Airflow to Cubic Inches
- Thread starter nes4pres
- Start date
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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 ?
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.
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.
Depends what level of builds were talking about here but most average guy builds generally don't even get over 2 hp per cfm.
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.
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.
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 .
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.
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 ?
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