David Vizard - Which to prioritize - PORT FLOW or PORT VELOCITY?
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Port Turbulence is a important component that
is often overlooked
is often overlooked
I'm not saying velocity not important, just for most part we have vary little control about it.
Plus look at the Cleveland and it 4 V head and 302/351 Boss these engines have to be one of the biggest examples of too much cross section for a given cfm for engines of small medium sizes generally not spinning the craziest of rpms.
Some will probably argue these engines especially the 302 are soft, but even if they are, these are some of the most extreme examples, the average small block Mopar can't even come close to over csa like the Cleveland. And there's got to be a ton of examples of strong running Cleveland based Fords out there. Not to say they wouldn't do better with an appropriate sized port. Point is a too big of port is probably better than to small for a given power. (within reason)
Most of us don't have the luxury to tailor match port cc, cas, cfm to each engine.
A stock X head can support 200-400+ hp engines, for a 200-300 hp engine a 273/318 head could easily do it too but gonna need way more cam over the X head which way is better ? On paper a stock 175 cfm 318 head should be able to make 400 + hp but is it the wises choice to do so?
On a single purpose race engine obviously the better you can match all this up the better the combo will be, a street engine that has multi-tasks to accomplish I feel velocity important but not at the compromise of everything else.
Plus look at the Cleveland and it 4 V head and 302/351 Boss these engines have to be one of the biggest examples of too much cross section for a given cfm for engines of small medium sizes generally not spinning the craziest of rpms.
Some will probably argue these engines especially the 302 are soft, but even if they are, these are some of the most extreme examples, the average small block Mopar can't even come close to over csa like the Cleveland. And there's got to be a ton of examples of strong running Cleveland based Fords out there. Not to say they wouldn't do better with an appropriate sized port. Point is a too big of port is probably better than to small for a given power. (within reason)
Most of us don't have the luxury to tailor match port cc, cas, cfm to each engine.
A stock X head can support 200-400+ hp engines, for a 200-300 hp engine a 273/318 head could easily do it too but gonna need way more cam over the X head which way is better ? On paper a stock 175 cfm 318 head should be able to make 400 + hp but is it the wises choice to do so?
On a single purpose race engine obviously the better you can match all this up the better the combo will be, a street engine that has multi-tasks to accomplish I feel velocity important but not at the compromise of everything else.
Often, We have gotten Greedy on the ports in both the heads and
intake and the turbulence has not only wiped out any gains but
really cost me power on the Dyno and on the Track!
I have turned some extremely good parts into only so/so stuff.
intake and the turbulence has not only wiped out any gains but
really cost me power on the Dyno and on the Track!
I have turned some extremely good parts into only so/so stuff.
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sometimes little in the right spot is better than more in the wrong spot when working a port.
But I think for most of us, that aren't bound by strict rules and highly competitive racing, the average performance guy. To build an engine with Slightly less inefficient HP is generally gonna be easier and less expensive to build.
A highly efficient 1.35-1.65 lbs-ft per cid is generally gonna be out of the range for most of us.
If you want 500-550 hp and torque with peak hp around 6000 rpms most don't think to build a highly efficient 360 to do it with only 220-240 cfms but a less efficient 408 with 275-300 cfms.
Even most mild engines would require less port volume than a 273/318 head has with more cfm then it generally does.
At Peak hp @ 5000 rpms these engine would need these port volumes and cfm to have optimal velocity, 273 = 99cc/167cfms, 318 = 115cc/195cfms, 360 = 130cc/220cfm, 408 = 147cc/250cfms so generally we would need to shrink the port while gaining cfm beyond most capabilities to have ideal velocities with a mild street engine.
A highly efficient 1.35-1.65 lbs-ft per cid is generally gonna be out of the range for most of us.
If you want 500-550 hp and torque with peak hp around 6000 rpms most don't think to build a highly efficient 360 to do it with only 220-240 cfms but a less efficient 408 with 275-300 cfms.
Even most mild engines would require less port volume than a 273/318 head has with more cfm then it generally does.
