Power Limiting Port Area

Written by: Neil Erickson aka Maxflow (Edits by tmoss)

In the small block ford world there is an amazing array of cylinder heads and bottom end displacements that can be put under them. One often-overlooked parameter is port area. This is important because of the medium we are passing through our cylinder heads - air. Air has mass and weighs .076 LB/cu ft. Each time the valve closes the air in the intake port is forced to stop. And conversely each time the valve opens it takes energy to accelerate it into the cylinder. This start and stop of the air stream creates an "inertial block" and sets the upper peak rpm limit of the engine. In a book published by the MIT press "The Internal Combustion Engine" studies suggest that if port velocities anywhere in the system exceed .55-.60 the speed of sound a limiting condition exists. What I am saying here is that port velocity is good, but too much will limit power - completely independent of CFM numbers.

So there does exist a minimal port area for each combination that meets your target for peak rpm. With almost every cylinder head (or intake) this power limiting port area is the constriction between the push rods (or the most limiting intake runner cross section). To make a long story short a formula exists to help you get the correct port area, or at least evaluate a cylinder head (or intake) for your combination.

To calculate the limiting port velocity: LPV=(.00353*RPM*S*B2)/CA Where:

S = stroke (in)
B = bore (in)
CA = minimum port cross sectional area in sq./in.’s
RPM = peak power rpm
LPV = limiting port velocity

For peak power at a target RPM the minimum port cross-sectional area can be calculated: CA = (.00353*RPM*S*B2)/690

Stock lower CA for 6250 RPM = (.00353*6250*3.00*4.002)/690 = 1.53 sqin

Stock lower sqin @ restricted head transition = .875x1.6875 = 1.476 sqin = ~1.2" round hole

Ported stock lower CA for 7000 RPM = (.00353*7000*3.00*4.002)/690 = 1.72 sqin

My ported stock lower = 1.125x1.875 = 2.11 sqin = ~1.5" round hole

Stock upper runner cross section = 1.7 sqin

Cobra = 1.625" round hole = 2.07 sqin

For the maximum RPM that a particular set of heads is worth: RPM = (CA*Kn)/(S*B2) Where:

Kn = Constant 184136 for endurance race roller cam
Kn = Constant 195558 for pro-stock type roller cam
Kn = Constant 177780 for flat tappet cams

An example would be a typical 5.0 engine with a mildly modified set of GT-40P heads. The intake ports measure 1.885 by 1.002 and equate to 1.889 sq./in.’s of port area. The equation would be:
RPM = (1.889* 177780)/(3.00*4.002) = 6,996

For evaluating intakes:

Stock lower RPM = (1.476*177780)/(3.00*4.002) = 5,466

Ported stock lower with stock upper (limiting cross section) RPM = (1.7*177780)/(3.00*4.002) = 6,296

Cobra Intake RPM = (2.07*177780)/(3.00*4.002) = 7,666

Put these same heads on a 392 and you get:

RPM = (1.889*177780)/(3.85*4.002) = 5451

Even if the port was super efficient and flowed 280 CFM on the 392 this head would kill power above 5451 rpm. Too often I see high rpm big inch small blocks with insufficient area or a 302 with way too much.