Jump to content
PeterC

SU theory: mixture spread

Recommended Posts

By way of introducing myself to Sideways I proffer this description of SU operation.

'Mixture spread' is a little known feature of SU operation.  It means that a given position towards the top of the needle will give different mixtures according to the butterfly opening, at the same air flow.

https://supertrarged.wordpress.com/2017/07/10/how-does-an-su-carburettor-work/

(pdf to download)

Happy to discuss on here as I dont use the wordpress facility

Peter

Share this post


Link to post
Share on other sites

Wow!  You've put some work in there.  I'm going need to read it properly...... several times....... to get to grips with the simpler bits...... and then work up from there.

You may set Alan (OTU) off as he's done alot of work with Stromberg CD carbs.  I've taken the easy way out and gone EFI.......

You mention the effects of pulsation and their importance, but as I understand it (please correct if wrong), you've learnt this to get to grips with fuelling your supercharged TR6 properly.  Assuming it's a suck-through system, does the supercharger not reduce the pulsation effects?  Or are you merely swapping the pulsation effects of the engine itself for the effects of a PD supercharger?

Nick

Share this post


Link to post
Share on other sites

Nick,

You're exactly right. Alan Allard the supercharger guru rated the SU highly but kept mentioning the 'reduced/mixture spread' caused by a blower sucking through an SU. I had no idea what he meant and he gave few details in his book as to how to get around the problem. Fortunately the clues are in Knight's paper and I worked from there.

The more frequent pulses from the blower rotors fed into one SU meant that they had little effect, whatever the butterfly position. So the carb runs rich even at cruise.  I have long been planning a device to lean the cruise mixture by applying 'back suction' over the fuel in the float chamber. SU made a device that did that, depicted in Wade's booklet. I've never seen one....

Peter

Share this post


Link to post
Share on other sites
10 minutes ago, Nick Jones said:

Ah, I see, the effect is working backwards from my first impression then.

Nick

Nick,

Its not only blowers than reduce mixture spread. For normally aspirated twin carbs the balance pipe will reduce the pulsing effect at the jets and hence reduce mixture spread.

Peter

Share this post


Link to post
Share on other sites

Interesting.  Might explain the issues that some seem to have had dialling in triple SUs fitted to Triumph sixes, which were helped by changes to the balance tube arrangements.  There's an ancient thread on the subject on here somewhere.

Nick

Share this post


Link to post
Share on other sites

For the Triumph 6s with firing order 153624 the pulses are not all equally spaced. The air flows are intermittent. In particular for 1 and 2, and for 5 and 6, there are stagnation periods of 260 and 20 degrees. While for 3 and 4 they are equal at 140deg . I suspect that during the 260deg of stagnation the air flow in the choke tube will slow, leading to a change in mixture -maybe leaner as the fuel flow up the jet, normally helped by its inertia, will slow in that 260 period.  SU must have devised a balance pipe for a triple-SU six 153624 somewhere - I'd copy those pipes as a starting point.

Peter

Share this post


Link to post
Share on other sites

Its one of the reasons that IMHO I think that a pair of correctly sized carbs on a good manifold on  a 153624 "6" are better than tripples, with odd pulsing. I think the reason that Triples have received good comments is mainly because 1) They are being used to replace undersized or battle set up twins 2) That replacing a pair or worn out twins with anything is going to be an improvement 3) Fitting Triples means a new manifold which if replacing the standard unbalanced factory item is going to improve things. 

So I'm always sceptical when I read of the improvements people claim of fitting tripple SU's/Strombergs, as we are really comparing apples to pears, and I'm fairly certain that if you'd spent the same money fitting a properly set up twin system the improvement would be equal or better.

On the balance tube issue, what I'd did prove with my pair set up with AFR sensors in each exhaust manifold, with an inlet manifold with about a 1/2" balance tube, is that at idle up to about 1500 rpm you can effectively turn of one carb by completely closing the throttle plate, and the engine will happily run feeding three cylinders only via the balance tube. I have sometimes pondered when setting up/balancing the carbs if I should block the balance tube, to ensure that I'm actually accurately setting the mixture of the carb I think I am and not looking at some hybrid result.

Sometime I will try setting up normally, and then block the balance tube and see what if any difference it makes.  Even tempting as I datalog AFR's from both carbs to see what if any difference it makes at higher rpm's.  So many interesting things to try, so little time...

Alan

Share this post


Link to post
Share on other sites

Alan,  There's nothing I could find on how SU engineers decided upon balance pipe sizing. And all the factory records were lost. But if you do block the bp and the butterflies are  open more than about 30deg the pulses lost through the blocked bp will make the needle you are tuning run richer. Because each pulse is not shared across the bp. There's no info on how much richer. Some measurements would be interesting. Peter

Edited by PeterC

Share this post


Link to post
Share on other sites

 

Thanks Peter, what a fascinating report.  It's not something I was aware of, and certainly another factor I'm going to have to take into consideration when finally setting up my triple SU arrangement.

It's been a long time, car now runs, but with 'random' needles and jets it needs to go on the rolling road soon.

