The Stovebolt Site Forums The Shop Area Engine Shop CFM Requirements

Hello all,
I have a fresh '69 307 w/327 heads out of a '69 C-10, that I'm getting ready to swap into my '69 C50. The old 350 is tired so it's going to get rebuilt or I'll find a tall deck 427 for it. However since the 307 will be powering it in the interim, I'm curious what carb y'all would run for decent power without abysmal mileage losses. It has a 600 CFM Edelbrock 4bbl on it currently and it runs like a banshee, but I'm not sure that it would be the best choice for my Big Bolt,... I tried to search for answers before posting this, so if I missed it, I apologize in advance. Any suggestions?

If it's running good now the best thing to do is leave it alone.


Pete

The 307 has about a 1/4" shorter stroke than the 350 which translates to less torque, so you'll need to wind it up a little tighter to make it move that big rig effectively. The 600 CFM carb is plenty big to flow enough air to get the job done. If you can find a tall-block 427, I think you'd really like what it can do for low-end pulling power. I'd pick that one over just about any way a 350 can be set up for load-pulling use.
Jerry

If you are going to use the truck to work with, the 427 tall deck was designed as a truck engine and has a lot torque and it will work. A 350 really never was a truck engine. If you use the vehicle just to drive around, a 307 is a cheap way of getting your vehicle on the road.

The 600cfm carb is plenty big enough but if it runs good without bogging at lower speeds I'd leave it. Through 60 years of messing with these old rides the most common performance killer has been "too big". Too big a carb, too big a cam, too big of exhaust, and too big of tires. A 307 at 5000rpm uses about 450cfm of air. There are tables at various carb sites but a handy formula is an engine turning 5000rpm (in this example) is doing 2500 intake strokes (the other 2500 are power strokes) so it's 2500 X 307 equals 767,500 cubic INCHES of air per minute which you divide by 1728 to give cubic feet which in this case is 444cfm.

and it runs like a banshee

It's not broken, don't fix it.

I wasn't sure of the calculation for that, so I appreciate that info! Either way I intend to find a Tall-deck as soon as availability and $$$ allows, but the 307 will have to work for now, as it's free and it's all I have. Thanks guys!

My Dad bought a 1981 C50 new and it came with a 350 with a 2 barrel Rochester on it. The truck would run 60 all day, loaded or unloaded. As mentioned earlier in the post a "truck 427" is the best route to go. Will the smaller engine work, sure, just a little slower.

There isn't any CFM "requirement", that formula calculates the largest carburetor for performance use on a single-plane manifold.
If the manifold is dual-plane, the CFM can be much higher.
If the carburetor is too small, the only bad result is slightly less peak power. There's no such thing as a small carburetor "starving the engine of fuel".

More CFM = loss of low RPM power

Less CFM = loss of Higher RPM

More CFM = loss of low RPM power

Why?

air velocity through the carb will be lower resulting in poor fuel atomization. less burnable fuel less power

How do engines run at lower speed?

Its not so much large cfm = equals less low rpm power, its more of a large venturi = less power at slow speeds.

This why GM installed Q-jets on all there engines. The small primary's kept the air velocity high during low speed driving and the big secondaries move the majority of the air at high speeds. Its all about fuel atomization the carbs ability do that all speeds. Large bore venturies just don't do a very good job of it at slow speeds.

Joe

Keep the 600 cfm Edelbrock.

Back in my younger, dumber days, I learned the "too big carburetor" lesson the hard way. My 1947 Ford coupe had a flathead V-8 engine that I believed to be a 239 cubic inch 100 HP model. Later, I tore it down and discovered an .080" overbore, making it actually a 252 cubic inch engine. It had the standard Ford/Holley 2 barrel carb with a 15/16" venturi diameter. I found a 2-BBL carb for a 1955 Ford 272 engine that would bolt on with minimum modifications, mostly tinkering with the throttle linkage. This carb had a 1 1/16" venturi size, not that much bigger- - - -right?

The engine never ran right with that carb, even after a rebuild, and bigger main jets! It had a horrendous flat spot off-idle that could only be avoided by gently opening the throttle to maintain enough venturi velocity to keep pulling fuel through the jets. Even a slight bit too much throttle opening killed the venturi velocity, and the engine went too lean to accelerate. That was usually followed by a series of backfires that would blow the air cleaner loose from the top of the carb. Putting the original carburetor back on it solved the problem. At highway speed, the engine ran fairly well, but I lost everything but full-throttle performance. Not an acceptable trade-off!
Jerry

A friend had a similar problem on a carb equipped Ford 300 straight Six that he 'warmed up' courtesy of Clifford performance,... I don't remember the size he initially went with, but it was horrible until around 2200 rpm, then it would sent the rear-end sideways if you weren't careful on a low-budget t-bucket,... he went to a smaller carb and played around with the jetting until he got it right. Then it was a monster off the line!

We had a competitor at the local dirt track with a deep-pocket sponsor who liked to run Ferd 300's. He could run a bunch of goodies the V-8 cars couldn't- - - -roller cam, lots of special head work, and 3 side-draft Weber carbs, one venturi per cylinder. That thing would scream once he got it wound up, but if it ever bogged down in traffic, it wouldn't get out of its own way. Too much carb only works one way- - - -wound tight and wide open!
Jerry

if you use the edelbrock setups (carb, manifold, and if you like, the cams as well) they have done all the work for you, I have seen a few of these set up on ford and chevy engines and they run like a deranged maniac.