At Peak hp @ 5000 rpms these engine would need these port volumes and cfm to have optimal velocity, 273 = 99cc/167cfms, 318 = 115cc/195cfms, 360 = 130cc/220cfm, 408 = 147cc/250cfms so generally we would need to shrink the port while gaining cfm beyond most capabilities to have ideal velocities with a mild street engine.
The short turn radius is what has bite us in the ***
more than any other area!!
more than any other area!!
If I remember right your mainly dealing with max performance out stock parts ?
I think that stuff pretty kewl, I like looking at old Pro Stock small blocks the crazy mods they would do to the heads. I was watching a video that some guy that collects Cleveland pro stock engines was showing the work that goes into those heads and a video on Hemi I think super stock heads by Darin Morgan and then came across DV's video inspire this thread.
From Chris Speier site
This is probably the most asked question I get. What size head do I need?? While head CC's are a by-product of a properly sized cylinder head, there are mathematical formulas to lead you to the correct answer. There are many reasons that choosing a cylinder head based off port volume is a bad idea, simply put there are 100 ways to arrive at a port volume!
First off, CC's or port volume measurements are “close guesses” to a ports average air speed only. It gives no insight what so ever into the ports velocity profile. CC's came about in the 60's when the heads where to small so the larger the port, the more power one could make. Head porters still use CC's as a quick dirty guess as the average air speed in the port, that’s all.
With three known things about any cylinder head, we can arrive at port volumes. Knowing this along with a program such as Pipemax, we are able to choose cylinder heads in a way they that is proper and logical. It's all about average velocity in the induction, 260fps is a great starting point!
Most cylinder head porters are working off a desired MACH speed of the port. 1116 feet per second = speed of sound @STP or .55 MACH. You can design ports slower than .55 MACH, but it's usually considered the speed at which "choke" or "port limiting velocity" occurs in a running engine.
- 613.8 fps = .55 MACH
- "average" intake port length (roof length + floor length from seat ring to opening) divide by 2
- port volume cc's
- MIN CSA = (bore x bore x stroke x RPM x .00353) / 613.8 (.55 MACH x 1116 fps)
Now let's turn your MIN CSA into a CC's for choosing a balanced cylinder head. Remember from above, I said you needed to know port volume and average runner length! Now let's put those into out formula:
- Port Volume CC's = MIN CSA x Port Length x 16.387
The key to any cylinder head is a balanced port. We are trying to balance it around a MIN CSA to achieve proper filling. SRH designs and ports based off velocity profiles. Although formulas are nice for "ballpark" sizing, localized and average velocity is KING!
I get this question PM'd to me all the time so I though I would take some time and show you how it's really all about AVERAGE AIRSPEED in the induction.
Two totally different cylinder heads, different flow curves, different port volumes, but yet the same average airspeed.
I'm going to use my CNC heads for the examples (because I know they are correct FPS vs AREA vs CFM)
Head #1. v2.70PF.. Flows 336@.700.. 250cc port.. 5.47 long
Head #2 v2.50PF.. Flows 320@.700.. 227cc port.. 5.45 long
Manifold Holley 300-110 in AS_CAST form.. short runner=4.625, long runner=5.500
short runner pours 215cc.. long runner pours 240cc
FORMULAS:
Average CSA = Port Volume CC/ (Port Centerline Length * 16.387)
FPS= (Flow CFM * 2.4) / Average CSA
Head #1 flows 320cfm through manifold and carb
Head #2 flows 305cfm through manifold and carb
Head #1= 465cc short runner.. 490cc long runner
Head #2= 442cc short runner.. 467cc long runner
Head #1= 10.08 short runner.. 10.95 long runner
Head #2= 10.10 short runner.. 10.98 long runner
Head #1= 10.08 x 16.387 = 165.2 / 465cc = 2.81 avg CSA short runner
10.95 x 16.387 = 179.4 / 490cc = 2.73 avg CSA long runner
Head #2= 10.10 x 16.387 = 165.5 / 442cc= 2.67 avg CSA short runner
10.98 x 16.387 = 180 / 467cc= 2.59 avg CSA long runner
Head #1= 320 x 2.4 = 768 / 2.81 = 273 FPS AVG VELOCITY short runner
320 x 2.4 = 768 / 2.73 = 281 FPS AVG VELOCITY lon runner
Head #2= 305 x 2.4 = 732 / 2.67 = 274 FPS AVERAGE VELOCITY short runner
305 x 2.4 = 732 / 2.59 = 283 FPS AVERAGE VELOCITY long runner
Induction Formulas and Helpful Sites
- Average_CSA = Port_Volume_CC / (Port_CenterLine_Length * 16.387)
- MIN CSA = (Bore x Bore x Stroke x RPM x .00353) / 613.8 (.55 MACH)
- Port_Volume_CC = Average_CSA * Port_CenterLine_Length * 16.387
- Port_CenterLine_Length = Port_Volume_CC / ( Average_CSA *16.387 )
- FPS = ( Flow_CFM * 2.4 ) / Average_CSA
- Flow_CFM = Average_CSA * FPS * .4166667
- Average_CSA = ( Flow_CFM * 2.4) / FPS
- cfm demand = cid x rpm x .0009785 / # of cylinders
perfacar
Well-Known Member
I attended a seminar put on by superflow,in 1999, harold betts was the speaker. bottom line he stated was, port shape and port velocity is everything, don't get hung up on flow numbers. port velocity fills cylinders' fast and creates more cylinder pressure, which makes more torque , also power , just food for thought.
