The Stovebolt Site Forums The Shop Area Engine Shop Carbs and cfm

I have a 307v8 and would like to put a four barrel carb on a free edelbrock manifold that I got from a friend.

When I went to Autozone, the "kid" was trying to sell me carbs that were 800 cfm to 1250 cfm, this seemed like way to much carb for a stock 307. When I told him this, he said it was related to horsepower. The shocking part was that the manager was also intent on selling me one of these carbs also.

I shook their hands and walked out the door. I was hoping that they had not been drinking or something worse.

I know that there is some math formula for figuring out 4-barrel cfm, but I forgot it.

Thanx
Dano

600 CFM is about all you want to use on a small
block street engine. I like to get a Q- Jet
at the swap meet & rebuild it. Makes great all
around perfomance. But if you are not up to
that the Edelbrock perfomer is real good right out of the box.

The equation is:

CFM = (RPM times CID) / 3456

This equation good for four cycle multi-cylinders engines with four or more cylinders at 100 percent volumetric efficiency.

CFM is required airflow in cubic feet per minute
RPM is revolutions per minute of the engine
CID is engine displacement in cubic inches

For what its worth, the constant 3456 is actually 12 times 12 times 12 times 2. The three 12's are necessary to convert cubic inches (on the right) with cubic feet (on the left). The 2 is necessary as the intake valve opens once for 2 revolutions of the crankshaft.

Volumetric efficiency of a stock 307 is probably around 70 - 75 percent.

Most apply the equation only to WOT (wide open throttle) which is the right approach if always driving at wide open throttle.

However, the equation can be applied for any RPM. If one attempts to maximize runner velocity at cruise RPM, one will normally find that Wrenchbender's suggestion of a Q-Jet (spread-bore) carburetor is quite accurate. While most Q-Jets are 750 CFM (more than you need total, but 150 CFM on the primary), the "on-demand" feature of the airvalve on the secondary side allows the carburetor to only deliver that amount of CFM needed by the engine.

Oh, and that 1250 CFM the parts guy was trying to sell you??? Just wrap that 307 to about 14,000 RPM and the carb should be just right!

Jon.

Dano,
If you decide to go with the 600 CFM Holly carb put .059 jets in the primary if you live under 2000 feet alt. and fine tune the air fuel with a vac guage.

T

I use a similar formula, but I convert the cubic inches to cubic feet diffrerently:

RPM divided by two, times cubic inches, divided by 1728, the number of cubic inches in a cubic foot. The result is theoretical CFM, with 100% volumetric efficiency, which never happens in the real world, especially on a street engine. Multiply the theoretical CFM times .85 or .9 to get a real-world estimate of actual CFM. Your 307 won't need over 600 CFM under any circumstances I can imagine, including wide-open throttle operation in first gear! (And if you do that very often, carry a basket to take the parts home!)
Jerry

Thanks Jon and Jerry for the formula......

What in your opinion is the efficiency factor in a stovebolt 6 like a 261????

From what I have calculated even at 4000rpm the single barrel carb is all it can handle.....

...then how do they manage to get more HP by using small 4 barrel or 3 single...???? is the volumetric efficiency increased by modifying the cam...??? and polishing and tuning the ports... then opening the otherside with tuned headers....?

Seems that a standard stock 6 almost has a built in rpm limiter....

Bob C.
....trying to learn!!!!!

Thanx to everyone, your help as always is greatly appreciated.

I was talking to my neighbor who has a 65 c-10 with a 230 L-6. He went in to the same a***zone and told them he wanted a rebuilt one-barrel carb and they tried to sell him the 800 cfm to 1250cfm carbs also. He explained that the manifold would not hold a four-barrel carb, but that still did not deter them from trying to sell him a monster carb.

Are those guys on commission?

Yep, they're on commission! Plus, most of the Auto Zone/Advance/Pep Boys/Etc. counter guys don't know diddly about fixing cars, they only know how to punch a computer keyboard and mis-read the part number!

That "free" manifold might be worth exactly what you paid for it, too! If it's a high-rise, single-plane type, it's only good for high-RPM, full-throttle operation. For street use, a "dual plane" masinfold is what you need, where 2 cylinders from each cylinder bank are supplied by the same side of the carburetor. For instance, the #1 and #7 cylinder, and the #4 and #6 cylinder will feed from one side, and the remaining cylinders will be supplied by the opposite side of the carburetor. If there's one big hole under the carburetor flange, with eight runners coming from it, RUN, don't walk, away from that installation!
Jerry

RAC1812 - I would GUESS the volumetric efficiency of a stock 261 at 70 - 75 percent.

Unless the engine is GROSSLY undercarbureted (or the original carburetor is not functioning properly); simply bolting on 2 or 3 one-barrels or a small 4-barrel is going to result in modest gains at best.

More substantial gains would be realized by also adding higher compression (more mechanical advantage), a longer duration or higher lift camshaft (more inlet air flow) in conjunction with better exhaust (if you have better inlet flow, you need better exhaust flow). Strengthening the lower end of the engine with the above for greater RPM will also raise the airflow.

Also, when applying the equation, remember that it is JUST an equation. If you get in the 95 percent or better volumetric efficiency (racing) the equation is not necessarily correct. Example - I own a "production" car that came from the manufacturer with (2) 500 CFM 2-barrel carbs. The engine is a 121 CID 4-banger. Car weighs 1900 pounds, has a 5.13 rear gear and will not pull itself in low gear below 2500 RPM. Always understand what the equation means, and what you are trying to do with it.

Jon.

I don't think that there is a simple formula for carb size. I remember a factory option on the Z28 Camaro: dual 600 cfm Holleys!

