mcollinstn

port flow at a given vacuum plotted on a lift curve will give you a START point for figuring the rest of your combo.

A high velocity port works for several reasons, but not necessarily for the reasons you might expect.

A high velocity port is more "active" which keeps the fuel atomized better (carb or TBI).

A high velocity port is more responsive to throttle plate changes, since the time it takes a molecule of air/fuel to get from the throttle plate to the combustion chamber is reduced.

A high velocity port dictates that the airflow maintains consistency (which cleans up low rpm performance a lot).

Tuning is a bit more critical with high velocity ports, cause you are dealing with a moving column of air with a higher inertia than a low velocity port. Pressure pulses have more energy which can either help you "more" or hurt you "more" depending on the timing of those pulses.

High velocity ports in and of themselves are not the universal answer, but for a given application with all else being equal, two heads with the same flow characteristics under the lift curve, the better choice will be the one with smaller port cross section (unless it's a drag motor).

As you can tell by some of the other answers, the question presented by this post is not answerable without a ream of supporting info.

mopower

Kaama,
A 540 has approx 68 ci per cylinder(volume) X 2900 rpm=115200 CI per minute going thru each cylinder. Now divide that by 908 to get cubic feet per minute or 117 cfm. If you times this x 8 you will get 936 cfm a rough carb size.
Now you can put heads on that engine that can flow 250 cfm BUT they won't because the engine is only pulling 117 @ 5800rpm. The result is because off the size of the port , the air is moving slower. The smaller port will cause the air to move faster giving it momentum so that for the split second the valve is closed the collum of air still wants to move and will pile up against the valve so when it opens you wind up "overcharging" the cylinder.Whereas a larger port is still only moving at 117 cfm and slower so the momentum is there and you get less of a charge in the cylinder.
I have this exact problem with two 540s right now. The Merlin heads are coming off and smaller Canfield heads are going on. I was looking at 5000/5200 top and they just didn't make the power I wanted. I went down in prop size and ran them 5800/5900 and they loved it and I picked up 5 mph. The higher rpm gave me a high port volocity and a better charge.
There's more to it than just this , port design , valve size , cam lift all come into play.When everything is matched accordingly you can gain HP. A good port should be quiet on the flow bench. A noisey port is cause by turbulance and that screws up flow.
My point is "bigger isn't always better" and there is no easy formula as to runner volume.
Sorry if I made an simple question difficult

johnnyboatman

boy am i lost

mopower

Just remember"If it ain't blown...it sucks"

mcollinstn

umm, you forgot that you got to get that 117cfm rate but you only keep the intake valve open about a THIRD of the time....

Sooo..... if you have an intake duration of 240@050", then that means that the effective "open" time is one third of the 720 degrees necessary to complete a 4 cycle combustion event.

So if it's open a THIRD of the time, then how can a 250cfm port flow 117 cfm average? Wouldn't it flow only 83cfm average? Well, yes, if it were bolted on a flow bench. But it's not. You got a moving column of air with dynamic interia. If tuned properly as far as intake runner length, etc, you can have the column of air all packed and ready to shoot in when the valve opens.

So once again, as you can see, it ain't all that simple.....

SeaRay Jim

LOL In this thread that really makes a statement! Of course it takes all the fun out of air the flow science if you just force induce!

Scary, but it's making some sense to me!

mopower

Searay, It's all physics

Mcollinstn...I knew that , but I figured I hit him with enough to confuse him

Michael1

Kaama,

I would go ahead and throw that formula out, unless you can find the person who created it, and he can tell you what exact engine he used to come up with it. You will not find this in any engineering book, I can tell you that. The port volume is going to change depending on the length of the port, which depends on the head. Right there, this formula is out the window. A better reference, but still empirical, would be port cross-sectional area relative to displaced volume. Even that's pretty bit shaky.

Also is your engine carbureted or injected? It makes a huge difference when it comes to port sizes. An injected engine will be very happy with big ports. With injection, there is no need to concern yourself with fuel coming out of suspension, unless the injector is a long way from the valve.

Unless you have access to a sophisticated computational fluid dynamics computer program, you'll have to rely on the head porters out there to give a well done head. There is no hard and fast answer, because a port has a complex shape.

Michael

mopower

Not to throw another wrench in the gears here but the same applies for exhaust ports and their scavanging effect

tomcat

I was going to stay out of this one...but I have to say one thing. Searay Jim, forced induction doesn't change any of this air flow science, all the same rules apply. The least restrictive/best tuned pathway will always beat a bad one when both are supercharged. In practical terms, supercharging a stock engine delivers wonderful gains without having to buy better heads, cam etc. But if you do both...look out!