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I kind of struggled naming this post, but finally decided on the title as it is probably the main point of this post. There is a book that I reference throughout this post, Engine Airflow by Harold Bettes, it is almost mandatory reading if one is accepting the challenges of learning how to port cylinder heads, and I give my highest recommendation for anyone modifying any engine. Where to start… After head disassembly, cleaning and inspection processes, you will need to measure and graph the ports and combustion chambers. To accomplish this task, a quality set of micrometers, dividers and calipers are required, along with a small level, and a cc’ing burette. There are plenty of options to use that help speed up this process like pouring a silicon rubber mold of the port, cutting the mold into sections and measuring the cross-sectional areas (CSA), or using the cc’ing method.
I use SolidWorks, a CAD program to model the ports and to accurately determine CSAs of the ports.
I also use Microsoft Excel to rapidly calculate the below example equations.
Any method that you choose however should be an accurate and repeatable process. You will need to gather the following information to calculate your first equation(s):
Bore Diameter (in inches)
Stroke (in inches)
Intended Maximum RPM Determine the Piston Speed (ref1) using this equation:
Ps=(S x rpm)÷6
Where Ps= Piston speed in feet per minute
S= stroke in inches
Rpm= Engine speed in Revolutions per minute (rate)
The VG30DETT would be:
Ps=(3.27 x 8000)÷6
Ps=4360 fpm After you have measured and documented the unmodified ports and chambers, you will need to determine the port velocities, using this equation:
Pvel=(PS÷60) x (B2÷Ap)
Where Pvel = Port Velocity in feet per second
Ps= Piston Speed in feet per minute
B = Bore Diameter in inches (squared)
Ap=area of port in square inches
Using a simplified, unmodified VG30DETT intake port CSA (the fuel injection “nozzle” dimension is omitted for sake of simplicity in this example) the port entrance measures 1.870” wide and 1.0625” tall, the fwd and aft ends of the port entrance are full radius, so the simplified port entrance area = 1.8136 in2.
So Pvel=(4360÷60) x (3.432÷1.8136)=471.392 fps or 321 mph or 0.43 mach Run this equation for all of your CSAs and document the results. You should have a CSA for any significant change in cross-sectional area or every .25” (whichever is greater).
Note on your port map(s) and pay particular attention to any CSA Pvel that is equal to or greater than 0.55 mach.
Why 0.55 mach? Experience shows that any area of a port that is equal to or exceeds this Mach number, the airflow cannot remain laminar and becomes turbulent.
For areas in the port for a 4 valve head down-stream of the divider wall, you may assume (at this point) that flow is divided equally between the two valves per port, and divide the bore diameter by 2 when running the above equation. After you have calculated the port velocities, you will need to study the combustion chambers and valve throat areas of the ports. Measure and Map out the chambers paying particular attention to anything or feature within 0.125” of the valve diameter. This would include the chamber walls, and plug bosses.
These features “shroud” the opening and create restrictions (restricting /obstructing flow).
Remember, air behaves like any other fluid, and will take the path of least resistance. If something is close enough to the valve to obstruct the flow, the air will flow around the blockage (if it has an easier path).
Determine the height of the obstructions, and determine in valve head angle percentage the amount of obstruction.
Any shrouding (or obstruction) has the effect of using a smaller valve or valve lift, which dramatically affects flow volumes and velocities.
The modified equation is:
Pvel=(PS÷60) x ((B2÷((Ap x (1-% VS))
Where Pvel = Port Velocity in feet per second
Ps= Piston Speed in feet per minute
B = Bore Diameter in inches (squared)
Ap=area of port in square inches
VS= Percentage of Port obstruction
As in our simplified example above lets assume that 33% of the port has shrouding or other obstruction combined total.
Pvel=(4360÷60) x ((3.432÷((1.8136 x(1-0.33))=703.570 fps or 480 mph or 0.65 mach Additionally, you are able to determine the flow velocity over the valve and through the valve opening by modifying the port velocity equation as follows:
VPvel=(PS÷60) x (B2÷Avo )
Where VPvel =Valve Port Velocity in feet per second
Ps= Piston Speed in feet per minute
B = Bore Diameter in inches (squared)
Avo=area of Valve opening in square inches (Valve Diameter x Pi) x Lift
The VG30 Nominal Valve diameter is 1.343” and we will use 10 mm for this example
So Avo=(1.343 x 3.1416) x 0.3937
Avo= 1.661 in2 x 2 valves
Plugging this area into our equation:
VPvel=(4360÷60) x (3.432÷3.322)=257.350 fps or 175 mph or 0.24 mach Re-Running this equation at different engine rpms and valve lifts will give you the data and trends to verify during baseline and modified port testing.
By modifying the above equation to determine the affects of shrouding within the combustion chamber, we determine the % of valve shrouding as in the earlier example and subtract this from the valve opening area as follows:
VPvel=(PS÷60) x ((B2÷((Avo x(1-%VS ))
Where VPvel =Valve Port Velocity in feet per second
Ps= Piston Speed in feet per minute
B = Bore Diameter in inches (squared)
Avo=area of Valve opening in square inches (Valve Diameter x Pi)x Lift
Multiply the Valve opening area by the number of valves per port (2)
VS= Percentage of Valve Head obstruction
VPvel=(4360÷60) x ((3.432÷((3.322 x(1-0.33 ))=384.104 fps or 262 mph or 0.35 mach By modifying the Port Velocity equation, you are able to determine exactly what affect this has. After you have documented the port and valve opening velocities on your port maps, now comes the time to partially assemble the head and prepare it for Baseline flow testing. During this test, you are establishing what the unmodified head flows (cfm, velocities, etc.) throughout the entire valve lift sequence, and verifying (using your port maps) the calculated port velocities, using velocity probes, flow balls and flow wands, clay etc.
Check, plot and measure the flow boundary layer on your Port Maps. The wands and flow balls will help you establish the boundary layer thickness, and alert you to any areas that contain turbulent flow. Note that up to this point, you will have accumulated a substantial amount of time, and you haven’t even touched your die grinder yet! However, the above process is the foundation for doing this job correctly. Be patient, observant and stay vigilant during these initial processes. By following the above procedure, you are now able to determine the affects of valve shrouding, O/S valves, port matching etc. and will serve as a “road map” in determining modifications to the head.
Porting cylinder heads is often referred to as an “art”, for those who do not understand the mathematical principles, and how vitally important it is to understand and apply the above equations, it is often referred to as a “Black Art”. Whether you are contemplating purchasing a set of ported heads, or if you have been porting heads for years, I hope you will find this information useful for you, and provides some insight as to why purchasing a set of ported cylinder heads is expensive. References:
Engine Airflow by Harold Bettes
Ref1: pg 11 Piston Speed equation
Ref2: pg 14 Port Velocity equation
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