tripple SU carbs

[marcusaurailius]

A friend has been trying for months now, with the help of others and some rolling road work done,to try and get the tripple Su,s he has put on his car to work

the car is a 2.0,litre , six cyl with a fire order of 1.5.3.6.2.4, and a 280 deg cam,

the prob he has got is blow back, /fuel stand off of the carbs.

some folk have come to the conclusion that because of the fireing order, then 3 sets of carbs of the SU type will not work

the standard set up is twin SU/Stromberg carbs

he has tried putting in a bigger balance pipe, but is no better

3 x twin side draughts, or FI will work, but these are different from the constant depression carbs like the SU

any one any experiance with tripples on an engine with this fire order


regards Marcus

 

[evelrod]

It's been many years since I worked with SU's and, I have not done three, but I have done several race cars with two. The "standoff" you speak of is quite normal and I doubt you would like the performance even if you could eliminate it. My normal procedure was to us air horns (trumpets) that were just long enough to catch "most" of the visible standoff mist. On an MG or Sprite, that was something like two to three inches...it varied with displacement/cam/rpm, etc.
Street machines always had an air filter and I assume the standoff problem was not as critical at lower rpm. On the race cars adding the trumpets made a big difference.
Perhaps you could use a K&N type filter over them.

Some things that come to mind (wow, that's a stretch!)...Using different viscosity oil in the dashpots and or springs could and sometimes did help. The richer the mixture the more standoff (well, DUH) and on these little engines the needles looked like "sewing" needles!!!


Rod

 

[marcusaurailius]

Thanks Rod, but he tried most of what you suggested too, got some trumpets on too

but the stand off is different on the end to carbs so he says, , and going by the pic and his discription, its still standing off at higher revs

this is whats leading folk to think that they wont work with SU,s due to the fire order

its 1.5.3.6.2.4, and the exhaust header is 6.3.1. with 1-6, 2-5, 3-4, linked together.

thanks any how, , Marcus

 

[patprimmer]

I have run triple SUs on such a configuration, no problems.

One was a 3.3 litre (202CI) with 3 by 1.75" SUs.

We had a longish manifold, small balance tube and about 1.5" big bell mouth ram tubes. The tubes were mounted inside a plenum and there was a Unifilter at the inlet to the plenum.

We set the carbies up exactly as per the BMC manual for these carbies as twins on an A series engine. I think twin 1.75s were stock on a Cooper S, but memories are now faded.

Some people ran as small as 1.5s and as big as 2". all worked, depending o engine size and rpm range sought.

1.75 worked well from 1000 rpm to 6000 rpm in the 3.3 litre push rod engine with 285 deg cam, Rhodes lifters and 12 port head. There was some stand off, but it never did any harm. Valve float limited the top end. It was a daily driver and my brother taught his 70 year old mother in law to drive in it. We never dynoed it, but it probably made about 150 hp. It outperformed the 5 litre V8 available in the same chassis. The V8s typically dynoed at about 140 to 145 HP and were slightly heavier

Regards
Pat
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[thruthefence]

Didn't at least one model of "E" Jaguar come with three SU's? from the works?

 

[WGJ]

I think the 4.2 Jag engines used triples as standard.

Bill

 

[marcusaurailius]

Thanks you lads , will report back to him,

E types, as far as i can remember they all had tripples,

the fire order is what some folk are saying, cos there are a few Cars over here, that dont seem to go that well with this combination,

Pat, was the engine you had tripples on, the same fire order !!, because it seems to be the middle two that is giveing the trouble

will try and get some pics for you,

and thanks again for your help lads

Regards Marcus

 

[marcusaurailius]

here are some pics of the set up, on me friends car,

http://i646.photobucket.com/albums/uu182/brucegt6uk/07032009320.jpg

http://club.triumph.org.uk/uploads/small_map_take_offs_on_car_and_balance_1495.jpg

http://club.triumph.org.uk/uploads/initial_breather_and_heating_639.jpg

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

this may help you Guru,s better!!,

regards Marcus

 

[gt6boy]

Hi Guys
Thanks for the input- I will try to put something detailed together for you over the next few days.
The car is off the road at present due to the cylinder head failing after I skimmed it too much.
Not withstanding the fuel stand-off, it was running so well when I broke it!

