Sonic Choke
Sonic Choke
(OP)
maxracesoftware, You have listed on this site the problems created by sonic choke, specifically, Dyno HP below levels predicted by the raw CFM numbers from the flow bench. Does this mean that veloctity, as measured by J type pitot tube, should not be above the numbers you have listed before. I am currently modifying a port. The velocity on the floor of the port along the short turn radius is 220 fps at 10 inches. The roof of the port in the same area is 155 fps. The velocity at the entrance averages 150-170 fps with the velocity higher on average along the left side or cylinder wall side. The port is rectangle 2.1" X 1.25" and 4.9" long, 2.02 valve, 45 degree seat. After moderate modifications, i.e. Combustion chamber, bowl, raising roof, and pushrod wall, produced little results, I then lowered the floor at entrance and widened short turn, which produced huge difference, but really increased velocity in the area of the Short turn radius. From your previous post you stated that velocity >209 fps at 10 inches can lead to Sonic Choke, is this value an average or does it mean any measurement, anywhere in the port? On average the port velocity is around 260 fps @ 28 inches @ .396 L/D. A second question, before .25 L/D, when the valve velocity and port velocity become the same, the calculated valve velocity is 350 fps at 28 inches .050 L/D, to around 220 FPS @ .198 L/D does this also relate to sonic choke? Thanks for any info.





RE: Sonic Choke
if most of the entire Port floor's Short Turn apex area is at 220 fps at 10 inches of Test Pressure ....you are very close or already into Sonic Choke on a live engine and may already be seeing HP losses
if you make a FlowBench CFM gain in this ST-area ,
but see a LOSS in HP when on a Live engine on the Dyno
...yes it went into Air/Fuel separation and possible sonic choke. The worse the Port-Axis -to- Valve-Axis centerlines the sooner in FPS it will go into choke or separation as you make the speed faster there.
The Port's velocity is more critical than the Short Turn velocity
you can loose 5 to 25+ HP on the Short Turn even at the same time you see FlowBench CFM gains
but you can loose over 75+ HP with too much Port velocity
on a 8 Cylinder near 600 HP output , almost 13 % Power
even though you see FlowBench CFM gains
<<<< A second question, before .25 L/D, when the valve velocity and port velocity become the same, the calculated valve velocity is 350 fps at 28 inches .050 L/D, to around 220 FPS @ .198 L/D does this also relate to sonic choke? Thanks for any info.<<<
yes, but its the ST velocity and more important the port velocity FPS that can hurt HP/Torque
don't worry about low-lift valve velocity fps
you need to solve any ST or Port velocity problems
as these spots that will affect the engine intake stroke when the piston is beginning to move the fastest speed down the bore all the way to 180 deg+...if the Intake Port goes into Sonic Choke earlier than this or at that period...pumping losses increase and there will be less air in the Cylinder reducing HP
Theoretically the Curtain Area = Valve Area at .25 L/D Ratio
before or at .25 L/D Ratio the Port velocity "does not" equal the valve velocity...the Port velocity is slower
typically on a ported head, the Port velocity is max near and from .37 L/D Ratio on upwards to .42 L/D Ratio
..and in this same range of L/D Ratios, the curtain area velocity is slowing down as the port velocity is increasing
usually, as you increase Valve Lift and if the Port keeps flowing more ..then Port Velocity is also increasing
and where to measure the Port velocity ?? and at what valve lift or L/D Ratio are the most important ??
---i've already posted way too much Public Info on this
you'll have to experiment to find out where and when :)
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Sonic Choke
to make decent use of Port Velocity, you should have a Cam with at least .25 L/D Ratio and upwards to "get on the Port"
a small block Chevy head with 2.200 OD Intake valve and .310" stem diameter
Flow Test
Valve-----CFM----------Curtain Area-------------Valve Area
Lift------@28"-----Sq.Inch---FPS---CFM/SqIn------CFM Sq/In
.200-----150.8-----1.049"---344.9--143.7------------40.5
.300-----214.9-----1.574----327.6--136.5------------57.7
.400-----268.1-----2.099----306.6--127.7------------71.9
.500-----314.2-----2.623----287.5--119.8------------84.3
.600-----353.6-----3.148----269.6--112.3------------94.9
.700-----380.8-----3.673----248.9--103.7------------102.2
lets say the Port entrance = 3.360" Sq.Inches Area
Valve------Port Entrance Velocity
Lift-------in Feet per Second (Theoretical)
.200-------107.7
.300-------153.5
.400-------191.5
.500-------224.4
.600-------252.6
.700-------272.0
2.200" OD times .25 L/D Ratio = .550" Valve Lift
to get decent Port Velocity you need at least .550" Lift
with a 2.200" OD intake valve (238.5 fps)
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Sonic Choke
The posts above are the most concise explanation of this topic I have seen.
RE: Sonic Choke