Single carb or more ?

[Bfg]

.

Just an idle enquiry.,  but I'm wondering if twin carbs 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 are rarely perfectly synchronized. ? 

It's not exactly the same but my Norton 850 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 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 gearing simply confuse any direct comparison. But on my Norton I felt no lack in acceleration nor pulling power when I switched to a single carb.  Seems to me that adding a second carb (OEM price £50 ?) was a sales ploy used by manufacturers to charge buyers another £1000 for a car with only half the interior trim.!

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

Thanks, Pete.

 

[Clive]

If you want the definitive research on a Triumph 4 cylinder single v twin SU differences, you need to find the article by John Thomason where he tries both on his spitfire.

This was a proper, scientific test on a std, but blueptrinted 1500 engine used in a production class race series, but he also used as an everyday car covering large mileages (he won the series, beating Alfas etc etc)

He has a set of twin HS4 carbs for the racing, and a single HS4 for the rest of the time (except to check MPG for a period of everyday use)

Upshot was single was a little more economical, the twins a little more power. Maybe 10%??

Everything was set up and optimised on a rolling road for both sets, so thorough and reliable results could be obtained. It is the only such research I have ever seen.

 

On a different, but comparable note, most modern cars use a single throttle body. Multiple throttle bodies are only used on performance cars with arguable wilder cam profiles. The other advantage is multiples give slightly quicker throttle response, but ultimate power is no different. But here fuel is injected by one injector per cylinder, carbs rely on good fuel atomisation and flow from the fuel jet/s

[Guest]

Hello Pete,

I can relate to this question.

Some time ago I owned a Sunbeam Alpine SV and that ran twin Stromberg's 150 - same as my Vitesse.

Later down the line I swapped over to a specific Webcon / Alpine induction conversion which replaced the Stromberg's with a single twin choke 32/36 DGVAS Weber and a purpose designed inlet manifold for the conversion.

The transformation was opposite ends of the spectrum and the Weber conversion was light years ahead of the OE set-up. Better performance, better tractability, fuel economy and far more responsive.

Logically twin carbs / triple carbs [dependant on application] give a better fuel supply over the cylinders for obvious reasons. Twin choke carbs [Weber] do an equally good job provided that their demand is not excessive.

As for tuning, maintenance and more importantly component wear a single carb or twin choke wins hands down; the flip side is that multi carbs give a better spread and increased performance provided that they are correctly balanced and tuned. 

The other important point is that for a base comparison the carbs must be in proper condition, otherwise the comparison is flawed to start with.

These are purely my observations on using both set-ups.

Regards.

Richard.

 

[Colin Lindsay]

24 minutes ago, classiclife said:

Some time ago I owned a Sunbeam Alpine SV and that ran twin Stromberg's 150 - same as my Vitesse.

Later down the line I swapped over to a specific Webcon / Alpine induction conversion which replaced the Stromberg's with a single twin choke 32/36 DGVAS Weber and a purpose designed inlet manifold for the conversion.

The transformation was opposite ends of the spectrum and the Weber conversion was light years ahead of the OE set-up. Better performance, better tractability, fuel economy and far more responsive.

Just as a query on an excellent topic - how 'like for like' would that have been?

Why I'm wondering is: if you replace a pair of older design carbs with a more modern version which is a) more expensive and b) more efficient you'll obviously improve things considerably; but if you replace a single Solex on a Herald with twin carbs of the same era, say twin SUs, how much of an improvement will you get without replacing, for example, the cam as well?  

Over the years I've replaced the Solex on Heralds with single Stromberg 150, twin Stromberg 125s on an Alexander manifold, and twin SUs on a Spitfire manifold, and sad to say I've no idea if they actually improved anything at all as I kept all of the other parts as standard. I still have to try a single Weber on a 1200 engine, and have manifolds adapted to take a single SU, but need to have a running Herald first... so just looking for opinion!

