This SS/IA RaceCar had the "slower velocity" #041x Heads on
and set the NHRA SS/IA Record at that time at
1.330 , 4.058, 6.377 , 8.402 , 10.132 ET 3330 lbs.
but couldn't fall down a "Mine-Shaft" and run
those kind of ET Times with the extreme Hi-Velocity
other pair of #041x Heads with the same FlowBench CFM Numbers.
Do you think the 041x heads would have different results had they been put on something like a 305 or something smaller where the velocity would have helped?
Yes, to a certain extent !!
the faster the Piston speed and the greater
the Piston/CID CFM demand,
the sooner and more intense the Intake Port
will go into Choke Condition relative to
Cross-sectional area or some too high velocity
spot inside a Port -VS- Cam Lift/Flow Curve
depending on how many crank degrees of rotation
the Cyl Head stays in Choke condition will determine
how much Torque/HP you Loose to pumping losses.
The Choke condition will also affect your "Tuned Lengths"
and upset TQ/HP gains.
looking at examples of the same exact casting Cyl Head,
with exact same valve size OD, Chamber and Port volume CC's,
like in Chrysler 318,340,360 cid engine sizes http://www.maxracesoftware.com/Volkman_ ... 00-600.jpghttp://www.maxracesoftware.com/milan_manningss.jpghttp://www.maxracesoftware.com/Sonny_St ... aytona.jpg
the 360 cid Engine will go into Choke sooner and is more sensitive
to Port Velocity Profile , and the 318 cid, is a little less sensitive to too high velocity, but will still Looose TQ/HP and suffer ET/MPH Times losses if
Port Velocity is too high especially as Choke point is farther away from the Curtain Area point
i'm working on my 6th design Port Shape..all at 162.0 CCs 1.880 Int OD valve
all 6 Port shapes hold 162.0 CC and all 6 Port Shapes flow the same
CFM FlowBench numbers ....but all 6 Port Shapes have varying degrees of Port Velocities
and Velocity Profile Shapes along the Port Lengths.
One Shape makes more bottom to mid range Torque, but less mid to hi-rpm numbers
One shape makes same HP/TQ numbers on Dyno at 600 RPM/SEC, but uses more Fuel
with higher BSFC numbers that you can't fix without loosing HP/TQ, and
even though it makes same HP/TQ numbers on the Dyno, runs a little slower down the DragStrip.
One shape makes very good TQ/HP numbers throughout and very low BSFC numbers,
very efficient and runs fast down DragStrip.
Another Shape does the same as above but entire power curve is shifted slightly higher
in RPM range by 300 RPM. and this combo has run the fastest down the DragSrip in the
318,340,360 Engine sizes. This shape has the slowest overall Port velocity of all the shapes,
but when i say Slower Velocity,..its still very Hi-Velocity and on verge of having too mucjh velocity..i wish i had another 5 CC's to work with, as i'm pretty sure i could get another 10 to 15+ HP more by slowing down the spots that are on the verge of too hi velocity.
Also, what readings are you getting with your pitot tube that make you say, "uh oh, way too fast"?
Ya when do you know when the velocities are getting to high? What should you look for?
If your 180 deg Shaped Pitot Probe Velocity pressure readings are
the same or higher than your FlowBench Test Pressure,
then there's a real good chance your Engine's TQ/HP
will not correlate to what you expect from your FlowBench Numbers.
Back in 1979 i purchased a Pitot Probe..but the Probe was straight shape,
i couldn't make Heads/Tails of correlating Performance with Pitot Readings,
so i abandoned Pitot Probe testing for a year or so until one day
i had a pair of Cyl Heads that had better Flow Numbers but wouldn't run as fast
down the DragStrip as another pair of same castings with worser Flow numbers.
So i grabbed the abandoned straight Pitot Probe and had an idea to
hand bend the Probe back into a 180 deg loop to check Intake Velocity Pressure,
retested both pairs of Heads against each other on the FlowBench and
immediately saw the Velocity differences between the Heads or more
importantly what parts of the Port were showing hi velocity or low velocity.
What stoodout between the 2 pairs was the Cyl Heads that had more Flow CFM also had much higher velocity at the pushrod area as this was almost polished stock OEM size there.
so just to experiment, that pushrod area restriction was enlarged a lot ,
and re-Flow tested. The FlowBench CFM numbers lost 2 CFM at .600" Lift
but the Velocity was a lot slower and the RaceCar ran faster down the DragStrip.
From that point on i started including Pitot Probe velocity readings at various locations inside Ports along with and while i flow tested Heads on the FlowBench....tried to create a Database
of Velocity and Flow Numbers in an effort to realize what was really happening inside a Port and how it related to DragStrip performance....and also how you could make a FlowBench correlate more accurately to DragStrip performance.
in 1983?? started working with B/FX Camaro and in 1984 with Port mods by using 180 deg Pitot Probe set NHRA Record in B/FX http://www.maxracesoftware.com/Meaux_Ba ... Record.jpg
so for about 20 years the Pitot Probe has been my friend
few years later on i saw Alan Lockheed's Engine Expert computer program and Manual
mentioned the Port Limiting Velocity aspects on Engine Performance potential.
and just recently, someone Emailed me articles/pictures of Pat Baer/ Sperry ??
articles on how to fashion and bend/strip a Pitot Probe to measure
Short Turn velocities and also Port velocity.
Nice to see other people discovering and verifying some of the
same things i've come across thru the years !!!
Darin Morgan also has previously mentioned the Pushrod Choke Area on other Posts here on SpeedTalk., and i've received a few Emails from others with different engines/heads
experiencing the same effects from Choke Conditions or Port Limiting Velocity.
What i thought i had discovered 20 years ago was brand new ,
was long ago discovered by other people...and a trip to
your local College Library , especially looking at SAE Papers
is a humbling experience !!
most everything you can think of , or any new bright idea you might have,
has already been thought about or discovered many years ago.
Sometimes material technology has to catch up to the Invention or Idea
for it to be practicle and useable.
Is the velocity being too high causing reversion?
No, its instead causing increases in pumping losses
as its taking more energy to create the too high Port Velocity
Another problem with too high localized velocity is
fuel/air separation with higher BSFCs and mixture quality suffers as well.
you might research Inlet Mach Index and/or Z-Factor on the InterNet
go to your local College Library and research thru SAE Papers