Are Triple Carbs Rubbish?

[oldtuckunder]

A bit of a controversial title, but looking for some feedback either positive or negative!

 

 

Whilst I'm looking at this in respect of a 2.0 Ltr Triumph 6, I think the same issue applies to the 2.5 engine.

 

I'm also talking about triple CD or SU carbs not triple Webber/Delorto's as they would have 6 choke/throttle barrels.

 

My conundrum is that I know that the twin CD150's on my 2.0 are close to their limit viz what the engine now needs, and also the adjustable needle choices available are very limited. So do I look at a better twin carb set up, or look at some triple carb set up.

 

Triple Webbers are out of my budget this year, so one of the options is a triple Stromberg or triple SU set up, as inlet manifolds are available.

 

However looking at the firing order of the 6, I think triple CD's or SU's could be a bad idea. I'm not saying that they may not be better than an under spec twin set and that fitting them might show performance gains, but that there could be issues in setting them up, that means that a better twin carb set up may actually be even better.

 

OK what am I going on about? Most/All triple CD/SU sets ups that I have seen in effect use 3 manifold pairs with an individual carb on 1&2, 3&4 and 5&6. OK now match this up with the firing order 153624, and where S is when a cylinder is actively sucking on the card and D is a dwell.

 

 

Twin 1 5 3 6 2 4

 

1-3 S D S D S D

4-6 D S D S D S

 

Triple

 

1-2 S D D D S D

3-4 D D S D D S

5-6 D S D S D D

 

As we can see on a twin carb setup each carb gets a regular S/D pulse pair, but on a triple set up each carb is running a different pattern. Well actually 1&2 are the same pattern as 5&6 but that isn't regular.

 

So my thinking is that with twin carbs there is a regular pulse pattern on each carb, that they are identical, and if there are any inlet resonance track pulse benefits or negatives it is identical on each cylinder.

 

Now compare that with the triple set up where pulses are all over the shop, and the carbs are seeing irregular demands for air and fuel.

 

My conjecture is that setting up triple constant depression carbs that have irregular and different air flow demand patterns on them could be difficult to say the least. i.e. the air flow through the carbs have different start/stop patterns and I conjecture that this could affect the mixture and possibly even the mixture on two cylinders on the same carb.

 

Of course this could all be utter rubbish and at idle rpm and above the cycle is so rapid that there is no effect at all.

 

So far I haven't found anything on-line that covers this or is particularly authoritative on the differences advantages/disadvantages of going single, twin or triple carbs.

 

What is better, a carb that is just big enough for the job running close to its maximum through put, or a bigger carb running well within its limits?

 

Anyone got any thoughts, suggestions, or can completely debunk my conjecture?

[graeme]

I am not an expert, but most of the manifolds I have seen have a balance pipe arrangement which would surely negate the pulsing that you refer to!

 

Cheers

 

Graeme

[Stag powered]

A similar problem occurs on the 4 cylinder cars with twin carbs with a 1342 firing order. IIRC the second sucking cylinder receives a slightly richer mixture than the first, but that didn't stop triumph using it on most of their cars.

 

Neil

[ianc]

For the 1962 RAC Rally, Triumph put triple SUs onto a 1600cc Vitesse, giving Vic Elford 95 BHP to play with. Unfortunately, the gearbox (the standard Vitesse unit) couldn't stand the strain!

Ian Cornish

[Peter Cobbold]

As youve spotted the air flows and fuel 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 richer.

I saw a very neat triple SU setup - on CNC machined billet manifold! - at Malvern years ago. I dont recall if it used 1 1/2 or 1 3/4 SUs.

[Peter Cobbold]

"What is better, a carb that is just big enough for the job running close to its maximum through put, or a bigger carb running well within its limits?"

With Webers its important not to oversize the venturis. But SUs dont appear to use venturi suction, the constant dperession from the piston providing the 'suction' on the fuel. Air velocity over the SU jet, and the lift provided by the bridge ramp, is important for atomising the fuel rapidly a so it distributes equally in a complex manifold. But with triple SUs its a straight run to the inlet valve so I would sacrifice some air velocity by going for bigger diameter carbs, light springs, and less pressure drop. That's for competition at mostly higher rpm. For road use I'd focus on keeping a decent piston lift at cruise to allow better atomisation and more accurate mixture tuning- so smaller carbs.

Others may disagree!

 

The length and diameter of the manifold from jet to inlet valve is important if your cam/exhaust is designed with overlap tuning in mind.

[Graham Robson]

For the 1962 RAC Rally, Triumph put triple SUs onto a 1600cc Vitesse, giving Vic Elford 95 BHP to play with. Unfortunately, the gearbox (the standard Vitesse unit) couldn't stand the strain!

