Jim Smart
November 1, 2002
Photos By: Mustang Monthly Archives

When we turn the key, what happens under the hood and why? How does an automobile engine work? What is that sound we hear and the propulsion we feel when the accelerator is depressed?

We know gasoline is flammable. It burns. And we know gasoline creates heat energy when it ignites. Heat created by the igniting gasoline and air mixture is what gives us the power necessary to propel our Mustangs. With all of this in mind, let's talk about internal combustion engines. "Internal combustion" means we burn fuel and air inside the engine to create heat and generate the power necessary to propel a car.

"External combustion," for example, is similar to that found with a steam engine. Fuel is burnt outside of the engine in a boiler to heat water that becomes steam and heat energy. With both internal and external combustion engines, heat energy (expansion) exerts force on a piston and connecting rod tied to a crankshaft to make rotary motion. In physics class, we call this turning linear motion into rotary motion.

Okay, so what does all of this "linear" and "rotary" stuff mean? Think about what you see when you're watching an old episode of Petticoat Junction. The Cannonball steam locomotive to nearby Pixley has a series of arms tied to pistons and wheels. Steam pressure moves the piston, which moves the arm, which turns the wheels to propel the locomotive. Pistons and arms make linear (back and forth) motion. Wheels make rotary (around and around) motion. The wheels are counterweighted to help momentum. "Chuga-chuga-chuga-chuga" motion that comes from steam pressure gets us down the track. Your Mustang's engine works on the same principle, only the "chuga-chuga-chuga" motion is inside the engine, invisible from the outside.

The crankshaft, like locomotive wheels, is counterweighted for balance and momentum. Around and around it goes-channeling energy to your Mustang's transmission, driveshaft, and rear axle.

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Burn, Turn And Roll

Mustangs are equipped with four-cycle gasoline engines. The four cycles are intake, compression, power, and exhaust. How do we make power from the four cycles? Air and fuel must first be mixed into a vapor or mist before entering the combustion chamber above the piston. Liquid gasoline, as a rule, doesn't burn. If you were to throw a lighted match into a bucket of gasoline (do not do this!), the liquid wouldn't burn; the fumes above the fuel ignite and burn. When gasoline becomes vaporized (mixed with air), it ignites with fury, which makes heat and pressure to do our work.

Fuel becomes a vapor two basic ways. Carburetors, common in Mustangs prior to 1986, mix fuel and air to create the vapor needed to support combustion. Electronic fuel injection atomizes (vaporizes) fuel under pressure either at a single entry point (central fuel injection) or at each intake port (sequential electronic fuel injection). Central and sequential fuel injection are both computer-controlled systems.

Vaporized fuel is drawn into the combustion chamber by the moving piston in the cylinder bore. An open intake valve provides the entry point. This is called intake stroke. When the piston reaches the bottom of the bore, the intake valve closes, terminating the entry of fuel and air. When the piston begins its journey back to the top of the cylinder bore, it squeezes the fuel and air against the closed valves. This is called compression stroke. As the piston nears the top of the cylinder bore, the spark plug fires, igniting the fuel/air mixture. The heat and pressure created during ignition exerts force on the piston, pushing it downward in the cylinder bore, applying pressure on the connecting rod and crankshaft. We call this the power stroke. This linear (straight line) force turns the crankshaft, becoming rotary motion. As the piston nears the bottom of the cylinder in the power stroke, the exhaust valve opens. The piston begins its journey back to the top of the bore, forcing exhaust gasses out through the open exhaust valve. This is called the exhaust stroke. Our engine's four power cycles are complete.

We have introduced you to the workings of a single cylinder. If Mustangs had only one cylinder, there wouldn't be enough power to get the job done. Since 1964, Mustangs have been getting the job done with four, six, and eight cylinders. From 1964-'73, Mustangs came standard with six cylinder engines-with six cylinders positioned in a row along a long crankshaft.

