Thread: Carb Sizes

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An X cubic inch motor will pull in X cubic feet per minute of air at X rpm's. It doesn't matter the size of the venturis or throttle bore, except to say that assuming the carb is large enough to provide X cfm, the smallest venturi will more properly meter the mixture because it is getting a stronger signal than a larger venturi. (same AMOUNT of air going through both, but the smaller venturi requires more VELOCITY to pass the same X amount of air).

When I encounter mind problems such as this, I take it to the extreme each way. Let's say the motor had a small 2-bbl on it that provided decent power, good mileage and good throttle response. You find a carb off a B-29 bomber that's about the size of a TV set and decide to adapt it to your world beater. It would easily meet the requirement for cfm of your X motor, but the venturis would be so big and the velocity of the slug of air going through them would be so slow that a proper air signal couldn't be established and I doubt you could get the motor started, much less have any kind of throttle response.

On the flip side, you decide to adapt the carb off your Briggs Rototiller onto your world beater. I suspect you could get the motor started with it, but little more than that. A 5-horse Briggs passes 14 cfm @3,600 rpm's. A 2-litre Pinto motor passes 17 cfm while idling at 500 rpm's. The tiny little venturi would be inadequate to provide enough air flow to feed a motor much larger than the Briggs. It would be like you running a marathon while breathing through a straw. Ain't gonna happen.

In my opinion, the smaller of 2 different carbs would provide a little better mileage simply due to the fact that the velocity would be higher through the venturis and the mixing would therefore be more complete and more precise.

For a real world example, look no further than the Rochester Quadrajet. When the GM engineers designed this carb (in my humble opinion, the very finest automotive carburetor ever built), they incorporated very tiny little primary venturis, just barely large enough to feed the motor through slow speed driving and small enough to provide excellent velocity, signal and mixing. They were much smaller than any of the 2-bbl venturis being produced at the time. To provide power, the Q-Jet used sewer-pipe sized secondary venturis. The best of both worlds. Ask anyone who has properly set up a Q-Jet and run it. Best gas mileage of any....ANY carb out there as long as you could keep your foot out of the secondaries.