Amal carbs CFM

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.

powerdoc said:

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.

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I don't and am definitely not an expert but can offer this.

At WOT, the main jet provides x cc/minute of fuel assuming the cfm will suck it. x is the size. So a 1972 750 Combat at sea level will have a correct A/F mixture with its 230 main jet (they were originally 220 but upped later to 230). So, if you know the fuel cc/minute and, A/F ratio and rpms, you can determine the cfm actually moved.

IMHO, it's nothing like a Holley 4 barrel on a 289 - you have one carb sucking at a time on a British twin with dual carbs for relatively long time until the next suction. On a single carb, it's still a relatively only time (but half of dual) until the next suction.

The cfm that could but put through the carb(s) has little to do with how hard/often the cylinder sucks and therefore the cfm that is actually moved through the carb. I'm sure the potential cfm would me many times what is actually moved.

I do not have the answer you seek, but I might be able to get you in the ballpark. I have been hunting these numbers for years and they are hard to come by. I have recently flowed a Mikuni VM32 and it is roughly 150 CFM at 28" on my buddies flow bench. I suspect the 32mm Amal concentric might flow a little better out of the box, that said I would rather have the Mikuni.

I’m sure Jim Comstock did this, and that the finding was that they’re good, but I can’t find the post…

For laminar flow, the flow rate is given by the formula,

Q ft^3/sec = Pi r^2 uavg ft/ sec
Where uavg = average mean flow velocity., and r is radius of carb throat in ft.

The flow velocity u, is maximum on the flow centerline, and is = 2 x uavg,
The maximum velocity on the centerline is limited by the sonic velocity Us, where
Us = 49 T^1/2 where T is absolute temperature of air flowing thru the carb, or 530 for 70 F air.
This makes Us = to 1127 ft/sec (approx).

Putting it all together, for a 30mm bore carb (1.5 cm radius), Q = 3.14 x (1.5 cm/ 30.48 cm/ft)^2 x 1127/2 = 4.28 ft^3/sec = 256 ft^3 / min.

This is the flow where "sonic choking" begins to throttle the flow. It is a theoretical upper limit and does not predict actual flow in the real case.

Slick

When I once suggested that gas in inlets and exhausts were sonic, I think Nigel denied that. I suggest the whole system resonates when the engine is running. I once found a helpful research scientist who was using a similar computer to the one I had in my job. He ran the transonic wind tunnel in the Aeronautical Research Laboratories at Fisherman's Bend in Melbourne. What you get under sonic conditions can be very different to normal flow, Sudden changes in port diameters can cause bigger effects than one might expect. In my ports, all changes in diameter are as gentle as possible - both exhaust and inlet.

Do you realise the factor which has the biggest effect on the noise of a two-stroke is the angle at the top edge of the exhaust port. Some idiots have it at right angles to the bore.

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

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?

Mart UK said:

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?

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Each revolution of the engine draws in and expells the displacement rating of the engine. A 5 liter V8 displaces 5 liters, not 5/8 liters.

Air fuel ratio is defined by mass, or weight, not volume.

Slick

texasSlick said:

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

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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:

texasSlick said:

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

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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.
When you have a single carb on a Y manifold, the gas in the carb and the bit before the Y must resonate at twice the frequency of the gas inside one of the ports. However in effect, the flow is almost continuous, but the pressure reinforcement from resonance is probably less. Single carb might need a different exhaust system.

maaseyracer said:

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

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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.

My feeling is that the single carb would experience a higher vacuum, which affects the needle jet. The needle jet cannot supply twice as much fuel so the mixture becomes leaner than it would be with twin carbs, so the bike would be faster. But by the same token, the exhaust system also affects the jetting.

I stand corrected. It requires two revolutions of a four cycle engine to sweep the displacement, as acotrel, maaseyracer, and Matt UK have alluded to. An oversight on my part, or is age to blame?
Sorry if I caused any confusion.

As to why the single carb does not produce the power at high rpm, I suspect the Y manifold dissipates much of the energy in the flow in the manner explained by acotrel, reply #12.

Slick

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.

acotrel said:

When I once suggested that gas in inlets and exhausts were sonic, I think Nigel denied that.

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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).

WZ507 said:

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).

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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.

At an ambient temperature of 40F, the sonic speed is 1095 ft/sec, less than twice the velocities you quote. Thus a local sonic condition is possible even though average or mean velocity is well below sonic.

It is interesting to note that no amount of external pressure "push" , such as from a supercharger, can overcome the choking effect of the sonic shock wave. The shock wave gets stronger in response to the pressure applied .

Slick

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.

maaseyracer said:

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.

Click to expand...

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.

I’d interpret your example as passing about 55 cfm at 7500 rpm (25.5*3750/1728 = 55). I suspect by now you've found your factor of 2X and how it came about.

Achieving volumetric efficiencies exceeding 1.0 are very challenging to obtain, so unless you undertake some extraordinary measures, they’ll be difficult to come by. As I mentioned previously, the old sages that make large valve ported CDO heads all seem to settle out on airflow just shy of 90 cfm @ 10” WC. If you get significantly more airflow than that, you’ve really accomplished something.

Heading off on a tangent ever so peripherally related to extraordinary airflow reminds of the late Ken Augustine, and his general response to compliments about remarkable heads he created (Harley XR 750, and many more) which was usually "I got in there and shined it up a little". The translation of which was he had spent 100's of hours welding, machining, resphering the combustion chamber, relocating the ports and guides, altering the rocker ratio, and machining through critical support structure along the way. That was "shining it up a little".

Anyway, good luck with your porting project and I wish you results beyond your wildest expectations.

WOW! I really opened up a can of worms. It seems like we still don't have the answer to why dual carbs will pull better at high rpm's by the mathematical calculations unless it's the design of
the manifold, it seems.