Carburettor(s) ?

[Bfg]

.

Just an idle enquiry.,  but I'm wondering if twin carburettors really makes that much difference ?  or whether with a suitable manifold a single carb might be that much easier to keep in tune ..and therefore in daily use just as powerful and perhaps more economical than two carbs which rarely are perfectly synchronized. ? 

It's not exactly the same but my Norton 850cc motorcycle (sold very recently) had a single carb conversion, as did the previous one I owned many years ago. Similarly Triumph (motorcycle twins) were spec'd with single or twin carbs,  ie., modeled as the Triumph Tiger and Triumph Bonneville.  Certainly my experience of Norton and Triumph twins is that the single car bikes are always a little easier to start and generally feel smoother around town. Perhaps that's because the air flow through their venturi is steadier at low engine speeds.?   I might hazard a guess that top speed might suffer from being a bit wheezy with a single carb (which has been sized for normal road use),  but as I have only ever driven or ridden over 120mph a dozen times in my lifetime, I can't see that is now a serious problem.

Similarly many sports cars have twin carbs whereas their kindred saloon, fitted with the same engine has a single carb.  And that is expected to provide ample pulling power for the heavier car and payload, as well as being more economical  ..and to be more reliable.     Of course it's not so easy to compare the performance of a sports car and a saloon with the same engine,  because vehicle weight and aerodynamics, camshaft & compression ratio, and even their gearing simply confuse any direct comparison.  But on my Norton I felt no discernible lack of acceleration nor pulling power when I switched to a single carb.  Of course that may because I also from old (possibly worn) Amal carbs to a Mikuni.   But it does seem as if ; adding a second carb (OEM price £50 ?) was a marketing ploy used by manufacturers to charge buyers another £1000 for a car with only half the doors, glazing, and interior trim.!

I note that car's like my Scimitar with a 3-ltr Essex engine (also used in the Granada and Capri ) had one twin-choke Weber,  as indeed did my flat-four ohc Citroen,  and so i wonder if one twin-choke might be easier to live with than twin carbs, their control linkages, extra plumbing, starting choke,  twin air filters, etc. ? 

Thanks, Pete.

 

[Nigel Triumph]

Um... Just seen this on the TSSC forum too, exact same question copy and paste. Perhaps we should have a combined forum as well as the excellent joint International Weekend?

Anyway, to get to the point. I have a 1970 Triumph Trophy 650, the single carb version of the Bonneville. The Trophy is less fussy, with better pick up at low revs. That may be why the police chose the Trophy over the Bonnie in the Sixties.

David Vizard wrote in the Seventies at a single 1.5" SU carb was often better on a BMC A series than twin 1.25" carbs, depending on the inlet manifold.

Perhaps it's down to gas flow in Siamese inlet ports feeding only one or two cylinders? Certainly the A series cylinder port arrangement is far from ideal, though it works well enough. The Triumph Trophy engine also has Siamese inlet ports feeding just two cylinders, if the tight inlet manifold is considered.

I'm not so sure a single carb would work so well feeding the six inlet ports of a Triumph six cylinder motor.

Nigel

Edited November 18, 2019 by Nigel Triumph
Typo

[RogerH]

Hi Pete, 

 not sure of the science but here are a couple of things to consider.

Constant airflow through the carb (as in one carb on 4 or 6 pots) is efficient.

Pulsing airflow (as in 2 or 3 carbs on 4 or 6 pots) is not so good.

Direct air flow through the manifolds ( as in 2 carbs on a 4 pot) is good so long as there is an element of swirl etc)

Long indirect manifold paths is less efficient.

So many pro's Con's for bot systems

 

Roger

[Nigel Triumph]

Oh, and the Ford V6 in the Scimitar is different again. The standard Weber 38DGAS carb has two synchronised chokes, effectively two single carbs in one body. Though the carb feeds into a single plenum, in effect, each choke feeds a single bank of three cylinders.

Comparable to twin carbs on a Triumph straight six?

Nigel

[Bfg]

 

40 minutes ago, Nigel Triumph said:

Um... Just seen this on the TSSC forum too, exact same question copy and paste. Perhaps we should have a combined forum as well as the excellent joint International Weekend?

Anyway, to get to the point. I have a 1970 Triumph Trophy 650, the single carb version of the Bonneville. The Trophy is less fussy, with better pick up at low revs. That may be why the police chose the Trophy over the Bonnie in the Sixties.

David Vizard wrote in the Seventies at a single 1.5" SU carb was often better on a BMC A series than twin 1.25" carbs, depending on the inlet manifold.

