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Old 07-21-2008, 05:40 PM
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Carb sizing

I'm hoping there are some carburetor/induction experts here who can help me out with this one.

So I know the basic formula for calculating the "correct" size for a carb, which basically converts an engine's displacement and max rpm into an airflow rate.

But does this work in all situations, with all types of (naturally aspirated) induction?

some logical thinking on my part tells me that it doesn't. But I could be wrong. I don't have enough experience with different types of engines to know.

What first threw me for a loop and made me wonder was what I read about hot rodding air cooled VW's. It seemed people were using not one, but two pretty big weber 2 barrels on small 4 cylinders. Some of these had as much induction as a mild big block.

What I eventually came up with was that two 2-barrels were acting the same as one 2-barrel. Since the intake manifolds were completely seperate, each carb only had to feed one cylinder at a time, since only one could be on it's intake stroke at once. The same as if there were one carb feeding all cylinders through a common manifold.

Is this conclusion accurate? Does anyone know for sure?

That made me think of the following:

On a v6 engine, with a dual plane manifold, wouldn't a single 4 barrel carb need to be the same size as a carb for a dual plane v8 with the same cylinder size as the v6? Wouldn't one "side" of the carb need to provide flow for a single cylinder filling itself at any given time in both cases?

Is this total rediculousness or am I on to something here?

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Old 07-21-2008, 08:20 PM
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What you may not understand about the Webers is that not only do they come in different throttle bore sizes, but the venturi sleeves (also called chokes) are also able to be changed larger or smaller. So, you could have a set of carbs on a big block and dialed in, then transplant them onto a small block and dial them in with a venturi sleeve change.

The casual observer may not be able to discern the difference between a 40 IDF using a 28mm venturi from a 48 IDF using a 40mm venturi.

Webers normally are not spoken of in terms of CFM, unlike the more popular 4-bbl carbs that us hot rodders are familiar with. Likewise, the throttle bore and venturi sizes are not usually given with the 4-bbls. I don't know why there is this difference in references. Maybe someone else can come up with the explanation.

Your comparison between the V6 and V8 lacks the TIME variable. Let's take it to the extreme. A one-cylinder motor that has 44 cubic inches will pass 54 CFM @ 85% efficiency @ 5,000 rpm's. An eight cylinder motor that has 44 cubic inches per cylinder (352 cubic inches total displacement) will pass 432 CFM @ 85% efficiency @ 5,000 rpm's. While it is true that the same carb will feed any one cylinder of the V8 or the one cylinder of the one cylinder motor for one intake stroke, motors don't operate on only one intake stroke. At the 5,000 rpm example given, a 4-stroke cycle motor will have 2,500 intake strokes.

Using the same logic and going back to your V6 versus V8 example, one barrel of the carb will be feeding three cylinders on the V6 and four cylinders on the V8. If the V6 also had 44 cubic inch cylinders and a total displacement of 264 cubic inches, one barrel would feed 132 cubic inches compared with one barrel feeding 176 cubic inches on the V8.

I hope I have answered your question. If not, just say so and I'll try again.
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Old 07-22-2008, 04:22 AM
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Thanks for the reply. This was certainly the kind of info I was looking for.

I had heard of the varying venturi sizes on idf's and ida's before. It sure seems to me like figuring out how to set those things up is a real pain...

I specifically left out the time variable in all those examples, though because I was trying to consider the peak flowrate through the carb in the split second it takes to fill the cylinder.

I know the average flow over an extended period will be less for the v6, dual plane or not. That makes sense. But I'm wondering if that's not what's important.

Shouldn't there be a "time" on the v6 when one side of the carb or other isn't doing anything? Compared to the v8, which will always have an open intake valve somewhere on each side.

Considering cylinder head numbers (and working backwards) might be another way to think about this. If the v6 head and valve flows 250 cfm per cylinder and the v8 head flows the same, and you run a 500 cfm 4 barrel carb on the v8 and a 390 cfm 4 barrel on the v6, dual plane manifolds on both, wouldn't the v6 be restricted? The 250 cfm intake runner and valve would only be provided with 195 cfm of flow from the carb. It seems to me that the only way to provide the head with all the air it can use would be to run the same size carb as the v8, or switch to a single plane intake...

Maybe I'm overanalyzing this, but I want to feel like I know how these things operate before I start working on any v6's. Something I have pretty limited experience with. All the cars I've owned have been v8 or 4 cyl.
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