How do you guys determine what size port volume is ideal for a given combination? What are the biggest factors etc?
Thanks
AL...
How to determine the ideal Cyl Head port volume?
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maxracesoftware
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Velocity FPS & Flow CFMHow do you guys determine what size port volume is ideal for a given combination? What are the biggest factors etc?
Thanks
AL...
Port volume is just a "byproduct" of the correct cross-sectional area
for a given engine CID and desired operating RPM range.
you need Flow CFM to help in achieving your desired RPM range
you need velocity for highest ram-effect and therefore, the highest Ve %
in your desired operating RPM range
Darin's CD available here on SpeedTalk has info on velocity fps numbers
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maxracesoftware
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from another Post ;
equation for determing "Minimum Cross-Sectional Area"
CA = ( Bore * Bore * Stroke * RPM * .00353 ) / FPS
lets say you want to turn a Chevy 355cid 7000 RPM
and most Engines have no more than 2000 RPM power band,
and really more like 1200 to 1500 wide Power Curve
so to work a Chevy 355cid from 5500 to 7000 rpm then;
2.2746 area = (4.030 * 4.030 * 3.480 * 7000 * .00353 ) / 613.98
so for decent Ve % , 300 fps port velocity ,..then
284.3 cfm = 300 * 2.2746 * .4166667
estimated Port Volume CC's = 2.2746 * 16.387 * Port_Length
(no corrections for corner radius included)
if your SBC Chevy head = 5.450 " CenterLine Length
203.14 CC's for 7000 RPM = 2.2746 * 16.387 * 5.450
the 203.14 cc port volume and the 284.3 CFM Flow number would be just a starting point or goal for 5500-7000 RPM for 355cid
to get a better idea, you would run all that thru a Piston-CFM_Flow_Demand program to see how much CFM the Engine is demanding every crankshaft degree or so, then knowing your target velocity fps, you would determine cross-sectional area
after you determined the necessary cross-sectional area,
then you would have to calculate the Tuned Length for the Induction System...and that would be the total Induction Volume
but even after all that, the calculated cross-sectional area won't be accurate enough, it will only be a starting point
the actual Port Shape will actually determine the real velocity profile inside the Port.
rearranging equations around
RPM = ( FPS * CA ) / ( Bore * Bore * Stroke * .00353 )
where;
RPM = point of desired Peak HP
FPS = Feet per Second
CA = Cross-Sectional Area in Square Inches (smallest measured)
FPS = ( Bore * Bore * Stroke * RPM * .00353 ) / CA
CA = ( Bore * Bore * Stroke * RPM * .00353 ) / FPS
Example=> ProStock 500 cid
4.115 Sq.Inch Area = ( 4.687 * 4.687 * 3.62 * 9000 * .00353 ) / 613.9758744
where 613.9758744 = .55 Mach
equation for determing "Minimum Cross-Sectional Area"
CA = ( Bore * Bore * Stroke * RPM * .00353 ) / FPS
lets say you want to turn a Chevy 355cid 7000 RPM
and most Engines have no more than 2000 RPM power band,
and really more like 1200 to 1500 wide Power Curve
so to work a Chevy 355cid from 5500 to 7000 rpm then;
2.2746 area = (4.030 * 4.030 * 3.480 * 7000 * .00353 ) / 613.98
so for decent Ve % , 300 fps port velocity ,..then
284.3 cfm = 300 * 2.2746 * .4166667
estimated Port Volume CC's = 2.2746 * 16.387 * Port_Length
(no corrections for corner radius included)
if your SBC Chevy head = 5.450 " CenterLine Length
203.14 CC's for 7000 RPM = 2.2746 * 16.387 * 5.450
the 203.14 cc port volume and the 284.3 CFM Flow number would be just a starting point or goal for 5500-7000 RPM for 355cid
to get a better idea, you would run all that thru a Piston-CFM_Flow_Demand program to see how much CFM the Engine is demanding every crankshaft degree or so, then knowing your target velocity fps, you would determine cross-sectional area
after you determined the necessary cross-sectional area,
then you would have to calculate the Tuned Length for the Induction System...and that would be the total Induction Volume
but even after all that, the calculated cross-sectional area won't be accurate enough, it will only be a starting point
the actual Port Shape will actually determine the real velocity profile inside the Port.