The Stovebolt Site Forums The Shop Area Engine Shop Why dual carburetors?

Hi guys,
Can anybody talk me about the use of the dual carburetors. What is the idea, what about the gas consumption, what about the performance, was a option?
thanks

What year/make/model truck are you asking about?

Has this article from the 1955 HRM been reviewed, especially the difference between tests 2 and 3. I do not recall ever seeing a good evaluation of the effects on fuel consumption. My experience with using both dual and triple carbs on a 261 indicates little or no effect on fuel economy.

Hoyt

In general, I think dual carbs is overkill for any unmodified engine. I have dual 1bbl carbs, but my 235 is bored .030 over and I have a mild cam, so dual carbs don't overwhelm the engine. I've seen four barrels on 235s, but I think that's overcarburetion. I did it for performance, and it paid off. The truck runs great, has plenty of power and takes off like a scalded dog.

My gas mileage is about 13 to 14 mpg, but I don't know what it was with a single 1 bbl and a stock engine, so I have no idea if that's better, worse or about the same.

The ideal way to run your dual carbs is to drop down the venturi and jet sizes when running two single stock carbs.
This better suits the air/fuel needs of the engine and will return good power as well as good economy.

Just two stock carbs on a stock engine will, as you say, be too much for the engine to run efficiently or economically.

If you want high performance looks but want to keep the gas costs down then get a 3 carb manifold. You only use the center carb and block off the other two. You can run plumbing to it but block the tubing where it would go into the carbs. Looks good, runs cheap!

Why? 'Cause it looks awesome.

I use my truck for work, I haul around 400lbs of tools everywhere, and often my 2000# tool trailer, plus there are the trips to the saw mill....

I run the dual W-1's, 547's, which have a smaller venturie than the later ones. My 216 is over bored, how much, I don't know, it was done about 20 years ago. I also have the Fenton headers.

What I have noticed, now that I have a few thousand miles on the set-up.

It idles much, much smoother. In fact, when at a stop light, next to a diesel, I can't tell if its running.

It does have more power, much more power. But, that's relative, stock I believe is what, 90 hp? So anything makes a dramatic change.

My gas mileage is exactly the same. Its two carbs, but each delivers half the fuel.

I do run way more timing than stock. The dizzy is 8 or 10 degrees ahead, I run regular gas.

I find I can run in forth gear where I used to have to run in third. So, my mileage has stayed the same, but I go faster.

Did I mention it looks awesome?

"Can anybody talk me about the use of the dual carburetors. What is the idea, what about the gas consumption, what about the performance, was a option?"

In addition to Mr. Lang's fine summary:

What is the idea, - see above; see below

what about the gas consumption, - the same or better, if you match up the carbs properly

what about the performance, - always better, unless you have not matched-up and tuned the carbs properly

was a option? - no, multiple carbs on a 216/235/261 was never a GM option on a truck. However, triple side-draft carbs on a 235 was standard on 235 Corvette in 53-55 - makes for good conversation at shows/cruise-ins.

Add dual-exhaust manifolds to enhance all of the above (if you have the carbs well-matched and well-tuned).


What is the idea, - see above; see below

- It looks good
- It sounds good (even with civilized, stock mufflers)
- It accelerates much better than stock (from start and at all speeds)
- It's a great conversation starter

I have run dual stock Rochesters and dual Carter-Webers (Motorcraft 740s)

I prefer the stock look but I'll live with this modification.

Tim

Back in the day there weren't as many options for carburetion. you couldn't get much bigger, so you simply added an extra (or 2 or 3...)! These days you can get a bigger carb. There are intakes for the old stove bolts that you can put a big four barrel on. Usually a single bigger carb will out perform multiple smaller carbs if the CFM's are the same. (Although some people will argue otherwise) Hot Rod recently ran an article about comparing performance between single and multi-carb setups, this article will confirm my points.

FACT: Single carbs will NEVER look as cool as multiples.

I apologize for hijacking this thread a little, but I was really taken by the air cleaner setup on Tim's 261. Could you provide a little detail on what you did to make that work, Tim?

