Trivia question that I’ve yet to have anyone be able to answer (including myself):

What are the CFM ratings (Cubic Feet per Minute) on the following Weber IMB carburetors:
- 24
- 26
- 28
????

My suspicions are that the model numbers correspond either with bite diameter and/or CFM (e.g. 24mm/240 CFM) or....I could be completely off base and those numbers were assigned similar to the “WD-40” history....(Water Displacement - 40th formula).... Weber 24th model....

Hoping to learn something new....

Naples500L said:

Trivia question that I’ve yet to have anyone be able to answer (including myself):

What are the CFM ratings (Cubic Feet per Minute) on the following Weber IMB carburetors:
- 24
- 26
- 28
????

My suspicions are that the model numbers correspond either with bite diameter and/or CFM (e.g. 24mm/240 CFM) or....I could be completely off base and those numbers were assigned similar to the “WD-40” history....(Water Displacement - 40th formula).... Weber 24th model....

Hoping to learn something new....

Click to expand...

the 24/26/28 numbering system is basically the "Barrel" size of the carb
the CFM of any carb will be based on the Venturi size fitted within this barrel...

so a 28 IMB will flow the same as a 28DCD (obviously this can be varied by the Venturi used...) hence why it is not always the best thing to fit a huge carb as you end up reducing the air flow with a smaller venturi

a 28 carb will flow exactly the same volume of air as a 28mm copper pipe, as basic physics means with all things equal you cannot push or pull more air through the same size orifice...
(obviously there are slight variations .. Polishing, smooth openings etc etc... but then things would not be equal!)

Agreed and understand the physics but Weber ultimately established a max airflow capacity for each combination (just like Holley or any other manufacturer)....which in turn can be used to induction modification calculations.

No doubt that the 24-28 come no where close to achieving maximum capacity with a stock 500....but what if I add a roots AISN-300 supercharger? Can they manage the pull? CFM plus boost at 4400 rpm is part of the formula (it’s a little more complicated )for that calculation.

“Determining Blower Carb Sizes

Dual Carbs are recommended for most blower applications. Choosing the correct size carb is essential for the proper operation of the blown motor. The following formula below will help you determine the correct size Carbs for any blown application.

[(DxR)/3456] x [(B/14.7)+1]=C

D = Cubic inch displacement of the motor

R = Maximum engine RPM

B = Maximum blower boost

C = Maximum CFM required”

Ultimately....the question is:

Can a 24-28 handle 70cfm (based on the calculation above?

Thoughts?

..from basics:

6.23 UK gallons of petrol weighs 21.21 kg.

1kg petrol requires 14.7 kg air to burn perfectly.

14.7 kg air = 5,902 cubic feet

You'll get about 332 miles from that amount of petrol

At 50mph that will take 6.64 hours

That's 888 cfh or

aproximately 15 cfm....


I stand to be shot down...

Naples500L said:

....................................but what if I add a roots AISN-300 supercharger? Can they manage the pull? CFM plus boost at 4400 rpm is part of the formula (it’s a little more complicated )for that calculation.

“Determining Blower Carb Sizes

Dual Carbs are recommended for most blower applications.
Thoughts?

Click to expand...

I was talking to a guy ages ago .. he worked for BMW.. on the design team when they were working out supercharging the MINI...
I said "what sort of carb would you need if you used that (pointing at the Eaton Blower on the bench) on a normal engine...
He just looked at me .. and said "carb??? my dad used to tell me stories about them"

fiat500 said:

..from basics:

6.23 UK gallons of petrol weighs 21.21 kg.

1kg petrol requires 14.7 kg air to burn perfectly.

14.7 kg air = 5,902 cubic feet

You'll get about 332 miles from that amount of petrol

At 50mph that will take 6.64 hours

That's 888 cfh or

aproximately 15 cfm....


I stand to be shot down...

Click to expand...

what's that in MPG?

Believe me....if I had the patience to retrofit a Harley fuel injection setup on to this little fella....I’d do it. And who knows maybe I’ll be inspired later....but for now supercharging 650cc is enough to satisfy this phase of the midlife crisis....

fiat500 said:

..from basics:

6.23 UK gallons of petrol weighs 21.21 kg.

1kg petrol requires 14.7 kg air to burn perfectly.

14.7 kg air = 5,902 cubic feet

You'll get about 332 miles from that amount of petrol

At 50mph that will take 6.64 hours

That's 888 cfh or

aproximately 15 cfm....


I stand to be shot down...

Click to expand...

I love it when you mix your units - makes me all misty eyed ....

Just don't do it if you're trying to land on, say, Mars

Chris

Bambino said:

I love it when you mix your units - makes me all misty eyed ....

Just don't do it if you're trying to land on, say, Mars

Chris

Click to expand...

Me and my mates at NASA manage OK like this:


"NASA has ostensibly used the metric system since about 1990, the statement said, but English units are still employed on some missions, and a few projects use both. NASA uses both English and metric aboard the International Space Station."

But it does sometimes have its problems as you imply:

"The dual strategy led to the loss of the Mars Climate Orbiter robotic probe in 1999; a contractor provided thruster firing data in English units while NASA was calculating in metric."

https://www.space.com/3332-nasa-finally-metric.html

Bigvtwin996 said:

what's that in MPG?

