Selecting carburator size

[sjrf]

Is there any simple way to select carburator size. Most non-american manufacturers do not have a CFM rating. The only recomendation I have found is in an old book (1940's), that recomended a mean air speed of 400 feet/sec. thru the venturi (4 cyl., 6 cyl. add 10% venturi area). Will this get me in the ball-park?

 

[stoveguy]

This should give you a ballpark idea, from

http://www.holley.com

http://www.holley.com/HiOctn/TechServ/TechInfo/P224A.html

 

[sjrf]

Thank's, but that was not what I was looking for. I might not have been to clear in my question. I want to now what venturi size to use in a non CFM rated carburator.

 

[patprimmer]

It really depends on a largely on several factors.

These include:-

Number of cylinders drawing on the venturi or venturis in question.
Speed range, capacity and intended use of the engine.
Manifold design.
Cylinder head design.
Cam timing.
Carby design.

As a real rough rule of thumb, use the Holley chart, then calculate the cross sectional area of the venturi of the Holley you choose. You will find some variables from Holley to Holley re their CFM rating vs venturi cross sectional area.

The larger the plenum, the larger the carby seems to act.

The lower the number of cylinders drawing, the smaller the carby seems to act, and it's not a linear progression.

The straighter and more downhill the route from the fuel discharge nozzle to the back of the inlet valve, the more forgiving the engine will be to over carburation.

The warmer the manifold, the more forgiving.

Constant depression or vacuum secondary carbys are more forgiving.

Float chamber, booster or secondary venturi and emulsion tube design also have an impact, as does idle and transition circuit design.

Regards
pat pprimmer@acay.com.au
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