Airflow to Cubic Inches

[nes4pres]

Is that formula ideal ? Don't know but it seems reasonable.

Definitely there's no ideal air flow just to cubic inches, you have to factor in rpm and volumetric efficiency for what I call dynamic displacement the size of the engine at any given rpm (cfm).

Basically cid x rpm / 3456 x ve% = cfm is what you engine roughly dynamically displaces but there's no easy way to figure needed head flow from that.

Are you trying to figure out head flow for any particular build ?

That’s why porting and flowing a head without porting and flowing the intake manifold is so important. A well ported RPM airgap will only flow 230 ish cfm. Its the total combination. If your port flows well but your air speeds vary too much in different areas of the port, you will lose hp. If you flow well but your port is too big, you will lose port energy and lose hp.

There's a formula cid x rpm x 0.0009785 / amount of cylinders = cfm

So 360 x 5500 x 0.0009785 / 8 = 242 cfm

To clarify. I am going to build a 440. I have a stroker kit to make it 493. JE dome pistons will have CR around 12.5 :1. Looking to produce IN THE RANGE of 675 TQ and 650 HP.

Therefore; 440 x 6200rpm x .0009785/ 8 = 334 cfm

Brodix; B1-BS - 2.20 intake @ 650 flows 295
Brodix; B1-MO - 2.30 intake @ 650 flows 382

Powerport 270; 2.19 intake @ 650 flows 347 cfm

440 SR; 2.19 intake @ 650 flows 360 cfm.

It did occur to me that you could have volume but that doesn't make it efficient. I have heard the B1-MO have a design that increases airspeed in to the cylinder. I do not have an understanding of the other heads (you know jack of all, master of none. Just enough knowledge to get into trouble). With the variances in CFM, relying on manufactures claims, technical aspects of heads, and my lack of experience, what head?

 

[PRH]

JE dome pistons for BBM will fit in the Indy heads, and possibly the TF heads.
Will not fit the others you listed.

Forget the B1 heads for what you’re doing.

SRcnc295’s or the TF 270’s will make the power you’re looking for, assuming the rest of the combo is correct.
Indy EZ295’s would also be fine.

Therefore; 440 x 6200rpm x .0009785/ 8 = 334 cfm

You said you’re building a 493……so the “440” is incorrect.

The real world numbers…….from over 20 years ago…….
14:1 493, std port SR’s flowing 320-ish, TG4500 intake, .660 roller cam.
630tq@4700/692hp@6800
2.16hp/cfm

A few years later, opened heads to MW size for a modest increase in flow(upper 330’s), paired with Indy 440-3 manifold.
Installed top end and cam on 14:1 557” short block.
770hp.
2.26hp/cfm

 

[273]

I wouldn’t shop for heads based on that formula maybe as a guide line, I shop based off known results the heads can produce with the combo you want to run, cid, cam, cr etc..

 

[PRH]

440 x 6200rpm x .0009785/ 8 = 334 cfm

This formula seems to indicate the heads need to be better than what I’ve seen in the real world.
For some of the combos I’ve tried, it’s almost 100cfm off.

 

[273]

This formula seems to indicate the heads need to be better than what I’ve seen in the real world.
For some of the combos I’ve tried, it’s almost 100cfm off.

That formula based on a VE of 125%

 

[PRH]

Well then, I’ll file that under “basically useless/does not apply for most builds”.

 

[nes4pres]

JE dome pistons for BBM will fit in the Indy heads, and possibly the TF heads.
Will not fit the others you listed.

Forget the B1 heads for what you’re doing.

SRcnc295’s or the TF 270’s will make the power you’re looking for, assuming the rest of the combo is correct.
Indy EZ295’s would also be fine.



You said you’re building a 493……so the “440” is incorrect.

The real world numbers…….from over 20 years ago…….
14:1 493, std port SR’s flowing 320-ish, TG4500 intake, .660 roller cam.
630tq@4700/692hp@6800
2.16hp/cfm

A few years later, opened heads to MW size for a modest increase in flow(upper 330’s), paired with Indy 440-3 manifold.
Installed top end and cam on 14:1 557” short block.
770hp.
2.26hp/cfm

 

[nes4pres]

JE dome pistons for BBM will fit in the Indy heads, and possibly the TF heads.
Will not fit the others you listed.

Forget the B1 heads for what you’re doing.

SRcnc295’s or the TF 270’s will make the power you’re looking for, assuming the rest of the combo is correct.
Indy EZ295’s would also be fine.



You said you’re building a 493……so the “440” is incorrect.

The real world numbers…….from over 20 years ago…….
14:1 493, std port SR’s flowing 320-ish, TG4500 intake, .660 roller cam.
630tq@4700/692hp@6800
2.16hp/cfm

A few years later, opened heads to MW size for a modest increase in flow(upper 330’s), paired with Indy 440-3 manifold.
Installed top end and cam on 14:1 557” short block.
770hp.
2.26hp/cfm

Sorry, I errored. 67 cast 440. I have a stroker kit that will make it 493 cubic inches. Therefore 493 x 6200 x .0009785/8 = 374 cfm.

 

[PRH]

Just disregard that formula, it has no relevance to what you’re building.

The EZ295, SR295, TF270 will all easily support your goals.

 

[rumblefish360]

Is there an ideal airflow to cubic inches?

Or is the most airflow a head can flow more desired?

Your question, probably unknown to you, but, is a loaded question. Let me see if this helps in a general fashion. Of course, this general answer may not fit what your thinking since the variables are always moving until a set plan is laid out and executed.

The easier an engine breaths the more power it will make. This works for a factory passenger car designed for grand-ma to go to the grocery store and church with up to top fuel.

No matter the engine size or performance level sought, it’s hard to feed an engine enough air in and out. There is a point where there is to much cylinder head. This is t a fixed size or number. I’d say this would be fairly obvious that a top fuel head on an otherwise factory built engine is too much. But where this balance comes in and where the, “OPS! Went to far!” Is, again, a variable dependent on engine size and performance.

Formulas and the math will point you to a certain criteria to look for in a head. Memeber @PHR has a bunch of B/RB experience. His suggestions are worthy to take note of.

All in all, you probably can’t get to much head on top of a 493 unless you go extreme.

 

[273]

This formula seems to indicate the heads need to be better than what I’ve seen in the real world.
For some of the combos I’ve tried, it’s almost 100cfm off.

Most of the formulas and or rules of thumb got to take with huge grain of salt, just curious what cid and rpm was off by 100 cfm's ?



Just in case your curious, you can modify the formula for any VE%, below
100% VE would be cid x rpm x 0.0007895 / 8 = cfm demand

"Cubic capacity * peak rpm * (variable) / number of cylinders = CFM target at 28” test pressure
The variable is a number that is derived from 1.01055 / fps / 2 * VE
The common variable is 0.0009785, represented by:
1.01055 / 640 / 2 * 1.2394 = 0.0009785
1.01055psi equivalent to 28” water column test pressure
640fps is in between our 2 max airspeed ranges
2 is needed because the intake cycle is every second crank rotation
1.2394 is the VE as a decimal, which is 123.94%

These numbers are used as a base because 640fps is around the average of our target maximum port speeds
and 123.94% VE is used because that is approaching the limit for the majority of naturally aspirated (NA)
engines. Therefore 0.0009785 is seen as a “best case scenario” target for us to work from. Now because the
(variable) is variable we can use it to cross reference future results from dyno testing or even give us an idea
of what the VE or port speeds may be in certain situations, This is going a bit beyond the scope for now but I
will revisit this another time."