Single carb or more ?

[Pete Lewis]

 Colin  my Cactus is a 1200   only 3 cylinders 130 bhp   fit one and keep the same capacity ?

[Bfg]

 

2 hours ago, JohnD said:

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.     

With all other things being equal  (ie.cylinder head inlet tract, valving, cam choice and exhaust flow being the same) ..surely the only difference between single and twin carbs is the inlet manifold, carburettor(s) and filter(s).   I recognise that each manifold would induce a different swirl effect,  and a crossover inlet manifold would be different again to that of the standard single carb type.  Is this what you mean by "air flowing through more effectively"  ..and because of improved swirl it uses more fuel  ? 

I'm not trying to being funny or a smart ass - I honestly don't understand ..yet. 

 

2 hours ago, JohnD said:

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.

So for the sake of discussion, if a single carb is 1-1/2" dia, and twin carbs are each 1-1/8".   The smaller twin carbs are  3/4 the radius of the single so their air-flow resistance is 12 times as great ..but there are two of them.    Tbh., I'm not knowledgeable enough to work the maths.  But as the resistance depends on the size of carbs used, then - if a BIG single carb is used rather than two smaller ones, there will then be less drag.  And I guess there is a balance point on the graph - where resistance is the same. 

But as very big carbs don't work so well at low engine speeds, then a two-staged / twin-choke carburettor is used  ..such as the 32/36 DGVAS Weber mentioned by Richard. ?

 

[rlubikey]

Bfg - Just to add to what Clive has said, the John Thomason article "Economy Spitfire" (I'll find the Courier issue for you later - it's missing from the magazine DVD archive index!) was based on the Triumph "Log" single carb manifold as used in the Triumph 1300 and 1500 saloons from 1971 on, not the "Cumberland sausage" used on some Heralds. He speculated that the Log manifold was an economy measure in response to the first oil crisis - or 2nd if you count Suez! Whereas Log manifolds generally have a bad reputation because of unequal flow to the different cylinders, the Triumph design has clever little "bumps" where the mixture goes around the corners to give you approximately equal flow/filling on cylinders 2 & 3 compared to 1 & 4. It also has smaller diameter runners to the ports which keeps the gas velocity higher, and the swirl near the inlet is meant to improve filling as the valve is closing. Well, that's what he said and it's the same argument as Vizard in one of his tomes, and it convinced me.

Anyway, John T's results showed slightly lower top-end power compared to twin carbs - a matter of 2- or 3bhp I seem to recall. However, the torque and MPG were slightly better, I forget by how much but it was a similar gain cf. the BHP loss. He also commented, as you and Clive have said, that there's no balancing issue as with twin carbs, so better MPG can be expected in the real world where things are prone to go out of adjustment - if they were ever "in" adjustment in the first place.

The bottom line is that he liked it for an ever day driver where economy was desirable, as was slightly better torque as a trade off for top-end power.

Cheers, Richard

[Chris A]

2 hours ago, Colin Lindsay said:

 

There has to be something simpler...

Yep, buy a differnt car 😀. A bit like buying a McDo then trying to make it into a Michelin 3* meal.

I've got a 13/60 because that is what I wanted, if I had wanted more power I would have bought a Vitesse or Aston (chance would be a fine thing).

As I said interesting thread no the less.

[rlubikey]

Found it. The article is "Economy Spitfire?" and is in Courier 216, June 1998, pages 8-14.

He got 51bhp at 5k & 5k5 rpm with twin carbs, but only 44bhp @ 5k with the single+log. He comments that you only see a difference in power *above* 4k rpm, as the single carb flattens off, while the twin set up power keeps rising. Below 3k5 the single carb has 1bhp more and he notes the idle is smoother.

For economy on a set route (his work commute), he got 33~34mpg with twin carbs but 36~37mpg with the single+log installation.

There was a follow-on article by Brett Denis in Nov '98 (Courier 221 pp. 22 to 24) but Brett decided to grind the runners out to the head port diameter, and I suspect he lost some of the torque without gaining any significant power.

