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Aren’t 3 carbs too much?

Whenever I post up pics of a 289 3x2 setup, there is always a few folks who exclaim, “that is way too much carb for such a small engine!!!”

Or “that is too much fuel!!!”

Recently, a guy on a Holley forum was ranting and raving about using a calculator to determine carb requirements for an engine.

The guy was completely unreasonable and wouldn’t listen to anyone at all.

Now a few things that need to be understood, truths if you will….

-you can NOT add cfm of each carb and think it equals the total. That is not how an engine sees it.

-a carburetor meters fuel into an airstream. An example. (An extremely simplified example)

If 1 cubic ft flows through a carb, it meters a fixed amount into that one cf of air. Flow two cubic of air, and twice as much fuel is lifted into the engine. (Please don’t pick this part, I’m just trying to prove a point)


Imagine if you will, one 2v carb. A 390ci will pull as much air as it needs to attain an rpm. The carb will see a large depression. The air needed for this engine will all flow through the single 2v carb.

Now…. Add two more 2v of the same size. What happens?

Does the engine suddenly flow three times as much air at the same rpm? Does it flow three times as much fuel?

No. Of course not. That air is split between the carbs to attain that rpm.

The carbs are metering less fuel each because less air is flowing through each one.

Also of note, each carb is seeing less pull at their fuel restrictions.

So on an FE, each carb is rated at 350cfm.

But add them up, 1050cfm!!! Way too much!

No. Absolutely no. For a few reasons. First, 2v are flowed and tested at a fixed depression which is greater than a 4v is flowed.

In 4v depression, these three 350s would be rated at more like 750cfm, much more reasonable.

In the case of the 289 setup pictured, the 4v equivalent would be 575cfm total.

Why are calculators bs?

If you rate a carb at .5, 1.5, 2.5, or 5in of depression, it will flow vastly different amounts.

Will the engine run when the carbs sees any of the above? Yes.

People like to say that the fuel is pulled into an engine because of the intake vacuum. This is not really correct.

Fuel is lifted by airflow through the venturi speeding up. Once the fuel is lifted it is drawn into the airstream.

Obviously a carb needs to see some depression otherwise the fuel won’t flow.

But it also wants to see a good bit of depression as the fuel remains in gaseous form once it is in a negative pressure environment (the intake manifold).

So a larger carb may see less and a smaller carb will see more depression with the same engine and rpm.

There is balance to be found depending on usage.

For a stock engine, a carb seeing higher depression and an intake being under greater vacuum is not bad, it may actually help clean burning at low flow situations like cruising. When is this bad?

If aiming for high flow, if a carbs airflow is maxed out, it begins to pull excessively at the boosters, the engine can become excessively rich and it may be difficult to lean it out.

For a performance vehicle, reducing pumping loses=more power.

To throw a wrench into the gears….

When a small carb is wide open, it cannot open or flow any more.

When a large carb is flowing too much, you can simply close the throttle.

This is why many tuners say: “it is easier to tune an overly large carb than it is an overly small one.”

Back to the 289 3x2 with progressive linkage. Lets say it flows exactly 600cfm at a fixed depression.

Now. Lets say a 4v flows exactly 600cfm at the same fixed depression.

Can you not see how with the 3x2 you have the best of both worlds? More throttle bores allow for more spaced out air entering the intake and provides way more throttle position options.

A concept often forgotten is an engine does NOT see a fixed depression. Valves opening and shutting make a plenum a very turbulent area.

Each cylinder doesn’t really pull from the carb. The cylinder pulls from the plenum.

The plenum pulls from the carb.

Air is elastic in nature, so the air in the plenum is constantly being pulled upon and tries to return to a neutral state by pulling on the carb. This functions as a shock absorber of sorts.

I’m really tired and wrote this at 3am, I may have mispoken somewhere in the above, sorry if I did, trying to make a somewhat complex situation very simple for comprehension purposes.

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