I don't and am definitely not an expert but can offer this.Does anyone know what the stock Concentric 1's CFM is? I've read lots of discussions of the 1 vs 2 carb solutions and I was just curious as the the maximum flow. If you buy a Holley for your Mustang that is about the first spec you see.
Isn't only one piston on intake, the other firing, per revolution, so 750/2?
As that is air and fuel mix, say at 14:1, that suggests (183/2)/15 = 6 cubic feet of fuel per minute. That's clearly rubbish. Where have I gone wrong?
While the above post (reply #5) calculates the flow POSSIBLE thru a 30 mm carb, it is illustrative to estimate the flow NECESSARY.
Assuming a 750 cc engine has a 100% efficient aspiration rate (that is, it draws 750 cc per revolution over its entire operating range), then the flow necessary is 0.75 liters x 60 cubic inches /liter 1/12^3 inches/ft x 7000 rpm = 183 cfm.
Thus, at maximum rpm, the 30 mm carb is only operating at 183/256 x 100% =71% of its theoretical CFM capacity
Slick
I like your calculation. But a 750cc Norton engine only draws 375cc per revolution. Mass transfer has both flow and pressure considerations. When a motor is running - the exhaust and inlet tracts are in harmony. When both valves are open at TDC the column of gas through inlet tract, through combustion chamber and the exhaust become one. The pulses in the gas column can interfere with each other, and they can complement each other, depending on tuned lengths and frequencies. It is not like water running down a pipe.While the above post (reply #5) calculates the flow POSSIBLE thru a 30 mm carb, it is illustrative to estimate the flow NECESSARY.
Assuming a 750 cc engine has a 100% efficient aspiration rate (that is, it draws 750 cc per revolution over its entire operating range), then the flow necessary is 0.75 liters x 60 cubic inches /liter 1/12^3 inches/ft x 7000 rpm = 183 cfm.
Thus, at maximum rpm, the 30 mm carb is only operating at 183/256 x 100% =71% of its theoretical CFM capacity
Slick
Where the motor makes it's power in the rev range determines the gearing which needs to be used. If there is a net loss, I would not use single carb.Based on your math I made a Google sheet to run this, however it does not take into account that the common single carb manifold does not make power over 6000 rpm. I think there is a bit of refinement to do to this sheet to help size carbs to port velocity, but thank you for starting me down this path.
This calculator might make this a bit more interesting as well:
Flowrate Calculation for an Orifice Flowmeter
www.efunda.com
And of course Nigel was correct. The charge in the inlet port can't move faster than the maximum instantaneous velocity of the piston, ratioed to the difference in area between the bore and the port. A stock 850 Commando running at 7000 rpm has a maximum instantaneous piston velocity of ~ 112 ft/sec occurring ~ 74 deg ATDC. The bore/port area ratio, with a 32 mm port, is 5.18. So the port can't flow faster than 112 ft/sec * 5.18 = 580 ft/sec, which is barely half way to being sonic, so coming up way short. The 580 ft/sec port velocity confirms that the engine is sized reasonably, since volumetric efficiency drops off significantly at instantaneous maximum port speeds exceeding Mach 0.6 (~ 660 ft/sec).When I once suggested that gas in inlets and exhausts were sonic, I think Nigel denied that.
Your analysis is correct when the velocities you quote are average velocities. If the flow is laminar, streamline velocities near the centerline are increased up to twice average velocity. Thus LOCAL streamline velocity may hit the sonic value, particularly with low ambient temperature. A shock wave results which prevents the local streamline velocity from exceeding sonic, and this shock wave chokes or throttles the flow, significantly dropping volumetric efficiency as you state.A perfectly streamlined 32 mm orifice will flow ~ 110 cfm @ 10” WC. A 32 mm Amal carburetor is not a perfectly streamlined orifice, so will flow somewhat less. From the reams of airflow data posted by Comnoz here on the forum, a stock 850 Commando flows ~ 75 cfm @ 10” WC, thus the 32 mm carb should support that airflow. The best big-valve ported RH10 heads flowed in the range of ~ 87-90 cfm @ 10” WC. In such applications the 32 mm Amal is likely getting on the small size. This is why owners with 850’s in a high state of tune (airflow, cam, compression, pipe) step up in carb size, e.g., opt for a set of 35 mm Keihin FCR carbs.
And of course Nigel was correct. The charge in the inlet port can't move faster than the maximum instantaneous velocity of the piston, ratioed to the difference in area between the bore and the port. A stock 850 Commando running at 7000 rpm has a maximum instantaneous piston velocity of ~ 112 ft/sec occurring ~ 74 deg ATDC. The bore/port area ratio, with a 32 mm port, is 5.18. So the port can't flow faster than 112 ft/sec * 5.18 = 580 ft/sec, which is barely half way to being sonic, so coming up way short. The 580 ft/sec port velocity confirms that the engine is sized reasonably, since volumetric efficiency drops off significantly at instantaneous maximum port speeds exceeding Mach 0.6 (~ 660 ft/sec).
When talking airflow results in the context of Commandos, we are generally talking about airflow of a single cylinder, not the entire engine. For your calculation you should use 25.5 cu inch for the displacement of a single cylinder, then realize that in 7500 strokes there are only 3750 induction strokes, which will result in considerably lower airflow.If the past I have calculated CFM of an engine by taking the displacement of the engine lets say an 850cc or 51ci, multiply it by peak power RPM (or peak desired rpm), the multiplying by engine efficiency, 100 to 120% ish. Then dividing it by the constant 3456 which is converting from inches 12 inches in a foot, so 12^3 1728, the part I do not understand is that I have always been tol you need to mutiply 12^3 by 2 to get 3456.
So my engine is (51ci *7500 peak RPM*100%)/3456 = 110cfm
If i get my porting sorted... I can maybe see getting upward around 120% efficiency, which would be (51ci *7500 peak RPM*120%)/3456 = 132cfm.
Compression, port shape, and a number of things are going to change things from here, also I suspect there is a pressure differential from carb entry to exit.
Also I am not an engineer, I am very happy to be corrected and once corrected, I would be happy to share that Google sheet with anyone if they want to plug in their engine.