For any American happy enough to want to use a Constant Depression carb, here is some info for you. It's all about finding the right needle, and is nothing to do with drugs. So DARE to be different, and listen up!
Most people wonder what the heck the 'Poms' were on when the devised a funny carb :-
*shaped like either a Budwiser bottle (SU or SK/Hitachi) or a small land mine (Zenith/Stromberg CD 125, 150, or 175).
*Then to add a brass needle to punch through a main jet to create a squirt of gas, well, that's pretty weird too.
* But the dashpot which holds an oil dampener with a spring which is linked to any alloy venturi. Zenith/Stromberg CD's have a composite diaphram too, just to keep you buying extra parts over its life...they do perish.
*And weird thang four...some versions have a biased spring which pushes the 1.625 to 2 inch long needle against the jet, so the jet wears out enough for you to consider changing it each 20 000 miles, well, thats a crazy idea too.
But the worst thing is "How the freak to I select the right jet for it?"
Here's the answer, in three charts to ensure your Constant Depression carb is Constant Source of Happiness. Each gives hp per carb for each thickness of needle, and allows for normal lean or richer settings. Most SU or Stroberg set-ups are multiple, so adding hp readings allows you to estimate actual hp very accurately.
Chart 1: For 90 thou Jet SU, and Zenith/Stromberg CD needles
NB: The needles are not interchangable between makes, and have a specific stamping number from which there is are about 13 to 16 width measurements available from reference data. Selecting valid jets requires correct spring and progression tunning profile to find the best choice.
|Max. Jet Thickness| Annular jet Area|Power at 15.2:1|Power at 12.5:1|
|-75 thou" thick at tip|-- 1944*E-6------|--30 Mean Bhp--|-25 Mean Bhp--|
|-70 thou" thick at tip|-- 2396*E-6------|--36 Mean Bhp--|-30 Mean Bhp--|
|-65 thou" thick at tip|-- 2847*E-6------|--43 Mean Bhp--|-35 Mean Bhp--|
|-60 thou" thick at tip|-- 3299*E-6------|--50 Mean Bhp--|-41 Mean Bhp--|
|-55 thou" thick at tip|-- 3750*E-6------|--57 Mean Bhp--|-47 Mean Bhp--|
|-50 thou" thick at tip|-- 4202*E-6------|--64 Mean Bhp--|-53 Mean Bhp--|
|-45 thou" thick at tip|-- 4653*E-6------|--71 Mean Bhp--|-59 Mean Bhp--|
|-40 thou" thick at tip|-- 5105*E-6------|--77 Mean Bhp--|-64 Mean Bhp--|
Chart 2: For 100 thou Jet SU, and Zenith/Stromberg CD needles
NB: The needles are not interchangable between makes, and have a specific stamping number from which there is are about 13 to 16 width measurements available from reference data. Selecting valid jets requires correct spring and progression tunning profile to find the best choice.
|Max. Jet Thickness| Annular jet Area|Power at 15.2:1|Power at 12.5:1|
|-80 thou" thick at tip|-- 2827*E-6------|--43 Mean Bhp--|-35 Mean Bhp--|
|-75 thou" thick at tip|-- 3298*E-6------|--50 Mean Bhp--|-41 Mean Bhp--|
|-70 thou" thick at tip|-- 3770*E-6------|--57 Mean Bhp--|-47 Mean Bhp--|
|-65 thou" thick at tip|-- 4241*E-6------|--64 Mean Bhp--|-53 Mean Bhp--|
|-60 thou" thick at tip|-- 4712*E-6------|--71 Mean Bhp--|-59 Mean Bhp--|
|-55 thou" thick at tip|-- 5183*E-6------|--79 Mean Bhp--|-65 Mean Bhp--|
|-50 thou" thick at tip|-- 5655*E-6------|--86 Mean Bhp--|-71 Mean Bhp--|
|-45 thou" thick at tip|-- 6126*E-6------|--93 Mean Bhp--|-77 Mean Bhp--|
|-40 thou" thick at tip|-- 6587*E-6------|-100 Mean Bhp-|-83 Mean Bhp--|
Chart 3: For 125 thou Jet SU, and Zenith/Stromberg CD needles
NB: The needles are not interchangable between makes, and have a specific stamping number from which there is are 16 width measurements available from reference data. Selecting valid jets requires correct spring and progression tunning profile to find the best choice.
