SU Needle stations.

It is amazing at the great amount of info there is on the SU carb on the internet.

I have been playing with needle profiles for my pair of 1.5" fixed needle SU's, one site that the tables work very well is...

http://www.teglerizer.com/suneedledb/index.html

Maybe the folks that have the site are on this board as I have tried without success to get a reply to a question. Here is part of the table for a #5 needle.


Needle size #5 (Stn1 890) (Stn2 850) (Stn3 814) (Stn4 785) (Stn5 758) Etc....

The chart will not reproduce here it all falls apart but the diameters and the stations are correct for this small part of the table.

...my question I have been tring to find is the distances of the stations...I assume they start at the root/fastening boss.

It is probably one of those things that everyone knows but me...a couple of diff sites I have visited mention the Stations but what are the longitudinal distances...they are probably a common repeating dimension... can someone help?

I believe the distance between each measurement is 1/8th of an inch.
Check here on pages 9 and 10:
http://www.bmcno.org/manuals/Tuning%20SU%20Carburetters.pdf
Greg.

It is 1/8" per station. starting at the bottom of the shank of the needle. Another good and easy to use needle comparison site is this one, http://www.mintylamb.co.uk/suneedle/ . A bit easier to use than others.

Paul Tegler has compiled a pretty good list of internet SU resources. Don't discount those sites put up by the Datsun guys, some of their stuff is, frankly, better than ours (britcar SU sites). {I put on my steel pot and am in my foxhole ready for the bricks to fly. ;) }

When I started mucking with my Hs4s and adding an A/F meter in the exhaust I thought i would need to do a compilation of station vs A/F.

To this end I discovered that a std dowel (forgot now...3/8"?) fits snug into the piston.

Colored tape could demark the stations. Simple enough. (and taken to a different level could be used as a synch tool)

And by running under load and observing both the tape color and A/f ration one could determine that station 8 under ?? load delivered xx A/F. But the transition between data to information (i.e. numbers to useful stuff) seems illusive. Not independent at all. Actually, this is part of the SU's charm -- it reacts to the load rather than to observable settings.

So I thought I'd just run it on a dyno. OK.

I did not do it. Because in designing the test protocol I decided that the data would be immense. The usable information would be virtually nil.

So I arrived at the conclusion: tune your car (valves, etc.) as best you can, document your system (K&Ns, compression, etc), call Curto and ask recommendations, and proceed. don't try to get more scientific unless you have specific issues.

mac townsend Wrote:

tony get some qtr scale paper and plot several needles youre playing with ,
<using different color or symbol for each needle > you can readily see the
differences fo tuning for performance/enocomy or whatever youre looking for..
in any standard engine the vacuum piston the needle is mounted to will
never be fully open..to check for performance improvement run car in hi gear
and check acceleration times from 2500 to 4500 rpm
stations 3 thru 6 are acceleration /cruise areas ,stations 7 thru 9 are top speed
stations

golf Wrote:

mac townsend Wrote:

A little elementaary plane geometry helps a lot. When you're calculating the function of a needle, you need to know the diameter of the needle at each station and the diameter of the jet. On an HS4, the jet is .090" in diameter. So, the surface area of the jet is pi x r squared where r=.045 so it's surface area is .0063617 square inches. The cross sectional diameter (using Tony's #5 needle) are .0890, .0850, .0814, .0785, .0758, .0733 .0705, .0680, .0653, .0627, .0600, .0590, and .0580. That makes the cross sectional areas of the needle at those same stations 0.0062211, 0.005674, 0.005204, 0.004839, 0.004512, 0.004219, 0.003903, 0.003631, 0.003349, 0.003087, 0.002827, 0.002734, and 0.002642. Then if you subtract the diameter of the needle from the opening in the jet at each station, you'll get the surface area of fuel exposed to the air stream at any time (the annulus of the circle). If you use that surface area as a proxy for fuel volume delivered (which assumes constant airflow capable of stoichiometric proportions (which is what a carb is all about) you can then divide the annulus (exposed area) by the exposed area at the previous station and see the percentage increase from station to station. They would be:

Station:
1 to 2 +388%
2 to 3 +68.46%
3 to 4 +31.46%
4 to 5 +21.50%
5 to 6 +15.83%
6 to 7 +14.76%
7 to 8 +11.06%
8 to 9 +10.35%
9 to 10 +8.68%
10 to 11 +7.95%
11 to 12 +2.64%
12 to 13 +2.53%

If you run these same calculations for all of your needles, you'll find out which ones will give you the bottom end you want (city driving), which will give you top end (racing), and which ones to use for midrange (freeway driving). By using the annulus calculation, you can tell which needle to switch to if you're too rich or too lean - just look for a needle with a similar annulus a half a step away (and raise or lower the jet .062"). This is why we took geometry in the 6th grade. I hope it helps some of you. Best of luck. Basil

Needle slection with all the avialable choices can get real confusing, real fast. I think the last statment in Mac's post is the key, if you treading into new ground not really traveled down by anyone before you, like alot of us were when the SCCA came out with limited prep engine rules, we had to explore because it was a new engine formula. However in the case of street performance MGB, this is a very well traveled road, you can take a HS4 fixed needle carb for example, and while 99% of these cars on the road with some performance enhancments will normally take either a #5 or #6 needle, you may find within the vast selection of needles more than one selction which are almost identical.

