Just this morning I flowed a stock head from .050 to .420 valve lift on a SF600. Stock numbers are always depressing to me, but at least the ratio is right around the magic .7 (.68 average is what I got both pre and post porting, though I think I would rather see around .75+ for a turbocharged engine).
For this (my own personal development and interest) work, all I worked on was the blending around the seat insert, long turn and bowl area of one intake and one exhaust port. Minimal material was removed, spent MAYBE 5-7 minutes on each port.
From memory all final flow numbers up to .250 lift lost between 3 and 7CFM, but after that they continued to rise to an overall gain of over 10CFM for both intake and exhaust. Keep in mind I did not touch the chambers, port inlet/outlet size any bigger or alter port volume substantially. Looking the data and thinking of stock vs aftermarket cam specs, there is really NO reason at all to lightly port the heads unless you plan on using aftermarket cams with longer duration and additional lift. The additional duration and lift will more than make up for the typical low lift losses you're going to see from basic porting (some of which can be regained through more extensive work which is what I plan to do over time). As I got to thinking about it, this is compounded even more by the timing of the peak pressure wave that is pushed through the port by the piston in mid stroke; you are at the peak airflow volume close to your peak lift and hence airflow numbers. Thus, the pressure drop through the port will be far less during the timing of the greater pressure "front" if you may. This partially negates the theory I had been taught time and time again which is the valve spends more time at lower lifts so more power can be had in the end because there is more airflow to be had with additional time. Actually, you may just end up encouraging backflow/reversion by having better flow numbers towards the lower lifts (and either end of your I & E durations, exhaust especially in turbocharged applications), because it's effectively doing the same thing as changing the cam lobe profile to be much steeper.
Feel free to comment, just some food for thought.
For this (my own personal development and interest) work, all I worked on was the blending around the seat insert, long turn and bowl area of one intake and one exhaust port. Minimal material was removed, spent MAYBE 5-7 minutes on each port.
From memory all final flow numbers up to .250 lift lost between 3 and 7CFM, but after that they continued to rise to an overall gain of over 10CFM for both intake and exhaust. Keep in mind I did not touch the chambers, port inlet/outlet size any bigger or alter port volume substantially. Looking the data and thinking of stock vs aftermarket cam specs, there is really NO reason at all to lightly port the heads unless you plan on using aftermarket cams with longer duration and additional lift. The additional duration and lift will more than make up for the typical low lift losses you're going to see from basic porting (some of which can be regained through more extensive work which is what I plan to do over time). As I got to thinking about it, this is compounded even more by the timing of the peak pressure wave that is pushed through the port by the piston in mid stroke; you are at the peak airflow volume close to your peak lift and hence airflow numbers. Thus, the pressure drop through the port will be far less during the timing of the greater pressure "front" if you may. This partially negates the theory I had been taught time and time again which is the valve spends more time at lower lifts so more power can be had in the end because there is more airflow to be had with additional time. Actually, you may just end up encouraging backflow/reversion by having better flow numbers towards the lower lifts (and either end of your I & E durations, exhaust especially in turbocharged applications), because it's effectively doing the same thing as changing the cam lobe profile to be much steeper.
Feel free to comment, just some food for thought.
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