The Stovebolt Site Forums The Shop Area Engine Shop cfm rating reloaded

We all know the formula for cfm rating:

Cubic inch * max rpm / 3456

So a 261 is rated about 377 cfm @ 5000 rpm right?
minus some VE let´s say 80% it is 300 cfm

Tom Langdon says in his catalog the Dual Offy Holley /Weber setup is the most streetable.

H/Ws are flow 250 each that would be close to 500 cfm for 2 (almost it is not linear) and a lot of poeple (including me) drive them, some even have 3 of them and are quite happy.

The litte Barry Grant 4 BArrel Road Demon flows 525 cfm and the manufactor website says:

"The new 525 Road Demon is for V6 engines and small V8s in the 260 to 400 cid range. "

WHAT?

Some guys even have TRIPLE WEBER 45 SIDE DRAUGHT CARBS on a 235 and swear that it is the most powerfull setup you can have.

Can we ignore the fact that all those combos flow way more than the 300 cfm? They seem to work fine.

Does the famous formula really work or is it rather a minimum than a maximum estimation?

On staged carbs with vacuum secondaries I would say thay flow what the engine can suck, but the Weber/Holleys or the Weber side droughts are ruled by foot.

Frank

I’ll make an attempt at this one.

Actually, the question was answered by Mark Twain many years ago; or at least the quote “figures don’t lie, but liars figure” is attributed to Mr. Clemens.

To answer the question, it is important to clearly understand the question. Wherein lies the derivation of the referenced equation?

An engine, if one removes the spark and the fuel, is an air pump. Do determine the size of the air pump, we simply need to write an equation about the air pump. So we write that:

Airflow is equal to displacement multiplied by frequency

Adding specific units of measurement we obtain:

CFM = (CFD x RPM)

Since very few engines are rated in cubic feet displacement, it is necessary to convert CFD to CID, thus:

CFM = (CID x RPM) / (12 x 12 x 12)

Or

CFM = (CID x RPM) / 1728

BUT

In a 4 stroke engine we have an intake and a compression cycle, so we must divide by 2 thus our final equation:

CFM = (CID x RPM) / (12 x 12 x 12 x 2)

Or CFM = (CID x RPM) / 3456

Now historically:

CFM should be considered a “vector” number, rather than a “scaler” number.

I do not know who published the first carburetor CFM charts, nor does it have any real bearing on the discussion. It is enough to know that charts were published at least by the early 1930’s. The state of tune of engines at this time pretty well established that at WOT (wide open throttle), the vacuum of the engine would be approximately equivilent to 3 inches of mercury. Thus it became the norm to “flow” and thus “rate” the airflow capacity of carburetors at a vacuum equivilent to 3 inches of mercury. FLOW RATING NUMBER 1!

Fast forward to 1952. Three carburetor companies offer 4 barrel carburetors in 1952. Up until 1952, automotive carburetors (for the most part) had been 1 and 2 barrel units. It was found that, under WOT with the 4 barrel, engines of the period would exhibit vacuum approximately equivilent to 1 and ½ inches of mercury. Thus it became the norm to rate 4 barrel carburetors at 1 and ½ inches of mercury. FLOW RATING NUMBER 2!

The difference between scales 1 and 2 is the square root of 2.

During the 1960’s one carburetor company began to flow carburetors “dry” (historically, ratings had been done “wet”). While seemingly obvious, wet ratings are done with a liquid with similar properties to gasoline mixed with air; and a dry rating is air only. FLOW RATINGS NUMBER 3 AND 4!

The difference between a wet rating and dry rating is approximately 8 percent.

So to summarize ratings which stayed pretty much the same up through the 1980’s:

(1) 1 and 2 barrel carbs wet
(2) 4 barrel carbs wet
(3) 1 and 2 barrel carbs dry
(4) 4 barrel carbs dry

How do these ratings compare? Lets take 4 different carburetors rated at 500 CFM (one from each scale) and re-rate them ALL using scale number 2:

The carb from scale one would be 500 / square root of 2 or 500 / 1.414 or 353.6
The carb from scale three would be 353.6 / 1.08 or 327.4
The carb from scale four would be 500 / 1.08 or 463.0

I picked scale two, as it is the scale I feel is most accurate; others may have differing opinions.

Summerizing the above, our 4 carburetos, each rated 500 CFM:

(1) 353.6
(2) 500.0
(3) 327.4
(4) 463.0

“Figures don’t lie, but liars figure”

Enter the 1980’s.

