The Stovebolt Site Forums The Shop Area Engine Shop cfm ratings

I've been reading alot of information/posts about carbs as I am deciding what to do with my 261. Aside from the computation I've seen to calulate how much cfm your engine needs (cu.in x maxrpm/ 3456)x(% v.e.),can someone give me the short story on the cfm rating of a 1 or 2 bbl being lowered if rating it like a 4 bbl? I've not heard of this. To me W.O.T. is wide open throttle. Sidebar: Where is the actual casting # on the 261 head? Manifolds side top foward or where? Thanks.

Well, four barrel carbs are flow rated at 1.5" of mercury, and two barrels and one barrels are rated at 3" of mercury. I don't remember enough of my fluid dynamics, but I don't think it is a linear relationship (flow vs. pressure drop) so I'm not sure how to answer your question. Maybe some mechanical engineer can answer.

Oh, the casting number on my 261 head is on the top manifold side toward the front. The numbers are about 1/2" high. Mine is 3836850. It has another number on the ignition side.

To convert 2bbl CFM to 4bbl CFM , divide the 2bbl rated CFM by 1.41. To convert 4bbl CFM to 2bbl CFM,multipy 4bbl CFM by 1.41. This formula is used by carb tuners,there are variables,only is accurate for the pressure drops 4 on the floor listed, but it's a good quesstimate.

Thanks for the info people. Now, if I've calculated the cfm needed for my 261 w/ M4F cam, fenton headers, stock intake to be 307 cfm, a single 32/36 Weber properly jetted and advertised to flow 330 cfm should be fine. But somewhere I read that it flows less than that when rated like a 4 barrel. What does that have to do with it if it has a published 330 cfm and its being used on a 1bbl intake. Man I'm dizzy!!

If you used a chart from Holley for instance to estimate CFM requirements,that would be a 4bbl.My 261 is equipped like yours,a 390 Holley 4bbl is just right.The Holley Weber,32/36 flows 280 CFM according to my Holley specifications manual, that's 200 cfm as like a 4bbl,not enough, 2 of those carbs works well.

Tony, I've used the formula posted in several places, as well as a few "plug your info in" sites and all of them say w/ 4250-4500 max rpm and 85-90% v.e. (I've tried different combos), the cfm comes out about 300cfm give or take. I guess that some vendors of the 32/36 Weber are over estimating the advertised cfm a tad. I think duals would work great as well but was hoping the performance of one would be better than adequate. At 280 cfm, it appears not.

If you're putting the Weber carb on a 261, you really need a pair of them. Biggest problem is the one bbl fitting, it's not big enough to allow proper flow. My friend's 261 which is in the 200-225 hp. range has the dual Webers, we're thinking about cutting the one bbl fittings off his manifold and welding on 2 bbl fittings, but that's one of those "someday" projects.

Will somebody post a link to a site that has this calculation? I see the way tan62 has written the math expression (cu.in x maxrpm/ 3456)x(% v.e.) = cfm, but another site I went to had the v.e. beside the 3456 in the denominator. That would make quite a difference in the result. Another related question, can I find somewhere the factory cfm rating for my original 1950 216 carburetor? GM "B" I guess. Thanks.

Frank, try this site from Holley, they are the performance carburator people
CFM calculator
I read somewhere,maybe on Inliner's, the larger Model B Rochester flows about 150 CFM, I don't know if that's factored like a 4bbl or if the information is accurate. Fred makes a good point, carb adapters can cause restricitions or fuel metering problems.I saw a 261 powered 47 Chevy car with 3 Holley Webers, ran very well.Maybe that's what Fred's friend needs for his engine.

Throwin all calculations out the window, I'm gonna see what happens with this setup on my 292:

[img]http://www.chevyasylum.com/64c10/64C10_056r4.jpg[/img]

It's a Carter 625 (supposedly) CFM unit with electric choke on an Offy manifold. I'll be using Headman Hedders and Purple Hornie mufflers with it. Otta make enough of a racket to [censored] off the neighbors.

I've had good success using the 600/625 AFB carbs on smaller motors (305, 307 "twin fours") and I figger with the vacuum-operated secondaries, it shouldn't overpower the motor like a mechanical secondary carb would. Now...just as soon as all this white stuff goes away, maybe I can actually start swingin wrenches.

I quit posting on these forums sometime ago, as the automatic censor made one of my posts trying to help a non-Chevy owner look like it was written by an idiot. As this post does not mention the Dearborn based company beginning with the letter between E and G, maybe the censor will permit the post.

The following is part of an article done some time ago for the Pontiac community. Hopefully, it will help some understand carburetor ratings.

Jon.

