David Vizard - Which to prioritize - PORT FLOW or PORT VELOCITY?
From Chris Speier site
This is probably the most asked question I get. What size head do I need?? While head CC's are a by-product of a properly sized cylinder head, there are mathematical formulas to lead you to the correct answer. There are many reasons that choosing a cylinder head based off port volume is a bad idea, simply put there are 100 ways to arrive at a port volume!
First off, CC's or port volume measurements are “close guesses” to a ports average air speed only. It gives no insight what so ever into the ports velocity profile. CC's came about in the 60's when the heads where to small so the larger the port, the more power one could make. Head porters still use CC's as a quick dirty guess as the average air speed in the port, that’s all.
With three known things about any cylinder head, we can arrive at port volumes. Knowing this along with a program such as Pipemax, we are able to choose cylinder heads in a way they that is proper and logical. It's all about average velocity in the induction, 260fps is a great starting point!
Most cylinder head porters are working off a desired MACH speed of the port. 1116 feet per second = speed of sound @STP or .55 MACH. You can design ports slower than .55 MACH, but it's usually considered the speed at which "choke" or "port limiting velocity" occurs in a running engine.
Now let's turn your MIN CSA into a CC's for choosing a balanced cylinder head. Remember from above, I said you needed to know port volume and average runner length! Now let's put those into out formula:
The key to any cylinder head is a balanced port. We are trying to balance it around a MIN CSA to achieve proper filling. SRH designs and ports based off velocity profiles. Although formulas are nice for "ballpark" sizing, localized and average velocity is KING!
I get this question PM'd to me all the time so I though I would take some time and show you how it's really all about AVERAGE AIRSPEED in the induction.
Two totally different cylinder heads, different flow curves, different port volumes, but yet the same average airspeed.
I'm going to use my CNC heads for the examples (because I know they are correct FPS vs AREA vs CFM)
Head #1. v2.70PF.. Flows [email protected].. 250cc port.. 5.47 long
Head #2 v2.50PF.. Flows [email protected].. 227cc port.. 5.45 long
Manifold Holley 300-110 in AS_CAST form.. short runner=4.625, long runner=5.500
short runner pours 215cc.. long runner pours 240cc
FORMULAS:
Average CSA = Port Volume CC/ (Port Centerline Length * 16.387)
FPS= (Flow CFM * 2.4) / Average CSA
Head #1 flows 320cfm through manifold and carb
Head #2 flows 305cfm through manifold and carb
Head #1= 465cc short runner.. 490cc long runner
Head #2= 442cc short runner.. 467cc long runner
Head #1= 10.08 short runner.. 10.95 long runner
Head #2= 10.10 short runner.. 10.98 long runner
Head #1= 10.08 x 16.387 = 165.2 / 465cc = 2.81 avg CSA short runner
10.95 x 16.387 = 179.4 / 490cc = 2.73 avg CSA long runner
Head #2= 10.10 x 16.387 = 165.5 / 442cc= 2.67 avg CSA short runner
10.98 x 16.387 = 180 / 467cc= 2.59 avg CSA long runner
Head #1= 320 x 2.4 = 768 / 2.81 = 273 FPS AVG VELOCITY short runner
320 x 2.4 = 768 / 2.73 = 281 FPS AVG VELOCITY lon runner
Head #2= 305 x 2.4 = 732 / 2.67 = 274 FPS AVERAGE VELOCITY short runner
305 x 2.4 = 732 / 2.59 = 283 FPS AVERAGE VELOCITY long runner
Induction Formulas and Helpful Sites
From Chris Speier site
This is probably the most asked question I get. What size head do I need?? While head CC's are a by-product of a properly sized cylinder head, there are mathematical formulas to lead you to the correct answer. There are many reasons that choosing a cylinder head based off port volume is a bad idea, simply put there are 100 ways to arrive at a port volume!
First off, CC's or port volume measurements are “close guesses” to a ports average air speed only. It gives no insight what so ever into the ports velocity profile. CC's came about in the 60's when the heads where to small so the larger the port, the more power one could make. Head porters still use CC's as a quick dirty guess as the average air speed in the port, that’s all.