 

[marcusaurailius]

AHHH welcome Bruce, , now you here, then you can tell the ladds all aboot it, as its your Baby

Marcus

 

[evelrod]

Thanks for the photos, Marcus. Picture/thousand etc.

First off, Bruce...You need to get some of the heat wrap paint, the silicon/heat resistant, whatever, stuff they sell. Use it and it will save you countless hassles down the line with the wrap coming loose and "stringing". Even comes in colors...smokes and stinks a bit on first startup but it goes away quickly. (I use the stainless steel ty-raps, but safety wire works just as well. Nice job, though).

Second...You REALLY need three DCOE's (OK, so I'm biased)!

Third...Like I said to start, a nice three inch trumpet is what the setup needs...betcha'!!!

Had a close encounter with a "flying" GT-6 at Riverside many years ago (flown by none other than Don Devendorf). Good story as far as close calls go.

Rod

 

[patprimmer]

The engines I put SUs on were Holden blue motor in line 6s. They had the same firing order as you stated.

There was some stand of, but it never created a problem.

Regards
Pat
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[gt6boy]

Hi Guys
The engine original spec was as follows:
2x 1.5" SU constant depression carbs.
Balance pipe fitted to heated manifold
2 litre straight six engine
CR 9.5/1
Cam 25/65 65/25

As now:
3x 1.5" SU constant depression HIF38 carbs.
Was an in-appropriate balance pipe arrangement.
Large bore cold air fed to air plenum box housing bell mouths.
Was an unheated manifold arrangement.
2 litre straight six.
CR 9.5/1
Cam 37/63 74/28. Correctly intalled now!
Has shallow depth MGB style bell mouth 'stub stacks.
6-3-1 exhaust manifold with un-evenly matched primary pipe lengths.
Mapable Megajolt ignition.
Gas-flowed cylinder head.


Unless I go for Alfa Romeo Spider style bent stub stacks, there is no space available to fit longer stub stacks due to the wheel arch being in the way.
I have been very careful to ensure the carbs are balanced at and off idle properly (i.e. gauges prove they simultaneously draw the same volume of air) and the float heights are correct to book. The carb pistons are all weighed and evenly matched. This includes the piston in dashpot drop time test and they all rise/fall at the same rate.
Where-as twin carbs draw evenly through each alternate carb, triple carbs exhibit a longer drawn out cycle where-by the outer two carbs draw a different sexuence to the middle carb. My outer two carbs(1+3) exhibit pronouned fuel vapour stand-off throughout the rev range, to the point that RR sessions are a definite health hazard!
I can't help but think that as the twin carb set-ups don't do this to anywhere near this extent(if at all), it frustrates me that the current set-up is busy pushing fuel vapour out of the carb throat when it should be drawing it in.
These cars put out an alledged 105bhp in the 60's. The car was putting out 120bhp at the rear wheels on the last session. It did drive much better than had been the case post build a few years ago.
Initially the camshaft wasn't timed in correctly.
The mapable ignition also greatly improved the torque and drive-ability of the car- far more torque.
The gas-flowed cylinder head was inadvertantly left with a CR of only 8.3/1! This was corrected with skimming it to 9.1/1, but that was ll it would do due to an inability to skim the head any further. The improvement to the CR also improved the performance incrumentally.
Getting better needles and lighter carb piston springs improved the situation too, to such an extent that after the last rolling road session, it was a dramatically improved car to drive.
A recent Trackday session at Cadwell Park showed the car to be very nice to drive- a good spread of torque, though even though the cam is only mild, it does have a marked powerband after the initial low down torque.
I've ditched he exhaust wrap. Unless there is an issue with under bonnet temperatures(there isn't at present), i'm not convinced of its merit for my installation.
I can see the relative merits for outright performance of side draught carbies, but the installs on GT6' are fraught with dreadful after market linkages and then there's the question and cost of finding the correct fuelling and getting a knowledgeable RR operator to achieve that throughout the rev range and not leave me with a gas guzzling monster that's got a miniscule power band at 4000+rpm!
I am happy to fit twin 1.75" carbs and a gas flowed inlet manifold in due course, but I want to just try to sort these out to a point i'm happy with before then.

