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  #1  
Old 04-11-2003, 09:51 PM
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Default rookie-can some one explain zero vacuum at WOT?

I'm having trouble with zero vacuum at WOT. What draws the fuel out of the bowls at WOT.
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Old 04-11-2003, 09:58 PM
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Suction. The air is passing thru the venturies [throat] of the carborator so fast that the fuel is sucked out of the passageways.

Then there are those people who will come with the idea of different pressures at work here. Something like the different pressures that cause wind between high and low pressure areas on earth that causes wind.
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Old 04-11-2003, 10:04 PM
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You have zero vacum at WOT? This cannot be, your enginewould have maybe 10 times that much or more at idle. What instrument are you using for the test. Theres a big differance in Zero gage, and zero absolute pressure wich would be 14.7 psi. Although this is not possiable. There is no such thing as a perfect vacum, maybe in space!

1 atmosphere = 14.7 psi Absolute
or 29.95 inches of Hg
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Old 04-11-2003, 10:04 PM
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Although the manifold vacuum is zero, there is plenty of air entering the engine through the carburator. The difference in size of the venturi in the carb creates a low pressure differential at the base of the carb in relation to atmospheric pressure so fuel is pulled into the engine. That's pretty much how the main metering works. Engine vacuum is really the engine wanting more air than the carburator will allow in, the engine vacuum is not what pulls fuel into the engine, it is the air passing through the venturi area of the carburator. Sort of the same theory of how lift is generated on an aircraft wing.
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Old 04-11-2003, 10:44 PM
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amx is right - with the butterflies wide open there is hardly any restriction to the incoming airflow so vacuum read at the manifold will be low, if present at all. What causes the fuel to continue to move into the airstream is the venturi effect. Technically a venturi is described as a passage so formed as to give the greatest discharge coefficient of any known shape. In English it means as the air flow is accelerated through the venturies it experiences a relative pressure drop which is enough to cause the fuel to move from the float bowls. The principle behind it is that moving air has less pressure, and the faster it moves the less pressure it has. It's the reason that planes fly - the airflow moving over the curved upper surface of a wing has further to travel to get to the trailing edge than the air underneath (which follows a straighter path) and accelerates. The faster moving air above the wing therefore experiences a relative pressure drop compared to the air beneath the wing, and this creates lift. Sorry if this got a little long winded, but I hope it makes enough sense to help you out.
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Old 04-11-2003, 10:48 PM
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Quote:
As air passes through the narrow center of the barrel, called the "venturi", it produces suction that draws spray from the cruising-circuit nozzle. The float-bowl level drops and causes the float to tip and the needle valve to open the fuel line.
A "venturi" is a tube with a restricted section. When liquid or air passes through the venturi tube, the speed of flow is increased at the restriction, and air pressure is decreased, creating an "increase in vacuum" (a reduction in ambient pressure). This causes fuel to be drawn into the barrel.

A quote from; http://www.musclecarclub.com/library...rburetor.shtml
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Old 04-12-2003, 12:57 AM
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Default As the pressure drops

Your engine is a big hole in the atmosphere, as x said above” it is the high to low pressure thing.
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Old 04-12-2003, 07:02 AM
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It's magic!!!

We size carburetors by hooking a vacuum gauge to the manifold, if we have vacuum at the stripe then we need a bigger or at least a higher flowing carb.

We always want the smallest venturis we can get and still maintain zero at the stripe, this ensures max velocity and the best atomization of the fuel.

Why are you having trouble with Zero?

What carb size and how many inches of motor do you have?
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Old 04-12-2003, 02:02 PM
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The venturi principle has been explained and the air pressure of 14.7 (at sea level) has also been mentioned. The 14.7 is acting on the fuel in the float bowl pushing it to the low pressure area created in the venturi. The greater the air flow the greater the pressure drop and as the 14.7 is constant more fuel flows to the increasingly lower pressure area.
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Old 04-12-2003, 04:10 PM
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As Mentioned before I agree. This is called Bernoulis principle states that when velocity is high. pressure is low. The carb the body (Venture) the air passed through it it causes a friction to maintane the constant velocity the flow must increase were flow is increased, the pressure drops, hints when veloctiy is high pressure is low, or when pressure is low velocity is high. The low pressure cause the fuel to move from the carb to the venture. Just like the air plane, the way the wing is made provides the lift being the top of the wing is curved and the bottom is flat. To maintain the constant velocity the air must travel faster over the top of the wing as it cuts to the air. The faster the air the lower the pressure, the pressure, now on the bottom. (air speed) is low there is more pressure so it over comes the pressure on top and lifts the plan. Thats easy, no let me explain how a helicopter works........yea right maybe next year.!LOL
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Old 04-12-2003, 04:26 PM
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wow Well lets see

The helicopter works the same way except the retreating blade angle of attack is increased so the ship doesn’t roll over, easy eh
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Old 04-12-2003, 11:48 PM
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The vacuum gauge in my old truck showed near zero all the way home today. The speedo showed max!!

Billy
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Old 04-12-2003, 11:48 PM
mtrv8n mtrv8n is offline
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What you are measuring, at the manifold, is suck. It is called vacuum, and measured in in the same increment as atmospheric values.

This does NOT mean you have a vacuum, any more than your VACUUM cleaner has a vacuum.

What you are reading is the amount of suction at that port. At idle,the venturi is restricted by the butterfly plates, and the engine is trying to suck the difference at any available opening.

The amount of "suck", or vacuum, is related to the efficiency of the motor at idle. In a situation where there is no effective valve overlap, no"vacuum" is lost to the exhaust port, and "suck is high, about 15-17, maybe more. This number has no relation to atmospheric pressure. In a high perf engine, there is a lot of overlap. At idle, this causes a pressure loss, read as a "Low Vacuum" at idle, maybe 5-8.

At WOT, the throttle plates are not restricting air flow at all, so the engine no longer needs to "suck" through the port to supply all the airflow that the venturi can pass. Therefore, the reading is zero, as in zero suck. No more air is being pulled through the little port hole since the venturi is allowing plenty in, unless, as Don stated, the carb is not big enough. That will cause a reading, though slight, 1- 1.5, at the port at WOT.


None of this has any relation to how fuel gets pulled out of the bowls: That is all handled by the aforementioned venturi effect in the throttle body, though the carbs also have"boosters" which are mini venturi's hung out over the main bore of the carb body.
The boosters help fuel get pulled through at lower air speeds.

The low vacuum of WOT explains why you get blowby at WOT, because the manifold is no longer sucking through the PCV valve.

hope I didn't muddy things...
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Old 04-12-2003, 11:59 PM
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I have to admit while the rest of us were get mired down in techno-speak, "suck" is about the best way I've ever heard manifold vacuum described.
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Old 04-13-2003, 12:22 AM
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Kinda cuts through the bells, whistles, smoke, and mirrors doesn't it?
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  #16  
Old 04-13-2003, 12:36 AM
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mirrors, mirrors?? no wonder I get turned around so easily!
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Old 04-13-2003, 07:15 PM
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19cuda67 19cuda67 is offline
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mtrv8n

THANX for the explination I think I understand this now.THANX Ken g.
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  #18  
Old 04-14-2003, 08:48 AM
mtrv8n mtrv8n is offline
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NP, though some of the deatils are not quite accurate, it's good enough for Gov work..
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