The Stovebolt Site Forums The Shop Area Engine Shop Multiple carburetors - reasons

There have been a number of threads on multiple carburetors. Often I have posted that mathematics and physics are the enthusiast's best friend. But believe it or not, history is also an enthusiast's friend.

When one goes to the license bureau to acquire a license for one's automobile (at least here in Missouri), one pays based on taxable horsepower (one can get some really interesting answers by requesting a definition of "taxable" horsepower from the license clerks)

But taxable horsepower dates back to the early days of the automobile industry in the USA, when a government official asked a representative of (A)merican (L)icensed (A)utomotive (M)anufacturer for a way to calculate horsepower. The internet states this occured in 1908. My sources say that A.L.A.M. contacted Cadillac, and one of the Cadillac engineers, in 1911, gave them the equation:

HP = D (squared) times N divided by the constant 2.5

Or

Horsepower is equal to the piston diameter multiplied by itself times the number of cylinders divided by 2.5

This was a very credible figure for horsepower for the low RPM engines of the time (either 1908 or 1911).

This equation was also adopted by the Royal Automobile Club of Great Britain.

So what does this have to do with multiple carburetors???

The British government since long ago (especially on tea ) has been very interested in taxes; and they were much more aggressive on taxing automobile engines, but they often used displacement. So the British manufacturers tried to squeeze more power from smaller engines (improve volumetric efficiency). Note that, while there are exceptions, most of the British factory built (and engineered) engines used setups where the number of carburetors was half the number of cylinders (a four cylinder had two carburetors, a six cylinder had three carburetors). Rarely, but it happened, a six cylinder would have two carburetors, AND THE MANIFOLD AND CYLINDER HEAD DESIGN WOULD BE ONE CARB FEEDING TWO BANKS OF THREE CYLINDERS! The idea with multiple carburetors was better A/F density to all cylinders, thus improving the volumetric efficiency. The carburetor size would be determined by the displacement, and the number of carburetors.

And while it may have occurred, I have NEVER seen one of these engines which used progressive linkage.

Just something to consider when considering the installation of multiple carburetors.

Of course you can also consider our government using a horsepower equation more than 100 years old!

Jon.

LOL That brings to mind the story about the diameter of the Space Shuttle Rocket Boosters. Something about the width of two horses' behinds.

How does one select a multiple carburetor set-up for a stovebolt?

Well, the first consideration is to look at the intake profile of the cylinder head. These are arranged in three banks of two. So, quite obviously, a manifold utilizing three carburetors (one per bank) is going to be more efficient than one utilizing two carburetors.

An example: as a child, did you ever throw a rock in a pond? And watch the "wake" from the splash? Which was uniform. Then throw two rocks in the pond, and watch the two "wakes" where they intersected with interference? The fuel/air mixture in the two carburetor manifold has the same effect, where fuel and air from both carbs are trying to feed the center bank. It might be possible to divide the runners to the center ports, but I have not seen that done.

And this arrangement is why Jerry (Hot Rod Lincoln) throws big rocks at two single barrel set-ups

If the two carb set-up is done PERFECTLY, both in selection, and tuning, there is a wee bit of gain, both in power and economy; maybe from 5~8 percent. If done incorrectly, one can certainly lose much more percentage over a well-set-up single carb. Generally speaking, most dual two-barrel set-ups gain is either due to: (A) the mind of the owner (eye candy), or (B) replacing a mal-functioning single carburetor.

So pick two or three, this decision is left to the owner. As far as carburetor selection, I wrote the following 20 or so years ago, and while they might need tweaking, I wouldn't change much:

Selecting 2 carbs for a 6 cylinder

Selecting 3 carbs for a 6 cylinder

Jon.

The poster child for the L6 with 2 X 1 or 3 X 1 is the Jaguar XK-E/Mk IX etc. 231" engine. The sedan engine uses 2 X 1-3/4" (or larger) S.U. carburetors, divided (as said) into separate banks, with 1 carburetor supplying the front 3 cylinders and the second the rear three. The L6 firing order (1-5-3-6-2-4) divided these groups evenly at 240 degrees. This is not possible with the stovebolt (or any siamese intake) engine because the 3-4 center cylinder pair shares a common port.
The performance car used 3 of the same size carburetors, but divided differently. 1 carburetor supplied each adjacent pair of cylinders: 1-2, 3-4 and 5-6. This didn't produce equal intervals, but each cylinder was nearly doing the opposite of its mate. It can be used on the stovebolt even though the ports are merged.

