talking about FlowBench CFM numbers for small Port Volume Heads

Thread: Max headflow acheivable while retaining the 170cc intake port

it depends upon the actual Port's CenterLine Length
to compare on an equal basis

here's an example of a relatively Short Port Length at 162.0 CC Port Vol Int Port Length= 4.948" inches long CL

a Chrysler SS 318/340/360 cast-iron Head at 162cc w/1.880/1.600 valves

at legal 162cc port volume ,
i've had that Head up to 280 CFM @28" w/1.880 valve

but NHRA Legal, it won't make any HP , or run fast enough down the DragStrip
and it adds another 1 week of very hard time consuming work
to get 280 cfm out of 162cc's w/1.880 int

at 280 CFM in 162cc port..the speeds way too fast in critical spots
it kills HP + TQ...a much lower 260 cfm Head makes way more HP
and turns 200-300 RPM higher

in another type Head with a different Port shape its possible
to run fast with 280cfm in 162cc

those are just things to think about,
check your Ports with Pitot Probe
when you FlowTest.

 

"Port velocity slower and more acceptable throughout the entire Intake Port "
yes that statement does seem unclear depending upon how you interpret it ?

Pitot Probe Velocity FPS "slower" in critical spots inside that Port
which were more acceptable "Pitot Velocities" throughout the entire Port's shape.
in other words... a better overall Port Velocity Profile as measured by Pitot Probe.

 

That above Quote was not by me..but Pasted from the reply to that Post,
it was supposedly a David Vizard quote about "Power Limiting Port Area"

Here's what i meant=>
1- i use the word "CHOKE" to mean the same thing as Vizard's "Power Limiting Port Area"

since it is a "Power Limiting Port Area"...its acting as a Choke Area as the Engine
will no longer make the correct HP for the amount of Flow CFM the Heads test on a Bench

Here's a good way to look at what i call "Choke" or Vizard's definition=>
"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"


2- "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 "

an Engine can be "Choked" by a certain size cross-sectional area 's FPS
if the Velocity FPS speed is too fast in that area

likewise, an Engine can be "Choked" sooner in RPM Curve , or more severe HP Loss,
if you still have the same too fast FPS cross-sectional area as you had before,
but now one of the Port walls is diverging away too quickly just right after
that same cross-sectional Choke area.
Sort of like difference between Ford and Chevy Intake port walls.
the Ford Port can have a Choke Point w/ too high Pitot Probe Velocity,
but just after that Choke..the walls diverge slower and relatively straighter

where as, at the SBC 's Choke point, just right after that point,
one of the Port walls turns and diverges much quicker than the Ford port

so the Ford and the Chevy can have the same exact Choking CSA FPS,
but the Chevy head will loose more HP and use more fuel
for the same Choke FPS speed

this is verified by my Dyno and Dragstrip tests so far.

 

So far, in these type SS Engines, they run faster with better mid -to high lift flow
than with better low -to mid lift flow.
They make very close to the same Peak Torque numbers with both methods,
but the Heads with better mid to high lift flow numbers
make a wider Power Curve with greater mid to hi rpm HP
and run faster down the Dragstrip.

i had a very interesting series of FlowBench + Dyno tests with a
Chevy 327 SS Engine w/#041x Heads 1.940/1.500 valves 165cc Port,...
one pair of Heads had great Flow Numbers on both Int + Exh sides,
but had a slight Choke speed FPS problem,
it like better Low to mid lift Flow Numbers to make HP

the only problem with this,..the Heads were illegally "Acid-Ported"
unshrouded around Intake Valves in Chambers to get that better
Low to Mid Lift Flow Numbers.

in 2006-2007 in NHRA, now its Legal, back then it was illegal

those acid-unshrouded #041x heads with better Low Lift Flow
made the same Peak TQ numbers, but were down almost 20 HP
from the other #041 legal ported heads without the Choke FPS problem.