I always tell people, if you're good at dyno tuning you will be able to increase horsepower with timing and fuel mixture changes, but at the end of the day the highest number you see may not be the number or tune state you want for your car's goal. For example sometimes a leaner fuel mixture and more advanced ignition timing may make the most power, but it may not be a reliable tune state for your car's mission whether it be street or track use. For years I've known alot of racer to play with lean fuel mixtures and alot of advance timing, but that's never really worked for me, in my case I always saw horseepower dropped when I tried this. Too rich is also a condition that play on people minds over the years, including mine. A few year ago with one of the very first SCCA limited prep Spridget ever built by us, we really didn't know where to begin with needles, we ended up choosing a needle way too rich for this engine. To be honest with you it would have been real easy to be duped at this car's first track test. With a #6 needle in the carbs we would start to miss at around 5800 rpm, the miss was pretty bad, so we would drive it at below the rpm where the miss started, so we didn't realy get any feedback from the EGT's or plug readings because we were keeping the car below the miss, rpm wise. After going to a #5 I notice that the rpm increased to 6200 before the miss startes, by then the test day was over. I left the test rather rejected but knew we were now atleast head in the right direction. I seen more than one person before me confuse this condition with thinking they were too lean rather than too rich, bottom no good data was achieved at the miss, because we simply didn't drive the car long enough at the miss to recieve any worthwile data. A chassis dyno for test and tune time always seemed rather high at $100-120 per hour but after just spending a day testing at the track at alot more than that, it was starting to look and sound cheap. We made a appointment for the car at the chassis dyno the following week, we went to the dyno with about every needle choice you could possibly ever want. The first run on the dyno and my AFR readings were not even on the chart they were so rich, I immediately switch to a #4 needle, the next run the AFR readings were now atleast on chart but still rich, ahha, we heading right now, the next run rather than leaning out the number 4 needle, I decided to keep dropping needles sizes until I saw where I wanted to be or a lean fuel condition and work my way back, so I installed a #3 needle, then next run and I was pretty close to where i wanted to be, maybe just a tick lean, so at this point I just adjusted the mixture richer with the same #3 needles and got real close to the AFR I was looking for, after that it was a couple more run with just jet height adjustment to hone in where we wanted to be. The end resutl was we started on the dyno with a car that starting missing a little above 6000 rpm, each change we made made it take more rpms for the miss to begin and we saw the horsepower rise with each change, at the end of a 45 minute dyno session we now had a car that pulled cleaned well beyond where we would race it at rpm wise, and saw a whopping 16 horsepower gain.
A chassis dyno may seem a bit much for alot of you playing with a MGB for street performance, but if you go to a chassis dyno with a good plan and everything you need to make changes, if changes are needed, then you will find out like me, you will seldomly if ever go into a second hour on the dyno. When you consider all we spend on our cars, what's a 100-120 buck to optimize it's tune, rather than spent countless hours fiddling around in the dark, and still never know if we are right or not. I don't think if you take a fairly normal running street MGB to the chassis dyno you will not find the numbers we found on the race car but you could very easily find 5 horsepower and maybe even increase your gas milage 4 or 5 miles to the gallon, that alone would be worth every penny you pay to go to the dyno.
I could go on and on , about my theories on timing and fuel mixture and what to look for in your street car to give you idea of where you're at, but I won't bore you with that, I'll close with this, if perfofmance is on your mind and part of your MGB's budget, then dyno time should be as well, basicly if you spend your time there well it will increase your learning curve compared to without a dyno by tenfold, and you know the ole saying, "time is money".

Amen, Hap and Basil.

Thanks Hap!

MGB FOR ME Wrote:

Sometimes, a rich condition under load shows that the jets are extracting themselves from the jet tubes. The choke mechanism should hold them in place (if it's properly attached) but if not, the key is to safety wire the jets to keep them from pressing out.

B-racer Wrote:

If you like the low end of your #5 and hate the midrange but the midrange gets better if you lean it out, you need a different needle. Look at the needle chart and find something that looks like a #5 for the first 3-4 stops and is leaner (fatter) for the rest of the way. That would describe a #4 perfectly :-)

Basil Adams Wrote:

Check out this chart for a graph comparison of needles:
http://www.mintylamb.co.uk/suneedle/

If you find what you need, you can always call Joe Curto for them! It looks like you may want a 24 or a 61.

B-racer Wrote:

What I think is really starnge is that a #5 or #6 usually will be perfect in your car. Going rich in the mid-range may be another problem and you might be wasting a lot of time treating a symptom rather than finding the cure. I would seriously check out Basil's recommendation to check the jets, as well as checking your air filters, and the intire intake system and igniton system to be certain its all 100% up to par before wasting money on needles. I bet Joe will also try to talk you out of buying the needles, as he kind of guards his stash for those who really, really need them. He is 3 hours ahead of you, by the way. You probably won't get a hold of him after 2pm. He might, on the other hand, try to sell you jets and a gasket set!

B-racer Wrote:

3 or 4 flats at that point in the range (as compared to the same change near idle) is a pretty small difference, and I'd be surprised if it is the primary reason for your problem; I suspect you are masking something else.
OTH, that's right where timing advance is most critical. You say centrifugal advance has been "checked", but against what? OE curves are not optimum, and any changes to the engine will have thrown them way off. If you know what the curve was they checked against, send it to Jeff or somebody else who actually knows what works; if not, send the distributor.
You could also experiment by changing your static timing a few degrees and seeing if the flat spot goes away, even if it is detrimental at other ranges.

FRM