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Jetting in the real world may not be what it says

Canyon707

Canyon707

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Getting closer to Wednesday evening for a test and tune night.
I just came across something I didn't know that I just wanted to share with you if you didn't know this. I am going to pin all my jets because what they say and what they are are not always correct. The results to follow in the morning. I did a couple of extra sets and both sets were not equal.
 
Okay, if you want. But I think you should know, they are rated by what they flow and not by the size of the hole in them.
 
IQ52 said:
Okay, if you want. But I think you should know, they are rated by what they flow and not by the size of the hole in them.
Click to expand...
So your saying two different size holes can flow the same? ?? I'm not an engineer but, to me that can't be possible. Garden hose versus fire hose.
 
In theory it would be completely understandable for a smaller orifice to flow more depending on shape of orifice. But in general I would agree a jets flow would be pretty tied to size. I believe IQ52 is right though. I think Holley flows them and marks them based on flow, not size.
 
if an orfice is .110 and the other is .109 and fuel pressure is equal then it would seem mathematicaly impossible to be equal. Comp jets are more accurate then holly jets.
 
I have a Holley jet set.When I make a move after a plug reading and looking at the racepak data,I go up or down according to what I see.I don't think it makes a diffrence what brand I'm using as long as they are the same manufacturer.

7.8.14 003.jpg 7.8.14 004.jpg 7.8.14 007.jpg 7.8.14 008.jpg
 
I would want that to be true but , I would think all orfices should be equal. No trying to say anything more than a .91x4 should all pin the same.
 
Canyon707 said:
So your saying two different size holes can flow the same? ?? I'm not an engineer but, to me that can't be possible. Garden hose versus fire hose.
Click to expand...
Well then, as you are not an engineer, I suggest you argue with Holley.

https://www.holley.com/products/fue...s_metering_rods_bleeds/main_jets/parts/122-73

"All Holley main jets are flow tested and then stamped with the correct number." (bold letters are mine)

You can pin them and be close, but they aren't final tested by hole size, but by flow, "in cubic centimeters per minute".
 
holley states that they rate their jets by flow rather than orifice size. they will have two jets with the same orifice, but different chamfers on the exit side. they claim the angle on the chamfers can aid flow. their jets are in 2% steps; so they claim. i don't believe the newer brass jets are made near as accurate as the older steel jets.

anymore when doing jet changes i figure the percentage of change using the orifice size as a baseline.
 
Well you may very well be correct. I am not arguing with anyone. Just curious about the differences. Appreciate the input.
 
I can vouch for the chamfer or radii making a difference on flow. I work at JD in the R&D department. See this play out on hydraulics as well.
 
I am an engineer with significant experience in hydraulic flows. The same size orifice does NOT directly relate to the same flow even at the same pressure. Radius & other manufacturing variances affect flow. Holley had the correct approach test the flow. Don't know if the new brass jets are different than the old steel jets. Like many years ago, you still need to do the testing to verify the jetting results.
 
The easiest correlation for us gearheads to understand would be to compare it to an intake port.... simply looking at the size / volume of the port doesn't even begin to tell the whole story. Shape and continuity is everything when it comes to port flow. The same physics that define that are also at work with fuel jets.
 
All this info is greatly appreciated. Always get good answers here.
 
LemonWedge said:
The easiest correlation for us gearheads to understand would be to compare it to an intake port.... simply looking at the size / volume of the port doesn't even begin to tell the whole story. Shape and continuity is everything when it comes to port flow. The same physics that define that are also at work with fuel jets.
Click to expand...

Very good analogy.
 
I have had air bleeds that were incorrectly stamped. We always pin them now. For the jets I check them and if they are close that's that. Refer to IQ52's post above. Also, I once was acquainted with a Top Alcohol dragster tuner and he told me they had a bit of rework when they changed jet manufacturers. Watch the air bleeds! 27 might be 72!
 
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