Both Heads around 62CC Chambers , same Peak Flow Numbers,
the Acid-ported + better Low-Lift to Mid Lift Flow number Heads
ran almost a Tenth ET slower + 1.5 MPH slower, basically same 60Ft

 

According to the FlowBench...both pairs are moving the same Air
According to the Dyno's Air Turbine ...both pairs are "not" moving the same Air

Same#041 165cc Port castings, same Valves swapped between Pairs, same Flow Numbers
as close as possible both Int+ Exh sides, at every lift point,
same valve job with Serdi Cutters..but thats the point the Pairs begin to differ=>

the Best HP 165cc Port head=> had "slower" Pitot Probe velocities in the critical spots
inside the 165cc Port = 581.4 HP @ 7300 RPM + 101.5 HP better than other #041x's

the Worst HP 165cc Port head=> had much "faster" Pitot Probe velocities in
the critical spots inside the 165cc Port = 479.9 HP @ 7300 RPM

Dragstrip the "slower Pitot Probe velocities" in the critical areas
ran 10.00's 130+ MPH @ 3330 lbs

the "much faster Pitot Probe velocities" in the critical areas
ran 10.80's first time out, later high 10.60's

 

"consistent" Pitot Probe velocities makes more HP period ?
--- the Key to making more HP is to eliminate spots where Pitot Probe velocities
are too fast ! 479.9 HP @ 7300 RPM -vs- 581.4 HP @ 7300 RPM 10.00's ET vs 10.60's ET

Head Flow on FlowBench is almost always not Laminar Flow
especially Flow in a live Engine ...its alway turbulent Flow, never Laminar Flow.

You are probably referring to a Port on on FlowBench being Flow Tested
with normal amounts of turbulence -VS- Port with severe turbulence.



I would lean that more likely it’s just pointing that although the port with the higher peak

velocity (larger range in velocity) flowed the same on the flow bench, the bench is simulating

an artificial condition that you do not see in the engine so the engine made less power with

that combination. The immediate, shoot from the hip answer is that in actual practice that the

higher velocity points really did become choke points, that in practice it flows less even

though the bench shows it flowing the same. I don’t like that answer (at least not as a 100%

description of what is happening) since it makes the BSFC numbers hard to explain. I’d suggest

(as much as I hate some of the way this data is being used now) that it may be something more

like the greater differences in velocity of different parts of the port are messing with the wet

flow of the head

Yes... "greater differences in velocity of different parts of the port are messing with the wet
flow of the head" ....that's true up to a point.

However, the SCFM the Engine is using , as measured thru the Air Turbine,
is showing that for the same FlowBench CFM Numbers on both pairs of #041x,
the Head that has the too high Pitot velocity spots
also uses less SFCM as measured,
the other Head that Flows the same, but slower Pitot velocties in critical spots,
also measures that its Flowing more Air on the Dyno's Air Turbine,
and verifies this with as much as 101.5 + HP greater or more
and runs 6 tenths faster ET's

That Head makes more HP moving more Air with about the same Fuel
so the BSFC's are lower. (200.4 Fuel avg. between 4500-7600 RPM)

The other Head makes less HP moving less Air with about the same Fuel
so its BSFC's are higher...also if you try to lean out jetting,
it kills TQ + HP even more...then that increases BSFC's even greater !
(206.0 Fuel avg. between 4500-7600 RPM)
if you jet it to get around same 200.4 Fuel avg.,
it kills HP + TQ further increasing BSFC's



Also greater velocities require greater Energy
and thereby , greater HP loss. The Crankshaft degree ATDC where peak CFM demand is occuring
is at a point where there is greater leverage,...so any reduction in Cylinder filling
requires greater Energy and looses more HP + TQ thru that greater leverage.

The points of Low-to-Mid Lift Flow are at times with much lesser Leverage
and the points of Mid-to-High Lift Flow are at times with greater Leverage,
so if there are too high Pitot velocity areas in that Port,
it will require greater Energy , and loose more HP

BSFC = Fuel Consumed / UnCorrected HP

if you loose HP thru a "Choke point" , it will show you higher BSFC's

if you loose HP from Air/Fuel Separation because of local FPS
too fast for Short Turn...it will show you higher BSFC's.


Here's another aspect to look at=>
in my FlowBench Pitot probe tests,
you can easily measure how fast the Short Turn Apex FPS velocity
is....and then you can measure lets say up around the pushrod area
where speed FPS may be just as fast or could be much slower .

the too fast Short Turn apex speed can hurt HP by around 20 HP or so
..the same or a little slower pushrod FPS can hurt you much more than 20 HP !
and the worst HP Loss i've seen is if you have very fast FPS speeds
at both Pushrod and Short Turn spots, but slower in between.


