SU carburettor needle comparison graph

[Hugo]

Anybody who's interested in playing around with different carb needles might find this interactive graph most useful. It gives the relative mixture strength at various throttle openings for all SU needles.

http://www.mintylamb.co.uk/suneedle/

[christopher storey]

Hugo : thanks for the very useful source of needle sizes. May I give a warning about the graphical representation , however ? It grossly understates the percentage increase in richness between needles, because it does not compare the size of the annulus between needle and jet. Thus for UM, UE and UO , ( 3 of the commonest E type sizes ) the graph shows full throttle/speed i.e. station 15 increase of 7% between UM and UE, and 5% increase between UE and UO, whereas the actual increases are 23% and 12% respectively

I have not worked out the relative percentages at lower stations on the needle , but the same principle applies

I used to have the formulae loaded into an old program called supercalc but I'm not sure whether I can still produce this since it was DOS based !!

[Hugo]

That's a very interesting observation. For some reason it reminded me of Gordon Brown's claim to have raised National Insurance by 'one per cent', when he increased it from 10% to 11% or whatever it was.
But at least the graph shows which direction each needle change will take you, even if the precise amount is off.

[Gfhug]

UO and UE needles are both richer almost all the way. But is there any way of working out which one to use except by trial and error? For example, if your car is set up correctly at idle but it's running a bit lean at higher rpm do you just buy and try these?
Has it been through such trial and error that UE needles have been found to be best if using the Maxogen filter?

Cheers

Geoff

[rfs1957]

This oft-quoted MintyLamb resource (quoted even by Burlen/SU on their web-site, see bottom right of page)

http://sucarb.co.uk/technical/

indeed does NOT give "relative mixture strength at various throttle openings" and I would suggest it is dangerous to think that it does.

As Christopher Storey has underlined above, the problem with these resources is that they express changes in needle diameters, and only that - proportional or percentage changes of needle diameters, which are not at all the same as the proportions or percentages of altered fuelling that the engine sees.

These are in fact dependant on the squares of the diameters (we're talking about surfaces), and they have to be taken in the context of their own proportion in relation to the jet diameter.

(If the needle were the diameter of a hair, doubling its diameter would not have much effect on fuel going through the main jet annulus - the proportions are thus important.)

It is the change in the surface area of the annular gap between the needle and the jet that determines any change in mixture strength, NOT the simple proportions of needle diameters, and neither MintyLamb or WinSU express their information in that way.

The 7% decrease in diameter of the needle (from 2.545mm to 2.375) between full-throttle on the standard UM to a UE is indeed just 7%, but the effect on the richness is to increase it by 23%.

Quoting an 11% change in needle diameter between full-throttle on a UM needle and the often-recommended UO is correct BUT in reality this equates to a 37% increase in fuel.

Anyone (like me)

viewtopic.php?f=3&t=10695&p=89123#p89123

adventuring into tweaking mixture and using exhaust-gas analysis (especially whilst driving, and not just flat out on the dyno) needs to be aware of these nuances.

Were Jaguar really that far wrong that our minor tweaks to exhausts and air-filters justify increases of the order of 23 or 37% ?

See the table below for fuelling comparisons between UM and UB, UZ, UI, UE, and UO, which seem to be the only plausible alternatives to UM amongst the 30 that are suggested by WinSU as being "similar".

Their increases over the UM are shown as multiplying factors in the RH column for each needle, and can be resumed as respectively +11%, +11%, +16%, +23%, +38% at wide-open throttle - or the 16th station.



This will zoom up to be legible with a couple of clicks ...........

Are there any other needles that contributors have used or have identified as "similar" and that may have escaped me ?

PS For our HD8 SU with the standard 0.125 main jet, the change between equivalent stations on Needle 1 and Needle 2 is best expressed as (10.08 - D1²) divided by (10.08 - D2²).

If there's a tweak to reduce (a² - b²) / (a² - c²) still further I'd be pleased to hear it.

[Hugo]

I like trial & error. Beats theory any day. How about putting different needles in each carb & seeing what colour the plugs come out?

[rfs1957]

Having read your many postings, Hugo, I'm confident that your approach will work wonderfully, for you, once you get your car on the road.

[Hugo]

I don't even have any carburettors yet, so it could be a while!

[rfs1957]

Hugo - my calculation is that 932 posts in just over a year represents 9.320 minutes (it must take time to think before you write) and since you presumably read maybe 4 times as much as you post that might make 37.280 minutes spent on the Forum, or 621 hours, or 78 days of nine-to-five.

So if you had spent just 10 per cent LESS time on the Forum, you could have sorted your carburettors many times over.

Just a thought.

Anyway, enough of the theory.