More on topic, I'm going to put a pair of C-W's on my stock 235 this Fall. I plan to take it to the dyno to see what improvement I get. But mostly I like the look.


regards,
Leon

Leon,

Tom Langdon makes and sells those oil-bath converted-to-cartridge air cleaners for the C-W carbs (most likely you are getting your carbs from him). His price is better than buying and converting your own. They fit perfectly.

Tim

Tim,

In your picture of the two C/W, you appear to be plumbed to the vacuum advance unit with both carburetors. Is there some reason to do this...maybe Mr. Langdons advice. Wouldn't one carburetor mirror whatever was going on with the other in regards to vacuum.

Stuart

Stuart,

You are correct in your observation, and correct in your understanding that connecting both carb's ports is superfluous.

It was easy to do both with rubber hoses and a tee-fitting; and, in doing it this way, the unused no-threaded port/pipe does not need to be blocked off (I'm lazy and I easily talked myself into thinking that it looks OK/better).

When I plumbed a set of dual stock Rochesters, I only plumbed one carb. The vacuum port on the other Rochester was easily sealed with a brass screw cap; and, I was able to use the stock all-metal vacuum line to one carb.

Tim

Hey, mine don't look too bad either.

Paul,

Your engine compartment is very 'sanitary'. Everything screams of neat, tidy and symmetry. You don't feel the need to reach in there and straighten anything!

Stuart

i have dual carbs on my 235 with fentons and 2.73 rear end and i am getting 12.5 mpg. a while back one carb became inoperative and the mileage jumped to 19mph. but i can't be sure of this. i do have plenty of power even with the 2.73 rearend. ron

I don't care whether they work or not, multiple carbs look good. They make the engine compartment look to be well organized and, as they say today, "tight". However, if you can keep the same amount of fuel moving to the engine but better distributed across the intake you have done well.

Paul.

Nice looking air cleaner cover. Tell us about it - the base, the filters, the cover (sheet metal or cast metal, and from where).

Tim

Here are a couple of articles:

http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

http://www.thecarburetorshop.com/Triple1barrels.htm

Most enthusiasts today go for multiple carburetors because of looks. As one earlier poster mentioned, multiple carbs on a stock engine will not help much. If you install PROPERLY SIZED AND CALIBRATED duals on a stock engine; expect 3~5 percent increase in HP and 3~5 percent increase in fuel economy (obviously not cost effective).

Put two STOCK carbs on a stock engine and expect 5~10 percent LOSS in low end torque and HP, and 15~20 percent loss in fuel economy.

The key is in the selection of the carbs.

If you truly believe that "one size fits all", try walking a mile in your spouses shoes.

Jon.

Let me interrupt for a moment, since I feel responsible for diverting this thread in the first place. We haven't heard from alfonso, who asked the original question. Did you get a complete enough answer, alfonso? As you can see from the responses here, its a popular modification.

regards,
Leon

Take a look at the may1953.ppt link in this link for a test by Frank McGurk that appeared in HRM in 1953. Two Stromberg BXOV-2 carbs did fairly well on an otherwise stock 216. As others have pointed out, the total venturi flow area must be properly matched to the size of the engine and the desired RPM range of comfortable operation, among many other considerations.

Hoyt

Hoyt

Thanks, Tim. I bought the air cleaner from O'Brien Truckers. If you click on the Products link, there's a picture of my air cleaner in the bottom right hand corner.

It's a two piece air cleaner; a finned aluminum cover with a plain aluminum base. I then had it powder coated black for looks, to match the valve and side covers. It bolts to the center mounting screw of each carb. The filters are "standard" five inch paper filters that seal to the base and the cover and encircle each carb separately.

The whole setup was not cheap. The air cleaner was $175 (if I recall correctly) and the powder coating was around $75.00.

Thanks, Stuart. I am a bit of a neat freak.

Hoyt....
try finding BXOV-2 Strommies these days!
Dunno what it is like in the 'States, but these particular ones were only ever released on a couple of model HOLDENS in the 50/60's.
All the rest were BXV-2's.
I spend hours at swap meets now trying to collect these things....who'd ever have thought!