Certainly No.1 carb (feeding cylinders 1 and 2) seems to suffer the imbalance of firing order most.

 - Nick, I recall the historical posts, it was Bruce I think who found that enlarged balance pipes were key to refining his set up.  We'll see on mine later this year.  I'll share the findings......

 

Share this post


Link to post
Share on other sites

The pulses on triples are

Carb 1   direct, secondary x2, secondary x1, secondary x2, direct, secondary x1,  recurring 

Carb 2   secondary x1, secondary x1, direct,  recurring

Carb 3  secondary x2, direct, secondary x1, direct, secondary x2, secondary x1,  recurring

Where secondary means a pulse via a balance tube. NB 1 & 3 are the same pattern note the x1/x2 sequence which indicates if the pulse comes from the adjacent carb or a carb once removed.

If you look at the above, it does start to suggest that actually a very large balance pipe, so that in-effect you are treating the three carbs as just a very large carb might be an approach to get some sort of balance across them. I'd love to see a video of the pistons on a triple set up to see if the damping is good enough to keep all three riding at the same height at low-cruise rev band.

3 x 175 on 2ltr would be gross over kill, I doubt that at 6K all three would have lifted the pistons more than half way, as 2 x 175 are only just reaching full lift at just above that figure,  3 x 150 might be nicer as 2 have run out of controlled flow at about 6K.

I still reserve my opinion that I think that a correctly sized and maintained pair on a good (not stock) manifold with153624 is likely to perform better than triples, unfortunately the only anecdotal evidence we have is normally from people who are replacing old worn out pairs on a bad manifold with refurbished triples on a better manifold, when in all likelihood a wet sponge full of petrol being squeezed in front of the manifold is likely to perform better than what they had.

I do however have to admit, that in the absence of triple webers, that 3 Strombergs or SU's  in a row does actually look pretty!

Alan

Share this post


Link to post
Share on other sites

Few people realise how knackered, as in utterly knackered, their carbs are.

I've a couple of sets of NOS 175 Strombergs, the difference between them and a 50 year old set is astonishing.

I rather suspect the fascination with replacing Strombergs with 'better performing' SU's was simply the SU's were just newer and less worn.

Share this post


Link to post
Share on other sites

Have also been fortunate in collecting a few sets of NOS 150 and 175CD's   as well as buying a few cheap Used (as in so totally shagged you wouldn't believe it) sets.

And the used sets some apparently just removed from running cars can be in appalling condition with completely mismatched springs and jets, I find it strange that people will spend a small fortune on nice bits, but totally neglect what was/is actually a high precision made assembly.

I also suspect that SU's gained greatly in popularity because some people knew how to tune them, but scratched their heads when it came to Strombergs, I'd bet a lot that a well set up pair of Strombergs would maintain their tune way longer than SU's, there's just so much less to go wrong and wear!

Alan

Share this post


Link to post
Share on other sites

Ohhh.... I'm trying to resist the SU or Stromberg argument!

 

Damn, can't hold back:

Stromberg was always a cheap copy of the SU - designed to allow Triumph to break away from the SU company.
Alan, I can't see why you think the Stromberg has less to go wrong and wear?  Almost the same number of moving parts, with the close fitting piston of the SU being replaced by the Stromberg's diaphragm (play the need to maintain tolerance against diaphragm deterioration....).

I do agree that many of both types have been messed with in the past, and both can have atrocious throttle spindle wear (my old Stromberg's were so worn you could almost pass the throttle plate through).

... and I think the SU is much better looking (which was a major decider in using them on the triple set up).

   Will.

 

 

Share this post


Link to post
Share on other sites

 

> and I love your reference to a wet sponge being squeezed Alan.

I recall riding on a moped behind a friend on his Excelsior Autobyke.  I watched as the carb fell off, and was prevented from falling on the floor by the throttle cable.

 - the bike continued to run as normal, I had to wave my friend down to stop him.  The fuel feed pipe had detached as the carb fell off, and the stream of fuel in front of the manifold inlet (less carb) was enough to keep the engine running sweetly.

  --  so much for the need of complex carbs!

 

Share this post


Link to post
Share on other sites
2 hours ago, flatter4 said:

 

 - the bike continued to run as normal, I had to wave my friend down to stop him.  The fuel feed pipe had detached as the carb fell off, and the stream of fuel in front of the manifold inlet (less carb) was enough to keep the engine running sweetly.

 

He didn't go on in life to invent the Lucas Fuel Injection did he? Many Similarities :biggrin:

Share this post


Link to post
Share on other sites
4 hours ago, oldtuckunder said:

He didn't go on in life to invent the Lucas Fuel Injection did he? Many Similarities :biggrin:

I am reluctant to devalue the high quality of this enlightening exchange, but I would proffer the observation from the simpleton's side of the fence, that the venerable carburettor was invented specifically to give automotive engineers (fnar! fnar!) the opportunity to claim that whoever rebuilt them/last worked on them/balanced them (delete as applicable) cocked the job up comprehensively.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...