CFM calculations and application

A question often asked is how large a carburetor should I install on my engine. The following information applies to carburetor application for street driven multiple cylinder (4 or more cylinders) four stroke internal combustion reciprocating engines. It does not apply to race applications, rotary engines, one, two, or three cylinder engines, nor engines with fewer or more than 4 strokes.

Most enthusiasts have seen the following equation:

CFM = (RPM x CID) / 3456

This equation will give CFM required at 100 percent engine volumetric efficiency. However, at this writing, no street engine will be 100 percent volumetric efficient. To estimate volumetric efficiency, compare your engine to one of the following examples:

1969 Pontiac RA IV engine approximately 90 percent
1969 Pontiac GTO (non-RA) approximately 85 percent
Pontiac 400 economy (low-compression) V-8 approximately 80 percent
Pontiac 230/250 6 cylinder approximately 75 percent.

These are estimates, and can change with the addition of headers, and other exhaust modifications.

Now lets break down the equation.

CFM = air flow in cubic feet per minute
RPM = engine revolutions per minute
CID = engine displacement in cubic inches
3456 = conversion constant
VE = volumetric efficiency (express as a decimal)

The conversion constant did not magically appear out of thin air. Note that air flow on the left side of the equation is measured in cubic FEET per minute; and that engine displacement on the right side of the equation is measure in cubic INCHES. As there are 12 inches in one foot, part of the constant is 12 x 12 x 12 converting cubic inches into cubic feet. This figure is 1728. Note also that the equation is listed for a FOUR STROKE engine. The crankshaft (RPM) makes 2 revolutions for each firing stroke, so a 2 enters the equation. 2 x 1728 is equal to 3456. So much for magic.

The equation is often misapplied by first time users. Note that the CFM is directly proportional to RPM. Many simply apply the equation at wide open throttle (WOT) and buy accordingly. This solution will work as long as the vehicle is driven only at WOT (the local constabulary frowns on this type of operation); but may result in an engine that is either severely over or under carbureted as normal cruise RPM. The successful engine tuner will install a carburetor which is large enough to satisfy WOT requirements, but has sufficiently small primaries to have maximum air velocity under normal cruise conditions.

Taking an example:

1969 Pontiac RA IV engine with desired 60 MPH engine speed of 2300 RPM, and maximum engine WOT of 5800 RPM

First calculate WOT requirements:

CFM = (5800 x 400) / 3456 or 671 CFM at 100 percent VE

However we stated that the RA IV engine had a VE of approximately 90 percent.

Actual required CFM = 671 * 0.9 or 604

Now, lets calculate cruise requirements.

CFM = (0.9) x (2300 x 400) / 3456 or 240 CFM

The Rochester Q-Jet from 1969 was approximately 150 CFM on the primary with a variable 600 on the secondary for a total of 750. Note that the 750 CFM is sufficiently large to satisfy moderate race conditions with open headers. The novice will now make another incorrect assumption regarding the primary. 4 barrel carburetors are rated at 1.5 inches of mercury. If this figure increases, so does the air flow. 1 and 2 barrel carburetors are rated at 3 inches of mercury. A good conversion constant from 1.5 to 3.0 ratings is the square root of 2 (1.414). To convert a four barrel rating to a two barrel rating, multiply the 4 barrel rating by 1.414. So if the primary of our Q-Jet were rated as a 2 barrel then 150 x 1.414 is 212 CFM and if the vacuum is even greater, then so is the CFM. At cruise RPM, the vacuum is significantly greater than 3.0 inches of mercury, and our Q-Jet has sufficient air flow. GM did modify the Q-Jet in the mid-1970’s to have approximately 200 CFM on the primary side for 455 CID engines.

Reviewing (and we will now include VE with our equation):

CFM = (VE x RPM x CID) / 3456

And converting 2 to 4 or 4 to 2 ratings.

CFM2 = CFM4 x 1.414
CFM4 = CFM2 / 1.414

Well, it passed the censor. In that case will try another.

ChevyAsylum - Carter recommended the use of AFB number 9400 or 9410 (w/EGR port) 400 CFM for use on the Chevy 283. While a 625 can be used, low RPM power and torque will suffer somewhat when compared to that produced using the smaller carb due to lower air velocity on the primary side.

Carter AFB CFM (aftermarket carbs)

400 = 200 P and 200 S
500 = 225 P and 275 S
625 = 250 P and 375 S
750 = 375 P and 375 S

Jon.

Thanks for the Carb info. I'd never seen it broken down into primary and secondary for Carters before. And a little low-end loss is sorta what I expected. If I can't live with it, I can always downsize.

When running multiple carbs, is the cfm simply multiplied by the number of carbs?

Six4me - when running multiple carbs, the design of the intake has much to do with the total CFM of the system; but under best case, the total CFM of the system will be less than the sum of the CFM of the carbs. There is no "ballpark average" in this area.

Jon.