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mrbones
RCTalk Addict
This is some stuff I found on line explaining what a venturi does.
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Different venturi sizes are used to increase run time and lower or increase power. Simply put, a large venturi allows for more air to enter the engine and you will then need to add more fuel for it to be tuned properly which will equal more power. A smaller venturi will reduce the amount of air entering the motor so in turn you will have to lean it out thus resulting in better run time and less power.
VENTURIS
Internal combustion engines require three things to produce power - fuel, oxygen and heat -- and they need them in specific ratios. Running in situations in which the air is denser (contains more of oxygen) an engine produces power with ease. In situations in which the air is less denser (less available oxygen), an engine will not be able to burn as much fuel as it does at higher density air situations. Less fuel means less power, and that means slower speeds.
What can you do to minimize the effects of less denser air. Compression creates power, and when the air is less dense, there is less fuel/air mixture available to compress. To recover some of the lost compression, you can: Use a fuel containing a higher percentage of nitromethane, or reduce engine-head clearance to increase the compression ratio.
To compensate for less density air, you need to adjust your needle valve to a leaner setting to maintain the proper fuel/air ratio. The opposite is true for higher density situations. The idea is to maintain the optimum ratio of fuel and oxygen by adjusting the fuel volume to compensate for ambient air pressure.
Now, if you have the approprriate compression, and still need some more fine tuning for lower density situations, that is where the different sized Venturis come in handy.
A venturi such as a model engine carburetor's works on the vacuum principle: as air rushes through the venturi, it accelerates and creates a vacuum at the spray bar. This vacuum draws fuel from the spray-bar and fills the crankcase with a mixture of air and atomized fuel; at very low density situations, less air accelerates through the venturi and past the spray-bar. This reduces the vacuum at the spray-bar, so less fuel is drawn through it, and the mixture becomes too lean.
There is a way to compensate for this: Switch to a venturi with a smaller area.
When you have a smaller venturi area, the airflow velocity through the venturi is increased. This also increases the vacuum at the spray bar, which, in turn, draws more fuel.
Obviously, you have to experiment to learn which size of Venturi best suits your ambient situation.
****************************************************************************************************************************
What is a Venturi?
A venturi is a tapered tube, designed to 'neck down' the in-rushing air and then allow it to widen out to the throttle bore causing a pressure difference and hence drawing fuel through the carb.
How do they work?
To get through the venturi the air must speed up, which reduces the pressure inside the the venturi to below atmospheric pressure. The difference in pressure allows the fuel system (HSN/LSN) to discharge fuel.
A small venturi will create a higher pressure difference at any given RPM than a large venturi. This a a VERY important aspect of setting up your carb and selecting the right size venturi for the task in hand:
If the venturi is too large (not usually a problem with R.C's as the fixed carb venturi is matched to the engine) then this could lead to delays in fuel delivery causing a lack of performance and flat spots in the engine acceleration. Smaller venturi's do not suffer with this problem.
Their Uses:
- For longer straights a larger venturi would be recommended as it allows the engine to rev higher but have a slower throttle response, this however increases the fuel consumption causing shorter run times.
- For a track with a mixture of long straights a mid size venturi would be recommended, giving a balance of quick throttle response with higher revs.
- For a tight, twisty track a smaller venturi would be recommended, this causes a snappier throttle response but limits the engines rev range, but as a result causes longer run times
******************************************************************************
Different venturi sizes are used to increase run time and lower or increase power. Simply put, a large venturi allows for more air to enter the engine and you will then need to add more fuel for it to be tuned properly which will equal more power. A smaller venturi will reduce the amount of air entering the motor so in turn you will have to lean it out thus resulting in better run time and less power.
VENTURIS
Internal combustion engines require three things to produce power - fuel, oxygen and heat -- and they need them in specific ratios. Running in situations in which the air is denser (contains more of oxygen) an engine produces power with ease. In situations in which the air is less denser (less available oxygen), an engine will not be able to burn as much fuel as it does at higher density air situations. Less fuel means less power, and that means slower speeds.
What can you do to minimize the effects of less denser air. Compression creates power, and when the air is less dense, there is less fuel/air mixture available to compress. To recover some of the lost compression, you can: Use a fuel containing a higher percentage of nitromethane, or reduce engine-head clearance to increase the compression ratio.
To compensate for less density air, you need to adjust your needle valve to a leaner setting to maintain the proper fuel/air ratio. The opposite is true for higher density situations. The idea is to maintain the optimum ratio of fuel and oxygen by adjusting the fuel volume to compensate for ambient air pressure.
Now, if you have the approprriate compression, and still need some more fine tuning for lower density situations, that is where the different sized Venturis come in handy.
A venturi such as a model engine carburetor's works on the vacuum principle: as air rushes through the venturi, it accelerates and creates a vacuum at the spray bar. This vacuum draws fuel from the spray-bar and fills the crankcase with a mixture of air and atomized fuel; at very low density situations, less air accelerates through the venturi and past the spray-bar. This reduces the vacuum at the spray-bar, so less fuel is drawn through it, and the mixture becomes too lean.
There is a way to compensate for this: Switch to a venturi with a smaller area.
When you have a smaller venturi area, the airflow velocity through the venturi is increased. This also increases the vacuum at the spray bar, which, in turn, draws more fuel.
Obviously, you have to experiment to learn which size of Venturi best suits your ambient situation.
****************************************************************************************************************************
What is a Venturi?
A venturi is a tapered tube, designed to 'neck down' the in-rushing air and then allow it to widen out to the throttle bore causing a pressure difference and hence drawing fuel through the carb.
How do they work?
To get through the venturi the air must speed up, which reduces the pressure inside the the venturi to below atmospheric pressure. The difference in pressure allows the fuel system (HSN/LSN) to discharge fuel.
A small venturi will create a higher pressure difference at any given RPM than a large venturi. This a a VERY important aspect of setting up your carb and selecting the right size venturi for the task in hand:
If the venturi is too large (not usually a problem with R.C's as the fixed carb venturi is matched to the engine) then this could lead to delays in fuel delivery causing a lack of performance and flat spots in the engine acceleration. Smaller venturi's do not suffer with this problem.
Their Uses:
- For longer straights a larger venturi would be recommended as it allows the engine to rev higher but have a slower throttle response, this however increases the fuel consumption causing shorter run times.
- For a track with a mixture of long straights a mid size venturi would be recommended, giving a balance of quick throttle response with higher revs.
- For a tight, twisty track a smaller venturi would be recommended, this causes a snappier throttle response but limits the engines rev range, but as a result causes longer run times
Roger A. Cucal Jr.
RC Newbie
- Thread starter
- #3
Thanks mrbones for the explanation this wil help me a lot. I am here in dubai and the weather now is a bit cool, we are racing in a bit compressed track. Base on ur explanation I think i would be fine if i use #6 venturi for my novarossi p5xlt in a bit lean race tune or else i'll do some experiment w/ different vunturi size. And w/ this engine i am using short manifold (low end power) & efra 2096 pipe. Thanks again Mrbones.
Last edited:
WoodiE
Kind of good admin
Administrator
mrbones
RCTalk Addict
Well, when Roger asked the question, I did not know the answer so I started looking. I once bought a used Novarossi .12 engine on Ebay and when it got to my house, it had a reducing venturi in the carb and I had no idea of what purpose it served. So I guess now I know.
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