[Clive]

It really is a minefield. The assumption that twin carbs will be better is very flawed. And in fact triple SU/strombergs on a Triumph straight six is very much a retrograde step. (I know somebody tried it, and despite knowing it would be an uphill struggle, eventually got it running pretty well, but the costs and effort would have been rather better spent on triple webers or EFi)

One of the issues with twin SU's on our 4 cylinders is that the inlet manifold is poorly designed. 

Firing order is 1342, or 2142 if you start in a different place. So the front carb gets 2 pulls, then the rear gets 2. Be better if they alternated.... but that is affected by inlet tract design and length.

So all very complex and in many ways we are best to leave it to the original factory parts, or pinch ideas from other manufacturers (Richards Alpine setup would be copied from a Ford, with the carb being  contemporary, or at least a version of it, for the period. )

[Bfg]

 

13 hours ago, clive said:

If you want the definitive research on a Triumph 4 cylinder single v twin SU differences, you need to find the article by John Thomason where he tries both on his spitfire. 

This was a proper, scientific test on a std, but blueptrinted 1500 engine used in a production class race series, but he also used as an everyday car covering large mileages (he won the series, beating Alfas etc etc)

He has a set of twin HS4 carbs for the racing, and a single HS4 for the rest of the time (except to check MPG for a period of everyday use)

Upshot was single was a little more economical, the twins a little more power. Maybe 10%??

Everything was set up and optimised on a rolling road for both sets, so thorough and reliable results could be obtained. It is the only such research I have ever seen.

Thanks Clive, I didn't know about John Thomason, but did a google search and see that he's published.  It would be of interest so perhaps one of the lads in my local group here in Suffolk has a copy they'd lend me for a weekend, otherwise I'll look out for a copy of that.

" single was a little more economical, the twins a little more power. Maybe 10%?? "    is in the region I'd expect,  which would knock the sharp edge off acceleration but be more noticeable at the top end.  I wonder what effect it had on low to mid-range torque ?  

 

13 hours ago, classiclife said:

Later down the line I swapped over to a specific Webcon / Alpine induction conversion which replaced the Stromberg's with a single twin choke 32/36 DGVAS Weber and a purpose designed inlet manifold for the conversion.

The transformation was opposite ends of the spectrum and the Weber conversion was light years ahead of the OE set-up. Better performance, better tractability, fuel economy and far more responsive.

Yes even though I couldn't tell you what the Essex engine's Weber was, except it had a collar for LPG which worked exceedingly well - I'd forgotten the Citroen's Weber was twin choke of different sizes.  I've driven the A-series Citroens a lot,  usually driving on a single choke ..with the positive feedback of the second choke's spring making driving 'economically' very much easier to gauge. 

 

13 hours ago, classiclife said:

The other important point is that for a base comparison the carbs must be in proper condition, otherwise the comparison is flawed to start with. 

agreed .. for direct comparison.  But of course there's also building an engine for either one or two carburettors.   I'm thinking for example when rebuilding an engine, a specific camshaft might be better for one carb rather than two.  So perhaps although more involved, the difference might be 'better optimized' than is apparent when simply changing the manifold and number of carbs.  ?  

Likewise it may be that an SU is a better carb in its twin carb setup than a Solex which for whatever reason seems to relishes being single ?  (or vice-versa, or Weber, Mikuni or whomever ?) 

12 hours ago, clive said:

It really is a minefield.

Indeed !

 

12 hours ago, clive said:

triple carbs on a Triumph straight six

does seem to complicate things, as JohnD recently pointed out. But it was used on the E-type and on cars like the Toyota 2000.  I'm thinking that it must have had advantages to have been introduced by those manufacturers.

 

12 hours ago, clive said:

Firing order is 1342, or 2142 if you start in a different place.  So the front carb gets 2 pulls, then the rear gets 2. Be better if they alternated.... but that is affected by inlet tract design and length.

2134  but your point is valid.  4 into 2 into 1 branch exhaust manifolds may be intertwined to balance this, but that's not practical with short inlet manifolds.  So I guess part of the issue with single carbs is that the middle cylinders have shorter induction tracts than the first and forth cylinders. ?