|Max. Jet Thickness| Annular jet Area|Power at 15.2:1|Power at 12.5:1|
|105 thou" thick at tip|-- 3613*E-6------|--55 Mean Bhp--|-45 Mean Bhp--|
|100 thou" thick at tip|-- 4124*E-6------|--63 Mean Bhp--|-52 Mean Bhp--|
|-95 thou" thick at tip|-- 4634*E-6------|--70 Mean Bhp--|-58 Mean Bhp--|
|-90 thou" thick at tip|-- 5145*E-6------|--78 Mean Bhp--|-64 Mean Bhp--|
|-85 thou" thick at tip|-- 5655*E-6------|--86 Mean Bhp--|-71 Mean Bhp--|
|-80 thou" thick at tip|-- 6166*E-6------|--93 Mean Bhp--|-77 Mean Bhp--|
|-75 thou" thick at tip|-- 6676*E-6------|-101 Mean Bhp--|-83 Mean Bhp--|
|-70 thou" thick at tip|-- 7187*E-6------|-108 Mean Bhp--|-89 Mean Bhp--|
|-65 thou" thick at tip|-- 7687*E-6------|-116 Mean Bhp--|-96 Mean Bhp--|
|-60 thou" thick at tip|-- 8208*E-6------|-124 Mean Bhp-|-102 Mean Bhp-|
|-55 thou" thick at tip|-- 8718*E-6------|-132 Mean Bhp-|-109 Mean Bhp-|
|-50 thou" thick at tip|-- 9229*E-6------|-139 Mean Bhp-|-115 Mean Bhp-|
|-45 thou" thick at tip|-- 9739*E-6-----|-148 Mean Bhp--|-122 Mean Bhp-|
|-40 thou" thick at tip|-- 10250*E-6----|-155 Mean Bhp--|-128 Mean Bhp-|
|-35 thou" thick at tip|-- 10760*E-6----|-163 Mean Bhp--|-135 Mean Bhp-|
|-30 thou" thick at tip|-- 11271*E-6----|-171 Mean Bhp--|-141 Mean Bhp-|
|-25 thou" thick at tip|-- 11781*E-6----|-179 Mean Bhp--|-148 Mean Bhp-|
Note 1: Peak power is based on having one carb per two cylinders, and assumes an efficient intake manifold with short intake runners.
Note 2: Power is calculated by annular jet area at wide open throttle (assuming correct spring selection) multiplied by 15194 for 15.2:1 stoichometric ratio, or 18417 for richer 12.5:1 power air fuel ratio. This is based on a 5.5 cc/min fuel flow per net hp, or a 6.6 cc/min fuel flow per net hp rating for the richer setting. Peak cc/sec is based on a few dyno runs by David Vizard tests on Mini engines. The peak hp readings of many British cars, converted to SAE net, was rated against jet area to determine flow. It could be argued that this is fairly nebulus, but it works very accurately.
Note 3: There is a change in flow rate at lower needle lifts, and there is a flow area change as the needle goes up and down. Given the astounding number of needles and the ability to change the needle area with minute or large passes of a jewlers needle file, there is no way a more specific jet selection can be made. But its a good start, in my opinion.
Most people wonder what the heck the 'Poms' were on when the devised a funny carb :-
*shaped like either a Budwiser bottle (SU or SK/Hitachi) or a small land mine (Zenith/Stromberg CD 125, 150, or 175).
*Then to add a brass needle to punch through a main jet to create a squirt of gas, well, that's pretty weird too.
* But the dashpot which holds an oil dampener with a spring which is linked to any alloy venturi. Zenith/Stromberg CD's have a composite diaphram too, just to keep you buying extra parts over its life...they do perish.
*And weird thang four...some versions have a biased spring which pushes the 1.625 to 2 inch long needle against the jet, so the jet wears out enough for you to consider changing it each 20 000 miles, well, thats a crazy idea too.
But the worst thing is "How the freak to I select the right jet for it?"
Here's the answer, in three charts to ensure your Constant Depression carb is Constant Source of Happiness. Each gives hp per carb for each thickness of needle, and allows for normal lean or richer settings. Most SU or Stroberg set-ups are multiple, so adding hp readings allows you to estimate actual hp very accurately.
Chart 1: For 90 thou Jet SU, and Zenith/Stromberg CD needles
NB: The needles are not interchangable between makes, and have a specific stamping number from which there is are about 13 to 16 width measurements available from reference data. Selecting valid jets requires correct spring and progression tunning profile to find the best choice.
|Max. Jet Thickness| Annular jet Area|Power at 15.2:1|Power at 12.5:1|
|-75 thou" thick at tip|-- 1944*E-6------|--30 Mean Bhp--|-25 Mean Bhp--|
|-70 thou" thick at tip|-- 2396*E-6------|--36 Mean Bhp--|-30 Mean Bhp--|
|-65 thou" thick at tip|-- 2847*E-6------|--43 Mean Bhp--|-35 Mean Bhp--|
|-60 thou" thick at tip|-- 3299*E-6------|--50 Mean Bhp--|-41 Mean Bhp--|
|-55 thou" thick at tip|-- 3750*E-6------|--57 Mean Bhp--|-47 Mean Bhp--|
|-50 thou" thick at tip|-- 4202*E-6------|--64 Mean Bhp--|-53 Mean Bhp--|
|-45 thou" thick at tip|-- 4653*E-6------|--71 Mean Bhp--|-59 Mean Bhp--|
|-40 thou" thick at tip|-- 5105*E-6------|--77 Mean Bhp--|-64 Mean Bhp--|
Chart 2: For 100 thou Jet SU, and Zenith/Stromberg CD needles
NB: The needles are not interchangable between makes, and have a specific stamping number from which there is are about 13 to 16 width measurements available from reference data. Selecting valid jets requires correct spring and progression tunning profile to find the best choice.