Why do I know all this? Because I was researching what to do about carbs on my Atlas where I'm sticking an 8-port 1500 engine where a 948cc used to be. Problem is, the side-draught carbs won't fit as the air filters need to be where my hip is. For a long time I was thinking of a solution along the lines of the Log manifold. But now I've bought something from Pierce Manifolds which should solve my space issues.

Cheers Richard

[Colin Lindsay]

McD is not a great example to use as you can go large.... !!  Almost everything these days from Broadband to Laptops has tips and tricks that you can use to squeeze a bit extra power from the basic model without having to use expensive add-ons; same as I want to do with my lowly 1200s, and I know it can be done very simply. 

As for buying anything else???? See Pete's post at the top of the page - as soon as he mentioned 'Cactus' I thought: 'Conifer.' It's too late for me, I'm afraid... 

 

[Colin Lindsay]

2 minutes ago, rlubikey said:

I've bought something from Pierce Manifolds which should solve my space issues.

Cheers Richard

Those look almost like mine for my 1200 engine, but more ports...! I've also got a blank Weber version that I'm debating adapting for the downdraught manifold... for no other reason than to see what happens. I'll be using a tubular exhaust manifold tho.

  

[rlubikey]

Hi Colin. There's another USA company - Cannon - which also does these downdraught manifolds. The Americans do like their downdraught carbs, don't they! The Pierce has the step down to give bonnet clearance, which my research suggested might be - ahem! - *marginal* with the Cannon! so do take care first time you close the hood (sorry) bonnet, just in case.

I'm using a 30' or 45' SU with a suitably angled adaptor. The SU will be angled back towards the rocker cover, which should give me the clearance I need in my application.

Cheers, Richard

[Clive]

2 hours ago, Colin Lindsay said:

McD is not a great example to use as you can go large.... !!  Almost everything these days from Broadband to Laptops has tips and tricks that you can use to squeeze a bit extra power from the basic model without having to use expensive add-ons; same as I want to do with my lowly 1200s, and I know it can be done very simply. 

As for buying anything else???? See Pete's post at the top of the page - as soon as he mentioned 'Cactus' I thought: 'Conifer.' It's too late for me, I'm afraid... 

 

Colin, just use a spit mk2 engine spec? Or failing that a better cam and a head skim? If possible the 4 branch manifold is a nice extra, totally in period, as would be a "better" carb or carbs.  With a tweek to the distributor to match. But do a complete package. Messing about just changing one thing rarely improves matters, often makes them worse.

[Colin Lindsay]

4 hours ago, rlubikey said:

I'm using a 30' or 45' SU with a suitably angled adaptor. The SU will be angled back towards the rocker cover, which should give me the clearance I need in my application.

Cheers, Richard

Morris Minor, by any chance? SU Carb is tilted but float chamber is level, so it can be easily fitted to a manifold adaptor for the Herald.

2 hours ago, clive said:

Colin, just use a spit mk2 engine spec? Or failing that a better cam and a head skim? If possible the 4 branch manifold is a nice extra, totally in period, as would be a "better" carb or carbs.  With a tweek to the distributor to match. But do a complete package. Messing about just changing one thing rarely improves matters, often makes them worse.

That's true, Clive. I'm just thinking out loud as usual... but it's all for the future as I haven't started rebuilding the 1200 engine yet. I'm still working out which of the current bits can be saved before buying anything new.

[rlubikey]

15 minutes ago, Colin Lindsay said:

Morris Minor, by any chance? SU Carb is tilted but float chamber is level, so it can be easily fitted to a manifold adaptor for the Herald.

Actually, an HIF off a Rover Metro/100. I can't seem to find a photo just now, but it comes with the angled adaptors at each end, and the HIF should cope with the exhaust heat in close proximity.

[Casper]

Quote

 (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 think it was 39 to 49.  I don't know about the advert but the basic difference was the 'hot' cam as used on the 12/50 and the late Spit 4 and early Mk 2 in Nov 1964 with greater lift for better torque.  Note the 12/50 manifold only gave 3 bhp.

Quote

The article will be 2005 ish,

Your memory must be going Clive, it was Courier 216 Jan 1998 and 221 Nov 1998.

