The Ford Barn

Terry, NJ · 08-25-2013, 07:01 PM

I've been thinking (Whoa, is something burning?) Bearing in Mind, Bernouli's princible of a venturi (The more choked down, the faster the air goes) Can we increase the air flow into a carb. by placing a tapered horn at the intake? And how long and how much taper is required? What would be the optimum size of the mouth?
Terry

Tom Wesenberg · 08-25-2013, 07:34 PM

Yes, you can. Aren't they called velocity stacks?

Raven · 08-25-2013, 07:37 PM

The way I understand the principle of a venturi is that the gas passing throught it slows down and its pressure increases....

Raven · 08-25-2013, 07:47 PM

Oops sorry should have typed pressure decreases and the gas flow increases..

montanafordman · 08-25-2013, 07:48 PM

Quote:

Originally Posted by Raven

The way I understand the principle of a venturi is that the gas passing throught it slows down and its pressure increases....

Actually the opposite. It speeds it up and the pressure decreases. Aircraft wings create lift this way, just like hose sprayer/dispensers with the little bottle below the nozzle are able to siphon whatever fertilizer, moss killer, pesticide that's in it. A good way to demonstrate this is to take a sheet of paper and hold it on each side letting the end droop down, now blow across what would be the top if it were stiff like cardboard and you'll see the paper drift upwards.

P.S. · 08-25-2013, 09:18 PM

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I'm an engineer (though that doesn't really help).

When you increase the speed of air through a venturi, you only increase the speed of the air through the venturi itself, and the pressure increases, assuming the pressure (or vacuum) on one side of the venturi remains constant.

However, in the case of your Model A carburetor, you have a vacuum on the motor side of the carburetor (the intake manifold) and a venturi is already used in the carburetor to assist with ionizing the fuel as it meets air and mixes in the carb just beyond the jets. If you add an external venturi to the atmosphere side of the carburetor, you will increase the vacuum on the manifold side of the carburetor which will reduce the engine's ability to draw in air/fuel mixture and will reduce horsepower.

If you had a turbo or blower to increase air pressure on the atmosphere side of the carb, you could certainly play with a venturi to tune the system.

Terry, NJ · 08-25-2013, 09:35 PM

I guess my question would be, if there was a venturi at the mouth/intake of the carb., would it increase the amount of air entering the mouth of the carb?
Terry

Quote:

Originally Posted by P.S.

I'm an engineer (though that doesn't really help).

When you increase the speed of air through a venturi, you only increase the speed of the air through the venturi itself, and the pressure increases, assuming the pressure (or vacuum) on one side of the venturi remains constant.

However, in the case of your Model A carburetor, you have a vacuum on the motor side of the carburetor (the intake manifold) and a venturi is already used in the carburetor to assist with ionizing the fuel as it meets air and mixes in the carb just beyond the jets. If you add an external venturi to the atmosphere side of the carburetor, you will increase the vacuum on the manifold side of the carburetor which will reduce the engine's ability to draw in air/fuel mixture and will reduce horsepower.

If you had a turbo or blower to increase air pressure on the atmosphere side of the carb, you could certainly play with a venturi to tune the system.

Tom Wesenberg · 08-25-2013, 10:02 PM

The velocity stack (if that's what it's called) is used, it isn't a venturi, but a trumpet shaped inlet pipe to smooth the flow of air entering the carb. You'd have to do a dyno test to see how much help it gives.

Mike V. Florida · 08-25-2013, 10:36 PM

Quote:

Originally Posted by Terry, NJ

I guess my question would be, if there was a venturi at the mouth/intake of the carb., would it increase the amount of air entering the mouth of the carb?
Terry

No, the speed may increase, the pressure would change, but the actual volume of air would actually decrease due to the smaller diameter.

I see only a few ways to increase the volume of air entering the engine. Increase the vacuum the pistons create when the "suck" in air fuel mixture, push the air in with a blower of some kind remove restrictions in the path on the way to the cylinders. Some will make greater horsepower, some not.