 

Thanks all.

[Guest]

Clive,

Do you think some engines are / were better suited for slight upgrades due to them being possibly under developed on leaving the factory ?? Thus giving the manufacturer [Triumph in this case] the opportunity of producing a better product without involving a new design option or spending loads of money to develop the engine further ??  Would Colin's Herald already have reached its maximum output potential on leaving the factory, barring significant re-engineering ??

Regards.

Richard.

 

[Guest]

6 minutes ago, Bfg said:

Yes even though I couldn't tell you what the Essex engine's Weber was

Pete,

I'm pretty sure the Scimitar Essex 3L ran with a Weber 38DGAS twin choke.

[Clive]

The 1200 engine produced anything from 39 to 50ish BHP in a herald, and 70? in a spit.

Yes the spit mk2 had a 4 branch exhaust, and twin SU's. But also a different cam and CR (bigger valves too I expect)

So most engines can be detuned or improved, whichever way you look at it.

 

John Thomason was the (superb) contributor to the TSSC mag over many years.  Along with Carl Heinlein, he was probably the best technical writers the magazine has had.  

The article will be 2005 ish, but if you find a stack of magazines from that era, his articles are the ones to read.

[Colin Lindsay]

11 hours ago, clive said:

The 1200 engine produced anything from 39 to 50ish BHP in a herald, and 70? in a spit.

Yes the spit mk2 had a 4 branch exhaust, and twin SU's. But also a different cam and CR (bigger valves too I expect)

So most engines can be detuned or improved, whichever way you look at it.

I know the Herald can be improved amazingly easily to gain half a dozen horsepower or so - as Clive says valves and cam, better manifold etc will make a huge difference but for a simple improvement I'm wondering how much a carb change alone would increase power. (I can't for the life of me remember what it was that adverts used to claim improved the Herald from 38bhp to 45bhp almost instantly with just a bolt-on replacement)

I had a Scimitar wayyyy back and the enthusiasts were always saying that replacing the original carb with a Holley or whatever made quite a difference.

[Clive]

What I am trying to say...on a 39bhp herald adding twin carbs will add naff all as the rest of the engine is the limiting factor. Which is where it gets complex...

some cars are restricted by the carbs, many a V8 benefits from a big 4 barrel carb (at the expense of economy)

On my ford engine I gained 6bhp by radiusing part of the intake manifold. Yes, it needed a bit more fuel, but previously extra fuel didn't add more power. Getting fuel into an engine is easy, it is getting the air in, that is the restriction to power (the answer to that being Nitrous/turbo etc.....)

 

The old TriumphTune manuals are hysterical. The power increases quoted have no bearing on real life. 

[Guest]

Hello Colin,

Not sure if you remember the Castrol Tuning Guide ?? It was a booklet which came with that excellent publication - Car & Car Conversions; it was THE magazine [CCC] in its day where owners considered vehicle modifications. I buy some of the mags, today still, when I see them at Autojumbles etc.

Anyway, back to my point - I have attached a couple of pdf's that you may find interesting relating to squeezing extra power out of small engine Triumphs. A bit outdated but in essence the theory and practicality remains consistent with some modifications today.

Enjoy !!

Regards.

Richard.

Castrol Tuning Guide - Triumph 1.pdf

Castrol Tuning Guide - Triumph 2.pdf

[Bfg]

12 hours ago, clive said:

Firing order is 1342, or 2134 if you start in a different place. So the front carb gets 2 pulls, then the rear gets 2. Be better if they alternated

Out of interest ; do all 4-cylinder in-line car engines have this firing order ? or have some been changed to balance out the induction pulses ?  Did it work ?

 

[Clive]

16 minutes ago, Bfg said:

Out of interest ; do all 4-cylinder in-line car engines have this firing order ? or have some been changed to balance out the induction pulses ?  Did it work ?