With three known things about any cylinder head, we can arrive at port volumes. Knowing this along with a program such as Pipemax, we are able to choose cylinder heads in a way they that is proper and logical. It's all about average velocity in the induction, 260fps is a great starting point!
Most cylinder head porters are working off a desired MACH speed of the port. 1116 feet per second = speed of sound @STP or .55 MACH. You can design ports slower than .55 MACH, but it's usually considered the speed at which "choke" or "port limiting velocity" occurs in a running engine.
- 613.8 fps = .55 MACH
- "average" intake port length (roof length + floor length from seat ring to opening) divide by 2
- port volume cc's
- MIN CSA = (bore x bore x stroke x RPM x .00353) / 613.8 (.55 MACH x 1116 fps)
Now let's turn your MIN CSA into a CC's for choosing a balanced cylinder head. Remember from above, I said you needed to know port volume and average runner length! Now let's put those into out formula:
- Port Volume CC's = MIN CSA x Port Length x 16.387
The key to any cylinder head is a balanced port. We are trying to balance it around a MIN CSA to achieve proper filling. SRH designs and ports based off velocity profiles. Although formulas are nice for "ballpark" sizing, localized and average velocity is KING!
I get this question PM'd to me all the time so I though I would take some time and show you how it's really all about AVERAGE AIRSPEED in the induction.
Two totally different cylinder heads, different flow curves, different port volumes, but yet the same average airspeed.
I'm going to use my CNC heads for the examples (because I know they are correct FPS vs AREA vs CFM)
Head #1. v2.70PF.. Flows [email protected].. 250cc port.. 5.47 long
Head #2 v2.50PF.. Flows [email protected].. 227cc port.. 5.45 long
Manifold Holley 300-110 in AS_CAST form.. short runner=4.625, long runner=5.500
short runner pours 215cc.. long runner pours 240cc
FORMULAS:
Average CSA = Port Volume CC/ (Port Centerline Length * 16.387)
FPS= (Flow CFM * 2.4) / Average CSA
Head #1 flows 320cfm through manifold and carb
Head #2 flows 305cfm through manifold and carb
Head #1= 465cc short runner.. 490cc long runner
Head #2= 442cc short runner.. 467cc long runner
Head #1= 10.08 short runner.. 10.95 long runner
Head #2= 10.10 short runner.. 10.98 long runner
Head #1= 10.08 x 16.387 = 165.2 / 465cc = 2.81 avg CSA short runner
10.95 x 16.387 = 179.4 / 490cc = 2.73 avg CSA long runner
Head #2= 10.10 x 16.387 = 165.5 / 442cc= 2.67 avg CSA short runner
10.98 x 16.387 = 180 / 467cc= 2.59 avg CSA long runner
Head #1= 320 x 2.4 = 768 / 2.81 = 273 FPS AVG VELOCITY short runner
320 x 2.4 = 768 / 2.73 = 281 FPS AVG VELOCITY lon runner
Head #2= 305 x 2.4 = 732 / 2.67 = 274 FPS AVERAGE VELOCITY short runner
305 x 2.4 = 732 / 2.59 = 283 FPS AVERAGE VELOCITY long runner
Induction Formulas and Helpful Sites
- Average_CSA = Port_Volume_CC / (Port_CenterLine_Length * 16.387)
- MIN CSA = (Bore x Bore x Stroke x RPM x .00353) / 613.8 (.55 MACH)
- Port_Volume_CC = Average_CSA * Port_CenterLine_Length * 16.387
- Port_CenterLine_Length = Port_Volume_CC / ( Average_CSA *16.387 )
- FPS = ( Flow_CFM * 2.4 ) / Average_CSA
- Flow_CFM = Average_CSA * FPS * .4166667
- Average_CSA = ( Flow_CFM * 2.4) / FPS
- cfm demand = cid x rpm x .0009785 / # of cylinders