Click to expand...

.......53.3mpg but this webpage suggests you can get 68.9mpg "extra-urban".

https://www.auto-data.net/en/fiat-126-650-24hp-6829

In theory, the total volume of air consumed will not vary because the same amount of fuel is eventually burned. But obviously the volume of air per minute will be reduced or increased according to how economically you drive.

Does it really matter??? Just bloody drive it.

Naples500L said:

Believe me....if I had the patience to retrofit a Harley fuel injection setup on to this little fella....I’d do it. And who knows maybe I’ll be inspired later....but for now supercharging 650cc is enough to satisfy this phase of the midlife crisis....

Click to expand...

Why hack a bike system when there are off the shelf EFI systems available for small twin cylinders..
Lots of bikes in the States have to retro fit EFI to get round the emissions....

Personally I cannot see the point of turbo or supercharging a 500.. the basic engine design is it's simplicity and bang per buck the tuning that most people do is probably more cost effective.. if you are aiming for even more power.. then what about everything else...


But if it is an itch that you have to scratch...

but as I always say each to their own.. your car...

Bambino said:

I love it when you mix your units - makes me all misty eyed ....

........

Click to expand...

You can't beat a good mixing of units..
and in the UK we can easily cope...
after all we have been doing it for years..
Metric engine sizing but imperial speeds and fuel
(it may say Litres at the pumps but we all still talk gallons)
I'm just off to buy some lengths of 2x4 and a couple of sheets of 8x4
despite the fact the merchants call them 40x90 and 2440mm x 1220mm

Bigvtwin996 said:

I'm just off to buy some lengths of 2x4 and a couple of sheets of 8x4
despite the fact the merchants call them 40x90 and 2440mm x 1220mm

Click to expand...

When you screw it together use either your 13mm (33/64") or for the larger bolts the 15mm (19/32"). For the smaller bolts you can use your 10mm socket (25/64") ???

Chris

PS: Oz has been completely metric since July 1974 and we've never crashed anything on another planet

Bigvtwin996 said:

I was talking to a guy ages ago .. he worked for BMW.. on the design team when they were working out supercharging the MINI...
I said "what sort of carb would you need if you used that (pointing at the Eaton Blower on the bench) on a normal engine...
He just looked at me .. and said "carb??? my dad used to tell me stories about them"

Click to expand...

Bigvtwin996 said:

-------------------------------
-------------------------------
-----------------

a 28 carb will flow exactly the same volume of air as a 28mm copper pipe, as basic physics means with all things equal you cannot push or pull more air through the same size orifice...
(obviously there are slight variations .. Polishing, smooth openings etc etc... but then things would not be equal!)

Click to expand...

.

But won't pressure difference between the 2 ends of the pipe/carb barrel affect the maximum flow rate?

In terms of how much air is drawn into an engine per revolution:-

On a naturally aspirated engine you're looking at the number of pistons moving downwards on the intake stroke per revolution (not the engine displacement!), which is then reduced by the volumetric efficiency of this engine - basically, due to various restrictions, port size, fluid friction, time lag/inertia of the fluid (air), throttle restriction etc., no engine draws in 100% of it's cylinder capacity, the real world figure might be say, max. 90% efficient. When people enlarge/smooth ports, fit larger valves etc, they're trying to improve the volumetric efficiency of their engine. Improving the V.E. means more air is drawn in, and therefore a corresponding increase of fuel can be drawn in and more power results (hopefully).

On a 'blown' i.e. turbocharged or supercharged engine, the air is pressurised by the turbo (driven by exhaust gases) or the supercharger (driven by the engine but there's some modern electrically driven ones nowadays).
Essentially this is trying to increase the effective V. E. of the engine. This always sounds like a 'free' power increase.
But both cost money to fit, require modifications, can affect reliability, shorten engine life, increase fuel consumption. etc.

Superchargers also have the downside of using some engine power to drive them. They often require the compression ratio to be lowered, (reducing the V. E. of the engine), they heat the intake, reducing potential power gains, unless an efficient heat exchanger is fitted. (not so easy to do on a 500).

I've sometimes wondered about the volume of air being pumped by the cooling fan around the engine under the cowling in order to cool the engine.
If this air was to be collected, channelled through a heat exchanger and then fed to the carb. intake, would this not be a sort of 'free' supercharging?

Reminds me of my little Honda motorcycle in the 1970's. I had the bright idea of fitting a 180 degree elbow to my carb intake and pointing it forward into the air stream. I'm sure that bike went much faster as a result. :devil:

Al.

Poiseuille's Law relates the laminar flow rate of an incompressible fluid through a tube directly to the pressure drop across the tube ends, the fourth power of the radius of the tube and inversely to the viscosity of the fluid and the length of the tube.

Whilst air is compressible and the flow may not be strictly laminar, the principles still apply.

So to maximise the CFM choose a short, smooth and wide tube and then raise the air pressure at the input end of the tube.

For a naturally aspirated engine, widening the diameter of the tube (eg: using larger auxiliary ventures for example) will result in larger air flows for a given pressure drop. There are however many other considerations when fitting larger carbies.

CFM is also dependent of ambient air pressure and if you drive at altitude, the air flow will decrease.

This all sounds logical and I hope my physics is OK
Feel free to correct any errors.


Chris