Air Flow 101 - applicable to engines
03-23-2002, 09:51 PM
#21
Okay, now that everyone has the basics under their belt, let's put this knowledge to use and get some perspective on air flow and pressure loss in cylinder heads compared to other "pipes".
For this comparison I will use actual data for three BBC cylinder heads (good, better and best) measured out of the box on a flow bench at 0.600" lift and 28" H2O, with 2.25" intake valves and ~320cc port volume. To get some perspective on these numbers we will compare the performance of these ports to that of a sharp elbow and an ideal bell mouth inlet with the same cross sectional area as the valve window (~2.3 square inches), and the same 28 "H2O of pressure difference applied.
Head........Flow....Velocity.....Ce.......VP.....L oss........K..
GM401......310.....10,526......0.50.....6.9...21.1 ......3.05
DartPro1...325.....11,035......0.52.....7.6...20.4 ......2.69
AFRCNC.....375.....12,732......0.60...10.1...17.9. .....1.77
Elbow........395.....13,412......0.63...11.2....16 .8.....1.50
Bell............612.....20,768......0.98...26.9... ..1.1......0.04
The velocity is the air speed (in fpm) through the valve window (very high). Ce is the coefficient of entry; when it's equal to 1.00, the conversion of static pressure to velocity pressure (flow) is perfect (no resistance due to turbulence). This is impossible, but a bell mouth inlet comes damn close at 0.98. You can see that VP and Loss always add up to 28 "H2O, the flow bench test pressure. See how small the resistance loss is for the bell mouth. That's why its K is so low. K is the number we are trying to reduce when we port heads.
A K factor of 1.5 is about as bad as we get when we design industrial ventilation systems. A sharp, square elbow has this K factor and therefore uses up about 60% of the available pressure difference to overcome losses (at this very high velocity). You can see that the new Air Flow Research CNC heads, as good as they are, do not flow as well as the worst element encountered in industrial ventilation.
This is not a criticism of head manufacturers or head porters. It is the nature of the beast. These AFR heads flow almost as well as a fully (hand) ported head (without increasing port volume). Further improvements may come with raised ports, different valve angles and better chambers. But as long as poppet valves are in the flow path, this is what we have to work with.
For this comparison I will use actual data for three BBC cylinder heads (good, better and best) measured out of the box on a flow bench at 0.600" lift and 28" H2O, with 2.25" intake valves and ~320cc port volume. To get some perspective on these numbers we will compare the performance of these ports to that of a sharp elbow and an ideal bell mouth inlet with the same cross sectional area as the valve window (~2.3 square inches), and the same 28 "H2O of pressure difference applied.
Head........Flow....Velocity.....Ce.......VP.....L oss........K..
GM401......310.....10,526......0.50.....6.9...21.1 ......3.05
DartPro1...325.....11,035......0.52.....7.6...20.4 ......2.69
AFRCNC.....375.....12,732......0.60...10.1...17.9. .....1.77
Elbow........395.....13,412......0.63...11.2....16 .8.....1.50
Bell............612.....20,768......0.98...26.9... ..1.1......0.04
The velocity is the air speed (in fpm) through the valve window (very high). Ce is the coefficient of entry; when it's equal to 1.00, the conversion of static pressure to velocity pressure (flow) is perfect (no resistance due to turbulence). This is impossible, but a bell mouth inlet comes damn close at 0.98. You can see that VP and Loss always add up to 28 "H2O, the flow bench test pressure. See how small the resistance loss is for the bell mouth. That's why its K is so low. K is the number we are trying to reduce when we port heads.
A K factor of 1.5 is about as bad as we get when we design industrial ventilation systems. A sharp, square elbow has this K factor and therefore uses up about 60% of the available pressure difference to overcome losses (at this very high velocity). You can see that the new Air Flow Research CNC heads, as good as they are, do not flow as well as the worst element encountered in industrial ventilation.
This is not a criticism of head manufacturers or head porters. It is the nature of the beast. These AFR heads flow almost as well as a fully (hand) ported head (without increasing port volume). Further improvements may come with raised ports, different valve angles and better chambers. But as long as poppet valves are in the flow path, this is what we have to work with.
Last edited by tomcat; 03-24-2002 at 12:09 AM.
03-24-2002, 02:35 PM
#22
It wasn't really fair to compare the multiple losses of the port and valve to the single element of loss in a sharp elbow. The head actually has four losses. Based on my information these are the approximate losses for each element in the AFR CNC head or any fully ported head that flows 375 CFM:
Element............K.........Description
Elbow..............0.25.....Curved port
Contraction.....0.25.....Just before valve
Obstruction.....1.00.....Open valve
Expansion.......0.25.....Into chamber and cylinder
Total................1.75.....Not bad at all!!!
The high loss for the obstruction presented by the open valve suggests that the area just before the valve, the valve seat and back of the intake valve, and the chamber immediately after the valve are the most important things to work on. This agrees with what I have been told about mild porting (concentrate on the bowl and the valve seat machining). More recently I see the importance of flow in the chamber being recognized in magazine articles.
Element............K.........Description
Elbow..............0.25.....Curved port
Contraction.....0.25.....Just before valve
Obstruction.....1.00.....Open valve
Expansion.......0.25.....Into chamber and cylinder
Total................1.75.....Not bad at all!!!
The high loss for the obstruction presented by the open valve suggests that the area just before the valve, the valve seat and back of the intake valve, and the chamber immediately after the valve are the most important things to work on. This agrees with what I have been told about mild porting (concentrate on the bowl and the valve seat machining). More recently I see the importance of flow in the chamber being recognized in magazine articles.
Last edited by tomcat; 03-24-2002 at 04:13 PM.
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