 

C.

[Clive]

11 hours ago, Casper said:

Your memory must be going Clive, it was Courier 216 Jan 1998 and 221 Nov 1998.

 

C.

Crikey. I had a quick google and found an article in a motorsport mag referencing Johns winning the series, that is where I got 2005 from.Maybe he has continued to race and still winning? No surprise, he is talented.

The devil is in the detail when making engine improvements. People don't get it. If you swap the cam, you will need to match the CR, and change the advance curve of the dizzy (plus it needs to be in good condition, distributor doctor or H+H can do the setting up to an engine spec).

If you skip those then you are unlikely to get more power but will use more fuel. I do have a T shirt or 2 for this.

[Colin Lindsay]

13 hours ago, Casper said:

I think it was 39 to 49.  I don't know about the advert but the basic difference was the 'hot' cam as used on the 12/50 and the late Spit 4 and early Mk 2 in Nov 1964 with greater lift for better torque.  Note the 12/50 manifold only gave 3 bhp.

It was something really really simple and annoying me that I can't remember it! I can remember the article stating: "The basic engine puts out 38bhp but simply doing ******* gives an immediate boost to 45..." (I've quoted the figures I remember to try to jog the memory!) Now I'm going to have to go through all of my old books to see where I read that...  

[Bfg]

22 hours ago, rlubikey said:

He got 51bhp at 5k & 5k5 rpm with twin carbs, but only 44bhp @ 5k with the single+log. He comments that you only see a difference in power *above* 4k rpm, as the single carb flattens off, while the twin set up power keeps rising. Below 3k5 the single carb has 1bhp more and he notes the idle is smoother.

For economy on a set route (his work commute), he got 33~34mpg with twin carbs but 36~37mpg with the single+log installation.

As I'm hankering after TR4a with O/D,  and the standard gearing is 4000 rpm @80mph without overdrive - I'm sure I would be content with loosing something in the  80-100mph acceleration range in exchange for having a little smoother idle,  plus another 3 mpg ..and the servicing / tuning advantages of just one carb.  Yes, I know the TR4's supposed to be a sports car,  but it's also a classic two-seater convertible with bags of torque for long-legged touring  ..so it needn't be driven at ton-up speeds to be thoroughly enjoyed.

I'm sure when I read this post yesterday evening - you mentioned higher torque at the lower engine speeds from the single carb.  I guess that was not wholly correct and so edited to correct ?

22 hours ago, rlubikey said:

I'm using a 30' or 45' SU with a suitably angled adaptor. The SU will be angled back towards the rocker cover, which should give me the clearance I need in my application.

I like the idea of these manifolds, which provide more-or-less-equally to all cylinders, but the TR4 bonnet is so low to the carbs that even with a 45deg adapter it (possibly) wouldn't fit.  It already has a bonnet bulge to clear the standard carb's dashpots.  However I'd recently spotted photos of the TR6 with a downdraft carburettor . .

Although this is a twin-carb twin-choke setup - the adapters to its standard (??) inlet manifold are off the shelf, and clearly fits under the TR6 bonnet line. 

But what if any are the advantages (or disadvantages) with down-draft versus side-draft carburettors ?  

Thanks.  Pete. 

p.s. If anyone can send me a digital copy of that article, it would be appreciated.  

 

[Bfg]

 

On 19/11/2019 at 11:48, 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.

It used to be said that the surest way to better performance is to increase the capacity (maximum re-bore size) and to increase the compression ratio (skim the cylinder head gasket faces + new pistons).  At the same time a few lumps in the induction tract would be smoothed out, and the gaskets and ports better matched to the size and shape of their manifold. The existing carb(s) would simply be re-jetted, and the air filters replaced with those which claimed 'free flow' ..or else open bell-mouths used instead.  Ducting (for cold air) might be run from in front of the radiator to point in the general location of the carb(s).  And, as the machining was needed on any old and worn (dry liner) engine anyway - the additional cost in achieving more torque would be negligible. 

