So through a stock 30mm Concentric it's about 143cfm @ .385" and about the same @.350".
Mark and Dave - How do you match up this flow rate to the actual needs of an engine and at what point do you determine that a larger carburetor is needed? I apologize if it’s an ‘idiot’ question, I am just trying to learn here.
If you consider a Lightning 650cc twin engine, running at 6000 rpm. That is 2x 325cc cylinders at 3000 intake cycles per minute but as each cylinder has a carb, you only need to consider one cylinder.
A 325cc cylinder is 19.83 cubic inches and at 3000 intake strokes per minute it needs (19.83*3000)= 59490 cubic inches of fuel/air mix per minute. There are 1728 cubic inches in a cubic foot, so that cylinder needs 34.43 cubic feet per minute.
What’s the relationship between the 34.43 cubic feet per minute the cylinder needs and the flow rate of 143 cfm at 28” of mercury you mention?
If that cylinder is breathing through a 30mm Concentric and the bore of the Concentric works out to 1.18 square inches, then at 6000 rpm the 19.83 cubic inches is inducted in (60/(6000x2)) seconds, or 1/200 of a second.
The speed through the choke is then (19.83/1.18)x(200/1)= 3361 inches per second or (/12) 280 feet per second.
Granted, that’s the average speed. Because the intake stroke is one half of a revolution, the peak speed is likely to be a pulse, lasting 1/400th of a second at (2x280)= 560 feet per second.
Given that air/fuel mix has mass, the energy needed to accelerate the mix into the cylinder (via the vacuum created by the descending piston on the intake stoke) is energy robbed from the output of the engine. However, because of that mass, there must be momentum at play here causing the mix to 'pressurize' around the back of the valve as it closes.
I am not well read on this, but the only place I have seen a discussion about the maximum air flow through a carburetor is in Phil Irving’s book ‘Tuning for Speed’. I have the 6th edition and this is discussed on page 14. Phil is calculating the intake speed of a 350cc single at 7200 rpm breathing through a 1 3/8” carb. I guess it’s an AJS 7R. Phil calculates that to be about 290 cfm (I stole his math for above). Phil thinks that about 300 cfm through the carb is about optimum for max power but goes on to say that there is no hard or fast rule. This is an old book, so might be way off now.
That said, if this was a 750cc Lightning, then the flow through the 30mm Concentrics isn’t much more at the same rpm. The A70 used the same 30mm units that the 650 did.
When and why would you consider fitting bigger carburetors to your custom engines?
Given that a 650 Thunderbolt is breathing both cylinders through a smaller carb (28mm) and so at least doubling the air speed through that one carb, does carb size really make much of a difference?
Thanks for getting this far..