A very old carburetor company is purchased by another company. The survivor company finds that the 600 CFM carburetor is the “standard” by which many carbs are sold, and they don’t have one (they have 500’s and 625’s). The 625 is given a second part number and rated 600 CFM (guess how these compare in horsepower ratings to the competitors 600).

“Figures don’t lie, but liars figure”

A new carburetor company comes to the conclusion (probably true) that racing engines do not produce a vacuum equivilent to 1 and ½ inches of mercury at WOT, and thus rate their carbs at a lower figure (producing higher numbers).

“Figures don’t lie, but liars figure”

An old established company issues a new line of carburetors with ratings similar to the ratings in the previous paragraph.

“Figures don’t lie, but liars figure”

Another established company not making carburetors purchases the rights from other carburetor companies to produce similar carburetors. Historical CFM ratings on these similar carburetors with the same venturi area are now increased.

“Figures don’t lie, but liars figure”

Personally, I pay absolutely zero attention to CFM ratings published in the last 20 years. I use (and for the most part recommend) carburetors produced BEFORE 1980. If I were going to purchase a newer carburetor, I would DEMAND to know the test criteria used to rate the flow of the carburetor. If the vendor/manufacturer could not (or would not) provide this information, I would look elsewhere!

And remember Mark Twain’s words. Amazing how much he knew about carburetors!

Jon.

WOW, carbking,,I'm gonna have to read this a few more times to try & get my head around it,,too early in the AM for me. I need a new carb for my 79 Ford 400,,& I have been studying on a Holley 2 bbl(advertised 350 or 500 cfm) for a replacement. Maybe you can also explain my son's Iowa Test of Basic Skills scores also. Brian

Higher CFM will improve the top end. Lower CFM will improve your low RPM band. For a street driver you want to keep the CFM on the low CFM side of things. Too big will be nothing to brag about for street applications.

There are guys out there with 350 cu.in. small blocks and 750 CFM carbs that I can beat with a 283 cu.in. and a 350 CFM 2 barrel carb.

When I talked to Tom about buying carbs for my 261 with a 3/4 cam, fentons and milled head, he reccomended the carter webbers, not the holley webbers. The CWs are supposed to be about 250/cfm each too, unless I misunderstood something along the way.

Thanks for the historic lesson John.
Your broad knowledge knocks me out of my socks every time I read your posts.

With anny respect, but all that don`t aswers my question how it is possible that we got many way overcarburated (at least on the paper) engines arround that run great and get good milage.

When I understand it right the different methods of messuring cfm result in variances up to 20%.

But we are talking about more than 200% (again on paper) with a side drought Weber setup for example.

And than there are carbs that seem to cover almost everything.

The 525 Road Demon -for example - is advertised for engines from 260 to 400 cid.

How can it cover this wide range? Is it the vacuum secondarys or bigtime rejeting?

I also heard that a Q-jet can be thrown on almost eny engine and performs great.

Frank

A few more things to consider for this issue.

First, as Frank mentioned in his original post, multiple carburetor setups are NOT additive! Thus two 600 CFM carbs are a dual 4 setup do not equal 1200 CFM. This is why I suggest that one will obtain BEST results by using a multiple carb setup such that the number of barrels is an even divisor of the number of cylinders.

Next to consider (again from the original post) is the concept of progressive carburetors. Here there are some HUGE misconceptions. Most would suggest that progressive carburetors came about in 1952 with the introduction of the 4 barrel. However, progressive carburetion was available as early as 1903 (the Schebler model D) where 2 inputs were used on a single barrel carburetor. The first input (fixed) was tiny, allowing high velocity of air for decent (for 1903) carburetion at low RPM, and a secondary (spring tension controlled) input would open as needed for additional air (and fuel) at higher RPM. There are other methods of early progressive carburetors.

Fast forward to 1952. The 4 barrel carbs were progressive, having a primary and a secondary side. The huge misunderstanding is the way many view these as either mechanical or vacuum secondary. This definition omits the "on demand" secondary.

Let us define the types:

Vacuum secondary carburetors generally have a diaphragm, usually but not always in conjunction with a spring; that control, based on a vacuum signal when the secondary opens to WOT.

Mechanical secondary carburetors open to WOT when the operator mashes the throttle.

