Estimated total engine Air Flow = (cid/2)*rpm*ve
Engine CFM demand at max piston speed = (cid * RPM * .000978474) / (number of cylinders)
Engine CFM demand Average = ((cid * RPM * .000978474) / (number of cylinders))*.6872
360 = ( 12 inches * 60 seconds ) / 2 Revolutions ...for 4 Cycle
282.7433388 constant = 360 * .785398163
.003536777 = 1 / 282.7433388
2.4 = ( 144 Sq.Inches / 60 Seconds )
.416666667 = 1 / 2.4
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
Speed_of_Sound_FPS = (( 459.67 + TempF) * 2402.625624 ) ^ .5
1116.319772 fps @ 59 F
.55 Mach = 613.9758744 fps
Speed_of_Sound = (( 459.67 + TempF) * 1.4 * 32.174 * 53.34 ) ^ .5
Speed_of_Sound = (( 459.67 + TempF) * 1.4 * 1716.567377 ) ^ .5
and the easiest equation is
Speed_of_Sound = (( 459.67 + TempF ) ^ .5 ) * 49.02
TempF = temperature in degrees Fahrenheit
459.67 = Rankine degrees
=============================================
In your calc. you use
"314.5 = Air velocity in Feet per Second " how is that measured, on the flow bench or engine?
its calculated or guesstimated
306.7 to 314.5 FPS range is about 1/2 the .55 Mach number (613.98)
its represents the average port velocity baseline starting point
..you could use a number of your own to correlate your data
on the FlowBench, you would never "calculate or guesstimate" this
you would instead measure Port Velocity with a 180 degree or "U" or "J" shaped Pitot Probe attached to a 48" inch or so Vertical Manometer
the Pitot Tube would accurately account for Port's corner radius effects
..you can use equations to account for that, but the Probe is better/quicker/accurate
if your Pitot Probe Pressure in Inches of water "exceed" or equal your Test Pressure..theres are good chance you are already in or close to being into Choke under live conditions
you can get into a Choked port "sooner" than your Test Pressure number
..just depends how far the Choke's smallest-area is away from the
valve , and what are differences in pressure between the Short Turn Pressure and the Choke pressure
on the Short Turn's Apex you can flip the Pitot Probe upside down and attempt to measure localized velocity there on the apex...but that area seems to be worth 5 to 25 HP Losses if too high...but if the Port's Velocity in the smallest cross-sectional area is "too-high" its worth 50 to 100+ HP Losses..just depends on how far this area is from the valve
just like the Valve's Curtain Area in the Lift Curve will be the final Choke,
the Short Turn's Apex speed/pressure will be the next choke point,
then further up the smallest cross-sectional area Choke point
if you have a CA Choke point..it will control or dominate the port,..it will cause greater HP Losses than the Short Turn apex localized velocity
the straighter the Port (like a straight piece of tube)
the higher the velocity fps can be without flow separation or choke
the higher the fps...the higher the Volumetric Efficiency attainable
heads like #292 Turbo, #034 Bowtie, #461, #462, etc
all had "potential" pushrod area choke problems
Can you please explain L/D ratios
the L/D Ratio is just
Valve_Lift / Valve_Diameter
.3732 Ratio = .724 Lift / 1.940 OD Intake valve
.4215 Ratio = 1.060 Lift / 2.515 OD Intake Valve
if you look at any Flow Curve...the Flow rate increase is pretty good until you reach .25 L/D Ratio..then the Flow rate increase really drops off
the older style heads with poorer design/shaped Ports, can't handle the air velocity as well as modern shaped ports...so increasing L/D Ratio above .37 to .39 usually shows no Dyno gains, and most times HP Losses above .39 L/D ...whereas, ProStock style heads like .41 to .42+ L/D Ratios
all you'll do with older heads above .39 L/D Ratio is slow down the Curtain Area velocity and increase the pressure differential between a possible choke point further up the Port ..like at the pushrod area ...causing the Port to go into Choke sooner in the Lift Curve.Meaux Racing Heads
MaxRace Software
PipeMax and ET_Analyst for DragRacers
http://www.maxracesoftware.com
http://maxracesoftware.com/bulletinboar ... cesoftware
Guru
Posts: 2069
Joined: Sat Feb 14, 2004 4:03 pm
Location: Abbeville, LA
Website