Eight-cylinder engines have always been optional, with eight cylinders in a "V" configuration on two banks of four cylinders each. Beginning in 1974, standard Mustang power was four cylinders in a row. When we line up four and six cylinders in a row, we call it an "inline" engine. Economy cars are traditionally equipped with inline fours and sixes. These engines make a buzzy sound. V-type eights, or V-8s, make a throaty sound much different than inline engines. If this doesn't make sense to you, think of it this way. NASCAR Winston Cup racing consists of V-8 engines that make that powerful roar. Busch Series and Grand National racecars get power from V-6 engines that buzz like a swarm of bumblebees. From 1974-'79, 1982-'86, and 1994-'03, six-cylinder Mustang power isn't inline, but instead a V-6, with two banks of three cylinders in a "V" formation.

Stop Cocks And Bumpsticks

How do we get the fuel/air mixture and exhaust gasses into and out of the combustion chamber? We do this with poppet valves (stop cocks). One valve allows the fuel/air mixture in and another lets hot exhaust gasses out. Poppet valves are shaped like large nails with huge heads. The tapered valve heads close against a tapered seat in the cylinder head to stop the flow. The closed valves seal the combustion chamber between intake and exhaust cycles.

How do poppet valves work? Poppet valves are held closed by springs known as valvesprings. The camshaft (bumpstick), a rotating shaft with a series of eccentrics or lobes, opens the valves in time with the crankshaft. The crankshaft and camshaft are tied together with a timing chain and gearset at the front of the engine. Camshaft speed is normally half that of the crankshaft. This makes perfect sense when we consider the four cycles that give us power. The crankshaft makes two complete revolutions for every revolution of the camshaft. Another way to look at this is we have two complete revolutions of the crankshaft for the four cycles. When the crankshaft is whirling around at 4,000 rpm, the camshaft is spinning at 2,000 rpm.

How do cam lobes open valves several inches away? They do this via lifters, pushrods, and rocker arms. The lifter rides on the cam lobe. The pushrod sits in the lifter and transfers linear (back and forth) motion to the rocker arm at the cylinder head. The rocker arm is a lever that takes the pushrod's linear motion and transfers it to the poppet valve. The poppet valve is opened by the cam lobe and closed by the valvespring.

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Light My Fire

To have power at all, we need a way to ignite the fuel/air mixture once it is inside the combustion chamber. We do this with a timed, high-voltage spark. The timed spark comes from the ignition system, which is tied to camshaft and valve timing. Mustangs prior to 1996 have a distributor, which times the spark at compression stroke as the piston nears the top of the cylinder. The distributor channels high-voltage electricity from the ignition coil (an electrical transformer) to each of the engine's spark plugs. Each cylinder (combustion chamber) has one spark plug. If we have a six-cylinder engine, we have six spark plugs. A V-8 has eight.

When we think of a multi-cylinder engine as four, six or eight individual engines, it becomes easier to understand spark timing. In a V-8 engine, for example, we have eight individual cylinders firing at eight different times in sequence. No two cylinders fire at the same time. They fire in a sequence known as the firing order. Your Mustang's 289ci V-8 engine, for example, has a firing order of 1-5-4-2-6-3-7-8. If you study this firing order, cylinders fire back and forth across the two banks. Cylinder 1 fires first. Then cylinder 5 on the opposite bank. Then back across to cylinder 4. Then across the same bank to cylinder 2. Then over to the opposite bank to cylinder 6. Then back over to cylinder 3. And finally cylinders 7 and 8.

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With eight separate engines working together on a common crankshaft, we have a continuous distribution of power from eight cylinder bores. The distributor makes it possible to fire these eight cylinders in proper order. When we tie the distributor to the camshaft and crankshaft, it all works together in near-perfect synch.

Understanding Old Sparky

We know the ignition system has to fire multiple spark plugs in a timed sequence. Each spark plug has a small gap between the center electrode and cross electrode. Vintage Fords typically have a spark plug electrode gap of .034-inch, suitable for low-energy, point-triggered ignition systems. To get a spark to jump any sized gap, we need high-voltage electricity powerful enough to jump the gap. We produce high-voltage electricity via the ignition coil, which is an electrical transformer that transforms battery voltage (12-14 volts) to high-voltage electricity (around 20,000 volts).

