Tri Carb setup with 2bbls?

[TheDude]

Hi,

I know this might be a crazy (or stupid) idea but I thought I would open it up for discussion. Would it be possible to fabricate a tri carb setup that used three 2bbl carburetors instead of three one barrel carburetors? Would this be too much air/fuel going into the cylinders? I assume this would work better on a highly modified six cylinder engine.

 

[MustangSix]

If it were built as an individual runner system with one venturi per cylinder, you could probably get away with three 500 cfm Holleys on a relatively mild 200. A rack of triple 45 DCOE Webers is roughly 1500CFM of carbs, if you flowed them continuously and simultaneously, yet they would be just fine on a 200. OTOH, if it were just three 2bbls on a log, almost everything would be too much carb.

 

[TheDude]

Thanks for the response.

If it were built as an individual runner system with one venturi per cylinder, you could probably get away with three 500 cfm Holleys on a relatively mild 200.

How would one set that up?

 

[Hot 6t Falcon]

Hey Dude,

The answer to your question is yes, no, maybe.

Yes, you can put them on. You could either put adapters on an offy 3x1, or you could build your own 3x1 and elongate the pipe stacks to match a two-barrel flange. Something like this.

I am currently working on a head that will use two Holley/Weber two barrels. That will be about 500 cfm from two carbs. Much more and I'd be saying "no, you can't do that" The advantage to a Holley/eber is that it works like 1/2 of a four barrel. The primary barrel opens to about 2/3 throttle and the the second barrel starts to open. They both hit Wide Open Throttle at the same time. So at part throttle it "Should" get good gasmileage, and it "should" create a few more ponies at WOT. Beside it will have a better eye appeal at cruise night.

So could you put three of these H/Ws on? No - well, maybe. On a 250 with a cam, compression, headers, ignition - maybe. On a stock 170 -never.

One more thought. Back in the day... I saw pictures of a 144 with three Stromberg 97's on it. If I recall, the 97's flowed about 100 cfm each (which is about the same as a 144 one-barrel), so that might be feasible, but do you know what those antique 97's are worth?

Good Luck

 

[rbohm]

8) one possibility is to use the smallest staged weber carbs so that you have very small primaries, each feeding essentially two cylinders, and when you have your foot down all six barrels would be open when you need it most. you would want to use something like a 26/32(iirc that is the smallest you can get, and you can get them off a late 70's chevette or something similar). and if you choose to use the stromberg 97's, remember to keep after them as they have a reputation for being serious leakers.

 

[xctasy]

The Dude pondered:-

I know this might be a crazy (or stupid) idea but I thought I would open it up for discussion. Would it be possible to fabricate a tri carb setup that used three 2bbl carburetors instead of three one barrel carburetors? Would this be too much air/fuel going into the cylinders? I assume this would work better on a highly modified six cylinder engine.

Yes, but it would be a screamer. The answer is that there is no problem "in theory".

The carb, as rated at by one kind of standard, is only a 227 cfm carb. That's not much, and tripling the number of carb set-ups doesn't triple the flow to 681 cfm because of the way the six cylinder engines firing cycles are aranged.

On a Pinto, revolution subjects the carb to a lot of time on duty. Four cylinders are sucking air from 121 cubes of engine.

On a triple carb six, with one carb per two cylinders, one carb is being vacummed by only two cylinders. In effect, one carb is feeding 66 cubes, so there is less "suck" imparted on them by the engine. They won't be as efficient unless the engine is designed to suck more air a nice big cam, header exhast, and some good Schendahl work on the head. It could then work really well.

Some examples of carb engines, with an idea on how few cubic inches of engine you can get away with, and still have the car run well.

Bristol/BMW 121 cid I6. Triple 34 1-bbls, 142 hp net on some Bristol or AC Acea sports cars. One carb per 40 cubes.

The V12 Jag had four 1.75" carbs. One carb per 54 cubes of engine.

A little 178 cube I6 Austin Healy can take three H4 SU carbs. They are 1.5" in diameter, with about 1.25" of actual flow area each when the carb is a wide open. One carb per 59 cubes.

The Australian 202 L6 Holden Torana XU1 GTR uses three 1.75" carbs, gave 195 hp gross, one carb per 67 cubes,

DB4 Aston Martin . 224 CUBES, triple SU's, and 74 cubes per carb

An early Jag 4.2 XK-E DOHC I6 has three 2" carbs. 265 hp gross.One carb per 86 cubes.

A set of three 32/36's would never be fully open until 2/3 rd throttle. In Europe a 97 cube OHC I4Cortina with this carb delivered 78 hp with one carb. One carb per 97 cubes

An Australian 32/36 carbed 121 cube OHC I4 Cortina gave 80 hp with one. Pintos gave a little more with the same carb and engine. One carb per 121 cubes.

