Carburetor Booster Vacuum

[motorsportsdesign]

Hi All,

I am trying to figure out the vacuum strength that is pulling fuel through a jet of a racing engine. The case is a drag raing big block chevrolet with 2 dominator carburetors.

I don't have a practical way to measure it on a running engine but I would be satified with a reasonable estimate.
The reason for my concern is that I want to figure out the effect of differing float/booster level on jetting. My assumtion is that if the vacuum is equal to many inches of gasoline that the impact of float level is not too important. Alternativly, if the vacuum equals 1 or 2 inches of gasoline, float/booster level matters a lot.

I was thinking that if I knew the ammount of fuel being consumed, then divided it by the number of jets (8) that the required vacuum could be calculted.

Alternativly if someone knows an approximate value, please let me know.



Any input would be appriciated.

Thanks

Jon Schmidt

http://www.motorsportsdesign.com

 

[patprimmer]

Jon
I have a couple of thoughts.

The vacuum varies directly proportional to airspeed. That's why carburetors work with one fixed main jet over a wide airflow range, so there is no fixed value for the vacuum.

At low speed the idle and transfer ports draw fuel, and they discharge a long way below the fuel level.

as an engine runs harder, the fuel level falls, as the needle must be further off the seat to allow for the extra fuel flow.

As a car runs, fuel is splashed all over the float bowl, and the float is being bounced around, seating and unseating the needle independantly of static fuel level.

Carbies are so imprecise and subject to so many poorly controlled variables, it's a wonder they run at all.

In my mind, to apply the level of science you propose is a bit like measuring a thou with a wooden rule.

Regards
pat

 

[motorsportsdesign]

Thanks for the reply, I know about the fluctuations, that is why I want to know the vacuum at the jet.

I'm interestsed in the vacuum behind the jet at wide open throttle at peak horsepower. I don't need a precise number, if I were within -50% to +200%, that would tell me what I need to know. If I know the vacuum, I can determine the impact of those fluctuations on mixture and decide if they are worth working to reduce. My guess is that the vacuum is high enough that the fuel level change doesn't matter much, but I'd like to know rather than guess.
(This is for racing that requires carburators)

Jonathan T. Schmidt

http://www.motorsportsdesign.com

 

[ShaunT]

Call The Carb Shop, or Call Davinci.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[techguru]

well, delivery trucks with GM motors and some dodge diesels have a vacuum guage on the air filter box. it records the highest vacuum and sticks so you know when to change the air filter. if you could hook it up to a hose and plug that hose to where you want to take a reading from that may work.
the guages are just plastic with a spring inside, but i think it would be close enough. i think you could get an exact reading off of it by hooking it up to a hand vacuum pump and checking to see how accurate it is, then just add or subtract from your reading.