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specialgreen · Posted: Mon Oct 10, 2005 3:24 pm Post subject: carburetor jet size calculator

I was playing with an orifice calculator at http://www.tasonline.co.za/toolbox/pipe/velorif.htm

I filled-in my stock main-jet orifice size (1.25mm), coefficient 0.98 and typical flow-rate of my motorcycle at cruising speed on the highway (0.0090909 gallons per minute per carburetor). I clicked to "calculate head drop", which filled-in the "head drop" field for me (whatever that means).

Then I increased flow rate by 30% (0.0118181 gpm) and selected "calculate orifice diameter", and got a new orifice size of 1.42mm... only 13% larger diameter than stock (1.25mm), to achieve a calculated 30% increase in flow rate.

That's interesting, since I've been running with 25% oversize diameter main jets for a couple of years. According to the calculator, they should be delivering 64% added flow rate over the stock main jets!

And even with those huge 25%-over diameter main jets, I had to increase idle jet diameter by 42% over stock to get a smooth idle (which seemed to be way too much to me). I've been suspicious that I was just using a huge idle jet to compensate for a too-small main jet.

But then, I had enlarged the idle jet by hand, which made for an imperfect orifice shape. The web form shows that the shape of the jet has more to do with flow rate than do moderate changes in orifice size. So who knows what size idle jet I really needed!

hotrod · Joined: 19 Apr 2005 Posts: 872 Location: Colorado

You are correct about the re-drilled jets. You can drillout a jet and actually end up with less flow than the original due to changes in the shape of the entry. If at all possible, use factory jets of the nearest calculated size.

If they do not exist, try to buff the entry of the new drilled out jet to have the same small radius on the entry of the hole. A very sharp entry edge causes a "Vena contracta " to form as the flow separated from the edge and you only end up using a portion of the actual hole size. very small changes in a sharp corner entry orfice make a very big difference.

So much so, that there is a measurable increase in air flow through a sharp cut hole as dirt/dust builds up on the entry side of the hole.

Simply buffing the entry of a hand drilled jet with a bit of fine steel wool to knock the sharp edge off the hole can make a noticiable difference in flow.

To calulate jet sizes you need to figure based on the AREA of the jet orfice.

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complete step by step

1.25 mm stockjet

1.25/2 = 0.625 = radius
0.625 ^2 = 0.390625 = radius squared
0.390625 * 3.1415927 = 1.22718463 == jetarea in mm^2

increase by 27% = new flow factor of 1.27

0.122718463 * 1.27 = 1.55852448 = new jet area

1.55852448 / 3.1415927 = 0.49609375 = new jet radius ^2
sqrt (0.49609375) = new jet radius = 0.704339229
0.704339229 * 2 = new jet dia = 1.408678459

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Short calculation

((old jet size dia) ^2 ) * (new flow factor) = x

(new flow factor would be 1.27 in my example above)

sqrt (x) = new jet dia.

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Notes

(y)^2 means the quantity (y) squared

sqrt (z) means take the square root of the quantity (z)

Larry

specialgreen · Posted: Tue Oct 11, 2005 11:46 am Post subject:

I'd been reading info on orifice/flow-rate relationship, such as http://www.mcnallyinstitute.com/13-html/13-12.htm . There are a bunch of variables, including the coefficient (shape) of the orifice and pressure difference across the orifice (or inches of vacuum, in the case of carbs), which is linked to original flow-rate. It looks like a jet change that makes 30% more flow-rate at WOT may produce a different percentage of additional flow-rate at quarter-throttle.

However, for highway cruising, your method produces about the same result as the fancy calculator, so it appears that for our purposes, just going by orifice area is sufficient.

hotrod · Joined: 19 Apr 2005 Posts: 872 Location: Colorado

quickd100 · Joined: 01 Oct 2005 Posts: 14

You can get a copy of the mean jet flow values of holley jets from holley at no charge or possibly on line. There should be no need to drill jets unless you're running something other than a holley. Dave
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1.595-60ft., 7.621-1/8, 12.19@105.88-1/4