Ideally, but practically we got about a half dozen heads to choose from.
Are any of the ootb heads available to us have velocity in the ideal range for there given flow vs port volume ?
Which I don't think any of them are which means your unable to ever build to optimal velocity, unless you modify the head for more flow for a given size or less size for a given flow. And say they all are built with optimal velocity that means whatever head you choose you'd have to build for that particular heads ideal rpm for that cid.
Which I don't think any of them are which means your unable to ever build to optimal velocity, unless you modify the head for more flow for a given size or less size for a given flow. And say they all are built with optimal velocity that means whatever head you choose you'd have to build for that particular heads ideal rpm for that cid.
There's many ways to run 12's with a 318, what velocity are those heads running at ?
So it's getting 110+ VE% making 1.4 + lbs-ft per cid it's at the optimal velocity for that displacement and rpm ? If not what would you even say is the optimal velocity for that engine?
So velocity is important we (you) just don't how much velocity is needed but were (your) definitely sure that smaller is definitely better cause it causes higher velocities and possibly/probably too high for some applications require, what application we (you) don't know but let champion the smallest port possible anyways not the ideal.
Its funny hearing people parrot what's too big and what's too small......Dan Smith runs incredibly quick with a head that everyone including yourself claims is too small for the CI.
There's more that goes on in a running engine than just air flow. What's the velocity in the port when the exhaust is pushing back into the plenum?
Here's a list of velocities don't know how accurate the description are, DVs in the video seem to run higher.
318 @ 5500 rpm head cfm need works out to be 214 cfm. Csa = cfm x 2.4 / fps
1.47" cas = 214 cfms x 2.4 / 350 fps too much velocity
1.65" cas = 214 cfms x 2.4 / 211 fps highest usable
1.80" cas = 214 cfms x 2.4 / 285 fps suppose to be ideal
2.28" cas = 214 cfms x 2.4 / 225 fps lowest usable
2.44" cas = 214 cfms x 2.4 / 210 fps too little
1.65" to 2.28" is a pretty big swing for a 214 cfm head.
318 @ 5500 rpm head cfm need works out to be 214 cfm. Csa = cfm x 2.4 / fps
1.47" cas = 214 cfms x 2.4 / 350 fps too much velocity
1.65" cas = 214 cfms x 2.4 / 211 fps highest usable
1.80" cas = 214 cfms x 2.4 / 285 fps suppose to be ideal
2.28" cas = 214 cfms x 2.4 / 225 fps lowest usable
2.44" cas = 214 cfms x 2.4 / 210 fps too little
1.65" to 2.28" is a pretty big swing for a 214 cfm head.
My point is your always champion the small, carbs, heads etc.. Always citing velocity but you don't seem to know what the ideal velocity.
Yes there's more to a running engine, the whole point of this thread velocity is important but it's at the mercy of what generally available/doable options and what's gonna give the most bang for the buck. Just cause a 318 head can perform doesn't make it the best option in every circumstance like you like to push, and don't mean it's never the right option either.
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