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 Post Posted: Sat Aug 04, 2012 10:12 am 
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working on a 23* sbc ex port, the accurate csa at the point near the exit thats being checked is 1.71 it flows 198 at this lift point with no pipe, the calculated way claims it should be 278fps < cfmx2.4/csa> but measured with 2 different pitots a water and digital mano im getting 350fps, which can you go by for accuracy, thats a big spread, or am i doing something wrong, thanks


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 Post Posted: Sat Aug 04, 2012 11:55 am 
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Are you measuring fps in many places spaced evenly around the opening and averaging? or is 278fps the peak or center fps?

It is hard to get good pitot readings on place in the port that is turning. The pitot tip and static ports of the probe are not at the same place. This makes little difference when the flow is fairly straight but can have a bigger effect when the flow is turning sharply or turbulent. The exhaust port exit can be really turbulent and have reverse flow back into the bottom of the port.

What bench? How does your pitot compare on an intake port measuring average velocity at a location vs measured total cfm?

Rick


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 Post Posted: Sat Aug 04, 2012 2:23 pm 
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hi rick, the pitot was about 3/8 in the center of the port , havent tried it on the in, its a sf600 with performance trends electronics, there dosent seem to be any backward flow but some areas at that spot do dip into the 170 range in the corners , is averaging more accurate than center point readings


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 Post Posted: Sat Aug 04, 2012 2:45 pm 
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zums wrote:
is averaging more accurate than center point readings


If you are comparing pitot readings to total flow cfm readings then yes, average pitot is the only way to compare since the total flow is spread out across the entire cross-section. It can be a lot different from one place to another.

On intake ports I am analyzing I do a 3x3 grid for 9 readings and they normally average very close to the cfm reading for the csa. I haven't done a lot of probing of the exhaust port because w/o a pipe on it the flow will be much different than with a pipe, both quantity as well as where most of the flow is at. With a pipe on it you can't get a pitot in the port.

Rick


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 Post Posted: Sat Aug 04, 2012 6:32 pm 
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ok , on your intake probe readings, you average out all 9 readings even though some areas may be close to dead and 100- 150fps slower than your fastest and you find that accurate enough to use as part of you cfm demand equation, what do you use for an accurate exhaust equation, -thanks, tom


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 Post Posted: Sun Aug 05, 2012 3:56 pm 
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zums wrote:
ok , on your intake probe readings, you average out all 9 readings even though some areas may be close to dead and 100- 150fps slower than your fastest and you find that accurate enough to use as part of you cfm demand equation, what do you use for an accurate exhaust equation, -thanks, tom

The pitot readings I use don't really have to do with cfm demand. They are for seeing how effectively the area is being used at a given point in a port. Shows how the shape of the port is working. Keep in mind a port that is too big for a given engine can read very high velocity readings on a bench. Pitot readings don't tell you how big a port should be for a given engine and rpm range.

When you questioned the reading on your pitot showing less than (cfmx2.4)/area - I was pointing out that a single reading can't be compared to what is an average velocity calculation. The highest velocity point (measured) should always show faster than the calculated average from the flow.

I determine the correct exhaust port size by using Engine Pro and/or pipemax. Then try to make the most efficient port of that size.

Rick


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 Post Posted: Sun Aug 05, 2012 5:47 pm 
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thanks rick


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 Post Posted: Tue Aug 07, 2012 4:19 pm 
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One thing I will add that's along the line Rick mentioned.

My pitot probe for the ex, has the static port over an inch from the impact port.
So when trying to probe the exit of an ex port the static port would be outside the port unless you had the end of the probe atleast an inch in the port. Then your not getting impact readings from the exit of the port, and on a 23* sbc ex port, the impact port would be at the short turn where (like Rick said) the air is turning and the velocity would vary a lot from top to bottom of the port. So really I don't think you can use a probe in a 23* sbc ex port to measure csa by averaging several readings.

I also wouldn't get all hung up on this 350 fps limit that gets mentioned a lot. If your working a 23* sbc intake or ex port, and chase the speeds in all locations down to 350 fps or less, you will either have a port that doesn't flow well, causing the speeds to be low, or a port that is too big. The short turn speeds on a good flowing 23* sbc head will be over 350 fps and in a non raised runner head it will be over 400 fps. I've seen heads ported by pros claiming how even thier speeds are, and they are as high as what i get.

Hey Zums, what head/project are working on?

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 Post Posted: Tue Aug 07, 2012 5:54 pm 
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hi randy, been a while, happy late congrats on emc i was impressed, all this carving ive spoke with you about months ago was on a mexico vortec, finally setteled on a decent int valve job form tool, now lately all work has been on the ex, this is the 5th port trying different vjs, csas and exit shapes, the head that will be used on this 373- 7800 shift point is the iron sm port vortec bowtie,i just posted on the pipemax site since this is the only program i have for csas, and if you go by suggested throat and exit fps the port seems extemely small- 400fps at the throat, 330 exit, i hear you on the speeds over the turn on the in, best i got was 390, the pitot pick up is outside of the ex port like you say but checking it with a mag gage the #s are real close to the pitot


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 Post Posted: Tue Aug 07, 2012 8:51 pm 
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So your using small port vortec bowties on the 373 and the factory vortecs on a different project?

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 Post Posted: Wed Aug 08, 2012 7:29 am 
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the spvb on the 373 - the factory heads were just practice to get shapes and v/jobs right


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 Post Posted: Wed Aug 08, 2012 9:40 pm 
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So how did the small port vortecs turn out?

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 Post Posted: Thu Aug 09, 2012 7:46 am 
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this is my last practice exhaust port, thats why i posted about the fps / area differences, the spvb are still in the box, once i get the form tool made i will start on them


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