I noticed that a lot of people want to try different carbs on their engines but don't really know if the change will be a performance improvement. I searched the postings for questions or recommendations people had about carburetors and then tried to find out the flow ratings of those carbs.

Obviously, Holley is a favorite choice for upgrades so I emailed Holley tech support and they gave me ratings of the carbs I asked about.

I emailed Edelbrock but their tech support did not have any flow rating information of the primary side of their 4bbls.

I looked up Rochester carburetor flow ratings in Doug Roe's book "Rochester Carburetors".

I did not have very much success yet with Carter BBS or BBD flow ratings and used the information in the Seven-L Engineering tech paper "Slant Six performance/Economy Modification" written in 1982.

I put the information on my web site and you can find it at the following location:
Carburetion

I hope it helps. If there are any errors or omissions or if you have any other information, let me know.

Frank
Raso Enterprises

Interesting, I think I'll need to read it a few more times to take it all in. I haven't really got the whole 1.5" Hg vers 3.0" Hg idea yet. If you explained it there, I didn't get it on the first read. I've never been much for swapping on non-stock carbs, but I'm hoping to learn.

Great work compiling CFM ratings (adapted for easy compare ) for so many different carbs.

For the sake of presentation, a few pics of carbs or fancy installations would be cool to break up all that tech.

_________________
1980 Aspen 225 super six

Cool.

That got added to the Mopar favorites with a quickness.

I wish we could compile an entire book of /6 mods & solutions and find a way to publish it... sort of a /6 bible. Hell, that's what /6.org is if you think about it

_________________
When in doubt, empty the magazine.
'72 Dart Swinger /6 - My Yard Dart!
'01 Ranger P/U 4x4 X-cab
We buy and sell MOPARTS & Ford Parts!!!
http://www.usironauto.com

Thanks for the feedback.

The reason we specify a pressure drop for a given flow is that simply there must be a reason for something to move from one place to another. If you want any fluid (water, air, etc.) to flow, you need to have some sort of pressure moving it from the high pressure area to the low pressure area. The greater the difference in pressure between the two areas, the greater will be the flow.

Picture two tanks of water, one tank 10 feet higher than the other and a pipe connecting them with a valve to stop the flow . The difference in pressure between the two tanks is 10 feet of water. Using a density of water of approximately 62 lbs / cubic ft, the pressure difference is 620 lbs / square ft or 4.3 psi. However, we don't need to convert to psi and we can rather refer to the 10 ft as "head".

If we were to open the valve, the flow through the pipe would be quite high. Intuitively, if the tanks were only 5 ft apart but still keeping the same length of pipe and the valve, you would expect the flow to be less than if the tanks were 10 ft apart. Engineers have relationships for flow and pressure and they know that the pressure drop in a pipe varies with the square of the flow. Conversely, flow varies with the square root of the pressure drop. That means that if we double the flow, we quadruple the pressure drop. If we double the pressure drop, the flow increases by the square root of two or 1.414.

Now putting this in terms of carburetors. the air pressure (or head of air) on the top of a 4bbl carburetor is greater than the bottom measured by a column of liquid mercury 1.5" high. The chemical symbol for mercury is "Hg" and an inch of mercury is equivalent to 0.491 psi or 3.386 kPa. This difference in pressure is what pushes the air through the carburetor which results in a flow rating @ 1.5" Hg. We can take the same carburetor and instead use a difference of pressure @ 3.0" Hg and get a 2bbl carb flow rating that is 1.414 times greater than the rating at 1.5" Hg because there is more pressure forcing more air through the carburetor.

I made the chart showing both pressure drop references because it makes it easier to compare the flow ratings of 1bbl & 2bbl carbs with 4bbl carbs. Because the reference points are different, you might think that some 2bbl carbs flow almost as much or more air than some 4bbl carbs. The only way to compare them fairly is from the same reference point. I also wanted to show that 4bbl carbs are not as large people might think because they are running on their primary barrels most of the time.

It's hard to write articles for a general audience on the internet because you don't really know who is going to be reading it. If you put in too much detail, it gets boring but if you don't put in enough it ends up going over people's heads. Now that I have a better feel for my audience, I'll be making some revisions. I'd like to put in more photos but I have to be careful about exceeding my storage allotment.

Frank

Frank,



I still have some questions.
I though carbuetors worked on vacuum? Is standard air pressure then considered the pressure forcing the air? That means the fuel would be considered under pressure as well? The whole reverse relationship throws me.

I don't mean to be difficult.

I did get a much better understanding of why a 4bbl is not too large and why it may actually improve fuel mixture throughout the rpm range.

