Ross B
13-02-2007, 16:08
Seeing as there are many more big power CA/SR engines being built I thought I would copy from paper to e-paper about how to estimate the necessary port size to obtain a certain amount of flow through the head ports.
This is all reworded plagiarisation (reworded for copyright purposes) so please feel free to argue if I'm wrong - no offence will be taken.
Anyway, to the practical.
If your working on the head and you've ported it and still require more flow which even a larger valve can't provide then you may well have reached the maximum flow the port can supply. You can also factor in the diameter of the valve stem as a reducer in flow. Certain aftermarket valves have a thinner stem allowing a little more flow around it.
If we assume 95% efficiency, we can calculate the flow ability of the ports using flowbench mathematics.
cfm = A x 0.95 x 213685.34
Where A is the area of the port in square meters. Use the narrowest point for this calculation. For example,
32mm diameter port, area is 0.000804mē
Therefore -
cfm = 0.000804 x 0.95 x 213865.34
= 163.3cfm
Or, including a valve stem into the equation (for this example, the stem will be 8mm in diameter),
Port Area - Valve Area = 0.000804 - 0.000050
= 0.000754mē
Therefore -
cfm = 0.000754 x 0.95 x 213685.34
= 153cfm
This shows that 153cfm is a (rough) guide as to how much the port will flow at any given time. You could put a huge valve onto this port, but due to the port shape/size, you would not get more than 153cfm through it.
You can apply the same science to the inlet side of the head also. If you know how much flow is needed, you can calculate the necessary size of the port. Sometimes working backwards this way can save time in the future. For example, if you want a 1000bhp CA, you can fairly quickly ascertain whether the CA head will be up to the job :wack:
As said above, the valve stem takes up space that could be occupied by air. If we wanted to know how much space would be required over a normal/ported head to compensate for the space taken up by the stem we can follow these calculations.
cfm = A x 0.95 x 213685.34 - rearranges to give:-
_______cfm_______ = Area
0.95 x 213685.34
If we use the previous example and say that the valve/port flows 153cfm and we want 163.3cfm, we need the difference in flow between the two values (163.3 - 153cfm) plus the current flow ((163.3 - 153) + 163.3).
Therefore -
______173.6______ = Area
0.95 x 213685.34
= 0.000855mē
To convert this to a useful number we use this equation to convert to metres.
Dia. in metres = 2√((0.000955 x 4) / 3.14)
= 0.03299 x 1000
= 32.99 diameter
Well I hope this proves helpful to someone. If you have any questions please feel free to PM me or email me and I'll try and rip off the book a little bit more for the answers.
I can honestly say I haven't needed to do this myself yet, so I won't even pretend I understand half of it, but it's better said than not.
Enjoy :thumbs:
This is all reworded plagiarisation (reworded for copyright purposes) so please feel free to argue if I'm wrong - no offence will be taken.
Anyway, to the practical.
If your working on the head and you've ported it and still require more flow which even a larger valve can't provide then you may well have reached the maximum flow the port can supply. You can also factor in the diameter of the valve stem as a reducer in flow. Certain aftermarket valves have a thinner stem allowing a little more flow around it.
If we assume 95% efficiency, we can calculate the flow ability of the ports using flowbench mathematics.
cfm = A x 0.95 x 213685.34
Where A is the area of the port in square meters. Use the narrowest point for this calculation. For example,
32mm diameter port, area is 0.000804mē
Therefore -
cfm = 0.000804 x 0.95 x 213865.34
= 163.3cfm
Or, including a valve stem into the equation (for this example, the stem will be 8mm in diameter),
Port Area - Valve Area = 0.000804 - 0.000050
= 0.000754mē
Therefore -
cfm = 0.000754 x 0.95 x 213685.34
= 153cfm
This shows that 153cfm is a (rough) guide as to how much the port will flow at any given time. You could put a huge valve onto this port, but due to the port shape/size, you would not get more than 153cfm through it.
You can apply the same science to the inlet side of the head also. If you know how much flow is needed, you can calculate the necessary size of the port. Sometimes working backwards this way can save time in the future. For example, if you want a 1000bhp CA, you can fairly quickly ascertain whether the CA head will be up to the job :wack:
As said above, the valve stem takes up space that could be occupied by air. If we wanted to know how much space would be required over a normal/ported head to compensate for the space taken up by the stem we can follow these calculations.
cfm = A x 0.95 x 213685.34 - rearranges to give:-
_______cfm_______ = Area
0.95 x 213685.34
If we use the previous example and say that the valve/port flows 153cfm and we want 163.3cfm, we need the difference in flow between the two values (163.3 - 153cfm) plus the current flow ((163.3 - 153) + 163.3).
Therefore -
______173.6______ = Area
0.95 x 213685.34
= 0.000855mē
To convert this to a useful number we use this equation to convert to metres.
Dia. in metres = 2√((0.000955 x 4) / 3.14)
= 0.03299 x 1000
= 32.99 diameter
Well I hope this proves helpful to someone. If you have any questions please feel free to PM me or email me and I'll try and rip off the book a little bit more for the answers.
I can honestly say I haven't needed to do this myself yet, so I won't even pretend I understand half of it, but it's better said than not.
Enjoy :thumbs: