I've seen several anecdotal accounts saying one clip position is equivalent to 3 changes in needle jet size so I decided I would measure my own 46-241 needles to see if that was true. I calculated the change in flow area for small increments across the full lift of the needle and then compared that with the increase in flow area resulting from a needle jet change.
It turned out to be relatively straightforward to do the calculations as the tapered section of the needle is a simple straight sided truncated cone that reduces from 2.45mm to 1.2mm along it's 23mm length. I plotted the needle diameter at 1mm intervals and calculated the change in area for 1 needle clip position.
The effects are non linear so the answer turns out to be more complicated than just saying it's a ratio of 3 although that is probably a reasonable average at mid throttle. Anyone who has tried it knows that 1 clip position is a big change but the flow area change turned out to be a lot bigger than I expected.
My factory settings are 2.66 needle jet and needle at position 2 so that is the base line for my comparisons as shown by the green column in the table of figures and graph attached. A summary of the results is as follows:-
Increasing the needle jet from 2.66 to 2.68 results in a jet flow area increase of 10% at initial lift reducing to 2% at max. lift
Likewise reducing the needle jet from 2.66 to 2.64 results in a jet flow area reduction of 10% at initial lift down to 2% at max. lift
Lifting the needle from P2 to P3 results in a jet flow area increase of 44% at initial lift reducing to 5% at max. lift
Combining a 2.64 jet with needle in position 3 still gives 34% increase at initial lift falling to 3% at maximum lift,
Base data for the calcs:-
Needle Dia 2.45
Tip Dia 1.2
Cone length 23
2.64 jet Area 5.47
2.66 jet Area 5.56
2.68 jet Area 5.64 |