Yep, TSSC also ..different viewpoints on here.  I was only going to post on here but the TSSC most likely has more members with Triumph saloon cars as well as one of the more sporty models, so perhaps more of a back to back comparison ?  Whereas here we have both y'all  and some particularly good contributors from Europe.

That was my experience with the single carb Triumph-twins as well.  The Bonneville was a sod for going off tune ..without it seemed either rhyme or reason.  The twin carb versions seemed to like lots of rev's  (which might be linked to Roger's post below)

Didn't know that D.Vizard wrote that about mini carbs. 

Thanks.

30 minutes ago, RogerH said:

Constant airflow through the carb (as in one carb on 4 or 6 pots) is efficient. Pulsing airflow (as in 2 or 3 carbs on 4 or 6 pots) is not so good.

That seem to make sense for very low engine speeds,  but I might only guess the pulsing effect lessens as rev's pick up. ?

Thanks.

 

Edited November 18, 2019 by Bfg

[Andy303]

The ideal setup is one venturi for each cylinder. Consider that many multi-cylinder bikes (i.e. Honda CB750) have one carb for each cylinder. I had an Alfa Spider with twin DCOE Webers. Each cylinder then had its own venturi for optimal air flow. Modern fuel injected engines use the same basic approach.

[Nigel Triumph]

2 minutes ago, Andy303 said:

The ideal setup is one venturi for each cylinder. Consider that many multi-cylinder bikes (i.e. Honda CB750) have one carb for each cylinder. I had an Alfa Spider with twin DCOE Webers. Each cylinder then had its own venturi for optimal air flow. Modern fuel injected engines use the same basic approach.

Indeed, That's the ideal setup, one venturi for each cylinder. Like the TR5 and TR6 PI system.

Nigel

[Lebro]

Just to throw in my twopennyworth.

On long trips I have got 42MPG on twin HS6's, once tuned up properly they should not go out of tune, & the car(TR3) drives very smoothly.

The T120 Bonnie (which have just sold  -  sob) again, once setup gave really good, & smooth  pickup from low revs.

Bob.

[ChrisR-4A]

2 hours ago, RogerH said:

Hi Pete, 

 not sure of the science but here are a couple of things to consider.

Constant airflow through the carb (as in one carb on 4 or 6 pots) is efficient.

Pulsing airflow (as in 2 or 3 carbs on 4 or 6 pots) is not so good.

Direct air flow through the manifolds ( as in 2 carbs on a 4 pot) is good so long as there is an element of swirl etc)

Long indirect manifold paths is less efficient.

So many pro's Con's for bot systems

 

Roger

That’s why most 2 into 4 inlet manifolds like the TR4 & 4A have a large Ballance tube built into the design, this equals out the vacuum from all the cylinders across the two carbs and also limits the negative effects of out of Ballance carbs and unsynced butterfly’s. Carbs like Webber’s offering one choke per cylinder only come into their own at higher revs, usually about 3500/4000 upwards. At lower revs they can actually slow the airspeed through the inlet manifold down and decrease the torque produced.
Chris

[stuart]

10 hours ago, ChrisR-4A said:

 like Webber’s offering one choke per cylinder only come into their own at higher revs, usually about 3500/4000 upwards. At lower revs they can actually slow the airspeed through the inlet manifold down and decrease the torque produced.
Chris

Really? Ive never found that, especially on the 4 cylinder where there is lots of low down torque.

Agreed about "A" series engines, Frogeye Sprites went better on a single 1 1/2" than two 1 1/4" but why strangle the performance on a TR engine which is a totally different animal, its a sports car after all.

Stuart.

Stuart.

 

[roger murray-evans]

1 hour ago, stuart said:

Really? Ive never found that, especially on the 4 cylinder where there is lots of low down torque.

Agreed about "A" series engines, Frogeye Sprites went better on a single 1 1/2" than two 1 1/4" but why strangle the performance on a TR engine which is a totally different animal, its a sports car after all.

Stuart.

Stuart.

 

A bit of pedantry here Stuart, but weren't Frogeyes on 2 X H1 SUs?

Roger M-E

[stuart]

3 hours ago, roger murray-evans said:

A bit of pedantry here Stuart, but weren't Frogeyes on 2 X H1 SUs?

Roger M-E

Well possibly, I havent had hands on one in 50 yrs so your more than likely right but same principle still applies.

Stuart.

Edit; on checking further they had 1 1/8" SU`s

Edited November 19, 2019 by stuart

[Bfg]

19 hours ago, RogerH said:

Constant airflow through the carb (as in one carb on 4 or 6 pots) is efficient.

