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well, what if you have a jacked up short turn that shows up at 28 but not at 36? does that mean that the short turn is ok now because your pulling so hard on the port that it doesn't show up?

I know Smokey went thru this when he decided that 28 should be the number. How many inches is the piston pulling once its past halfway down the bore?

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If it shows up at 28 as a SMALL issue it may or may not get any worse but if it does get worse in my testing the issue needs addressed. I can only go to about 34 inches on my bench. Lots of discussions on this but it still comes down to the fact that a valve is at .500 twice and .700 once, that is if you have a 750 lift cam by time lash and pushrods deflection is factored in.

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Thinking about it dynamically you would think lower lift should be tested at lower pressure because the piston is not moving very fast or forcefully compared to 1/3 to 1/2 way down the bore when the valve is open and piston speed is high. Also if the ex valve is open it is just scavenging depression causing the flow witch surely is not 36 inches.

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I disagree, low lift is when it is moving the fastest during the overlap period and right after the exhaust valve closes, after that

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Thinking about it dynamically you would think lower lift should be tested at lower pressure because the piston is not moving very fast or forcefully compared to 1/3 to 1/2 way down the bore when the valve is open and piston speed is high. Also if the ex valve is open it is just scavenging depression causing the flow witch surely is not 36 inches.

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I disagree, low lift is when it is moving the fastest during the overlap period and right after the exhaust valve closes, after that

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well, what if you have a jacked up short turn that shows up at 28 but not at 36? does that mean that the short turn is ok now because your pulling so hard on the port that it doesn't show up?

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If it shows up at 28 as a SMALL issue it may or may not get any worse but if it does get worse in my testing the issue needs addressed.

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I keep reading on SpeedTalk where folks talk about ports looking just fine at 28", but then go all to kwap at 36-40". Supposedly, even if the flow #s don't increase at 28" after correcting the issue, the engines run better once the issue found at the higher test pressure is resolved.

I know I've had some heads on my "toy" bench that didn't sound good even at 10". Would be really interested in knowing what they do at 28", and then even higher.

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I'm sure this is a stupid question, but why not test flow at a vacuum level similar to the actual port at wide open throttle? Like 1 inch? WOuldn;t that be a more realistic test of the ports performance in the engine?

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Thinking about it dynamically you would think lower lift should be tested at lower pressure because the piston is not moving very fast or forcefully compared to 1/3 to 1/2 way down the bore when the valve is open and piston speed is high. Also if the ex valve is open it is just scavenging depression causing the flow witch surely is not 36 inches.

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I disagree, low lift is when it is moving the fastest during the overlap period and right after the exhaust valve closes, after that

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Well, you're kind of(?) both wrong.

This graph from David Vizard's most recent porting book may help you understand what's going on w/ the in-cylinder pressures and seat / port velocities:
1. Highest is during overlap (at higher RPM, at least) due to exhaust-assisted low pressure
2. Lowest is as the intake cycle is closing; might even go in reverse as you get into reversion if the valve doesn't close soon enough
3. Peak after the overlap cycle has ended is shown around the 70-75* ATDC point where, in this particular graph, the pressure differential is bouncing around the 50" H20 mark

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I'm sure this is a stupid question, but why not test flow at a vacuum level similar to the actual port at wide open throttle? Like 1 inch? WOuldn;t that be a more realistic test of the ports performance in the engine?

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Thinking about it dynamically you would think lower lift should be tested at lower pressure because the piston is not moving very fast or forcefully compared to 1/3 to 1/2 way down the bore when the valve is open and piston speed is high. Also if the ex valve is open it is just scavenging depression causing the flow witch surely is not 36 inches.

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I disagree, low lift is when it is moving the fastest during the overlap period and right after the exhaust valve closes, after that

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Well, you're kind of(?) both wrong.

This graph from David Vizard's most recent porting book may help you understand what's going on w/ the in-cylinder pressures and seat / port velocities:
1. Highest is during overlap (at higher RPM, at least) due to exhaust-assisted low pressure
2. Lowest is as the intake cycle is closing; might even go in reverse as you get into reversion if the valve doesn't close soon enough
3. Peak after the overlap cycle has ended is shown around the 70-75* ATDC point where, in this particular graph, the pressure differential is bouncing around the 50" H20 mark

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Well if thats the case, it looks like you should be using much higher vac. According to Vizard's graf and comments

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28 has been used as a industry standard for so long because it works and is trusted in the high performance market. Go to Stan Weises website and every head is at 28 inches. WHY Super Flow sets their benches up for 25 inches of depression always bothered me and is one reason I would never buy one of their benches. Even their high output models test at 25 inches.


http://www.superflow.com/Flowbenches/

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I'm sure this is a stupid question, but why not test flow at a vacuum level similar to the actual port at wide open throttle? Like 1 inch? WOuldn;t that be a more realistic test of the ports performance in the engine?

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When we measure flow on a flowbench, we are measuring at 28" h2o. The vacuum measurements you take at the carburetor are measured in "hg
1"hg is close to 13.6" h2o.
When Smokey came up with 28" for a standard he was assuming 2" h2o was the average and converted that number.
Keith

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Great info, thanks!

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I'm sure this is a stupid question, but why not test flow at a vacuum level similar to the actual port at wide open throttle? Like 1 inch? WOuldn;t that be a more realistic test of the ports performance in the engine?

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The 1.0" to 1.5" WOT pressure at the carb isn't the same as the pressure seen (felt?) by the head & cylinder during the intake & exhaust cycle. Same engine, but very different conditions being experienced in different locations of the induction system.

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My bad... also forgot about the Hg vs. H2O difference that Keith D. pointed out.

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Just because 28 is the defacto standard means nothing. What looks good at 10" won't work at 28" (as seen by early ported stuff when a superflow bench top was the cats buttocks).

So...if you flow something at 28" and then test it at 32" (or more) and the port doesn't keep up mathematically...you need to fix it.

It's that simple.


FWIW...the higher the test pressure the better.

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As usual a lot of people put way to much into the
flow numbers... we have all seen this... big flowing
heads that SUCKED on the track... you can spend all
sorts of time TRYING to come up with the perfect set
of heads on a flow bench... but.. I've never pulled
up to the line next to a flow bench.. and everyone
will agree that the best dyno is the track