So, I have seen numbers of 25" of water, 28", etc. Work uses 62mBar (25")
What is recommended for pressure and why?
What is recommended for pressure and why?
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What is the Best Pressure to use for Flow Bench Testing?
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PRH
Well-Known Member
It’s not super critical what the actual test pressure is, but if you’re comparing numbers from different services/benches, any difference in test pressure between them needs to be accounted for…….. as in a correction factor applied.
Superflow calibrates their benches using 25” test pressure, but most of the published test results are for 28”.
Superflow calibrates their benches using 25” test pressure, but most of the published test results are for 28”.
25" is fine, the correction factor for the reading @25"H2O is × 1.0583 to get the common @28"H2O value.
What is 'best', there's a lot of tall grass to wade thru', there are a number of opinions on that. I test using floating, and more than one static, and reverse.......and a couple of other 'things'. Best thing is for You is to have a common baseline item w/a predictable flow flowed on the bench, then test the modded/aftermarket stuff. There are so many documented examples of say stock 906's w/a decent valve job, that flowing one 1st lets You have a relative perpective to other bench results.
Talking with folks at work, it seems that the more fine tuning you do in port shape/size, the more PSI you want to check at. Some are capable to go up to 100". This will show very minute changes being made. Some very large ports need 40+" to flatten out turbulent flows
PRH
Well-Known Member
Try not to get lost in the idea of chasing flow numbers.
Bigger flow numbers don’t always equate to higher power output.
Bigger flow numbers don’t always equate to higher power output.
I am mostly curious as to never having flowed heads before and these have been worked over. I know you can mess a set up by just hogging them out without knowledge of how air flows. I do enjoy the Youtube Channel Bain Racing, he does a great job
PRH
Well-Known Member
Imo, for a NA 470, 440-1’s with 2.25” valves flowing in the 350’s are plenty big enough for you to reach your power level goals.
If there are deficiencies at the SSR that are holding the numbers back, that could be rectified with minimal material removal………depending on whether or not the current/previous porting left enough to work with, that’s often the trouble spot when those heads are coming up short on flow.
If there are deficiencies at the SSR that are holding the numbers back, that could be rectified with minimal material removal………depending on whether or not the current/previous porting left enough to work with, that’s often the trouble spot when those heads are coming up short on flow.
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I have heard it said, if the flow isn't butchered, you can typically double the CFM and that is the HP it can support. What are your thoughts on that?
TT5.9mag
Two atmospheres are better than one
To answer your original question, the industry standard is 28” of water column. All other test pressures are “factored” or “corrected” to obtain results consistent with 28”. There is definitely a bunch to learn flowing at different pressures and in reverse. Also I’ve heard more than one head porter say they listen to the port and that tells them more than any number from a machine.
2hp per cfm is consider a reasonable goal/capability even if the head is less than stellar, with the right parts you should be able to get there, Pro Stock heads can get like 2.5 hp per cfm, but most street performance engines fall into 1.4 to 1.8 hp per cfm some getting into the 2's. With enough cam and cr should be able to get 2hp per cfm.
TT5.9mag
Two atmospheres are better than one
As far as what pressure is best? I’d say whichever pressure simulates the conditions the port will see in a running engine. But we all know how dynamic the airflow in a port can be at different rpm ranges and loads so there has to be some kind of standard. I wonder how they settled on 28”?
I know it would have to be much higher than 28" but if the standard is 28, then that is best to use, I think. I read some things where Ford was using 60" when fine tuning ports back in the day. One article said 100" to pick up the finest of changes. So, there seems to be a correlation between pressure and fine tuning what you are changing.
PRH
Well-Known Member
That’s def an “it depends” proposition……
I would refine the parameters a bit more…….
For a racing application, when all the parts are on the same page, for a v8 engine………. It’s really not that hard to take the flow at 28” for one cylinder, and have the engine output reach or exceed 2x that number.
Especially when the heads err on the correct/smaller side for the displacement.
Example, 300cfm rpm heads on a 446……600hp, no problem.
360cfm cnc ported SB Victor heads on a 273?
Not going to be easy to get 720hp out of that.
I’ve seen 800+hp out of 360cfm EZheads……. Big cubes, high CR.
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From what I see flow is overall easier to get and the power per cfm is fairly wide, It's mainly about cross sectional area, minimum cross sectional area (air speed) and shape (how well it can deal with that air speed) you got those two things it should be flowing enough to turn the rpm and make the power you want.
People used to use 3" and 10" back in the day and did great things, pretty sure Bob Mullen was using 3" when designing W2's.
Yet they failed miserably trying to do a duplicate treatise to a B/RB head,..... & I thought Mullin used 5 & 9 inches. I'd have to go back to those 'old' books & check again tho'. Smokey did so much extensive testing with His shop partner, & discovered that once You achieved the velocities created by 28"H2O, it always correlated to track performance........more flow @28", more power/speed on the track, and once He declared this 'rule', almost everybody followed it....'cause He was Smokey. We know more now for sure, but it's not a bad rule, just an incomplete one.
Earlie A
Well-Known Member
On a Superflow bench, pulling 400 cfm at 28” depression takes almost 30 amps at 230 volts. If you double the depression to 56”, the flow would theoretically increase by 42%, but the power required to move the air would nearly triple. That means close to 90 amps. That’s lots of equipment cost and lots of electricity cost.
As Killer6 said, Smokey determined that 28” on the bench correlated pretty well with the running engine.
As Killer6 said, Smokey determined that 28” on the bench correlated pretty well with the running engine.
I'm just saying 3" is better than none
I mean porting without any kind of bench, I don't know if the OP has one, building one or whatever, but if all he can afford is a shop vac or two using the block as a bench to professional one that can do 28" plus at his needed flow, something is better than nothing.
We are going to flow test at work. We have a Superflow Bench. Not sure the capabilities until next week. Not my department, so I need to wait for them.
If you're just looking for flow that means nothing without knowing the air speed. A little less opening can increase air speed. We put a little bit of clay on the bottom of my Hemi ports and it increased flow and air speed. Give your heads to someone doing good competition work, such as stock and superstock heads and you should get good results.
You also have the opposite problem, too much air speed.
We have an old super flow that's 15 inches.
He said back in the day it would barely keep up with a stock camel hump.
It's mine when I want to take it home and upgrade it... Figure Timmy and I can have some fun with it.
He said back in the day it would barely keep up with a stock camel hump.
It's mine when I want to take it home and upgrade it... Figure Timmy and I can have some fun with it.
My first experience with having my heads flowed was pretty cool, I was surprised how long it took to get up to 28 once the valve got opened up to .750
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