RAC1812, three carbs and headers are not a sure bet for more horsepower. I installed three carbs and header on my 250. I ran great, started good, and got decent fuel milage. I took it to the drag strip for some fun and testing. The next week I went back with the stock intake and exhaust with a good running stock Monojet one barrle carb. The stock set up was quicker by .02 seconds and the three carbs were faster by 1/2 mph. I now run the stock set up full time. Joe

Fran and Gord, the dual carbs on the Z-28 was an option for racing where thew engine will be modified and see over 7000 rpm.It was over carbureted on purpose for that reason.
My actual experience on 261's says adding the equivilent of an additional carb with no other changes adds about 10-15 HP above 3000 rpm with no loss in fuel mileage,that's a 10 percent increase in power.

Dano,

Consider a Carter (Edelbrock) AFB 500 CFM. Carb will give your 307 excelent throttle responce and it is tuneable. Summit Racing should stock them and the tuning kits.

Well hotrodlincoln you're right! The manifold is a lowrise single plane. At least it was free or should I have had the owner pay me to take it off his hands? What type of application is this type of manifold for?

Too bad too, I just bought a Holley 600 cfm carb with electric choke on ebay for $10.51, and a rebuild kit from the local speed shop.

Live and learn. back to the original 2-barrel manifold, with a 2-barrel rochester.

Single-plane manifolds are used with camshaft, valve, and cylinder head modifications, to let the engine breathe better at high RPM's. If you're going to turn it up above 3,000 RPM a lot, and you're willing to invest about $1,000.00 in trick stuff to make it work at that speed, that manifold is just the thing! The 307 engine is getting very popular with some guys who run special classes at the drag strip, something about having the right weight per cubic inch to stretch the rules a little. It was a pretty lousy combination back when the 327's and 350's were the high HP options. The 307 sort of replaced the 283 as the economy V-8.

The engine will run OK, not great, with the single plane manifold, and that 600 CFM carb will be about right for your application, but you can expect some low-to-midrange sluggishness with the single plane manifold. Once it's wound up, the power increase should be pretty noticeable. It looks cool, though, especially with a big chrome air cleaner!
Jerry

I don't worry about CFM formulas and such. Throw a quadrajet on there and you'll be happy no matter what your driving style. I know of people with quadrajets on buick 231 V6 pigs, and people with the same (albeit modified) quadrajet on 650 HP 455 oldsmobiles. They'll do it all, you just need to make sure with the high performance stuff, that you keep teh float bowl full. On a 307, a q-jet is perfect. I have one on my (oldsmobile) 307, and with the computer controlled everything, that 800CFM qjet gives me 25 MPG on the highway. For the past 2 months, I haven't seen below 15 MPG. Buddy of mine had an olds 307 with a non-computerized qjet in a 73 omega for a while. with 2.41 rear gears and a TH350 tranny, he got 30 MPG!!! You can't beat quadrajets. Besides, they're cheap and availible in nearly any pre-1980 GM car, truck, van, whatever in nearly any junkyard in the world.

52PU:
If I go 0.059 on the primaries, what should the secondaries be?
Thanx Dano

Sorry I got lost in all this(A.D.D.)

Anyone mention the the CFM HAS TO BE MEASURED AT THE SAME .....INCHES OF WATER?????

Edelbrock measure the 2brl and 4 brl at different vacuum
the two don't relate 500 cfm 2b is not a 500cfm 4b

Different manufactures do this a bunch

So you need some 'Day Standard' to really compair them

Oh AND I agree....parts kids don't know squat....

J.H. - now why in the world would you want to compare everything the same way??? That would make things way too easy.

Actually, one and two barrel carbs were always rated at 3 inches Hg, beginning back in the '20's and '30's. This seemed to be a good average representation of the vacuum of these engines at WOT.

When four barrel carbs came into being, it was found 1 1/2 inches Hg more closely represented the average vacuum at WOT. To convert the 3 inch Hg to the 1 1/2 inch Hg scale, divide the 3 inch scale rating by the the square root of 2 (1.414).

Then in the 1960's one manufacturer started rating carburetors "dry" (air only), vs "wet" (air and liquid mixture). There is about an 8 percent difference in these ratings (a 500 "wet" actually flows about 40 more CFM than a 500 "dry").

I am not sure anyone really understands :confused: the ratings of the last few years, other than they flow markedly different than the same castings did ten years ago. It must be the new math :mad:

Jon.

A vacuum gauge inside the vehicle or hanging on the outside rearview mirror can help with carb selection. When you floor board the throttle, the vacuum goes to zero,as the engine rpm increases, the vacuum will normally rise if the carb is too small for top end power.So if the vacuum reads 3 or 4 inches as the engine nears the point of maximum power, a larger carb can be fitted.

Tony

On level ground? How much head wind -crosswind?

Who gonna pay the ticket

Bet you gotta go 100 before the three pot starts up

And all this time I thought CFM meant Can`t Find Motor

Hey Luke!!
Jim, level ground, second gear is fine, all 3 open together

Dano,
I honestly can't remember the secondary jet on the Holly 600 for the 307ci Cheby. I do remember the primary at .059 sticks with me because that was the exact size of the stock rochester 2bbl had as a jet. It took much fooling around with jets to realize this. Maybe Carbking can steer you closer to your answer.

52pu

The stock jets on a Holley 600 4bbl with vacuum secondaries are #66 primaries and #76 for the secondaries for a 4150 carb or the equivelent for the metering plate on a 4160 carb.Holley jets larger than mid 60 numbers are not a drill size,but a stock number.
The stock Holley 600 primary jets are just a tad rich for a mildly tuned small block V-8's,going down a few sizes is ok.But if you change jets more than 6 or 8 sizes the carb is incorrect for the application.And reducing or increasing jets sized affects the high load mixture provided by the power valve.