Stovebolt (and other L6 siamese) size selection:
2 X 1 or 2 X 2 must use a common plenum manifold, which means that all carburetors feed all cylinders. Easy to use too large carburetors since the engine sees only the total of both.
3 X 1 or 3 X 2 may use a common plenum manifold, with the same warning: easy to choose too big, progressive linkage helps.
3 X 1 or 3 X 2 may also use 3 individual manifolds like the Corvette (sometimes with a small vacuum connection to steady the idle), in which each carburetor only feeds 2 cylinders. Too small is possible here.

The famous CFM formula: CFM = D X VE X RPM / 3,456 is only nearly correct as the total for carburetors joined on the same manifold.
3 X 1 individual carburetors are NOT 1/3 of this total, but MUCH LARGER.

Panic - glad you chimed in.

For those who have not seen it, panic has a wonderful description of intake manifolds, I send customers there frequently. For those do have not read his article, I urge you to do so:

Panic's intake manifold comments

Again, mathematics and physics are your friends.

And to take Panic's comment about the CFM equation one step further: that equation is for a 4-stroke multi-cylinder engine of AT LEAST 4 CYLINDER. As Panic mentions, it is for carburetors on a single manifold. Using multiple manifolds (or an individual runner style) forces one to use the equations for engines of less than 4 cylinders.

Jon.

So... from what I am reading in this thread. More Carbs is better, sometimes.

If the manifold breaks things up evenly then its good, if not its difficult to tune correctly.

While this thread is specifically targeting inline 6's, does the same apply to V8's? I saw a couple of 6 carb intakes for a V8 like this here INTAKE. I would never get one because I admittedly am not great at tuning carbs but it looks interesting. What are thoughts on this? While the interfering waves analogy is easy to see for I6's its not for V8's. Does perfect tuning become more or less important with more carbs?

Too bad someone couldn't come up with a simple, self-tuning fuel injected throttle body set-up to replace carbs like what's been available for years on V8's, that could be made to work on any old engine. Surely it's quite possible. I would buy one for sure. Chevy used that set-up way back in like 1998.

The same mathematics and physics apply to V-8's.

2 x 4 or 4 x 2 run the best of the more common set-ups

Then 2 x 2

Then a single four, followed by a single two.

3 x 2 bring up the rear, unless it would be 6 x 2.

I have also (rarely) seen 8 x 2 and 3 x 4.

All of the above assume that carburetors for any configuration are properly selected (check the second line in my signature block).

Many internet pixels have been disturbed (unjustly, they should have left to sleep) about hot rod carburetion.

One of the most interesting comments (at least to me) is: "the hot rodders in Southern California starting using 3 x 2 because their were no single carburetors flowing sufficient air and fuel".

How big does the carburetor need to be?????

Considering Stromberg alone (the hotrodders used Stromberg 97's): by 1934 Stromberg had a production automobile 2-barrel of more than 500 CFM capable of feeding a racing Duesenberg 420 CID engine with a supercharger; and if you consider aircraft engines, Stromberg had a four-barrel carburetor as early as 1932. So did the automobile carb engineers for Stromberg not know about the four barrel aircraft carburetor? Must be the old left-brain right-brain thingy, as the same engineers designed BOTH systems (I have the Stromberg records to prove this).

The reason the small Stromberg 97 was chosen was cost. The junkyards were full of wrecked Ford's with V-8's and Stromberg 97 carbs. Many of the hotrodders were long on talent, and short on money! As these carbs were too small, various manifold manufacturers designed 2, 3, and 4 carburetor manifolds.

And as far as the 2 x 4 or 4 x 2 vs the 3 x 2; or the 3 x 2 vs a single 4-barrel: Pontiac paid Carter to sabotage the AFB so the standard 4-barrel GTO engine would not outrun the more expensive tripower engine. Hint: if you like Carter AFB carbs, take a look at the various hot idle compensators used in different applications, both functional and dummy.

EDIT - Spartan since you pictured a six deuce manifold, here is one with carburetors: Log 6x2 setup

Jon.

Lots of us prefer to play with things mechanical rather than things electronic

Jon.

I saw on U tube where some one used an electric throttle body of a 4.3 v6 with some success .
personally i prefer the fewer wires the better .

i think this might work for someone who wants to modernize the original engine:
Atomic EFI Throttle Body Master Kit

https://www.holley.com/products/fuel_systems/fuel_injection/atomic_efi/atomic_efi_tbi/parts/2900

it will be an expensive experiment, perhaps put the money directly towards the fuel bill would be easier
-s

This one's more fun- - - - - - -and there's no computer to mess with!



