I thought this was pretty interesting from Pipemax's Larry Meaux, was his answer to why run higher fps then 285 fps, I put the Chrysler example 1st.
"another example : Chrysler SuperStock #308 castings 1.880/1.600 Valve Combo
with a 162.0cc max Intake Port - NHRA Limit
i spent 3 extra Days hand sanding Floor's epoxy shape to get to 285 CFM @ 0.750 Lift in a 161.5cc Port 1.880=Intake valve OD
so i dupilcated that Port shape in the other 7 Intake Ports
and because its a casting ... only that one Port=285 CFM , the others came out between 277 t0 280's
on the Dyno that 371cid lost HP and made Peak HP 100 to 200 RPM lower
ran slower down the Dragstrip , so from that point on , i gave my Customers around 260 to 265 CFM max in a 161.5cc Port at 4.94800 CenterLine Length
at 265 CFM at 0.750 Lift we set a bunch of National Records with 318,340,360 Engines
Note : these are PipeMax v4.70 calculated FPS ... and NOT Pitot Probe FPS values
277 + 285 CFM = 281 CFM average basically
343.0 FPS 1.99417 CSA 161.7cc 285.0000 CFM ... way too fast !!!! lose HP ... run 9.90's ET
334.0 FPS 1.99042 CSA 161.4cc 277.0000 CFM ... still fast ! ... run 9.80's to 9.90's ET
Look at this :
307.0 FPS 1.97550 CSA 160.2cc 252.7000 CFM ... this has set Records too ! 9.70's to 9.80's
320.0 FPS 1.98750 CSA 161.2cc 265.0000 CFM ... about the highest useable FPS with a 3-Speed Automatic Trans
run "big MPH" with 260's CFM with correct Intake Port FPS and Shape = 137+ MPH 9.5:1 CompRatio w/ThermoQuad 162cc Legal Limit
371cid GT/CA has been as fast as 9.67 ET at 137+MPH ... and has set the NHRA MPH Record at 137.??? years ago"
"its more than just a choosing choice !
instead , you should be measuring both Intake and Exhaust Port Volume CC's, with a "Class A" Buret
and also measuring both Intake and Exhaust Port CenterLine Lengths
then measuring both Intake and Exhaust Port Flow CFM at your Cam's Maximum Valve Lifts
use these 2 Equations to calculate both Intake and Exhaust Port "FPS" velocities :
CSA Velocity FPS = ( Intake Port CFM * Intake Port CenterLine Length * 39.3289536 ) / Intake Port Volume CC
CSA Velocity FPS = ( Exhaust Port CFM * Exhaust Port CenterLine Length * 39.3289536 ) / Exhaust Port Volume CC
PipeMax's "CSA Velocity FPS" are calculated FPS
and a Pitot Probe is actual FPS ... mostly what Chad Speier uses, same as i do too
however, you do not input the Pitot Probe FPS into PipeMax , instead PipeMax needs the calculated FPS from 2 above Equations
You can just enter 285 FPS for both Intake and Exhaust ( 285fps-Default value in PipeMax )
285 FPS size Ports let you have a pretty good safe Port design in those Port Lengths + CFMs
to use higher than 285 FPS successfully , you need to be experienced Cylinder Head Porter that
can grind a Shape that can handle Port speeds higher than 285 fps
There's another critical part to this being :
always use these 2 Equation results :
CSA Velocity FPS = ( Intake Port CFM * Intake Port CenterLine Length * 39.3289536 ) / Intake Port Volume CC
CSA Velocity FPS = ( Exhaust Port CFM * Exhaust Port CenterLine Length * 39.3289536 ) / Exhaust Port Volume CC
find out what are your CSA's FPS values , see if they are too slow , or too fast
too slow : more than likely your Engine won't make the correct amount of Peak TQ or TQ overall
too fast : very possible you will loose HP from pumping losses , more than you gained from increased Flowbench CFM gains !
examples of "too fast" are here #041x -vs- #462 with same Flow CFM numbers
#041x with 165.0cc limit ... 7600 RPM = 570.3 HP
#462 with 162.0cc Limit ... 7600 RPM = 521.8 HP ... looses 48.5 HP at 7600 RPM because FPS is too fast + pumping losses increase
... yet both castings show same TQ and HP at the starting RPMs
basically identical TQ and HP at 4700 to 4800 RPM
also look at VE% number differences"