Swapping to a four branch 'extractor' exhaust, along with contact-less ignition, and possibly an improved inlet manifold - were the next stage in 'home tuning'.  And only then, were things like replacing the carbs considered, upgrading the camshaft, altering the advance curve, and actually paying someone   to tune the car on a rolling road.   Next tuning stage included lighter engine components, re-balancing and more advanced porting to safely achieve higher engine revs.

Why.., have things changed since I were a teenager ?   ..in the world of Triumph Heralds ?

 

[rlubikey]

6 hours ago, Bfg said:

I'm sure when I read this post yesterday evening - you mentioned higher torque at the lower engine speeds from the single carb.  I guess that was not wholly correct and so edited to correct ?

I think I mentioned it in my first post. I hadn't re-read the article then and was going from memory. While John T. doesn't quote torque, his dyno plots do show power through the rpm range. Since torque is rotational force and power is force times distance, I think I'm right in saying that if the power is higher at a certain rpm then the torque will be higher. You would just have to do some maths to find out how much higher. Someone please correct me if I'm wrong!

I can thoroughly recommend the Courier Archive dvd collection for research like this as, despite the index not being as reliable as one would like, it still makes reading these old articles a cinch! Even if the index lets you down you can always ask here and some nerd like me will probably know where to look.

Cheers, Richard

[Bfg]

Thank you Richard. I understand your conclusion ..derived from their direct relationship. This is also my understanding. 

Power is the work done in any unit of time, it may therefore be a measure of ;  force x distance / unit of time.  (NB. 'distance' may be linear or around an axis)

Torque is the gauge of force x radius  (..with neither distance-moved nor time coming into this equation). 

But as  'force'  is directly proportional to the results in both equations - it must be true that if the force is greater in the single-carb's power curve, then it would be proportionally so in the torque curve.   Of course from the result suggested, a 2% difference is unlikely to be felt in everyday road use.

Interesting nevertheless.

Cheers.

[Bfg]

On 19/11/2019 at 15:16, rlubikey said:

Found it. The article is "Economy Spitfire?" and is in Courier 216, June 1998, pages 8-14.

OK.    ...being a newbie  (..actually a returnee to the club after a few decades away ) I didn't know about this resource nor how to access it.  But I have just explored the TSSC website,  found  COURIER  in the left hand side bar,  clicked on it and found 29 years worth of excellent courier magazines and their articles in pdf format, freely available to us.!  - FANTASTIC  !!  

And Thanks

Please,  where might I find the index ?

 

[Colin Lindsay]

Oh Lord... those wonderful pre-Internet days! The size of the classified ads section in the mid-1980s issues speaks volumes...

[Herald948]

My practical, real-life experience with twin-carb installations on otherwise stock Herald 1147cc engines seems to parallel that of others here, as well as the "conventional wisdom" I've read in several places over the years. Only adding the dual SUs (without any other modifications) does seem to be good for a bit of extra power at higher revs, but the price for that is a bit of a loss of "low-end grunt"! I actually have the opposite problem: One of my Heralds came to me with a Spitfire Mk2 block and head installed with the original "12/50-style" manifold and single Solex. It's fine, but I have been meaning to throw some proper Spitfire dual carbs and manifolds on to see what I might "gain back" from doing so!

[Casper]

 

Quote

29 years worth of excellent courier magazines and their articles in pdf format, freely available to us.!  - FANTASTIC  !!  

And Thanks

Please,  where might I find the index ?

Sorry.  No index.  Various efforts have been made to index it but it's a mamoth task.

JT's contributions to the Courier included a series on the 4 cylinder (SC) engine describing its development from 803 cc  in 1953, through 948, 1200, 1300  including variants to the 1500 in 1980.

Starting Courier 174 Dec 1994. 

The last of the series (Courier 177 Mar 1995) included a chart showing cc, power, torque, CR, cam timing, cam lift, valve sizes and carb details.

Very instructive 

He also looked at the 1600 engine in Courier 108 Jun 1989

C

[Bfg]

On 18/11/2019 at 21:17, clive said:

John Thomason was the (superb) contributor to the TSSC mag over many years. 