LOL!
Jerry

And for better gas mileage, you could use one of the new low-flow models!

Jerry - is that a CNG model?

Methinks my thread has been low-jacked

Jon.

It is timely that I stumbled across this thread!

Last year, I bought a '61 Apache, with a Jasper-rebuilt 235 in it. By looking up the casting numbers, I discovered that it's a '58 truck block, with the higher-compression head and mechanical lifters. With all that, it's very similar to the engine setup on the early 'vettes. Unfortunately, it has an after-market Carter YF with no 'S' model-number in the casting.

I have been thinking of getting a 2- or 3-carb manifold setup and getting rid of the oddball YF. The discussion here is steering me toward a 3x1 setup; I think that's what the first 'vettes had as well. I don't know how many junkyards in central Missouri would have any stuff this old...I'll probably have to start looking online.

I'm in no particular hurry. The engine is running well after I disassembled and cleaned the carb, although out of the overhaul kit I ordered, only the accelerator pump diaphragm and gaskets fit...everything else was different inside.

Any suggestions on a preferred manifold/carb combination?

Thanks for the education!

-Kevin

You've got the best possible combination already, including the Carter YF. What's "oddball" about a carb specifically designed to solve the problems the Rochester B created? Unless you make your own manifolds and use tiny carburetors designed for a motorcycle, three carbs is two too many. If you simply must have the Corvette 3-carb setup, I hope you're independently wealthy- - - - -a good-working setup like that will set you back thousands of dollars, and then you'll discover it won't fit a truck engine compartment without a lot of modifications. Aftermarket 3-carb setups are also hard to fit, but if you make the two outboard carbs non-functioning dummies you can impress the rubes who don't know any better and still get good performance from an original-equipment center carb.

Get a correct rebuild kit from Jon (Carb King) and put the YF in good running condition, and don't even consider screwing up a good fuel system just because you like eye candy. Very few multi-carb setups work well, and even the ones that don't give constant trouble usually result in less power and torque than a good single carb system.
Jerry

Look at what Clifford has to offer.. They also have some youtube videos.

Is this what you are referring to?

This Clifford assembly gives you 120% hp increase.

Here is their wimpy-er assembly.

I wonder if that performance is guaranteed?

Didn't Mark Twain have something to say about lies- - - - - -and statistics?
Jerry

This with out exhaust headers, I have those already.. Figures often beguile me, particularly when I have the arranging of them myself; in which case the remark attributed to Disraeli would often apply with justice and force: "There are three kinds of lies: lies, damned lies and statistics."
- Mark Twain's Own Autobiography: The Chapters from the North American Review

Kevin--would you sell the YF?-need to replace my Rochester 7004522--Thanks Bill

Sorry, I don't think you want this oddball thing. From what I've read, this is an 'aftermarket' YF, which makes sense with the Jasper engine...they seem to be quite a bit different internally than model-specific carbs.

After a good cleaning, it's running much better now. I'll run it until I decide on a multi-carb setup, then probably keep it for a spare. I may remove it and take it down to The Carburetor Shop to see if they have any wisdom as to what it is.

-Kevin

The Carter "replacement' carbs for Chevy engines were developed after Chevy stopped buying Carter carbs for assembly line installation in the late 1940's/early 50's and started making their own carbs- - - -the infamous Rochester B's. The CEO of Carter vowed to sell a million replacement carbs for Chevy- - - - -and they did! Several years later, the 1-millionth Chevy replacement carb came off the assembly line- - - -got gold plated- - - - -and was presented to the CEO of General Motors! Now, that's payback- - - - -in spades!
LOL!
Jerry

I talked to Jon at The Carburetor Shop today on the phone; he was very knowledgeable and helpful. He indicated that without a tag or S-number on the housing, it would be exceedingly difficult to determine exactly what model of Carter YF it is.

I'll just keep using it until I find a multi-carb setup.

-Kevin

maybe here.. Or this.

I went this way. mounts on a 4 bbl intake.

I don't care if the engine won't run at all with that setup. That has enough eye candy appeal to warrant pushing the truck to wherever you need it to be just so others can gawk.

This is my mock up engine, the real block at Gaerte's, more engine bling and my rear end.