But, in my opinion, the most important class for street use, is the "on demand" secondary. Representative of this type would be the post-1960 Carter AFB with the weighted airvalve, or the Rochester Q-Jet and Carter TQ with the spring-tensioned airvalve. These units are designed to open ONLY to admit the amount of air that the engine can use; maybe NEVER actually opening to WOT. It is this class of carburetor which allow huge (in terms of WOT) carburetors to be used on smaller engines, because regardless of the pressure the operator places on the throttle, the carbs will only open as necessary for the engine demand.

Frank, you entered another issue in your second post; that of the sidedraft Webers.

Without getting into the mathematics (I had to edit my first post, as I initially divided by the square root of three rather than the square root of 2 - had a "senior" moment!) the equation from your first post is valid for a multiple cylinder 4 stroke engine of at least 4 cylinders fed by a common plenum area!

Most sidedraft Weber setups do not utilize a "common plenum area". The barrels (or carbs) are connected by a balance tube to minimize pulsing, but by and large each barrel feeds an individual cylinder. Single cylinder 4 stroke engines require TWICE the air as provided in the equation above. By not using the common plenum, the engine, for the purposes of carburetion, must be treated as a single cylinder engine.

Finally, engines are flexible, and many (most?) enthusiasts do not perform sufficient testing to determine if their combination is merely adequate, or highly tuned.

I always suggest that prior to any modification a "baseline" is determined. If your goal is increased acceleration, then run an acceleration test. Have a friend time you from zero to some fixed speed (not to exceed the posted limit) with a stop watch. Run three (or more) runs and average the times. Now make your modification and run the test again. You now will have actual proof of whether or not your modification actually improved your time.

If your goal is fuel economy, do a tuneup, drain the fuel system and introduce a fixed amount of fuel. Drive the vehicle at a fixed speed until the fuel runs out. Make the modification and redo the test. Again, you have real proof of the results.

Many enthusiasts love their aftermarket carburetor because they removed a worn original, possibly with a stuck power valve, or leaky float and replaced this unit with a new aftermarket unit. Generally, even the very worst of aftermarket carbs or one that is incorrectly sized is going to win in these circumstances.

Jon.

Jon, if two 600 carbs do not equal 1200 cfm, what do they equal?
The reason I ask is, I have three Model H Rochesters on my 250 Chevy, I have been told the "H" is about 78 cfm. I run all three together as one carb on a open plane intake, all three runners are hooked to one plenum. I have always considered this set up to be about the same cfm as my original Mono-jet, approx. 220 cfm plus or minus some.

I have run both, stock and modified at the drag strip making three to four runs each to get a good solid base line, each set up was the same, 17.90's at 73 mph. I like the three carbs for the better throttle feel and fuel milage, at least 2 to 3 better then the Mono-jet. The Mono-jet was tuned to the best I could get it using air-fuel monitor in the tail pipe and extensive test runs and milage runs. The three H's were treated to the same tune up. I bet I changed jets and float settings 15 times or more.

So what can I tell someone who ask, What cfm are they? Joe Hand, Lees Summit Mo.

Joe - one honestly cannot tell without a flowbench. A good guess would be that two 600 CFM carbs are maybe equivilent to 900 CFM.

As to the triple H setup; the answer again would require a flow bench flowing the manifold, but since the setup works, the real answer is the CFM is a moot point.

Say hi to your Dad for me.

Jon.

O.K. now I got it. Thanks John.

Frank

Regarding Weber carbs (of which I'm a huge fan, also being an I-car guy) - they are adjustable to the point that it is mind-boggling. You can choke them down to run on just about any engine, and can team them up to power just about any engine, too. Adjustable / configurable parts on Weber DCOE sidedrafts are:

* Air Correctors
* Main Jets
* Needle valves
* Idle Jets
* Main chokes or "main venturis"
* Auxillary venturis
* Accelerator pump jets
* Accelerator pump inlet valves
* Emulsion tubes (18 - 20 sizes here alone)

and probably some other stuff that I'm missing. I have an old Alfa that runs dual DCO sidedrafts on a 1600cc motor, but they are choked down and the car does not run rich at idle, and it doesn't run lean at WOT. The twin carbs (one individual throat per cylinder) are tuned perfectly for the car. I haven't yet seen a Weber setup for a stovebolt, but I wouldn't be surprised. They could deliver excellent tractibility for even a stock motor if configured correctly.