S.U. and Stromberg CD 150/175 Needles for any engine.

[xctasy]

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.

 

[xctasy]

An example of how it works. Say you have a 200 or 221 cubic inch engine which couls hit the 220 hp mark with the right carburation.

Eg. 1
What jet size do we need if we were going to use three CD175 Strombergs (a 1 3/4" carb) or tripple SU HD8's (A 2 inch carb).

Easy. Take 220 hp and divide it by 3. Check Charts 2 and 3. Each of the carbs will produce 73 hp at a 12.5:1 air/fuel ratio. These carbs use a 100 thou jet as a CD175, and often 125 thou (Chart 3) as a SU HD8. Looking at the 100 thou Chart, a 73 hp requrement would be served by either a 60 thou jet (71 hp) or a 55 thou jet (79 hp). So any 100 thou jet needle which has a 60 to 55 thou needle diameter at the last section should be considered as a candidate.

In the case of the SU HD8's, look at the 125 thou chart 3. 95 to 90 thou gives 70 to 78 hp. Try a needle of about 95 to 90 thou. Then look at the needle charts in the SU factory or Haynes manuals, and pick a candidate.


Eg.2: What if we have a stove hot turbo 200 Mustang with a draw through turbo? It lookes like it might give 300 hp. We've decided two SU HS8's, with 125 jets, is the best option. Look for a needle with 150hp capability. But we want to run it rich so the excess fuel will cool the combustion process. From Chart 3, at a 12.5:1 air/fuel ratio, then a 25 thou needle will only just make 148 hp, or 296 hp working as a pair. But its the only option there.

Eg.3: A kid and his engine builder dad looke at an old Ak Miller article in Hot Rod on working a 200 cube six. They decide two 1.5" SU carbs is a good idea for a cool streeter. They want 150 hp. What sort of carb needles is needed, and will they get away with a smaller 90 thou jet items form an early MG? They check Chart 1, and find a 40 thou needle will give an HS4 carb 77 hp each with a 90 thou needle. Enough for 148 ponies. From Chart 2, a 100 thou jet carb can use a different needle of around 55 thou at the tip.

So there you go. The formulae used is very slightly conservative. I've seen 115 hp + come out of a 100 thou jet and the thinest needle. It won't get you into trouble!

 

[addo]

I feel like you're looking over my shoulder... Have to do a deal on those other carbs!

 

[xctasy]

Yes, Adam, I'm quite taken by these carbs. They have lots of adjustability!.

As we discussed, I'm fairly sure I've got no need for a set of Vanden Plas Princess HS8 carbs with UV needles which have a 102.9 thou tip. By my fomulae, it has about 60 hp per carb. That would be a minimum of about 180 hp for three carbs. For the two carbs in stock form in the 3.912 liter Rolls Royce engine, they claimed 175 hp gross. The way it all works is to take the area of the 125 thou main jet, which is always 12272 x10-6, and deduct the area of the tip of the needle 2 inches down from the shoulder. This is 102.9 thou form my Haynes SU carb book, dating back to the mid 1970's. This gives an area of 8316 x10-6 inches. Deducting the big area form the little one, thats about 3956 x 10-6 inches of anular area for a 102.9 thou jet. Discharge is proportional to area exposed.

The normal jet sizes for whatever carbs you have can be found here at Burlen. In your case, these are the jet details:-

http://www.burlen.co.uk/burlen-user/default.asp?Category=burlen&Service=CarbSpec&Veh=2332

The really good thing is that the carb is very responsive to engine vaccum changes, and can lean cruise with the best of them. And you can't over carb much with them, the variable cross section of the piston and the housing make sure of that. It just doesn't open up fully if you've got one too big. It's estimated it is 5 to 10% more economical than a Weber or Dellorto carb.

 

[twentyover]

Another assist in fine tuning is Haystack and and WinSU. Haystack lists all - what, 375 needles for .090 jet and 250 or so fo .100 jet.

But this is most helpful getting an approximate starting needle.

And the need thing is-

When you get a needle close enough, you can richen it in various zone by filing or rotating agains a grinding wheel (I has our machinist chuck it in a collet on a surface grinder.

 

[xctasy]

Wow, twentyover!

http://haystack.lcs.mit.edu/

http://homepage.ntlworld.com/sbeavis/winsu/winsu.html

My goodness, you were right on in everything you've said! Thankyou!

At work, we use Acess and Oracle-based databases to catalogue our maintenance intervention strategies. I've often thought of just running Lookup tables like we do in those environs. Seems the peak power concept is a very healthy initial guess, and the WinSU program ties it all together. I have a passion for details, and usually remember things I'm really interseted in. But it sucks when you have to rationalize your choice.

That program is quite a treat, because it takes away all the talk and gives clear results from a known combo!


http://www.bikerlifestyle.co.uk/su-carb ... le-finder/