Patrick L. · 08-26-2013, 05:53 AM

Aaron is right. There needs to a camber to the venturi which causes the pressure change. Look at an airplane wing or propellor. The air passing over the camber increases its speed which decreases the pressure allowing atmospheric pressure on the fuel in the bowl to overcome the lower pressure and fuel is then introduced into the venturi area.

Tom Wesenberg · 08-26-2013, 06:16 AM

Quote:

Originally Posted by Terry, NJ

I've been thinking (Whoa, is something burning?) Bearing in Mind, Bernouli's princible of a venturi (The more choked down, the faster the air goes) Can we increase the air flow into a carb. by placing a tapered horn at the intake? And how long and how much taper is required? What would be the optimum size of the mouth?
Terry

I'm sure Terry has a velocity stack in mind.

http://en.wikipedia.org/wiki/Velocity_stack

Mitch//pa · 08-26-2013, 06:37 AM

add a tornado

P.S. · 08-26-2013, 08:37 AM

FYI- I meant "atomize" but for some dumb reason, typed "ionize". Sorry for the error. Thanks, Skip!

Terry, NJ · 08-26-2013, 11:04 AM

Yes, Tom. Very much like that. Longer and more tapered, not bell mouthed. I was thinking of increasing the velocity at the mouth of the carb and then the carb's venturi would increase it some more. That was the idea anyway. Give the air velocity a boost, get it up to speed before it enters the carb. It seems like it would increase the amount of air entering the carb. I'm sure I'm n ot the first one to think of it. Pretty elementary, yet I've never read of it.
Terry

Quote:

Originally Posted by Tom Wesenberg

I'm sure Terry has a velocity stack in mind.

http://en.wikipedia.org/wiki/Velocity_stack

BILL WILLIAMSON · 08-26-2013, 02:24 PM

Just mount a BIG, BIG, funnel on the front of the car & duct it to the carb with a flexible hose! Now, wasn't that EASY?? (OH! & paint the inside of the funnel, RED!!)
Bill W.

Allanw · 08-26-2013, 02:27 PM

The short answer is: it would increase the velocity, but decrease the flow - as above. The venturi is a restriction, so you would end up with slightly less air travelling at a faster rate, through a smaller hole.

The bell mouth (velocity stack) increases flow, and removes the turbulence at the mouth of the carb - the bell allows for smoother transition of the air in the carb mouth, as shown in the wiki article mentioned above.

1931 flamingo · 08-26-2013, 03:04 PM

How about a crank driven blower??
Paul in CT

Bruce Lancaster · 08-26-2013, 03:10 PM

Yes, venturi is a restriction...the restriction serves to provide the motivation for the gas to flow in. Like the airplane's wing shape providing lift...that is a drag on the airplane, but again a useful one, since lift is REAL handy in airplanes.
This is a point of superiority for fuel injection, which provides the motivation for the gas to move by a pump that doesn't influence flow. From what I have read, though, the pressure drop of the venturi helps with atomization and so is perhaps an advantage in another way.

Mitch//pa · 08-26-2013, 04:55 PM

turn the carb around so the fan can blow the air into the throat..

John · 08-26-2013, 05:43 PM

Let me go out on a limb here and make a few statements: Bernoulli's Equation is actually a mass and energy balance around the venturi. Since the mass rate of air flow in must equal the mass rate of air out, simply pounds in equalls pounds out thus nothing is lost, then any other changes of energy such as velocity must be balanced with a an appropriate change in another energy such as pressure energy.

When the air enters into the large area venturi at a given flow rate, it must speed up in the smaller area of the venturi and thus the velocity energy of the air increases with respect to the velocity energy at the entrance to the venturi. When this increase in velocity energy happens, it must be balanced by a corresponding decrease in another energy of the fluid, in this case the pressure energy of the fluid to maintain an energy balance.