My initial plans are a new cylinder head with a CR of 10.5/1.
Trial the much enlarged (and improved) balance ppe arrangement, with the newly installed heating arrangements for the inlet manifolds.
Try longer ram pipes on the RR- irrespective of them not being able to be fitted with the bonnet down!
The, have a power mapping run to maximise the power throughout the rev range.

So, any thoughts on my plans?
Regards
Bruce.

 

[Artisi]

Turbo with 1 big carb. will solve your problem.

 

[evelrod]

I had a two inch SU on a Judson supercharged 1950 MG-TD...Ran just fine and with it, I could finally blow off those damned VW bugs.

Rod

 

[Artisi]

Nothing has changed over the past 50 years it's still the same as I was told when I first started playing with hotting up (or trying to) car engines.

Of course I am only referring to fueling an internal combustion engine in this instance.

1. get it in - as fast as you can
2. squeeze it - as hard as you can
3. get it out - as fast as you can

 

[patprimmer]

I don't see the purpose of a bigger balance tube or the lighter pistons. SUs set up stock on a similar port layout will work OK. Go back to exactly as the most similar carby is when used by OEM with one carby per siamesed port. Better still but not essential if it has a similar divider to the manifold and similar number of degrees of crank rotation per inlet stroke. OEM spec is always a good starting point. Tuning away from OEM most often creates more problems than it solves. Make sure the needle and seat are concentric, the correct needle is at the correct height and the jet and float are at the correct heights. Use stock piston and spring. Can you shorten up the manifold to make room for ram tubes. Can you rework the inner guard to make room, being careful not to interfere with suspension and tyre interference with all combos of travel and lock.

The only purpose I see for so called balance tubes is if the carby is to small and each cylinder can pull a little from the nearby carbies, or else mainly for allowing fuel enrichment on one carby for cold start and to a smaller extent for closed throttle.

So long as the idle throttle settings are reasonably close, and the linkage is not ridiculously bad and they all get full throttle when you stomp on the pedal it will run OK. It may not be optimized but it will be 95% of optimum.

Maybe you are trying to solve a problem that does not matter and overlooking one that does.

I have seen a lot of standoff problems along with poor performance created by valve train problems.

Regards
Pat
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[gt6boy]

Thanks Pat.
The bigger balance pipe is actually a balance reservoir in effect. The previous install wasn't correct in its design- or even my implementation of that design. I haven't tried the new install yet, as the car is awaiting its new head at present.
I understand and agree with the sentiment about OEM spec's. The guys who designed these set-ups were after all full-time Engineers.
However, this firing order appears on other straight 6's which seem not to suffer with the stand-off mine has- something is intrinsically wrong here, I'd like to put that right, rather than run the white flag up the pole at the first hurdle, as it were.
Carbs are pretty much sorted for state of tune. I haverebuilt them very carefully (new throttle bushes, plain throttle discs, thinned down throttle spindles etc. I break outhe micro-meter when I set the jet heights(all of which are new). The fitting of lighter springs and alterations to the needle were fine tuning of a non-standard set-up, which improved the engines behavior and performance considerably.
The throttle linkage is all new and works well(no slop or lost motion between the carbs) allowing uniform full travel/opening of throttles.
The valve train is entirely standard- albeit the 'go-faster' cam has a higher lift than standard. I set my valve clearances to .017"
I will try longer stub stacks on the rolling road- as the bonnet/wheelarch will be raised and out of the way.
"Maybe you are trying to solve a problem that does not matter and overlooking one that does".
I suspect there maybe something in wht you say!
Regards
Bruce.

 

[bigpaul]

Does this problem manifest itself when you drive the car,
we had similar poroblems with older 4 cylinder engines
when you hold the power on the rollers @ a costant
speed you get stand off. If you do a sweep off at least
300rpm per sec no problem. One other thing springs to mind
check camshaft base circle run out as this will also cause the problem if it is cronic, try opening out the tappet setting on the offending cylinders then do a run

 

[gt6boy]

Hi Paul.
Forgive my ignorance- but what is cam shaft base circle run out? Opening the tappets will in effect retard the valve timing if my understanding is correct?
Regards
Bruce.