Hope that helps.

 

[CobraSix]

On a triple carb six, with one carb per two cylinders, one carb is being vacummed by only two cylinders. In effect, one carb is feeding 66 cubes, so there is less "suck" imparted on them by the engine. They won't be as efficient unless the engine is designed to suck more air a nice big cam, header exhast, and some good Schendahl work on the head. It could then work really well.

Yes and no. Maybe you simplified and left this point out. 1 carb to 2 cylinders is kind of a simplification. while it will pull mainly from the closest carb, the other carbs will still have a slight vacuum pulled against them from thos two carbs.

UNLESS, you do like jack said and divide the plenum into 3 individual chambers.

If you get small enough 2Vs, you can run it. I mean, some people with the aussie head are running 600CFM, so what's 681?

Slade

 

[MustangSix]

An IR system would be one cylinder per barrel, like the Mikunis in my avatar. Cool looking, responsive, but a real PITA to tune. More carbs = more hassle.

The CFM requirement in an IR setup is computed differently. When a carb venturi is slaved to a single cylinder, it doesn't flow continuosly. It has to flow enough air for a single cylinder in one big gulp in a very short period of time, so what looks like overkill in a plenum system might not be sufficient in an IR system.

I haven't tried it, but I think that an ideal system on a 200 would be two 32/36 Webers on a split manifold. That same carb setup was great on my TR4 and Proline sells something similar for the Slant 6 Mopar.

 

[Anonymous]

this is a dumb stupid question but imma ask it, see how on the aussie 2v head on the intake manifold it looks like it can separate from the 6 intakes to the one for the carb? is it possible take that carb piece of and run 6 1 barrels?

 

[rbohm]

this is a dumb stupid question but imma ask it, see how on the aussie 2v head on the intake manifold it looks like it can separate from the 6 intakes to the one for the carb? is it possible take that carb piece of and run 6 1 barrels?

8) yes you could run 6 1bbl carbs and create the independant runner system. the problem comes in that, like jack said, you need to have a carb large enough to handle a sudden demand for airflow, but small enough to get proper fuel draw, and atomization. the trick is tuning the independant runner system for good performance and efficiency.

 

[xctasy]

CobraSix replied:-

Yes and no. Maybe you simplified and left this point out. 1 carb to 2 cylinders is kind of a simplification. while it will pull mainly from the closest carb, the other carbs will still have a slight vacuum pulled against them from thos two carbs.

You're right. It sucks six sandwhiches similtanouesly (<sp! )


I've seen a 202 with 725 cubes of Qjet 4MV and a 265 with 650 cfm of double pumper...seams like your right , Slade.

 

[Anonymous]

I don't know if the idea is crazy or stupid, but here that concept is already used and seemingly it works.

 

[rickwrench]

Augh! The Argies and that ME head. I MUST have one. Let's see three Weber 45 IDAs will cost me...
Rick(wrench)

 

[Anonymous]

Hey X-

BMW 750cc opposed twin motorcycle motor- 22 cubes per carb. Ha!

The thought of a 6 Bing CV carb set-up has crossed my mind. Tuning would be a nightmare, but possible with a good set of carb sticks. Dunno about the debate between CV vs. flat slide motorcycle carbs. Bing CV's are the devil I know, so they came to mind first.

Finding 6 identically jetted carbs would be hard. But, on the brighter side- the jetting from a BMW R100 might be close (based on cubes similarity- no clue on things like flow, and valve lift/size). Still more research to do.

And the throttle linkage would be...interesting.....

--mikey

 

[xctasy]

You do not like 6 BingCV's well......

 

[ski4evr]

How would you set up the carbs to "gulp" when it's cylinder(s) are ready for a shot of fuel?

How would you set up TBI's vice carbs?

How would you segregate the cylinders to operate in 3 sets of two or as six individual cylinders with a log head?

Sorry for the many how's.

ski

 

[ski4evr]

As soon as I posted it, I think I figured it out - the gulping thing, that is. The cylinder or bank of cylinders would draw a vacuum and pull in the air fuel mixture when needed - just like they do already, duh.

But, what is a good way to segragate the cylinders? slots with plates jammed in the log?

ski

 

[xctasy]

Pull the frost/welch plugs out each end, and shove a nice big 1" broom handle in each end. Put your flanges on the log, then mark out the gaps with a marker pen. Then make an alloy bar and die grind it out to follow the wooden broom handle.

You could dowl the alloy spacers in with a cotter pin or use a jointing compound.