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.
---SteveS

Posts=>


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
1.940/1.500 valves
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)

RPM---TQ----HP----Fuel Lbs
4500-419.3-359.3--178.4
4700-438.1-392.1--171.5
4900-449.1-419.0--177.2
5100-451.0-437.9--169.8
5300-445.8-449.9--174.1
5500-443.2-464.1--188.3
5700-441.0-478.6--209.8
5900-429.6-482.6--222.1
6100-424.3-492.8--227.4
6300-413.9-496.5--214.9
6500-412.7-510.8--200.8
6700-407.6-520.0--210.3
6900-388.8-510.8--221.9
7100-363.7-491.7--236.1
7300-345.3-479.9--239.8
7500-325.1-464.3--233.4
7600-312.6-452.4--226.1

Avg=>406.5-464.9--206.0

--------------------------------------------------------------------------------

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

RPM---TQ----HP----Fuel Lbs
4500-449.5-385.1--166.1
4700-444.2-397.5--164.5
4900-455.2-424.7--177.0
5100-456.2-443.0--158.3
5300-464.1-468.3--169.6
5500-471.1-493.3--192.9
5700-470.2-510.3--199.2
5900-468.2-526.0--199.3
6100-465.0-540.1--204.5
6300-459.8-551.5--209.7
6500-456.6-565.1--216.3
6700-442.6-564.6--223.1
6900-432.8-568.6--217.2
7100-426.6-576.7--215.3
7300-418.3-581.4--224.5
7500-401.8-573.8--238.2
7600-394.1-570.3--231.0

Avg=>445.7-514.1--200.4

================================================== =

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.

RPM---TQ----HP----Fuel Lbs
4500-443.0-379.6--168.3
4700-441.8-395.4--159.8
4900-450.6-420.4--164.4
5100-456.9-443.7--169.8
5300-459.2-463.4--183.9
5500-465.9-487.9--190.6
5700-464.1-503.7--192.0
5900-463.0-520.1--195.2
6100-460.6-535.0--196.6
6300-454.9-545.7--206.5
6500-446.8-553.0--216.1
6700-438.0-558.8--225.1
6900-428.4-562.8--220.8
7100-422.0-570.5--223.9
7300-410.1-570.0--219.5
7500-395.3-564.5--226.4
7600-388.8-562.6--231.1

Avg=>440.6-508.1--199.4


Quote:
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 leverage/velocity, etc.)


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.
---SteveS

Note=> a lot of this stuff is on Darin's CD also
and SAE Papers in your College Library

Originally Posted by 1LESSZ28

… I want to increase port velocity for maximum torque production….

Originally Posted by automotivebreath

I went back and read 1LESSZ28’s original question and I think I get Bret’s point. The LT1 has a high velocity port, you don’t want to increase velocity the way Larry did with his test of Super Stock heads; rather you want maximize flow while maintaining the high velocity characteristics of the LT1 port.

Originally Posted by WS Sick

How far did you open the pushrod pinch?

Originally Posted by automotivebreath

What I do for a mild porting is measure the thickness of the port wall at the push rod to determine how much can be safely removed. I then remove material from the inside of the port in that specific area until the wall is 0.60" thick, that is plenty enough to assure no break through.

At that point I measure the width of each port and adjust by removing material from the divider wall until all ports are identical in width, I don't remove excessive material from the divider wall.

If you would like I have a finished set in the shop I can measure port width at the pinch.

Originally Posted by SStrokerAce

Actually if you read thru that, Larry lowered the velocity of the 041 castings and made MORE power. It had lower BSFC numbers and less timing and made more power.

Originally Posted by grammerman

I think "hi velocity" may not make clear an important part of the benefit provided by lower port volume.

It's fairly obvious that when the intake valve closes there is a moving column of air that stacks up behind the valve. The smaller high velocity type port is going to have a smaller average cross sectional area so there is more momentum (because of the higher velocity) contributing to this pressure rise. Another side of this is that a 165cc port is going to bleed off less of this energy and will produce a higher pressure rise behind the closed valve than a 190cc port. The smaller port will kick things off more briskly when the intake valve opens again because of this extra boost. If your 165cc port is well designed and manages the airflow effectively then it can potentially be more effective than a larger port with higher flowbench numbers assuming the port velocity doesn't reach a high enough mach number to blow up the efficiency. Of course port velocity will be all over the place in different regions of the port so it's more difficult to manage the hot spots on a smaller port with a good bit higher average velocity. A quality port job that manages this higher velocity airflow well would be important if using a healthy cam.

For a daily driver I suspect that these effects of a smaller port volume improves transient throttle response, BSFC and power at small to moderate throttle openings ( mid to high manifold vacuum) disproportionately in a positive direction. These are often important considerations for someone who wants a quick, snappy and torque rich driver rather than a race car.

You can do some calculations based on your intended RPM range, cylinder displacement and volumetric efficiency and get a rough approximation of the average port velocity for a given cross sectional area.

I agree with automotive breath that it's not very conventional to go with a 160cc port on a 7500 RPM, 580 HP engine. Not very likely that this is what he's trying to do so the dyno graphs above would not be nearly as relevant if the goal is a 6000 RPM, 450 HP build because of the major decrease in port velocity. Certainly seems reasonable that he increased the power a bunch in an unusual application that was setup specifically to test the influence of too high mach numbers on power. If using a very well done 170cc port with manageable velocity numbers it's possible you might actually lose power by decreasing port velocity significantly on a moderate output engine.

Later, Michael