Since air entering is, let's say 14.7 psi absolute at sea level, then the pressure of the fluid in the narrow part of the venturi must be less than 14.7 psia. due to the increase in velocity. If the gas in the float bowl is at 14.7 psia, same as the entering air pressure, then it will flow from the jet located at the narrow part of the venturi into the airstream.

Changes in the airstream prior to the venturi will just disrupt the hopefully smooth flow into the venturi throat thus reducing the effectiveness of the venturi.

This same thought pattern shows why the Model A carburetor will force the fuel air ratio to become more rich when an air filter is used.

Hope this helps rather than confuses.....

John Cannon

08-25-2013, 07:01 PM	#1
Terry, NJ Senior Member Join Date: May 2010 Location: Bucks Co, Pa Posts: 3,749	Any Engineers? Bernouli's principle. Venturis I've been thinking (Whoa, is something burning?) Bearing in Mind, Bernouli's princible of a venturi (The more choked down, the faster the air goes) Can we increase the air flow into a carb. by placing a tapered horn at the intake? And how long and how much taper is required? What would be the optimum size of the mouth? Terry

08-25-2013, 07:34 PM	#2
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: Any Engineers? Bernouli's principle. Venturis Yes, you can. Aren't they called velocity stacks?

08-25-2013, 07:37 PM	#3
Raven Member Join Date: Jul 2012 Location: Houston, Texas Posts: 37	Re: Any Engineers? Bernouli's principle. Venturis The way I understand the principle of a venturi is that the gas passing throught it slows down and its pressure increases....

08-25-2013, 07:47 PM	#4
Raven Member Join Date: Jul 2012 Location: Houston, Texas Posts: 37	Re: Any Engineers? Bernouli's principle. Venturis Oops sorry should have typed pressure decreases and the gas flow increases..

08-25-2013, 09:18 PM	#6
P.S. Senior Member Join Date: Jul 2012 Location: Hazzard County Posts: 1,921	Re: Any Engineers? Bernouli's principle. Venturis Sponsored Links (Register now to hide all advertisements) I'm an engineer (though that doesn't really help). When you increase the speed of air through a venturi, you only increase the speed of the air through the venturi itself, and the pressure increases, assuming the pressure (or vacuum) on one side of the venturi remains constant. However, in the case of your Model A carburetor, you have a vacuum on the motor side of the carburetor (the intake manifold) and a venturi is already used in the carburetor to assist with ionizing the fuel as it meets air and mixes in the carb just beyond the jets. If you add an external venturi to the atmosphere side of the carburetor, you will increase the vacuum on the manifold side of the carburetor which will reduce the engine's ability to draw in air/fuel mixture and will reduce horsepower. If you had a turbo or blower to increase air pressure on the atmosphere side of the carb, you could certainly play with a venturi to tune the system.

08-25-2013, 10:02 PM	#8
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: Any Engineers? Bernouli's principle. Venturis The velocity stack (if that's what it's called) is used, it isn't a venturi, but a trumpet shaped inlet pipe to smooth the flow of air entering the carb. You'd have to do a dyno test to see how much help it gives.

08-26-2013, 05:53 AM	#10
Patrick L. Senior Member Join Date: May 2010 Location: Largo Florida Posts: 7,225	Re: Any Engineers? Bernouli's principle. Venturis Aaron is right. There needs to a camber to the venturi which causes the pressure change. Look at an airplane wing or propellor. The air passing over the camber increases its speed which decreases the pressure allowing atmospheric pressure on the fuel in the bowl to overcome the lower pressure and fuel is then introduced into the venturi area.

08-26-2013, 08:37 AM	#13
P.S. Senior Member Join Date: Jul 2012 Location: Hazzard County Posts: 1,921	Re: Any Engineers? Bernouli's principle. Venturis FYI- I meant "atomize" but for some dumb reason, typed "ionize". Sorry for the error. Thanks, Skip!