I tried the smart method recommended, just looking at the spark-plugs, but they seemed to change colour every time I stopped the engine, depending on what I'd been doing, and for how long I'd been doing it.

Furthermore I realised I had no clue what the colours meant anyway, since the arrival of un-leaded fuel means that all plugs look sooty nowadays, making those reassuringly straightforward Champion mixture-interpretation pictures a waste of time.

Worse still, I couldn't equate the colours I was seeing with the 16 stations that there are on every SU needle.

Then I woke up from the nightmare and remembered that I'd known all that for over 30 years.

So in desperation I resorted to the dumb method of thinking things through, and decided to choose a new needle (UB) according to a bit of maths and some Excel tables, and not according to bar-room heresay.

Happily, these hitherto un-sung* needles have at a stroke solved all the problems of pre-ignition and weakness that I had - unwittingly, until recently - been living with for 15 years.

The Lambdas are now perfect from idle to thrash, the pinking has gone, at ANY and ALL throttle openings, and the car is ................ a different car.

PS One the engine has been re-built (this winter) I'll try fine-tuning this still further, with extensions to the SU float pistons, so that the needles' actual positions are visible.

* just one discreet mention on the Forum, from KingRichard, back in 2014. Respect.

viewtopic.php?f=3&t=6583&p=47740&hilit=needle#p47740

[Hugo]

Hugo - my calculation is that 932 posts in just over a year represents 9.320 minutes (it must take time to think before you write) and since you presumably read maybe 4 times as much as you post that might make 37.280 minutes spent on the Forum, or 621 hours, or 78 days of nine-to-five.

So if you had spent just 10 per cent LESS time on the Forum, you could have sorted your carburettors many times over.

Just a thought.

And how long did it take you to count them all? You must think I spend all day glued to a computer. I don't. I'm actually very busy rebuilding my E Type and all the other cars I have in the workshop - but I will pop in from time to time to see what's going on with the forum.
And I wish I had some carburettors to sort! I ordered a set from SU a year ago but they keep putting the date back.
Just in the middle of rebuilding my engine right now. Crank in; pistons in; head going on shortly. And also working on getting the bonnet to fit. And waiting for Quaife to come up with some details on their new E Type gearbox.
Onward and upward!

[Heuer]

And how long did it take you to count them all?

No need:

[Woolfi]

I have put a V12 with 6l form a 93 limousine , 4 original Stromberg carbs and the original front silencers with 48 mm wholes longline. No back muffler. 58 mm pipes instead of the 32mm-trumpet leading to the air filter.
The mixture was much too weak rom 10% part throttle to full throttle.
I have mounted a simple wide band lambda measuring system from innovate (roundabout 160 pound) into the car. On this gauge you can see the mixture just in time perfect.
Then I started to grind the 4 needles very little bit, mounted them back to the carbs and made a test ride, reading the lambda.
After this I have continued this, until I reached a mexture quality, I und the motor would like to have.
AFR 14 at iddle. AFR 15,5 to 17 at aprt throttle and Afr 13 to 13,5 from 80% throttle up to full throttle.
Iddel is pefect, gas consumptiuon is low and power is high. That's it.
I think to do it like this, is the only way to reach a good result.
Regards Wolfgang Gatza

http://www.ebay.co.uk/itm/INNOVATE-3918 ... SwlyJZbY2q

[Hugo]

You GROUND the needles down? You're a brave man.
Just in case anybody's interested in my carburettor saga, I ordered a set from Burlen/SU a year ago. They told me it would November before they did their next production run. Then it was going to be March this year.
I had just about given up on them, when lo and behold I got a call today saying they were ready!
Good timing as I almost have my engine back together.

[JagWaugh]

You GROUND the needles down? You're a brave man.

Actually, the usual way is to file a flat on one side. Much easier to see where you're making the change, and duplicate on the other needles.

[rfs1957]

And just what kind of file do you use when it's rich ?

[JagWaugh]

And just what kind of file do you use when it's rich ?

Then you need the excel file. The filename is "Parts to order".

Once you've taken too much off you have to start again, regardless of if you've been messing things up with a lathe or a file. If you use a file to remove material then you can afford to purchase a great many more needles, and you'll save a lot of time. Even if the lathe is on the same floor as the car, and you've got a collet chuck, I can take a couple of swipes off a station and have the needle back in before you've hung the collet closing wrench back up after releasing the needle (and a toolpost grinder is a bit fiddly to set up in the first place).

[PeterCrespin]

PS For our HS8 SU with the standard 0.125 main jet,...

Slight tweak needed Rory:

HS are the XJ (& MG / Mini ?) spec carbs with sliding jet tube and flexible hose to the float bowl.

HD are the E-Type & MkX/420G diaphragm carbs with fixed float bowl connection to the carb body.

[rfs1957]

Ta Pete, done.