Look at it this way,
1 pair of Heads runs 10.00's ET
1 pair of Heads runs 10.60's + ET

i'd much rather the 10.00's ET #041x Heads with slower Pitot fps in critical areas !

 

Larry,
Wow, thanks for taking the time to decipher all of this and providing detailed analysis. NHRA forces you to do some strange things sometimes.

The LT1 cylinder head is an impressive piece with good port shape that responds well to very basic modifications. I have found by removing less than 5cc, developing flow numbers in the 250 -260 range, can produce a good low budget street strip type engine in the 500HP range. I don't have access to a pilot tube so I don't know port velocities. Attempting to build more HP with the same head has provided less impressive results, my guess is elevated port velocity are hindering further performance gains.

Thanks again. The engine we discussed months ago is built, I am not able to proceed with testing at this time. I apologize for not following through, that's not like me at all.

 

MaxRaceSoftware, my head now hurts and is spinning at the same time. But in this case it's not a bad thing. Even if I can't apply what you've stated myself, (which is a given) I would like to think I might one day be able to ask enough questions to a head porter to figure out whether or not he knows what it takes to port a set of heads with what you have explained. So here is a question if you don't mind. If one was to have a decent set of heads for a 355ci engine, would these same ports work on an engine that was increased by 30 to 40 ci? As in, they might be lacking slightly for total high rpm HP but if the car was street driven also they might be more prone to have some more decent street manners then a set that only preformed well at the higher rpm level? Hopefully you know what I'm asking here. At times I have a hard time trying to word the question correctly. Thanks again for sharing your knowledge, Rick.

 

I don't have access to a pilot tube so I don't know port velocities. Attempting to build more HP with the same head has provided less impressive results, my guess is elevated port velocity are hindering further performance gains.

Take a look at this Pic



its my old SF-110 around late 1970's to 1980 ?
Notice the plastic U-Tube bent around a Yard Ruler ,filled with Red Fluid ,
i hooked one end to a straight piece of tubing to measure Port pressures
in different areas.

i was Blessed to live in an Era when commercial FlowBenches were being
made available to the Public , and another "Key component" was the
small block Chevy Head back then was capable of moving a lot of CFM
at a relatively small Cross-sectional Area and Port Volume.

The first couple of years of using the SF-110 and Porting Heads,
just about everything i Ported ran faster down the Dragstrip,
but as i learned to get the Heads flowing even more and trying to keep the
Port Volume low...i started noticing a few of my best flowing Heads
didn't really run much faster than a lesser CFM Head,
and i had one pair of Heads that had a little lower CFM numbers,
run the fastest at the Dragstrip -vs- the other Heads.

So i made that plastic U-Tube and homemade Probe to tell what was
going on ??? Probing around with that straight piece of tubing
you instantly see the Velocity Pressure is radically different inside the
Port at different locations..... but a straight piece of tubing doesn't
tell you enough correct information ! ...i ordered a "real"Pitot Probe,
(Tube within a Tube)....it came in straight....so i tried using it straight,
not much useful information...so i bent it into a J-Style 180 degree Shape
and Probed the Lower CFM Head that ran faster than the High CFM Head,
in just a few minutes with J-Pitot..you instantly saw the Speed/Pressure
differences....the higher CFM Head was smaller at Pushrod area and
Speed there was a bunch faster ...the other Head that ran faster
was much taller and wider at the pushrod area and the speed was slower.
From that point on i tried correlating how much FPS speed was necessary
to give you great bottom-end Torque but still not too much speed FPS
to hurt hi-rpm HP .

The first time you Probe the Short Turn Apex floor area inside a Port,
you will be surprised...and very lucky if you can pull out the Pitot Probe
fast enough before you "Peg" the Gauge Oil out the top of your
Manometer

automotivebreath..my advice to you is just make up a U-Tube out of plastic
tubing and hookup a straight piece of tubing and probe around the Port
while flowtesting ....you instantly see the variations in Port Pressure !!
Then , if you like what you see and are interested ??
purchase a real J-Style 180deg Pitot Probe for Intake Ports
and a long 90deg Bend Pitot Probe for Exhaust ports !

