effect of changing several jet sizes with even a 1 psi variation. So trying to stabilize/equalize the pressures across the float bowls is probably an area for experiment.

[AJ]

Great stuff so far, I will be rereading it so it sinks in, but as my used to be mechanic said, "it shouldn't even run like that". Blocked off tubes that is. Eric has suggested that he runs the tubes up under the seat near the battery. I'm sure I'll hear from him if I stated that wrong and edit to suit, but why do I get the effect with blocked tubes and would shielding the pods from the offending crosswinds. This is not a problem I come across often, I'm just trying to learn more.

Your mechanic is technically correct. However, it can run with the tube blocked off for a couple reasons. Some ideas:

1) the seals on those air tubes connecting the carbs together may leak enough to equalize the pressure.

2) the negative pressure (from the venturi sucking gas out of the bowls) is relieved every time the gas level drops enough in the float bowl to open the little fuel valve, added to by the pressure of the fuel flowing down from the tank (think "water tower").

3) your bike is inhabited by magical pixies that keep it running properly.

Maybe there's another explanation, but it might be one of those three.

[AJ]

Glad you point that out . While I was positing before but without conclusion I was starting to surmise that if there was anything that you could do it would be to avoid the differential power variation (between carbs/cylinders) due to the localized pressure variation at the vent port. My guess was that a local pressure variation for a pod was probably not as big of an effect as the vent port. The pods are kind of mini plenums with sufficient surface area to flow sufficient flow to meet the engine need. That big surface makes them less susceptible to local pressure changes. On the other hand the pressure vent is quite small in comparison and can have the effect of changing several jet sizes with even a 1 psi variation. So trying to stabilize/equalize the pressures across the float bowls is probably an area for experiment.

Agreed. Also the smoothing of the airflow due to the filters per the K&N link above.

[posplayr]

There was a great tech note about this I read a few years ago, but cannot find it anymore. It was about turbulence on the pod filters - specifically on the GS1100E - increasing 1/4 mile times by tenths of a second. It was on one of the big names in drag racing websites, like Schnitz or Vance & Hines.

Anyway, I did find the following on K&N's website (http://www.knfilters.com/filter_facts.htm) about running open (unfiltered) velocity stacks on car engines:

Given that, it seems to me that it would be good to add some kind of filter to the end of the float bowl vent tubes to reduce turbulence in the tube. Maybe one of those little filters people put on crankcase vents would work. (plus I have a spare one from my Harley!)

Revert back to what the factory would try to do. Create a plenum for pressure equalization across the bowls.

[AJ]

Revert back to what the factory would try to do. Create a plenum for pressure equalization across the bowls.

Yep, that would probably help. And it has a very nice "Model T" look to it.

Actually, there are only two lines, as the vent line connects the two carbs on each side. I can't recall if all four carbs actually join across the carb bodies.

[Agemax]

Your mechanic is technically correct. However, it can run with the tube blocked off for a couple reasons. Some ideas:

1) the seals on those air tubes connecting the carbs together may leak enough to equalize the pressure.

2) the negative pressure (from the venturi sucking gas out of the bowls) is relieved every time the gas level drops enough in the float bowl to open the little fuel valve, added to by the pressure of the fuel flowing down from the tank (think "water tower").

3) your bike is inhabited by magical pixies that keep it running properly.

Maybe there's another explanation, but it might be one of those three.

not forgetting the fuel tank is subjected to atmospheric pressure by the vent in the gas cap, which would allow the fuel to flow into the bowls and overcome any pressure lock