Ian Cornish

 

And for one brief period, Harry Webster was hoping to persuade Leyland to approved the launch of a triple-SU/2-litre Vitesse 'homologation special' with a TR4 gearbox. But then the Lotus-Cortina came along, and changed everybody's minds ....

[oldtuckunder]

 

And for one brief period, Harry Webster was hoping to persuade Leyland to approved the launch of a triple-SU/2-litre Vitesse 'homologation special' with a TR4 gearbox. But then the Lotus-Cortina came along, and changed everybody's minds ....

 

From that I can assume that they had done some serious testing with triple carbs and were pleased with the results? any idea if they were using triple 1.25" or 1.5", from everything I have read twin 1.75" are actually too big for 2.0 Litres, so I can assume that they were not looking at triple 1.75's?

[Peter Cobbold]

http://club.triumph.org.uk/uploads/carb__stub_stack_1_618.jpg

 

http://www.sw-em.com/SU_vs_disp.jpg

[Andy Moltu]

The triple SU 6 is Reg Bowler's.

 

The firing order does, to an extent, limit the benefits of triple SUs. Webers, injection and Dellortos effectively give an independent feed to each cylinder.

 

A triple set of the special SUs fitted to the TRS would have got round that.

Is that what they planned to fit to the Vitesse project before it was shelved?

[Dave Larnder]

For what it is worth, I had triple 1.75 S.U.s on a 2.5 litre Austin Healey 100/6 with a full stage III Raymond Mays conversion.

The Healey was clocked on a race track at 125MPH when new and set up accurately. I say accurately as it is very difficult to keep them balanced using a balance pipe inlet on a road car and the fuel consumption NEVER got above 18MPG.The only trouble was the roadholding, a TR2 on a windy road could leave me standing only to be caught on the straight.

BTW that was back in 1963.

 

Dave

 

 

 

[oldtuckunder]

http://club.triumph.org.uk/uploads/carb__stub_stack_1_618.jpg

 

http://www.sw-em.com/SU_vs_disp.jpg

Interesting the SU's are HIF 4's 1.5" with the MGB filter stub stacks. From the picture that was at build up stage, I wonder what the final results was like.

 

Thanks for the chart from the SU Tuning Manual, first time I have seen it, not having played much with SU's in the past.

 

So many options! where to start? and throw Mikuni's into the mix and one could spend a lifetime experimenting. Unfortunately don't have a whole one of those left :-)

[Nick Jones]

Some discussion here

http://www.clubtriumph.eu/cgi-bin/forum10/Blah.pl?m-1103288330/s-0/

 

and here

 

http://sideways-technologies.co.uk/forums/index.php/topic/6372-gt6-triple-150cd-strombergs/page__hl__%2Btriple+%2Bsu__fromsearch__1

 

http://sideways-technologies.co.uk/forums/index.php/topic/1813-inlet-manifold-balance-pipes/page__p__23603__hl__+fuel%20+standoff__fromsearch__1?do=findComment&comment=23603

http://sideways-technologies.co.uk/forums/index.php/topic/763-fuel-stand-off/page__hl__%20fuel%20%20standoff

Some of the pics have become detached when the forum was moved a while back.

I think that the bottom line is that the triples can be troublesome, especially with wilder cams.

There are other, certainly simpler, certainly cheaper, to get similar results from twins. You can fit 175s to the standard manifold. You can fit 175s to the standard manifold and open it out/port match. You can use the TR250/TR6 manifold with longer runners and 175s (my favourite - could be bonnet clearance issues with the front dashpot though). Do be aware that some (but not all) US market TRs have different port sizes and spacings so check any TR manifold you buy matches your head before parting with cash.....

 

Nick

 

PS. I like electronic injection best!

Edited November 3, 2013 by Nick Jones

[oldtuckunder]

<snip>

 

I think that the bottom line is that the triples can be troublesome, especially with wilder cams.

 

There are other, certainly simpler, certainly cheaper, to get similar results from twins. You can fit 175s to the standard manifold. You can fit 175s to the standard manifold and open it out/port match. You can use the TR250/TR6 manifold with longer runners and 175s (my favourite -

 

Hi Nick

 

Many thanks for taking the time and posting the links.

 

Just about confirms in spades what I was thinking that "Triple Carbs might be rubbish" but probably better than a worn out twin set on a restricted std manifold.

 

Fortunately I don't have that second problem, so at least if change anything I'll be comparing Apples to Apples.

 

I have a pair of 175's sat on the shelf, so I think the next step is a refurb and try those. I have some adapter plates that mate the 175's to the inlet manifold. Making a guess at some initial needles to match the rest of the engine could be fun though.

[Peter Cobbold]

 

Hi Nick

 

Many thanks for taking the time and posting the links.

 

Just about confirms in spades what I was thinking that "Triple Carbs might be rubbish" but probably better than a worn out twin set on a restricted std manifold.