By the time I went to bed last night - it felt as if I had been in lectures all day   ..after reading what John Thomason wrote about about the single v twin carb on his Spitfire and then following through to his previous discussions on the why and wherefores of manifolds. 

216 - June 1998.pdf

111 - Sept 1989.pdf

112 - October 1989.pdf

107 - May 1989.pdf

It was well written and a great read - I've learnt a great deal.   I can't honestly say that the Dolomite 1300 manifold looked to be the most refined in world,  but it did make for an easy conversion (back from single to twin carbs in 20 minutes !) ..and most likely was dirt cheap too !   Very noticeable also was that the standard 13/60 Herald air filter trumpet does look very restrictive for a tuned 1500 Spitfire.   Neither of these things take away from the results achieved but who knows - they might imply that slightly better results might be achieved with a single carb setup.? 

Thanks All - great thread.

 

I look forward to going back into the archives and reading more of his reports.

Pete

 

[Clive]

1 hour ago, Bfg said:

By the time I went to bed last night - it felt as if I had been in lectures all day   ..after reading what John Thomason wrote about about the single v twin carb on his Spitfire and then following through to his previous discussions on the why and wherefores of manifolds. 

216 - June 1998.pdf

111 - Sept 1989.pdf

112 - October 1989.pdf

107 - May 1989.pdf

It was well written and a great read - I've learnt a great deal.   I can't honestly say that the Dolomite 1300 manifold looked to be the most refined in world,  but it did make for an easy conversion (back from single to twin carbs in 20 minutes !) ..and most likely was dirt cheap too !   Very noticeable also was that the standard 13/60 Herald air filter trumpet does look very restrictive for a tuned 1500 Spitfire.   Neither of these things take away from the results achieved but who knows - they might imply that slightly better results might be achieved with a single carb setup.? 

Thanks All - great thread.

 

I look forward to going back into the archives and reading more of his reports.

Pete

 

Worth mentioning......

John Kipping (Triumph guru, now in NZ but his company is now Canley Classics) did a LOAD of work with different carbs, often after economy. 

One of their favored setups was to use a single SU 1 3/4 carb, on the dolomite inlet. Somewhere in the ether you will find the details.  

[Casper]

Quote

John Kipping (Triumph guru, now in NZ but his company is now Canley Classics) did a LOAD of work with different carbs, often after economy. 

One of their favoured setups was to use a single SU 1 3/4 carb, on the dolomite inlet. Somewhere in the ether you will find the details.  

Herewith, from another board:

Message: 21

Date: Fri, 30 Aug 2002 09:50:51 +1200

From: "John Kipping" <johnkipping@******.nz>

Subject: Re: 13/60 mild engine mods

 I wouldn't use a K & N filter, but if you do it must be fitted at the same time as all the other mods as on its own it will require a different carb needle to a paper filter. 

 13/60 cams are usually perfect, using a Spitfire III profile will help with bhp at the top end so that using revs will give a quicker car, but still retaining a decent tickover.

 Whatever set up you choose I would suggest the Dolomite 1300/1500 and Spitfire 1500 distrubutor, ie the later Lucas type, as I think the centrifugal advance curve is better.  A Spitfire III one would be OK if you can find a decent one which isn't easy.

 With the Canley exhaust manifold a 2 inch Y piece can be ordered and then a Vitesse II system fitted (full system is X21S), quiet with a shiny 2 inch outlet pipe.

 If you fit this set up with an 1 1/2 SU carb you are on your own with a needle requirement as I don't know anybody who has this.  If you can get an 1 3/4 SU (one off a 2.5 S is fine, any HIF type on a more modern car??) then use a BAF needle. The hole in the manifold needs opening up so the butterfly opens but the four mounting holes for the carb are all drilled and tapped.

 This set up will give all the power of a Spitfire III engine, the ease of a single carb, and the best overall fuel economy.

 John Kipping

Not specifically mentioned in THIS post but, yes, a Dolomite manifold and a good quality paper air filter.  I have such a manifold tucked away and I'm keeping it until I find myself with a 1300 engine in need of work.  Not so easy to find now.

C.