Posted on: March 9, 2009

Carburetion – Street Driving Modification

Most performance increases are accompanied by a compromise somewhere along the line. This modification is probably an exception because (unless you put your foot on it) gas mileage should not change. There are several manifolds available for the Ford 260/289. Most are raised higher than the cast iron manifold that they replace and may present hood clearance problems. From my experience, I have found that the least expensive modification that you can do to your stock 260/289 and get the best performance increase from, is to install:

1 – Ford cast-iron 4-barrel manifold
2 – 465/500 CFM Holley 4-barrel or AFB (Carter)
3 – A Ford Dual-Point Distributor-Ford part #C502-12127-E
4 – Good Copper/Steel Spark PLug wires-Ford part #C402-12259-C

Don’t misunderstand the above statement to mean “best”. I am taking into account the cost, also.

A Cast-Iron 4-Barrel Manifold can be found in plentiful supply at junkyards for less than $25.00 as opposed to $100.00 plus for a new aluminum manifold. With a small 4-Barrel Carb (465-500) CFM-any make), you will be impressed at the new boost your stock motor will get. The Ford dual point distributor (join c.A.T. and get one for a fair price) and a good set of spark plug wires (steel/copper, not resistive type) makes for a super low budget stock set-up.

I have heard only bad things about the Ford Cast-Iron 4·Barrel Manifold, but for stock driving, you can’t beat it. The Cast-Iron 4·Barrel Manifold was not designed to operate in the 7000+ rpm range, but it was designed to operate in the same range as your 2·Barrel Manifold and (if you haven’t forgotten) your factory hydraulic camshaft (4400 rpm). What you will get is a nice boost from 2000 up to 4400 rpms. If you stay with a small 4-Barrel Carb, throttle response will not suffer.

A carburetor indirectly delivers fuel to the engine. As the “gas” pedal is depressed, the throttle opens, allowing more air into the engine. As air rushes through the carburetor constriction (called the Venturi). the air speeds up, and at that point a vacuum is created. This vacuum point is where gas from the fuel bowl is fed into the air stream. The faster you go (or should Isay the more air you allow into your engine by opening the throttle), the more fuel is “pulled” into the air stream from the fuel bowl. The problem with the carburetor throttle is that when you open up the throttle, the air that was rushing through a small opening, is now moving more slowly through a larger opening. The air stream velocity drops, vacuum drops, and not as much fuel is pulled into the air stream. The car would stumble or stall unless it had a helping squirt of gas (from your accelerator pump) available to overcome this “momentary” change in air velocity . Ifyou understand what is happening, then you can see that a bigger carburetor (bigger than is needed, that is) compounds this problem. A bigger carburetor has a slower air velocity through the venturis, and any incremental change in gas pedal position in results in an even bigger drop in vacuum when you step on it. When you hit the gas, all of the sudden what was going through 2 barrels, suddenly has to go through 4 barrels. Velocity drops way down, vacuum drops and the car lacks response, stumbles or stalls. If you don’t believe this, try driving your car without an accelerator pump.

Now, if you can believe this logic, read on. Assume that your 289 engine has to rotate twice to induct 289 cubic inches of AIR into your cylinders. It sounds silly to think otherwise, but at very high rpms, restriction in the intake ports.incomplete emptying of exhaust gases due to muffler back pressure may, and I say may, prevent 100% cylinder filling.

Let’s assume you have a 289-271 HP engine rated at 271 horsepower at 6000 rpms. Since your engine is a 4 stroke engine, it takes 2 revolutions to induct 289 cubes (one down stroke is the power stroke, the other down stroke is the induction stroke). Therefore, every other downstroke (or 6000/2) equals 3000 rpms. At 6000 revolutions per minute, your 289 inducts 3000 times (or 289 x 3000).
867,000 cubic inches of air per minute at 6000 rpms
1 cubic foot = 1782 cubic inches
867,000 = 501.7 Cubic Feet of air per minute at 6000
1728 rpms., assuming 100% cylinder filling

If you read Ford’s specs on the 289·271 HP engine, you will see that the stock carburetor is 480 CFM’s. Pretty close to the calculated 502 CFM’s. If you have a stock 289 that peaks out at 4400 rpms, then look at this:
635.800
289 x 2200 = 1728 = 267 cubic feet of air per minute

The stock 2-Barrel carb on your engine flows 280CFM. Since a465 Holley is about the smallest 4-B arrel available, this should be an ideal setup on most street cars. The $100.00 saved on an after market aluminum manifold should be well spent on a dual point distributor and a good set of steel or copper wires.