 

Fortunately I don't have that second problem, so at least if change anything I'll be comparing Apples to Apples.

 

I have a pair of 175's sat on the shelf, so I think the next step is a refurb and try those. I have some adapter plates that mate the 175's to the inlet manifold. Making a guess at some initial needles to match the rest of the engine could be fun though.

No need to guess! Measure AFR and piston lift on the road at wot, various rpm, then fettle needle to suit.

Peter

[Graham Robson]

 

From that I can assume that they had done some serious testing with triple carbs and were pleased with the results? any idea if they were using triple 1.25" or 1.5", from everything I have read twin 1.75" are actually too big for 2.0 Litres, so I can assume that they were not looking at triple 1.75's?

 

Memory fails, but from the look of a picture I have (sorry, don't know how to post it), the SUs on this car (407VC, the 1.6-litre Vitesse used on the RAC rally) were triple 1 1/4ins - I believe because such things were already being used on the Spitfire.

 

'Serious testing' ? No, not yet. Neither of that 1.6-litre, or of the proposed 2-litre. At the time that car was built, it was all very new ....

 

Ian Cornish has the 'master' of that picture, and may be willing to send you a copy.

 

Hon. Pres.

[mdave]

Triple carbs on a 6 are not rubbish - unless you call all 6 cylinder E-type Jaguar engines rubbish?

Like all carburettion, the results are in how you design it and set it up.

[Andy Moltu]

Triple carbs on a 6 are not rubbish - unless you call all 6 cylinder E-type Jaguar engines rubbish?

Like all carburettion, the results are in how you design it and set it up.

 

What's the firing order on the Jag & the Healey?

There's nothing intrinsically wrong with triple carbs - the real question is are they (SUs) a good option on the Triumph 6 cylinder injection where the firing order doesn't get the best from a 3 carb set up. How well they function on other cars is an entirely different issue as are triple Webers.

[Tony Millward]

The firing order on an E type is exactly the same as a TR6. Triple carbs WILL work fine if set up correctly.

 

Cheers

 

Tony

[johnny250]

looks interesting, http://www.goodparts.com/shop/index.php?categoryID=2 bonnet clearance may be a problem with SU's?

[Stanpartmanpartwolf]

The Goodparts kit's nice, like all Richard's TR parts.

However, I think the main issue here is the rear carb fouling a RHD steering column & this is something to consider on any potential triple SU HD/HS/HIF or Stromberg conversion.

The triple carb Healey 3000 went well enough, though its linkage was interesting, to say the least.

Jaguars, Astons, tuned Fords et al spring to mind.

[Peter Cobbold]

The triple SU 6 is Reg Bowler's.

 

I am pretty certain his is RHD. So issues with steering column and servo are not insurmountable.

And maybe he still has the CNC programme?

[stuart]

 

What's the firing order on the Jag & the Healey?

There's nothing intrinsically wrong with triple carbs - the real question is are they (SUs) a good option on the Triumph 6 cylinder injection where the firing order doesn't get the best from a 3 carb set up. How well they function on other cars is an entirely different issue as are triple Webers.

153624 however on the Jaguar engine no6 is front

Stuart.

[stuart]

The triple carb Healey 3000 went well enough, though its linkage was interesting, to say the least.

 

You aint kidding there! Especially the choke linkage.

Stuart.

[oldtuckunder]

Triple carbs on a 6 are not rubbish - unless you call all 6 cylinder E-type Jaguar engines rubbish?

Like all carburettion, the results are in how you design it and set it up.

 

My title was a bit extreme. But from an awful lot of reading over the last 24 hours, it appears that whilst triples on a 6 with a 153642 firing order are almost certainly an improvement over a bad standard inlet and worn out twin set, and for a road/fast road spec engine can provide a measure of improvement. They also have a nice “bling” factor. However the more I have read when people have been trying to get them working with highly tuned engines, the more almost insurmountable problems they seem to have hit. It does also appear that a properly set up twin set does actually deliver more power at the end of the day, Yes even on the Jaguar 6 and Healey according to several threads.

 

So I guess the real question (if you don’t already have triple CD/SU’s fitted) is why would you bother. If what you have is worn out and you have a standard or mildly tuned engine, and you like the “bling” factor, then OK. However if you have a highly tuned engine and your looking for an improvement over a well set up carefully fettled inlet and perfect twin standard carbs, (and you wish/have to exclude fuel injection/Supercharging), then it appears that its either triple webbers, or a better twin set.

 

I guess I’m slightly disappointed at the answer as I think triple CD’s or SU’s would have looked real cool. However what I’m after is a measurable benefit over what I have (which is working well) that once set up I can use, rather than spend the next two years tinkering with something that fundamentally can’t work as well as the other options.