Port square inches



Ok, I feel like a moron having to ask this, but does anybody know how to find the area in square inches of a port? Specifically, a 1 1/2" round one. I am trying to determine if I will have any loss of velocity going from a 1 1/2" runner to the intake port on a 300-6. I believe the intake port is around 1.2 square inches, so I would guess that anything over that would be acceptable.

A 300 with round intake ports? Mine are rectangular with big radii at the corners. Anyway for round ports:

A=.7854 x D^2

Where A = area of a circle
and D = port diameter

The port area should be the same as the flow area of the valve which is given by:

3.14159 x D x L

Where L is lift at valve and D is the valve diameter

It should also be the same as the cross-sectional area of the flow bowl which is given by:

A = .7854 x ( D^2 - d^2)

Where D is the bowl diameter and d is the valve stem diameter.

I've got all this set up in a spreadsheet, so if you give me your valve diameter and lift, it will spit out all the relevant numbers.
Well, what I am really talking about is going from a round upper intake runner, to the rectangular port on a 300-6, using an EFI lower intake. I am using 1.5" stainless tubing for the upper, and want to know if the air is going to slow down as it enters the port, or if it will speed up. Speeding up is ok, slowing down is not. I want velocity, because velocity=swirl=better combustion. Also, faster airflow across the injector makes for better atomization of the fuel.

Any change in area is bad. If you increase area, the velocity obviously drops but if you decrease area and increase velocity you decrease pressure which reduces cylinder filling. You want to get to a constant cross section and maintain it as nearly as possible throughout the length of the intake tract.

Ricardo's experiments showed that volumetric efficiency was maximized at a mean intake velocity of 140 FPS and remained acceptable within a range of 40-240 FPS.
On a stock 300 with 1.78" intake valves and .398 lift working at 70% VE, the 140 FPS figure comes at 2908 RPM with the 40-240 range being 831-4985 RPM. Change to the C***y 1.94/1.60 valves and the VE peak is at 3530 PM and the range becomes 1009-6052 with a required valve lift of no less than .435".

The runner inside diameters which correspond to these numbers are 1.60" for the stock valves and 1.77" for the bigger C***y valves. IIRC the runners in the stock EFI manifold are 1.62" I suspect your 1.50" OD tube (which is only going to have on ID of approx. 1.38") is going to be way too small and will end up strangling the engine.
Good stuff, SR... 8)

BTW, your equation above for the "flow area of the valve" appears to describe the volume of a cylinder, in which it should read 3.14159 x R^2 x L, right?
Nope. The flow area of the valve is the surface area of a cylinder defined by the valve circumference and it's lift. That's the area that the incoming mixture flows through to get into the cylinder. FWIW Ricardo gives a rule of thumb that lift should be at least one-fourth of the valve diameter which works out nicely if you ignore the area of the valve stem and the difference in diameter between the valve itself and the bowl. I had too much time on my hands one afternoon so I loaded all his stuff into a spreadsheet.

I sometimes wonder how I do anything without loading it into a spreadsheet. :roll:
Okay, that makes sense... :oops:

Can ya PM me the spreadsheet? :D 8) :roll: