Head CFM figures to Carb CFM figures?

[stretch]

Doing some research on the net I have found figures that quote that a 48 IDA with a 44mm venturi will flow 311 cfm, or a Dellorto 40 DRLA will flow 266.4 cfm, with various other carb combinations in between.

Looking at the figures that CB quote for their ultra wedge port head, 224 cfm, why would anyone use a carb bigger than a Dell 40?

Or am I massively over simplifying things?

An explanation that's not too technical would be great, I struggle to get my head around complex equations.   

[Erlend / bug66]

The flow numbers from CB are a bit high. Just so you know it.

[stretch]

The flow numbers from CB are a bit high. Just so you know it.

If that's the case then why use any more than a 36mm carb?

There must be more to it than the simple maths that i'm looking at.   

[Bruce]

All flow numbers are meaningless without the depression quoted.

[stretch]

All flow numbers are meaningless without the depression quoted.

I don't know what that means.  Any chance of an explanation please?

[TexasTom]

'Depression' = lift at the valve

[Zach Gomulka]

Depression- how many inches of water. Usually 25" but not always.

[mrmut]

this is my opinion , carbs is chosen according to RPM and CC





Here a difference of 40 and 45
shown here for dcoe but I do not think the is different  IDF and DRLA
I think you will gain representation on the CFM.

[Frallan]

All flow numbers are meaningless without the depression quoted.

I don't know what that means.  Any chance of an explanation please?

Let me try and at the same time ask for better expertise.
The pressure difference in the measured media and in our case it is almost always air.
25" and 28" are very common figures.
Now I have recently seen 5". You can imagine that this is not comparable flows.

Look what I got out of Wikipedia in spite of todays blackout (which I support)
The test pressure manometer is teh one giving the depression in inches ". Or millimeters or other defintion, if someone prefers milliibars. The main thing is to compare apples with apples and not anything else.

[stretch]

Thanks Frallan.

So we need the figures for a head & carbs to be measured at 25" or 28" or even 5" as long as they are both measured at the same scale.

So basically the figures quoted for the carbs are meaningless unless we know the criteria used when testing.

[TexasTom]

So, getting back on topic ... I've searched and cannot find the sources I once knew for this information.
Could have sworn I had see a calculator of some kind for Carb cfm vs. engine size & use. They are All Over the Place for plenum-style manifolds, single & dual plane, but the individual runner manifold info escapes me.
I think the source may be a 'desktop-dyno' type software I have ... somewhere ... unfortunately, it doesn't show HOW the results are calculated.
Certainly, there are those formulas used for venturi sizing ... that's a form  of calculator that can be expanded upon, I suppose. Obviously you can't use a 42mm vent for a 40mm throttle plate.   Then again, this is part of the 'calculation'/choice ...
So, if anyone runs across this information, please pass it along ... love to see the formula and how it is generated.

TxT

[Simpsonshoe]

 

20√ V/1000*N/1000

Where:

 V = the size of one cylinder in cc’s.

 N = the RPM where peak HP is reached.

 

Then, it’s √ of V/1000 X N/1000 *20.

For example, a 3.0L Porsche 911SC would be:

499/1000=.499

5900/1000=5.9

.499*5.9=2.9441

√ of 2.9441=1.715838

Times 20=34.31676 or a 34mm venturi is called for.

 

The above formula is from Bruce Anderson's book, “Porsche 911 Performance Handbook” which is an excellent addition to any Porsche lovers bookshelf.