Airflow to Cubic Inches

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Using the bore size, the piston speed in fps at every degree through the 180 degree intake stroke, the average area of the intake port (determined by port length and cc volume) and of course the peak rpm of the engine to calculate the average air velocity through the port. The maximum air speed that can be utilized is determined by the intake port design. The higher the velocity up to the maximum the head can handle the better. If the port area is too large for a given engine/rpm range then the engine can't create enough pressure differential to get the air speed up where it needs to be to make good power. Intake ports are designed and/or matched to an engine combination based on air speed not cfm.
 
Hysteric said:
That's a great analogy :thumbsup:



Piston demand and piston speed is what fills the cylinder not the heads ability flow. Like I've pointed out earlier static flow on a flow bench is not representative of what happens in a real running engine.
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If the cylinder head can not provide as much as the piston demands at the optimal piston speed then is it still efficient? And if so, efficient at what?

It all works hand in hand.

Port velocity is a wicked thing until it becomes a choke.
 
74 Cuda said:
Using the bore size, the piston speed in fps at every degree through the 180 degree intake stroke, the average area of the intake port (determined by port length and cc volume) and of course the peak rpm of the engine to calculate the average air velocity through the port. The maximum air speed that can be utilized is determined by the intake port design. The higher the velocity up to the maximum the head can handle the better. If the port area is too large for a given engine/rpm range then the engine can't create enough pressure differential to get the air speed up where it needs to be to make good power. Intake ports are designed and/or matched to an engine combination based on air speed not cfm.
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Great post!
 
If the port is too small (to much air speed) then the engine will just peak at a lower rpm
 
74 Cuda said:
What he's saying is regardless of how much food is on the table a person can only eat so much.
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What I'm saying is a cylinder head upgrade and headers will make an engine more efficient without changing anything else.
 
"Potential" and "lower than it can be" are relative terms. If the ports is too small it peaks earlier. If the port is to big it peaks later so you would have to rev it higher. The issue here is where do you want peak rpm. If you want it to peak at 6k then you would pick a head, cam, manifold etc that will work. If the cam and manifold are sized for 6k but the port isnt then its not going to work quite as well. How well it works or doesnt depends on how far off the mis-match is.
 
74 Cuda said:
"Potential" and "lower than it can be" are relative terms. If the ports is too small it peaks earlier. If the port is to big it peaks later so you would have to rev it higher. The issue here is where do you want peak rpm. If you want it to peak at 6k then you would pick a head, cam, manifold etc that will work. If the cam and manifold are sized for 6k but the port isnt then its not going to work quite as well. How well it works or doesnt depends on how far off the mis-match is.
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I had no problem making a 370 ci engine with a 140cc intake port runner volume reving to 6K+
 
The cam also affects the port velocity. Lets say you're sucking on a straw. Not sure how to word this without sounding dirty. Your mouth would be the piston and the straw the port. Changes in how hard you suck on the straw can be compared to rpm changes. Now your finger tip will be the valve which is controlled by the cam. The faster you move your finger off the straw and the further you move it away from the straw affects the air speed through the straw.
 
74 Cuda said:
The cam also affects the port velocity. Lets say you're sucking on a straw. Not sure how to word this without sounding dirty. Your mouth would be the piston and the straw the port. Changes in how hard you suck on the straw can be compared to rpm changes. Now your finger tip will be the valve which is controlled by the cam. The faster you move your finger off the straw and the further you move it away from the straw affects the air speed through the straw.
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Its amazing what a small 222/234 hydraulic cam can do.
 
Hysteric said:
I had no problem making a 370 ci engine with a 140cc intake port runner volume reving to 6K+
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And I am sure it was very responsive on the pedal and a blast to drive but at the same time, that same head would not work well or be efficient on a 410 stroker at 6500 rpm.
 
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