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Too much CFM in carburetor?

9K views 14 replies 10 participants last post by  Huskinhano 
#1 ·
What problems are incurred when installing a carburetor with too much CFM? Can anything be done to reduce the CFM of an existing carb?
 
#3 ·
Poor throttle response would be the primary drawback.

Unless the carb is a Demon RS with changeable venturi sleeves the only thing you can do to lower the cfm is use some type of throttle stop or in the case of a 4 barrel carb limit the opening of the secondary butterflies or disconnect the secondaries altogether.

The best advice is to use the proper carb for the application.
 
#4 ·
Poor throttle response would be the primary drawback.

Unless the carb is a Demon RS with changeable venturi sleeves the only thing you can do to lower the cfm.....
".
Some Weber carbs, like the IDA series have replaceable venturies as well. Without that option, if the carb is too large the air speed is going to be so slow that the fuel falls out of suspension = poor throttle response as TommyK points out.


Z
 
#6 ·
Here's what I have: A 260 with a C9OZ-6250-C cam 2 1/4" dual exhaust and an Edelbrock dual plane (?) intake. It had an Autolite 4100 and am thinking about a newer 600 CFM carb. Is that suitable?
 
#10 ·
Probably not as suitable as 480 CFM 4100 for a stockish street driven car but lots of folks are happily running 600CFM carburetors and like them fine. Edelbrock makes a 500 CFM carburetor that works VERY well.
As GypsyR Points out, yes it would work, but not as well as a carb that is closer to the Ford carb's 480 cfm. People are generally much much happier with the better low and midrange throttle response of the 480 cfm carb vs. a slightly better high rpm performance you'd get with a 600 cfm carb.

Just think about what type driving you will do. If most of time the engine will be used under 4,500 rpm, as most street cars are, then do not be tempted to go with anything larger than 500 cfm. Again, in my view, if you don't think your current carb is repairable, then replace it with a good Autolite 4100. The other choices are not an upgrade.

Z
 
#7 ·
Probably not as suitable as 480 CFM 4100 for a stockish street driven car but lots of folks are happily running 600CFM carburetors and like them fine. Edelbrock makes a 500 CFM carburetor that works VERY well.
 
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#9 · (Edited)
arbys,
Carburetors work off of vacuum which all engines create on the intake stroke. Venturis ( also called chokes in Webers) are necked down areas above the butterflies to speed up air passing through it to allow fuel to be "siphoned" out of the fuel supply tubes (boosters in the center of the venturi). A carburetor that is "too big" will still ONLY FLOW as much air as the engine vacuum/flow being made, same for the smaller carb. Where problems crop up is the calibration in a "big" carburetor is designed for MORE air flow in order to be efficient , so when less air is flowing , it doesn't "pull" fuel into the engine like the carburetor was calibrated for and performance "can" suffer. Changes to the engine (cams, big heads , compression and stroker cranks) can cause the need for a larger carburetor and have a big affect on performance. Proper "sizing" makes the engine operate more efficiently and more enjoyable to drive.
Randy
 
#13 ·
most street driven cars you see out there over over carb'd and run fine

the question is by how much, there is a limit.
 
#15 ·
IMO another very important consideration of a carburetor is the type of venturi booster. You hear guys recommending the Autolite 4100, Edelbrock or the Summit because they all have annular boosters which are larger and more precisely meter and atomize the fuel mixture far better in most conditions then a straight or down leg. The annular gives sharper throttle response and better fuel economy. The straight and down leg are better at high speeds where the throttle is around wide open.
 
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