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Old 03-19-2010, 07:27 AM   #1
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Default why do you need a carb insert, when

you can adjust the slide on your carburetor to open however much you want?

And you can adjust the low needle separately to what you want?

Couldn't you then adjust the airflow to the same amount as a insert? And have the correct corresponding needle setting?

I have wondered this for a while. Am i missing something?






Of course we all know i am
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Old 03-19-2010, 07:36 AM   #2
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Carb inserts are also called Venturis. They create a pressure differential and a bunch of other physics type stuff someone way more qualified than me can explain. If I understand it correctly it's like a water hose, the air will travel into the engine with more velocity than without it.

Slap me if I'm way off.
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Old 03-19-2010, 07:39 AM   #3
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These motors will runn just fine without an insert, but they will allow much more aire into the carb which in turn will need much more fuel to achieve the correct ration for it to run. The main reason we use aan insert is for fuel mileage. This is the basics, maybe someone else will in detail describe how a carb works-the venturi effect and what not-i am not patient enough.
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Old 03-19-2010, 07:41 AM   #4
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that makes some sense
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Old 03-19-2010, 08:10 AM   #5
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This is right up my alley, I'm a mechanic by trade. The venturi is normally located either right at or right before the main jet in a carb. The idea is to create a pressure drop (and a consequent increase in speed) right at the point where fuel is introduced into the air stream. Doing that helps to snap the fuel/air mixture into the engine and increase throttle response, you wont notice a difference at mid to high engine speeds without a venturi due to the mixture velocity already being at a high speed. At low engine speeds you might see a difference in throttle response and efficiency depending on the carb design (whether or not a venturi is present to begin with). Bottom line, a small venturi will increase low speed efficiency and throttle response (more torque), a large venturi will increase total flow (more horsepower) with a subsequent loss of low speed efficiency and throttle response.
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Old 03-19-2010, 08:16 AM   #6
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How much will a venturi save you on gas? you a small one give you more acceleration but a big one give you more top speed? So would that small one take away top speed than? Thanks
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Old 03-19-2010, 08:25 AM   #7
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How much will a venturi save you on gas? you a small one give you more acceleration but a big one give you more top speed? So would that small one take away top speed than? Thanks
Yes, a small insert would limit your top speed because at some point in the engines RPM range you wouldn't be able to cram any more air into the engine through a small insert. You want to size up your insert to the RPM range that you use on your particular track to maximize the efficiency of the engine, that's what is going to give you the best economy for specific engine.
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Old 03-19-2010, 09:06 AM   #8
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Ok, so lets say a 100 x 100 track with tripples and doubles, would you use a red (8) or blue (9) insert for the v spec?
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Old 03-19-2010, 09:22 AM   #9
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Originally Posted by mudboger07 View Post
Ok, so lets say a 100 x 100 track with tripples and doubles, would you use a red (8) or blue (9) insert for the v spec?
I would use a 5.5 or 6.5 in a V-Spec. The V-Spec has great snap to make the jumps, so going smaller on a insert will give you the fuel mileage. 5.5-7.0 is normally were I run with all my engines.
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Old 03-19-2010, 09:41 AM   #10
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Quote:
Originally Posted by dreaux View Post
you can adjust the slide on your carburetor to open however much you want?

And you can adjust the low needle separately to what you want?

Couldn't you then adjust the airflow to the same amount as a insert? And have the correct corresponding needle setting?

I have wondered this for a while. Am i missing something?






Of course we all know i am
The venturi creates a smooth path for the air to flow into the engine. Simply limiting the throttle pull will create a turbulence around the carb slide. When you limit the throttle pull, you are leaving some of the "flat" surface of the slide exposed which creates a problem for air flow. The smooth surface of the venturi allows a smooth flow of air through the carb. The result is crisper throttle response. There are also other factors involved that help the venturi accelerate the air. The cone-like shape of the venturi creates a kind of "ram air" effect which drives even more air into the engine.
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Old 03-19-2010, 12:30 PM   #11
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Originally Posted by dreaux View Post
you can adjust the slide on your carburetor to open however much you want?

And you can adjust the low needle separately to what you want?

Couldn't you then adjust the airflow to the same amount as a insert? And have the correct corresponding needle setting?

I have wondered this for a while. Am i missing something?






Of course we all know i am


if you just limit the carb opening with epa on your radio then the lsn would always stay in and never would you get full throttle. It will be like a single needle controlled engine (lsn). The hsn would have minimal effect.
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