I little info on the 041X
Sometimes class rules make you do things that are out of the ordinary. Such
is the case with NHRA Super Stock engines. Read more about some testing
done by Larry Meaux on 162-165 cc intake runner SBC heads.
Could it be in the case of the bike heads the original ports were too large?
Originally Posted by maxracesoftware
here's some Threads from a couple of Posts that might explain
CHOKE to you
here's a very good quote from STEVES
that explains the general idea and reason why Choke
or too fast FPS hurts HP+TQ
just read rest of Posts with that in mind .
As I understand it, ports don't actually go into sonic choke at .55 Mach - but
at this point (approx.) we reach the trade off where the energy required to
move the air through the port becomes higher than the power increase
(cylinder filling) that comes from higher velocity.
From a previous Thread
too fast Velocity FPS can be a total disaster
Note=>all 3 of these Heads were tried on
the same Short Block with all the same pieces
and Dyno Tuned for best possible HP/TQ Curve
with those pieces.
#041x SBC Heads = 165.0 CCs
these are the very Hi-Velocity Heads
with too much velocity everywhere inside
the Intake Port, same FlowBench numbers
and the "BEST" Dyno test with them
600 RPM/SEC , i don't have the Sheets
that we started at 3000 RPMs and all the
rest of the Sheets , but only kept the
Copies that stood out, and these are every
200 Hundred RPM increments as its too much Info to type
every 100 RPMs, but it should give you enough Info ?
note=> at 600 RPM/SEC you get a little Needle/Seat
action showing up especially with small gas bowl
chamber in Q-Jet, so look at Fuel Lbs/Hour trend
as well as rate (Same Q-Jet Carb all Dyno Tests)
with #041x Heads back-to-back on same Short Block
same basic Flow CFM Numbers, same valves, same CC's
but with Port Velocity slower and more acceptable
throughout the entire Intake Port
with #462 castings 1.940/1.500 162.0 CC Ports
differences just 3 CCs can make when ground out in the
correct places, again FlowBench CFM between the
#462 and the other 2 #041x Heads were very close
and CFM numbers don't indicate the HP/TQ differences observed
and Ports had different Velocity Profiles.
Same ShortBlock and all pieces the same.
Very well put - I have thought for some time that "Choke" is actually a
misleading term here. David Vizard has called it "Power Limiting Port Area",
a more accurate description, but unlikely to catch on.
i just use the word "Choke"
because sometimes the Engine will be Choked by an Area
and sometimes by the same cross-sectional, but now has one of the walls
with too much local velocity FPS and/or diverging too quickly on 1 wall
in the above Dyno Test examples the
one extreme hi-velocity #041x SBC Heads
is using more Fuel, but if you try to lean it out, you loose even more Torque
and HP...notice it makes Peak TQ and Peak HP lower and runs out quickly
with rapid rising BSFC numbers as rising RPMs show Choke problem even more.
the 2nd pair of #041x heads
make more Peak TQ & HP and at higher points,
and don't lay over top end.
the #462 castings with 3 less CC's
make Peak TQ at same point, but past Peak HP point start to layover
more than the #041x
Fuel consumption is about identical
Same FlowBench CFM Numbers
but different Intake Port Pitot Probe profiles/velocities
note thats a 117.9 HP "LOSS" for the extreme hi-velocity Heads at 7600
yet..on a steady-state FlowBench test,
"BOTH" Heads flowed almost as exact CFM as you could possibly
make them be equal on both Intake + Exhaust sides.
even used and swapped the same exact Valves out of both Heads
for those tests
same #041x castings , both same Chamber+Port volumes
what i call the extreme velocity FPS #041x Heads were;
every possible portion of that Intake port that could have
Epoxy added to it, and that Flow CFM was not reduced at all,
was epoxied up.
and the rest of that Port was enlarged just enough
to hold the same Port Volume CC's
the Short Turn Apex speed was to the moon
and so was the pushrod area...and just about every where else in the
Port....the Floor had some "Ski-Jump" shape to it also...as it kept
the CFM Numbers up and the velocity sky-high
i guess you could call it an experiment to see how far
you could "shrink" certain CSA areas of a Port
and not reduce the FlowBench CFM numbers .
pretty evident from Fuel Consumed Numbers -vs- Dyno HP/TQ Numbers
that Intake Port could not handle that much speed FPS
without Choke or Separation
you can also see why just about everyone i know
will Run the #041x heads over the other Legal #462 castings,
those 3 more CC's can be used to "SLOW DOWN" the already
too fast FPS
one other thing that stood out in some of the Tests, was the very hi-velocity
too fast FPS Heads that had a choke problem,often liked "more" low to mid
lift flow.The engine's being fed sooner and more, so the cylinder depression is
lesser until Choke occurs...and you still have good low-lift to take advantage
of high velocity at end of stroke
also the velocity FPS is slower in the smallest CSA areas, in the low to mid lift
portions of the Flow/Cam Lift Curve...pumping losses working thru rod angle
leverage in early and latter parts of stroke are going to be lesser than at Peak
Piston CFM demand point where leverage is great and Choke makes more losses.
a Closed Intake Valve has "ZERO Port Velocity FPS"
at Max-Lift , typically the Cyl Head has its best FlowBench CFM Number
or about in that vicinity...so Port Velocity FPS is highest at Peak Lift or
so. as the valve starts to move towards max lift, Port FPS is increasing...
also the minimum csa area FPS is starting to really increase or any too
fast FPS area is also increasing in FPS (add to that max Piston CFM
demand in vicinity of 70-80 deg ATDC and volume CCs increasing till
BDC, + Flow Lag Times, pumping losses working thru Rod Angle
FPS = (CFM * 2.4 ) / CSA
if Head Flows 127 CFM at .200" Lift = 156.6 fps @ 1.948 csa
if head flows 260 CFM at .700" Lift = 320.3 fps @ 1.948" CSA
but in reality there will be CSA spots in Heads that will be
smaller than 1.948 sqinches, so the FPS will be higher than 320.3
other CSA will be larger than 1.948
and other localized spots can have too high FPS
even though your Average CSA of 1.948 says its only 320.3 fps
the Port Volume is the same in both cases,
the FPS changes up or down inside the Port
in relation to the Lift/Flow Curve -vs- Piston CFM demand
picking up the Low to Mid lift flow in the too-fast-velocity heads
helped...but it still didn't run as fast down the DragStrip.
the best way i've found so far is to slow the FPS to as close
to reasonable speed as possible, as long as its not too slow,
take the choke CSA out of the picture as much as possible.
As I understand it, ports don't actually go into sonic choke at .55 Mach -
but at this point (approx.) we reach the trade off where the energy required
to move the air through the port becomes higher than the power increase
(cylinder filling) that comes from higher velocity.