 

Good question, and I wouldn't be surprised if there have been alternative used.  Triumphtune made a crossover inlet manifold for using a single DCOE carb on a 4 port head. Must have been worthwhile.

Of course, after the mid 80's everything was EFi and the problem disappears. Carbed car owners don't seem to worry too much, often pleased that the car runs well....

[Bfg]

33 minutes ago, clive said:

On my ford engine I gained 6bhp by radiusing part of the intake manifold. Yes, it needed a bit more fuel, but previously extra fuel didn't add more power. Getting fuel into an engine is easy, it is getting the air in, that is the restriction to power (the answer to that being Nitrous/turbo etc.....) 

I recall reading about shared induction tracts on motorcycles ; as two cylinders alternate in their opening - the air-fuel flow is repeatedly reversed.  ie. when one cylinder's inlet valve opens the air is sucked from behind the other (now closed) cylinder's inlet valve.  Then flow is reversed again as that other cylinder's inlet valve opens.  The reference endorsed sharpening the divider in the shared tract     

Sunbeam 500cc in line twin cylinder motorcycle shared induction port, seen here with its standard (rounded) divider between the two cylinders.  Air/Fuel is expected to change / alternate direction dependent on which inlet valve is opened, and as one opens the airflow to the other is reversed as air is sucked out of the void behind the valve.  The blunt radius of that divider is also a surface for the fuel particles to impact and so loose their 'suspension in the air flow'.   The advice was that this edge should be sharpened to lessen that impact face and to minimise the air reversal.  

I did that on this engine but the result was inconclusive because so many other factors were changed at the same time (the engine was rebuilt with rebore and new rings etc.).

 

[Bfg]

2 minutes ago, clive said:

Triumphtune made a crossover inlet manifold for using a single DCOE carb on a 4 port head.

Sounds interesting - would you happen to have any photos of that ?

 

[dougbgt6]

 I had a standard SU on my old mini, I upgraded to a larger one, more omph but less mileage. Upgraded to twin SUs and a little quicker still, but very much worse mileage. The gain was really not worth the money and effort and I rapidly returned to the big single SU.

And the thing only briefly mentioned, SUs seem to need constant tweaking 😡 Strombergs, just set them and leave them alone. Strombergs are OK in a twin situation, but my heart sinks when I lift the bonnet and see SUs.

Doug

 

 

[Clive]

5 minutes ago, Bfg said:

Sounds interesting - would you happen to have any photos of that ?

 

https://www.google.com/search?q=triumphtune+weber+crossover+manifold&rlz=1C1CHBF_en-GBGB830GB830&tbm=isch&source=iu&ictx=1&fir=G-aiefhVPWx9pM%3A%2C9yhXwVkwvqFGeM%2C_&vet=1&usg=AI4_-kR6mWzbikzrP7dHnJUzpm4FVal_AQ&sa=X&ved=2ahUKEwjW2Jvi_fXlAhXxtHEKHYlQBqoQ9QEwCHoECAkQBg#imgrc=G-aiefhVPWx9pM:

[Bfg]

That's really interesting, I was not aware of 'crossover' inlet manifolds.  Unassuming little blighters aren't they ! ..but see Lynx  still make them for Ford 1600 and the Volvo P1800.  I'll have to research / see what Lynx claims performance wise.  Following on from Clive's excellent prompt ..  < here >  is Lynx's explanation. 

I like it !

Thanks.

 

[Bfg]

I'm probably boring you all now but I'm enjoying learning about this, so here's another question . .

Why do multiple carbs use more fuel ?  ..when the engine is of exactly the same capacity and the optimum air-fuel mix must remain the same ..let's say 14:1 for the sake of this discussion.  

I can imagine the mix / swirl effect might be more effective, but that doesn't alter the air : fuel ratio. 

So a logical interpretation may be that twin carbs offer more ram effect (if that's the right terminology for this context ? ). ie. both more air and more fuel (at the optimum mix) gets to fills the cylinders, which when ignited gives more powerful combustion. Is that right ?  