Will you add any info about the Webber carb CFM ratings?

Nice work!

_________________
1980 Aspen 225 super six

Regarding Webbers,

I've been studying the webber carb option. They (At least those for our applications) have the interesting ability to have their venturi changed so that each carb can provide (again for our applications) between 200 and 400+ CFM.

I've seen all kinds of cool manifolds for webbers, 1x2 / 2x2 / 3x2 / hell, even a 6 x 1 (not sure if those were webbers though)!

I started a thread earlier to ask about their reliability... not too many people responded yet, but from what other sites have to say, webbers are quite reliable and extremely tuneable.

_________________
When in doubt, empty the magazine.
'72 Dart Swinger /6 - My Yard Dart!
'01 Ranger P/U 4x4 X-cab
We buy and sell MOPARTS & Ford Parts!!!
http://www.usironauto.com

Slant6Ram,Technicaly they say there is really no such thing as a "vacuum",just a difference in pressure. Atmospheric pressure at sea level is 14.7 psi and that is what "pushes" the air through ,because the piston lowering in the cylinder creates a pressure differential.That is why when you drive up in higher elevations you loose power ,because there is less atmospheric pressure to help push air through the carb.

Thanks Frank, your data jives well with some 'observations' I see. Currently I run a stiff secondary spring in my Carb and at 'drag' throttle I get a vacuum reading of 5" Hg (need to open the secondaries more at WOT)...this coincides with the BBD topping out somewhere at 7" Hg and the Holley 390 cfm at 3" Hg (with all barrels wide open...)....

Thanks for posting the data it makes for a good reference.

-D.Idiot

These ideas are sometimes difficult to understand. Skidderdriver is right about pressure and vaccuum. Pressure is measured from a reference point and we commonly measure from the atmospheric point. When we say that a gauge reads 0 psi, it means that the pressure is the same as the air pressure surrounding us. However, the air pressure surrounding us is really 14.7 psi (more precisely 14.696 psi or 101.325 kPa) above absolute vacuum. To be more specific when we talk about pressure, we can say that atmospheric pressure is 0 psig (g is for gauge) or it is 14.7 psia (a is for absolute).

We can talk about vacuum too and this is pressure below atmospheric. Vacuum is most commonly measured in "inches of mercury" in the USA and "Torr" (mm of mercury) elsewhere in the world. 0" Hg vaccum or 760 Torr is atmospheric pressure. Absolute vacuum is defined to be 29.9" Hg, 0 Torr, or 0 kPa but is only theoretically achievable. Even deep space has an extremely small amount of pressure. There is lots of information about pressure and vacuum on the internet if you want to learn more.

Carburetors do operate on vacuum but that only means that the pressure below the carburetor in the intake manifold is less than the pressure above it outside the air cleaner. The difference in pressure is what causes the air to flow into the engine through the carburetor. Water or air or any other fluid always moves from the area of higher pressure to that of lower pressure. If the throttle valves are wide open, there is very little restriction in the airflow and the pressure drop across the carburetor will be minimal.

The key to maximizing the power produced by an engine is to get as much air into it as possible. All the modifications people do to get more horsepower involves filling each cylinder more completely with air. Power is proportional to RPMs but it becomes harder to fill the cylinders at higher engine speeds. The restrictions in the air cleaner, the carburetor, the intake manifold, the valves, etc. reduce the amount of air that could potentially get into the cylinder. Some of these factors are directly related to the carburetor because, for example, only a 1bbl air cleaner will fit a 1bbl carb (unless you have the hood clearance and adapters for another type) and the 1bbl carb only mounts on a 1bbl manifold. Once you modify the heads and install the camshaft, you can install any carburetor and intake manifold.

Whether you have a 1bbl, a 2bbl, two 2bbls, or one 4bbl, the pressure difference between the part above the carburetor and the part below the carburetor will be a function of the design of the carburetor and the flow passing through it. The more restrictive it is, the more it reduces the volumetric efficiency of the engine. The volumetric efficiency is a percentage ratio of the actual amount of air that gets into the engine compared with the theoretical maximum amount of air that could get in. A stock 1bbl carburetor will probably be the most restrictive and would significantly limit the amount of air that could get into the engine. A pair of 2bbl carbs on the other hand would probably be very unrestrictive and would maximize the amount of air getting into the engine.

Regarding the question: "because there is more pressure forcing more air through the carburetor", this is something that happens in the laboratory and not the real world. Like I tried to show with the two water tanks, if one tank is 10 ft (3.0" Hg) above the other it will flow more water (air) than if one tank is 5 ft (1.5" Hg) above the other. Carburetor manufacturers rate 1bbl and 2bbl carbs with 3.0" Hg difference in air pressure (or vacuum if you like) while 4bbl carbs are rated with only a 1.5" Hg difference in pressure. The carb with a 3.0" Hg rating will flow 1.414 times more air than if you had rated it at 1.5" Hg.