Inside conventional point-type distributors, the ignition points act as a switch for the ignition coil's task of producing high-voltage electricity. When the points open, the ignition coil releases the store of electricity to the distributor's high-tension lead. Because this is a timed sequence, the distributor rotor will be positioned on a given cylinder's spark plug wire lead when the points open. With a V-8 engine, this happens eight times in two revolutions of the crankshaft. With electronic ignition, a magnetic pickup module inside the distributor takes the place of ignition points and condenser. This is oversimplified, of course, because there is also an ignition amplifier separate from the distributor that works hand-in-hand with the pickup module.

The four-cycle power process would be easy to handle if all we had to worry about was firing a spark plug when the piston reached the top of the cylinder. But if we fired the spark plug when the piston reached top-dead-center (TDC), we wouldn't make the most of the fuel/air mixture. Because fuel takes a given amount of time to ignite and burn, we have to fire the spark plug before the piston reaches the top of the cylinder. Fuel and air don't "explode" in the combustion chamber. The fuel/air mix ignites in a "quick fire" that begins at the spark plug and roars across the top of the piston. This is called a flame front.

As engine speed increases, the spark has to occur earlier in the compression stroke. Because we're locating the spark earlier in the compression stroke, this is called "advancing" the spark. How do we advance the spark? We do this with two functions inside the distributor-vacuum advance and centrifugal advance. Centrifugal advance works with the speed of the engine. Vacuum advance works when we lean on the throttle. Both work together to adjust spark timing in line with engine power demands.

Centrifugal advance works with engine speed via springs and flyweights that position the distributor rotor. The faster the distributor turns, the further out the flyweights move against spring tension, positioning the rotor earlier in the compression stroke. Spark timing happens in degrees of crankshaft rotation and piston location. Crankshafts rotate 360 degrees in a full circle. As the piston makes its way up the cylinder bore on compression stroke, this happens between zero and 40 degrees of crank rotation before the piston reaches the top of the bore.

At idle speed, we want the spark to occur at 6 to 12 degrees (crank rotation) before the piston reaches the top of the cylinder. This is called BTDC (Before Top Dead Center). When the engine revs to 3,500 rpm and higher, the spark needs to happen earlier around 36 to 40 degrees BTDC. This helps our engine make the most of the fuel/air charge. When we move the spark any earlier than 40 degrees BTDC-and this is pushing our luck-we risk premature combustion (pinging and detonation) that can do serious engine damage.

Cooling It

All that heat energy released in each combustion chamber during power stroke can be destructive if it isn't controlled. We control the heat with water jackets around the cylinders and in the cylinder heads. Water jackets contain the coolant mix that keeps our engine temperature stable. This is done with coolant, the radiator, a thermostat, hoses, water pump, and a fan that pulls outside air through the radiator.

Coolant flows into the engine's water jackets via the bottom radiator hose. Ideally, we have water jackets that are completely full of coolant, without air bubbles that cause hot spots. The thermostat is a flow-control valve that opens when the coolant temperature in the engine reaches either 180 or 192 degrees Fahrenheit. Older Mustangs have a 180-degree thermostat. Newer, computer-controlled Mustangs are 192 degrees.

When the thermostat opens, it allows hot coolant to leave the engine and flow into the radiator through the upper hose. Coolant in the radiator is displaced by the incoming hot coolant from the engine. Fresh coolant in the radiator enters the engine, which closes the thermostat. The cycle begins all over again when coolant inside the engine reaches 180 or 192 degrees.

Lube It Or Lose It

Another engine survival tool is the oiling system. Engine oil is stored in the oil pan at the bottom of the engine. When we check oil, we're pulling a dipstick that shows us how much oil is in the pan. The camshaft we mentioned earlier also has the busy task of turning the oil pump that keeps our engine alive. It does this through a shaft splined into the distributor shaft.

Our Mustangs have what is called a "G" rotor oil pump that draws oil from the pan and forces it into the oil galleys that feed moving parts like crankshafts and camshaft bearings, pistons and cylinder walls, lifters, rocker arms, timing gearset, and more. Oil is fed to these critical moving parts under pressure, which keeps them from rubbing together. Moving parts float on a layer of engine oil. Think of engine oil as a barrier that protects moving parts from certain destruction.

Another thing engine oil does is cool hot engine parts. As oil flows through the engine, it carries heat away from the hottest parts, such as bearings, pistons, rings, and even valve stems. Engines fail whenever oil pressure is lost and moving parts grind themselves to an unpleasant halt.