Ok, I'm thinking as I write that "ram effect" is an incorrect term because that mix has not pushed / rammed / forced into the cylinder, and is only drawn-in by the descending piston.

So might I conclude that induction through twin carbs (or twin chokes) simply offers less resistance than a single (..even larger) carb. ?  

 

[JohnD]

What an interesting conversation!

On why more fuel used by multiple carbs, I'd say that is because they allow the engine to use the air flowing though them more effectively than a single.      Cubic capacity times revs does not equal engine flow, due to the dynamic effects of the inlet tract, valving, cam choice and exhaust flow.      Flow through a duct to a single cylinder will be more efficient than through one that shares many cylinders, where as you say, BFG, there is pulsing and flow reversal.     More air into a cylinder, will carry more fuel, and allow more power. 

Air resistance itself contributes.    The resistance to flow of a duct varies as the fourth power of its radius.     Halve it, and resistance rises SIXTEEN times!   Two carbs will each have a duct diameter  a little smaller than the single they replace, but a lot less drag.

And as you say, there is no 'ram' effect, either from the inlet ducts themselves, or from their being shown to the oncoming air flow, which just stalls.    The duct length and joining can be chosen for resonance advantage, just as can exhaust primaries, or fresh, cool air ducted from the front to a larger air plenum, where the pressure rises as flow diminishes, by Bernouille.

John

[Pete Lewis]

there are  some  examples of  4 cyl    webber  manifolds in the club museum 

[Colin Lindsay]

2 hours ago, classiclife said:

Hello Colin,

Not sure if you remember the Castrol Tuning Guide ?? It was a booklet which came with that excellent publication - Car & Car Conversions; it was THE magazine [CCC] in its day where owners considered vehicle modifications. I buy some of the mags, today still, when I see them at Autojumbles etc.

Anyway, back to my point - I have attached a couple of pdf's that you may find interesting relating to squeezing extra power out of small engine Triumphs. A bit outdated but in essence the theory and practicality remains consistent with some modifications today.

Enjoy !!

Regards.

Richard.

Castrol Tuning Guide - Triumph 1.pdf

Castrol Tuning Guide - Triumph 2.pdf

Is that the Richard Hudson Evan's book? Yep, got that... in fact got two and don't think I've completely read either. He starts at the bottom end, moves through cams and pistons to flywheels, and seemingly never once mentions carbs (unless I've missed that bit!). Does this mean he reckons the standard carbs are already up to the job? 

I also have the Gareth Thomas 'Tuning Manual for Standard Triumph Cars' but he starts with the Spitfire MK3 engine and discounts the 1147cc almost completely.

Most of them go way over the top and talk about cams, boring the engines, porting manifolds etc as though the Herald (or usually Spitfire) is a race track car. I'm just wondering about the lazy-man's way of squeezing a few HP out of the engine of a factory-standard Herald and gaining better response, maybe a slight increase in fuel economy, rather than beating anyone away from the lights.

[Chris A]

58 minutes ago, Colin Lindsay said:

I'm just wondering about the lazy-man's way of squeezing a few HP out of the engine of a factory-standard Herald and gaining better response, maybe a slight increase in fuel economy, rather than beating anyone away from the lights.

As there is plenty of room in the engine bay of a Herald just bolt on a tuebo or supercharger 💨😁

Very interesting thread, most of which I understand ( if not all the techy detail). Clearly a subject close to everyone's heart as there has been no thread drift. Damn, was that thread drift - Call the moderator 😱

[Colin Lindsay]

4 minutes ago, Chris A said:

As there is plenty of room in the engine bay of a Herald just bolt on a tuebo or supercharger 💨😁

 

You've got that book, haven't you? "Everyday Modifications for your Triumph Spitfire, Herald, Vitesse and GT6?"

'You can fit a Toyota or Mazda engine but this will require substantial modifications to chassis, gearbox, drivetrain, rear axle and brakes...' Yep, the sort of thing we do everyday.

There has to be something simpler...