SU Needle Station spacing vs Richness Tuning Conundrum

[zKars]

You can tell I’ve been drinking and thinking again...

If the SU needles have richness “stations” (Progressive diameter changes) every 1/8th of an inch, why does moving the richness adjusting knob by as little as 1/4 turn make a significant and measurable change in AFR? The thread Pitch on that adjuster is 1.0 mm, so it should take 3.5 turns to make a one station shift.....

Just saying, what the heck is going REALLY going on that no one wants to talk about...

Is there more to the amount of fuel that is allowed to leave the jet tube than the simple needle OD/tube ID difference right at the top of the bridge or top of the jet below the bridge? Or a complex function of the various ID/OD changes throughout the jet tube? 

Does this revelation also lead one to addition tuning engineering? Or just more reasons to drink?

Let’s see whatcha’ all come up with on this one...

Edited June 3, 2020 by zKars

[Captain Obvious]

I believe it's simply ID and OD.

The small change might seem immaterial, but I bet it's quite significant if you look at it in percentages.

[Captain Obvious]

Oh, and I forgot this!!!   >>>     

[240260280]

 

 

Here is a quick calculation based on the following general assumptions:

• Air Flow over a puddle of fuel draws fuel into the air flow
• The larger the puddle, the more fuel is drawn
• The air does not saturate with fuel
• Air flow is constant
• Depression over exposed fuel  is constant

and here are the SU assumptions:

• venturi effect of bridge is ignored
• effect of piston raising is ignored
• air flow is constant over exposed fuel puddle
• fuel depth in well is ignored
• exposed fuel puddle is proportionate to diameter of fuel well (2.54mm)  minus the area blocked by the needle's diameter at the surface.
• Station one Top needle dia is 0.095"
• Station one Bottom needle dia is 0.094"

 

Here are A:F Ratio calculations for lowering a jet for an SU carb with:

• Top of station (Jet drop = 0) AF is 14 with N27 needle dia of 0.095" reducing to 0.094" at bottom of station.

The math seems to match observations ?

 

 

 

 

 

Edited June 3, 2020 by 240260280

[240260280]

https://www.youtube.com/watch?v=ljfJkPode_k

 

[zKars]

Interesting. Perhaps the root misconception is that the needle is actually more of a continuously tapered shape, it is NOT a series of 1/8” Long tubular sections. Thus you ARE changing the Needle OD continuously as you raise and lower it by small amounts.

Even though I can find drawings with both concepts, the first one here is more the correct representation of the actual shape

More like this;

Not like this:

[Captain Obvious]

Right. The slope of the taper (rate of change) may be different across the stations, but it's always a "slope" and not a step.

Looking at the .095 vs .094 example above in a different way: 

If the needle is .095 and is in a .100 hole, you have 90.25% of the hole filled up (with needle) and you have 9.75% of the available area open to fuel flow.

Changing the needle diameter to just .094, you have 88.36% of the hole filled which results in having 11.64% of the available area open for flow.

So changing just one thousandth like that results in an approximate 2% increase in the area available for fuel flow.

[Patcon]

17 hours ago, Captain Obvious said:

Right. The slope of the taper (rate of change) may be different across the stations, but it's always a "slope" and not a step.

Looking at the .095 vs .094 example above in a different way: 

If the needle is .095 and is in a .100 hole, you have 90.25% of the hole filled up (with needle) and you have 9.75% of the available area open to fuel flow.

Changing the needle diameter to just .094, you have 88.36% of the hole filled which results in having 11.64% of the available area open for flow.

So changing just one thousandth like that results in an approximate 2% increase in the area available for fuel flow.

2% of the whole but 25% richer...

Correct?

[Captain Obvious]

Yes... 2% of the hole.  

Considering the hole (nozzle) to be 0.100 inches diameter, the "whole hole" area would be 0.00785 square inches. The difference in the amount of the hole open for fuel flow is 2% larger using a .094 obstruction as opposed to a .095 diameter obstruction.

As far what the direct impact is on richness, I don't know. I doubt it's a simple calculation and don't know if it could be modeled easily.

In other words, the area calculations are easy... The exact IMPACT on the bottom line (mixture ratio) is beyond my pay grade.