08-26-2013, 02:24 PM	#15
BILL WILLIAMSON Senior Member Join Date: Dec 2010 Location: FRESNO, CA Posts: 12,560	Re: Any Engineers? Bernouli's principle. Venturis Just mount a BIG, BIG, funnel on the front of the car & duct it to the carb with a flexible hose! Now, wasn't that EASY?? (OH! & paint the inside of the funnel, RED!!) Bill W. __________________ "THE ASSISTANT GURU OF STUFF"

08-26-2013, 02:27 PM	#16
Allanw Senior Member Join Date: Oct 2012 Location: Whangarei, New Zealand Posts: 298	Re: Any Engineers? Bernouli's principle. Venturis The short answer is: it would increase the velocity, but decrease the flow - as above. The venturi is a restriction, so you would end up with slightly less air travelling at a faster rate, through a smaller hole. The bell mouth (velocity stack) increases flow, and removes the turbulence at the mouth of the carb - the bell allows for smoother transition of the air in the carb mouth, as shown in the wiki article mentioned above. __________________ Allan '29 Tudor, Canadian RHD Whangarei, NZ "Duct Tape can't fix stupid, but it can MUFFLE the sound"

08-26-2013, 03:04 PM	#17
1931 flamingo Senior Member Join Date: May 2010 Location: new britain,ct 06052 Posts: 9,428	Re: Any Engineers? Bernouli's principle. Venturis How about a crank driven blower?? Paul in CT

08-26-2013, 03:10 PM	#18
Bruce Lancaster Member Emeritus Join Date: May 2010 Location: Madison, NJ Posts: 5,230	Re: Any Engineers? Bernouli's principle. Venturis Yes, venturi is a restriction...the restriction serves to provide the motivation for the gas to flow in. Like the airplane's wing shape providing lift...that is a drag on the airplane, but again a useful one, since lift is REAL handy in airplanes. This is a point of superiority for fuel injection, which provides the motivation for the gas to move by a pump that doesn't influence flow. From what I have read, though, the pressure drop of the venturi helps with atomization and so is perhaps an advantage in another way.

08-26-2013, 04:55 PM	#19
Mitch//pa BANNED Join Date: May 2010 Location: Bucks County, PA Posts: 11,454	Re: Any Engineers? Bernouli's principle. Venturis turn the carb around so the fan can blow the air into the throat..

08-26-2013, 05:43 PM	#20
John Senior Member Join Date: May 2010 Location: Richmond, VA Posts: 214	Re: Bernouli's principle -- Conservation of Energy Let me go out on a limb here and make a few statements: Bernoulli's Equation is actually a mass and energy balance around the venturi. Since the mass rate of air flow in must equal the mass rate of air out, simply pounds in equalls pounds out thus nothing is lost, then any other changes of energy such as velocity must be balanced with a an appropriate change in another energy such as pressure energy. When the air enters into the large area venturi at a given flow rate, it must speed up in the smaller area of the venturi and thus the velocity energy of the air increases with respect to the velocity energy at the entrance to the venturi. When this increase in velocity energy happens, it must be balanced by a corresponding decrease in another energy of the fluid, in this case the pressure energy of the fluid to maintain an energy balance. Since air entering is, let's say 14.7 psi absolute at sea level, then the pressure of the fluid in the narrow part of the venturi must be less than 14.7 psia. due to the increase in velocity. If the gas in the float bowl is at 14.7 psia, same as the entering air pressure, then it will flow from the jet located at the narrow part of the venturi into the airstream. Changes in the airstream prior to the venturi will just disrupt the hopefully smooth flow into the venturi throat thus reducing the effectiveness of the venturi. This same thought pattern shows why the Model A carburetor will force the fuel air ratio to become more rich when an air filter is used. Hope this helps rather than confuses..... John Cannon