|Max. Jet Thickness| Annular jet Area|Power at 15.2:1|Power at 12.5:1|
|-80 thou" thick at tip|-- 2827*E-6------|--43 Mean Bhp--|-35 Mean Bhp--|
|-75 thou" thick at tip|-- 3298*E-6------|--50 Mean Bhp--|-41 Mean Bhp--|
|-70 thou" thick at tip|-- 3770*E-6------|--57 Mean Bhp--|-47 Mean Bhp--|
|-65 thou" thick at tip|-- 4241*E-6------|--64 Mean Bhp--|-53 Mean Bhp--|
|-60 thou" thick at tip|-- 4712*E-6------|--71 Mean Bhp--|-59 Mean Bhp--|
|-55 thou" thick at tip|-- 5183*E-6------|--79 Mean Bhp--|-65 Mean Bhp--|
|-50 thou" thick at tip|-- 5655*E-6------|--86 Mean Bhp--|-71 Mean Bhp--|
|-45 thou" thick at tip|-- 6126*E-6------|--93 Mean Bhp--|-77 Mean Bhp--|
|-40 thou" thick at tip|-- 6587*E-6------|-100 Mean Bhp-|-83 Mean Bhp--|
Chart 3: For 125 thou Jet SU, and Zenith/Stromberg CD needles
NB: The needles are not interchangable between makes, and have a specific stamping number from which there is are 16 width measurements available from reference data. Selecting valid jets requires correct spring and progression tunning profile to find the best choice.
|Max. Jet Thickness| Annular jet Area|Power at 15.2:1|Power at 12.5:1|
|105 thou" thick at tip|-- 3613*E-6------|--55 Mean Bhp--|-45 Mean Bhp--|
|100 thou" thick at tip|-- 4124*E-6------|--63 Mean Bhp--|-52 Mean Bhp--|
|-95 thou" thick at tip|-- 4634*E-6------|--70 Mean Bhp--|-58 Mean Bhp--|
|-90 thou" thick at tip|-- 5145*E-6------|--78 Mean Bhp--|-64 Mean Bhp--|
|-85 thou" thick at tip|-- 5655*E-6------|--86 Mean Bhp--|-71 Mean Bhp--|
|-80 thou" thick at tip|-- 6166*E-6------|--93 Mean Bhp--|-77 Mean Bhp--|
|-75 thou" thick at tip|-- 6676*E-6------|-101 Mean Bhp--|-83 Mean Bhp--|
|-70 thou" thick at tip|-- 7187*E-6------|-108 Mean Bhp--|-89 Mean Bhp--|
|-65 thou" thick at tip|-- 7687*E-6------|-116 Mean Bhp--|-96 Mean Bhp--|
|-60 thou" thick at tip|-- 8208*E-6------|-124 Mean Bhp-|-102 Mean Bhp-|
|-55 thou" thick at tip|-- 8718*E-6------|-132 Mean Bhp-|-109 Mean Bhp-|
|-50 thou" thick at tip|-- 9229*E-6------|-139 Mean Bhp-|-115 Mean Bhp-|
|-45 thou" thick at tip|-- 9739*E-6-----|-148 Mean Bhp--|-122 Mean Bhp-|
|-40 thou" thick at tip|-- 10250*E-6----|-155 Mean Bhp--|-128 Mean Bhp-|
|-35 thou" thick at tip|-- 10760*E-6----|-163 Mean Bhp--|-135 Mean Bhp-|
|-30 thou" thick at tip|-- 11271*E-6----|-171 Mean Bhp--|-141 Mean Bhp-|
|-25 thou" thick at tip|-- 11781*E-6----|-179 Mean Bhp--|-148 Mean Bhp-|
Note 1: Peak power is based on having one carb per two cylinders, and assumes an efficient intake manifold with short intake runners.
Note 2: Power is calculated by annular jet area at wide open throttle (assuming correct spring selection) multiplied by 15194 for 15.2:1 stoichometric ratio, or 18417 for richer 12.5:1 power air fuel ratio. This is based on a 5.5 cc/min fuel flow per net hp, or a 6.6 cc/min fuel flow per net hp rating for the richer setting. Peak cc/sec is based on a few dyno runs by David Vizard tests on Mini engines. The peak hp readings of many British cars, converted to SAE net, was rated against jet area to determine flow. It could be argued that this is fairly nebulus, but it works very accurately.
Note 3: There is a change in flow rate at lower needle lifts, and there is a flow area change as the needle goes up and down. Given the astounding number of needles and the ability to change the needle area with minute or large passes of a jewlers needle file, there is no way a more specific jet selection can be made. But its a good start, in my opinion.