Pulsing airflow (as in 2 or 3 carbs on 4 or 6 pots) is not so good.

Doesn't a single carb crossover inlet manifold even the pulses out ?

 

[Lebro]

3 hours ago, stuart said:

Well possibly, I havent had hands on one in 50 yrs so your more than likely right but same principle still applies.

Stuart.

Edit; on checking further they had 1 1/8" SU`s

I can verify that, I had 2 X 1 1/8" SU's on my Morris 1000 (1098CC, 13:1 comp ratio, gasflowed head). They were from a frog eye.

Bob.

[Peter Cobbold]

A single carb feeding four cylinders needs a finer spray from the jet to avoid mixture bias. On an SU or Stromberg that means a stiffer dashpot spring. And that will marginally increase the 'constant depression'.  A single carb also sees, per rev,  double the pulses at the jet as each inlet valve opens. Each pulse richens the mixture at butterlfy openings above  ca 30deg. This means that  the single carb tuned to give a lean mixture at cruise ( bfly less than 30deg) wont give much more fuel at wide open throttle. In other words the "mixture spread" will be greater with twin carbs and is factory-detuned with the balance pipe.  There is much more going on in an SU than meets the eye, its not operating as a venturi for starters.

Peter

[Bfg]

^

Thanks Peter, 

Tbh that's pretty much over my head at this moment.  I'll need to study how an SU carb works before I might better understand your post and possibly ask anything-like-sensibly questions.    My poor ol grey matter is confuddled with flooding of a simple Amal 276 carburettor ..and the MC45 dynamo on the same 1948 m/c not doing what its meant to, so that study period will have to wait a day or two.  But still I appreciate your prompting me in that direction.  Cheers,

Pete.

[Peter Cobbold]

2 hours ago, Bfg said:

^

Thanks Peter, 

Tbh that's pretty much over my head at this moment.  I'll need to study how an SU carb works before I might better understand your post and possibly ask anything-like-sensibly questions.    My poor ol grey matter is confuddled with flooding of a simple Amal 276 carburettor ..and the MC45 dynamo on the same 1948 m/c not doing what its meant to, so that study period will have to wait a day or two.  But still I appreciate your prompting me in that direction.  Cheers,

Pete.

Pete,

I spent a lot ot time seeking to understand how an SU works, and gave a talk at an IWE:

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

It is inevitably tricky to explain but I hope it helps.

====

An  Amal is different. The needle is carried by the air-valve and it lacks a butterfly. When the air valve is controlling rpm the air flow under it will be "choked" ie at the speed of sound, Mach1 !!   In choked air flow the pressure is always fixed at 1/1.89 of ambient pressure. ( https://en.wikipedia.org/wiki/Choked_flow ) So if ambient psi is 14.7 then the jet will see 7.77 psi. This depression of ca 7psi below atmospheric pressure is far greater than in an SU ( around 0,25 psi: see talk) and means that the Amal needle will need far greater precision to get the mixture right. Mikunis likewise.  When the throttle is opened to accelerate the engine the choked condition goes away and the depression over the jet drops such that opening the throttle too fast can induce a lean stall. Hence the need for the throttle pump.

So although Amals and SUs look superficially similar their fluid mechanics are very different.

Peter

Edited November 23, 2019 by Peter Cobbold

[BlueTR3A-5EKT]

A line of Reece Fish (MinnowFish) carbs instead?  No diddling about with needles and jets is required with them.

 

http://minnowfishcarbs.co.uk/full-history

https://www.imps4ever.info/tech/carbs.html#fish

 

Peter W

[Peter Cobbold]

40 minutes ago, BlueTR3A-5EKT said:

A line of Reece Fish (MinnowFish) carbs instead?  No diddling about with needles and jets is required with them.

 

http://minnowfishcarbs.co.uk/full-history

https://www.imps4ever.info/tech/carbs.html#fish

 

Peter W

Hi Peter,  More complex than it looks I fear.  No adjustment possible as it uses a metering groove instead of a needle.  Depression is provided by the butterfly, at the spindle. It will be around 7 psi below about 30 deg opening. At wider bfly  openings the depression will fall away , the exact value depending upon local air flow around that spindle. The metering groove will have been factory-sized to give a mixture for that range of openings. It is likely that, even with bfly part open, pulsing from the inlet  vlaves will richen the mixture and that would have to be allowed for too at the factory. Then any changes to the  cam etc would alter the pulsing and hance mixture. Re-tuning that groove looks a challenge , compared with sadpapering an SU needle !

And that tiny fuel pick up hole looks prone to blockage, whereas SU are almost immune to that.

 

Peter