CFM rating and pressure drop

[Harte3]

Being mostly technically challenged I am attempting to understand carb cfm ratings.

4v carbs are rated by the factory at 1.5 pressure drop. Is there a definitive relationship/point between this and manifold vacuum? For example, when cruising down the freeway and the vacuum guage is showing 15 hg, at what point, if any, will the guage read when a 1.5 pressure drop has occured when applying pressure on the throttle and manifold vacuum drops...even pegging the guage at zero momentarily at WOT and slow speeds?

I have been through Vizard's discussion of pressure drop v cfm a number of times but somewhere I think am missing something or trying to read something into it that isn't there?

 

[80broncoman]

That 1.5 pressure drop just happens to be where they measure the flow of a 4bbl carb.
To make things even more interesting, a 2bbl is measured at a differnt pressure drop. So a 500CFM 2bbl will not flow as much as a 500cfm 4bbl.
A holley 4412 500cfm 2bb is exactly the same asthe front half of a 3310 750cfm 4bbl.
When measured the same way as a 4bbl the 4412 flows 356cfm IIRC.

 

[wallaka]

Broncoman is right; it is just a standardized point at which to measure. Like the "at .050 lift" with cams. It helps keep the manufacturers honest, otherwise they would be measuring at much higher pressure levels (which would give more flow).

 

[Harte3]

Thank you. And I understand it is an arbitrary point determined by the factory. My question is...is there an actual point where a 1.5 pressure drop occurs that is measurable with a vacuum guage reading or am I pursuing a mythological phantom? Or, how does one determine if and/or where the rated flow of the carb occures if it can be determined at all?

 

[Lazy JW]

I think of it like this: if you have a big enough engine and it is running fast enough at WOT at some rpm it will start to "pull" vacuum in the manifold because the engine wants more air than the carb can deliver. When it gets to 1.5" hg then a 4 barrel carb rated at 450 cfm would actually be flowing the full 450 cfm.

At slower speeds but still WOT (think heavy truck going up hill) the vacuum will drop to zero and actual cfm flow will be something less than rated cfm because the engine simply cannot ingest as much, therefore the manifold is completely "full" of air (almost)

At even higher speeds (still WOT) the engine wants more air but the carb cannot deliver enough (too restrictive) to fill the manifold so even more vacuum "builds up" (larger pressure drop) which will actually cause more than rated CFM to pass through the carb (due to the higher pressure difference). This is the point at which a larger carb can help.

Remember, there is always one "atmosphere" of pressure pushing, pushing, pushing, constantly trying to get in to any empty spot we create. The piston moves down creating an empty spot and this 14.7 psi (approximately) is always at the ready to fill it up as long as we open the throttle enough to let it in (if the carb is big enough).
Joe

 

[gb500]

hmmm - the rating of a carb is at the pressure drop ACROSS the carb ie inlet to outlet of carb and measured with a differential pressure guage.


this 1.5 " is as stated before is a std measuring point - so sure if your engine can pull more than a 1.5 " pressure drop ACROSS the carb then it will flow more than rated CFM




the manifold vacuum can be more than the actual carb vaccuum

and depending on where it is measured ie tapping point the manifold vaccuum will be a summation of the pressure drops for the aircleaner/intake piping /carb/manifold bends/ manifold runners and anything else that is in the inlet to the engine.

brett
( who may or may not have been an HVAC commissioning eng)

 

[XPC66]

The vacuum in the plenum is essentially the drop across carburettor assembly. The upstream stuff like filter and snorkel are fairly minor.

Carbs are rated as either 3"HG or 1.5"HG at WOT by convention. The carburation is pretty much independent of manifold vacuum, because the bowls are held at the same pressure as the horns and the venturies are interested in velocity pressure.


The 2V carbs are generally matched to small intake valves while 4V is matched to larger intake valves. The idea is to maintain authority over the flow throughout the driveability range. The 2V is invariably a "3" HG" jobbie, which results in a poorer VE because of the increased restriction, but it isn't simply a matter of saying it will cut flow by 1- SQRT(1.5/3) = 30%, because the 4V and the 2V are matched to different flowing heads.

The question as I understand it, is what the reference flow is for the various Ford engines. This would be easy if they just provided a capacity index, but of course they don't. My further understanding is, the flow that corellates to the pressure drop selection is based on maximum power. So if your engine is 200kW peak power with a 2V carb, you would expect a pressure drop across the WOT of 3"Hg @ ~ 424 CFM. There really isn't any sense to select it at any other point because the flows drop off either side.

 

[Harte3]

Thanks to you all. What I have concluded is that the actual cfm flow of the carb cannot be determined at any particular point of engine operation simply using a vacuum guage.

 

[XPC66]

Not unless you want to plot the the pressure drops versus flow at various throttle positions. Of course speed density algorithms on fuel injected engines use/used manifold pressure and rpm with secondary reset via throttle position.... works well with turbochargers.

 

[Harte3]

I think of it like this: if you have a big enough engine and it is running fast enough at WOT at some rpm it will start to "pull" vacuum in the manifold because the engine wants more air than the carb can deliver. When it gets to 1.5" hg then a 4 barrel carb rated at 450 cfm would actually be flowing the full 450 cfm.

That is what I was leaning towards thinking...that when a vac guage shows 1.5 hg this is where the rated flow of the carb would play. BUT, when I think of pressure drop I ask "drop from where?" The 14.7 atmospheric? The drop from say a 18 hg vac reading at idle? Plus there is the conversion factor to psi in the manifold depending on the "vacuum" (which is still pressure...just less than atmospheric.) And VE is part of the equation too.

I think when I retire I will go back to school and get a late education in math and physics...just for my own amusement

 

[XPC66]

It's the drop between atmosphere and the manifold. 14.7 psig tells you it's gauge pressure to atmosphere reference. The 1.5"HG drop equates to ~0.74 psi drop or 14.69 - 0.74 = 13.95 psia (psi absolute).

The 18"Hg (5.85 psia) is there because the throttle plate is closed placing a restriction to flow. The engine itself can only pull a vacuum within the constraints of ring seal, valve seal, etc and most importantly the inlet valve closing angle.

 

[Lazy JW]

Thanks to you all. What I have concluded is that the actual cfm flow of the carb cannot be determined at any particular point of engine operation simply using a vacuum guage.

Yup. Lots of things come into play here. Barometric pressure, temperature, humidity, etc.

The main concept is that the carb must be at WOT and you have to be pulling the air away from the carb's outlet fast enough to create the 1.5" of pressure drop, then have a flow meter in line that will measure CFM.

I reckon they test these things on a flow bench with a BIG air pump.

Seems like I recall someone saying that One-barrel carbs are tested at 6" hg or thereabouts.
Joe