Airflow to Cubic Inches

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nes4pres

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Is there an ideal airflow to cubic inches?

Or is the most airflow a head can flow more desired?
 
nes4pres said:
Is there an ideal airflow to cubic inches?

Or is the most airflow a head can flow more desired?
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There's a formula cid x rpm x 0.0009785 / amount of cylinders = cfm

So 360 x 5500 x 0.0009785 / 8 = 242 cfm
 
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 ?
 
Its a deep subject. You want the most cfm flowing through a port of the smallest size to maximize port energy. Optimal air speed is 300. Learn alot on YouTube at David Vizard’s and Charles Servedio’s channels.
 
ch1ll said:
Its a deep subject. You want the most cfm flowing through a port of the smallest size to maximize port energy. Optimal air speed is 300. Learn alot on YouTube at David Vizard’s and Charles Servedio’s channels.
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I wouldn't necessarily say smallest but the right size for the application.
 
For a well thought out combination and ported heads, figure 2.4 horsepower per cfm of the port. Your major restriction in the head is the valve area and then the intake manifold.
 
273 said:
I wouldn't necessarily say smallest but the right size for the application.
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Right, alot of truth to that. Like I said it’s a deep subject. Even top head porters have a difference of opinion.
 
ch1ll said:
For a well thought out combination and ported heads, figure 2.4 horsepower per cfm of the port. Your major restriction in the head is the valve area and then the intake manifold.
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Depends what level of builds were talking about here but most average guy builds generally don't even get over 2 hp per cfm.
 
You basically got a handful of head choices so unless you build an engine to take full advantage of one the available heads you looking at a bunch of compromises.
 
273 said:
Depends what level of builds were talking about here but most average guy builds generally don't even get over 2 hp per cfm.
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Yeah, it has to be well thought out, purpose specific. Sometimes alot of $$$ and experience. Alot of guys don’t have that and I’m not sure I even do. Parts testing, dyno pulls and experimentation limited by whats available for your platform of choice. No one but you can build the engine you really want but you and it’s a long road.
 
fishmens67 said:
Most are lucky to get 1.25 hp and very few achieve 2 hp per
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If I was to build a fairly serious street strip engine I'd shoot for 1.8 cfm per hp as a goal any hp above that be a bonus. Eg.. 500 hp / 1.8 = 278 cfm so I'd would want a head that flowed at least 280 cfm around .500-.600" lift to try make 500 hp. (Obviously with a decent overall curve .300-.600".)

If looking for something more mild 1.6 hp per cfm.
 
fishmens67 said:
Most are lucky to get 1.25 hp and very few achieve 2 hp per
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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.
 
Oldmanmopar said:
Wonder why they put 1200 cfm six packs for 340's and 850 TQ's on 340's at the factory? I use 750 carters on all mild 340's .
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The 6 pack if rated like a 4 bbl is more like a 950, they probably could get away with it easier with both of those because most of the % of cfm (less restriction) came in only when kick down. Plus I don't think engines are as sensitive to cfm (less restriction) as some make them out to be, I wonder what vacuum level they pull at full throttle ?
 
ch1ll said:
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.
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You may like this

 
Here is the head flow on a 345 inch LS that I’m going to drop in my beater scamp…these are home ported cathedral…I don’t think it will be making no 2hp per CFM…

IMG_2926.png
 
IMG_9441.jpeg


I would like to put these X’s on a 3.79 340 for my Duster.
I think it could do it
 
In very general terms, my experience wrt performance oriented American v8’s is……… the lower the head flow numbers are, the easier it is to exceed the typical flow based “predictions”(limitations).

300cfm heads on a 9:1 318……..not easy to make 600+hp.
However, 300hp from 150cfm heads on the same combo…….not nearly as hard.
 
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273 said:
You may like this

Click to expand...

Thank you, I missed this one. I’m a 1/3 through this(had to rewatch some of it) and I won’t comment too much on it till I watch the whole thing. Darren Morgan is well respected by the porters on you tube I follow, seems like his content I have found is limited. Maybe I just haven’t been looking in the right places. Everything I have alluded to in my posts on this thread he has stated so far as I have gotten in his conversation except he is talking about single plane manifolds and on this thread I have been talking about dual plane manifolds.

I’m going to have to watch this 4-5 times and I’ll still have boatloads of questions. His depth is the epitome of deep.

Thanks again for posting this.
 
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