In the real world, if you do like D.Idiot and mount an vaccum gauge on your intake manifold, you will see a manifold vaccum reading at wide open throttle (WOT). This reading will show that the engine is drawing in air through the air cleaner and carburetor and the restrictions across these things cause manifold pressure to be less than atmospheric. In D.Idiot's case, he found the pressure drop to be 3.0" Hg with the Holley 390 CFM carb. When he measured the BBD, the WOT reading was around 7" Hg. This means that there was 4" Hg more pressure drop with the BBD carb and, although he didn't say it, his car probably had a slower 1/4 mile time. His RPMs might have been the same when he compared the manifold vacuums but the BBD with higher pressure drop caused the engine's volumetric efficiency to be lower and thus produced less power.

So, in the real world, the atmospheric pressure is pushing air into your carburetor because the cylinders are drawing it in. Although the carbs have ratings at a given pressure drop (1.5" Hg or 3.0" Hg), you will actually get a pressure drop that is different and specific to your engine.

Fuel flows to the venturi in a carburetor barrel because of Bernoulli's principle. In simple terms, the pressure of any moving fluid is less than tthat of a stationary fluid. The venturi speeds up the airflow (exactly like the way air speeds up over the top of a wing to cause a airplane to fly) and lowers its pressure. The fuel flows to the low pressure area in the venturi and the greater the airflow, the lower the pressure (or higher the vacuum), and the greater the fuel flow. This is a somewhat different concept than the flow rating of the carburetor so just remember that any carb will supply the right fuel mixture if it is tuned properly.

Slant6Ram, when you are confused about the whole reverse relationship, I presume you mean the part about doubling the flow quadruples the pressure drop while doubling the pressure drop increases the flow by the square root of 2. We are using the number 2 because 3.0" Hg is twice 1.5" Hg. This is a mathematical relationship that looks like this written out:

pressure drop (dP) varies with the square of flow (CFM):
dP ≈ CFM²
dP2/dP1 ≈ CFM2²/CFM1²
but CFM2 = 2 x CFM1
dP2/dP1 ≈ (2 x CFM1 / CFM1)²
dP2 ≈ dP1 x 2²
dP2 ≈ 4x dP1
Therefore, doubling the flow quadruples the pressure drop .

The squiggly equal sign (≈) means "is proportional to". Including a factor or constant would allow us to say "is equal to" but the extra symbols would make the relationships a bit less clear.

flow (CFM) varies with the square root of pressure drop (dP):
CFM ≈ √dP
CFM2/CFM1 ≈ √dP2/√dP1
but dP2 = 2 x dP1
CFM2/CFM1 ≈ √(2 x dP1) / √dP1
CFM2 ≈ CFM1 x √2 x √(dP1/dP1)
CFM2 ≈ 1.414 x CFM1
Therefore, doubling the pressure drop increases the flow by the square root of 2.

I don't profess to the expert on any brand of carburetor but I have some understanding of how they work. I have not included about any information about Webers because I don't know enough about them and because I didn't see alot of postings talking about trying out the different kinds. It seems to me that most people tend to buy a prepackaged kit from Proline if they want Webers. BTW, the postings I have read about Proline's kit are all positive and Chris seems to offer an excellent carburetion alternative. I'll add some information about Webers when I learn some more about them.

When you are stuck in the mud, you either get stuck deeper or you get out. I hope I haven't been getting myself deeper in the mud with the lengthy explanations but I think your questions deserve a complete answer.

Frank

Frank, I don't know what kind of Weber info you are looking for. There is a book about the different Weber models printed by HP or one of the other auto publishing houses. But a book about carbs in general that I really like is "How to Build Horsepower, Volume 2, Carburetors and Intake Manifolds". It has the basics of how carbs and manifolds work and has enough theory to make it interesting for anybody interested in building an intake system. The author is a fan of isolated runner systems and he gives the basics of tuning Webers as used in an isolated runner system. It also has chapters on tuning Holleys, Quadrajets and Carters. Good book for a general overview of performance carbs and intake systems.

Thanks for the advice, Marc! I hadn't planned to work much with Webers but I might just pick up one of those HP Books Weber books for some light bed-time reading.

For the time being, I was just going to look up the flow ratings of the carbs that people might want to put on their slant sixes. Maybe one of the local libraries has a copy of a Weber book.

Frank