Example=>
You can have (2) 170 CC Port Heads that Flow the same Numbers,
but have radically different Port Velocities in areas inside the Port.

 

we try out or test the same pair of Heads on various sized Engines
all the time . The Heads loose HP/CID Efficiency as you put the same Head
on larger and larger CID Engine => like the example you see
in NHRA 500 cid ProStock -vs- IHRA larger CID Engines

if you added 30 to 40 cid size increase with same Head,
at same CompRatio,.... majority of times you'll still see an increase
in HP , especially Torque numbers,..but the HP/CID Ratio will be decreasing

then sometimes if the Head's Port velocity was too slow for
one engine size and you put that same Head on a larger CID size,
its very possible to make much more HP + TQ and better HP/CID Ratio,
it just depends on a lot of interelated factors,
thats what makes working with Engines interesting

 

My situation is for from ideal but improving. For me porting cylinder heads has been a hobby for many years. Only in the last few years have I ventured out to do this for others. The majority of my work has been done with out access to a flow bench; on occasion I bring my finished work to someone that has a bench to get a feel for where I am. The performance is there but I know much more can be done.

My shop was not big enough for race cars and a flow bench. This year I was able to build a garage for the cars opening up room for a flow bench and such to advance my knowledge. I can't wait to get a bench and a pilot tube, I feel like I have been working in the dark.

When I get the pushrod pinch as wide as it will go with in the confines of the cylinder head, I start making the port taller, no problem. The part I have not mastered is what you call the ski jump, when to do it and what to watch out for?

Then comes reducing flow velocity at the short turn, so much to learn, so little time.

 

its not as easy as just making the "pushrod pinch" area larger.
you have to consider the Camshaft and desired RPM Range also !

here's a Tip=> multiply .87% PerCent times your Cam's Theoretical
max Intake Valve Lift...then put that Head on a FlowBench with the valve
opened to that .87 x Lift and then check out Pitot Probe Velocity FPS
Profile thru that Port.

in other words,..it doesn't do you any good to design for
.700" Lift Flow's Velocity Profile
if all you have is a .550" Lift Cam

so if i had a .550" Lift Cam,
i would multiply .87 x .550 = .479" ,
and open valve to .479" Lift on Flow Test,
then measure Port's Velocity FPS Profile,
then if at that Lift point you see your Pitot Probe pressure
equal or exceed your Flow Test Pressure,
then that area will be a Choke, and the Flow Number you see
on the Bench will not correlate well to real-world HP .

if thats the case,
then you open up that Choke area,
and slow down the speed there to more acceptable level

on the Short Turn Apex Floor's surface's speed FPS,
9 out of 10 times it will be way over 350 fps @ 28"
....until you gain experience,..i'd leave the
Ski-Jump speed FPS alone even it was 435 fps.

if its over 435 fps ...widened the Port there,
if you can't widen..then raise the Roof there,
if you can't raise the roof,
then lower the Ski Jump.

Don't slow the Ski-Jump 's FPS lower than 350-380 fps until you've
had experience with Dyno test results with that Head.

 

You can only imagine how difficult it is to make big HP numbers with the LT1 head when the only flow bench access is two hours away. Even then I don't get to touch it only read the numbers.

I'm working with a 383, stock LT1 heads, 1.94 valves, production intake manifold & EFI. The car has run 9.90 on the best day. MPH is low indicating low HP numbers likely due to port or intake manifold choke (or both). I need 0.2 to 0.3 seconds more. Like I said, I'm working in the dark.

How much floor space do I need for the needed equipment?

 

Thanks so much, I'm saving this information along with this, another problem I think we are having with the 383.

"with a Choke Problem, increasing Cam max Lift, is "decreasing" Curtain Area velocity and "increasing" choke area velocity duration and thereby increasing the pressure differential between those 2 points"

 

How much floor space do I need for the needed equipment?

you could put a SF-600 Bench in your Bathroom

basically you could,
but with all those Blower Motors,
there's a lot of Heat to get rid of.

i'd say a Room 10x10 or so would be plenty,
with an A/C Unit,
and best would be to make ducts to duct out Blower Heat
outside !

if you like Porting Heads,
sooner or later you'll have to buy a FlowBench
to be able to "measure" what you are doing.

The quicker and more accurately you can measure a Change,
the greater your chance should be to discover the Solution