Just what is a stroker anyway? It is an engine with increased or decreased stroke, which is the distance the piston travels in the cylinder bore. Changing the stroke changes when and how the engine makes power. By increasing an engine’s stroke, you gain displacement. By the same token, when you decrease an engine’s stroke, you lose displacement. Short-stroke engines like high RPM, where they make the most torque. Our focus is about increasing stroke in order to achieve greater amounts of torque and horsepower.
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“Stroking” an engine does more than just increase displacement. It increases torque by giving the engine more of an internal mechanical advantage. When you increase stroke, you increase the length of the engine’s crankshaft arm (or lever), which makes the most of a combustion cycle. The longer the stroke, the greater the torque or twist.
The stroke number is the length of the crankshaft’s rod journal arm, doubled. You double the length of the crankshaft arm because that arm moves in two directions: to TDC then to BDC. For example, if the arm is 1.5 inches (measured from the crankshaft centerline to the end), you have a 3-inch stroke.
So how do you get more power from a stroker? Power comes from the greater mechanical advantage of a longer crankshaft arm. You are also filling the cylinder with a greater volume of air and fuel, which gives more power all by itself. From stroke (and cylinder swept volume), you get torque. Torque is the truest measure of an engine’s power output.
So what exactly is torque? Think of the crankshaft’s arm as a simple lever. Torque equals the downward force of the stroke times the length of the lever (or arm). At a 351C engine’s 370 ft-lbs of peak torque, each cylinder bore is producing 740 pounds of pressure on each power stroke. Remember, you increase torque when you increase the length of the arm. And when you increase the length of the arm, you increase stroke.
A stock 351C engine’s arm is 1.75 inches long, which means the 351C engine has a 3.50-inch stroke. If you add 1/4 inch to the arm, you increase the arm length to 1.875 inches. That means you have 3.750 inches to achieve 377 ci with the standard 4.000-inch bore. This gives you 40 additional foot-pounds of torque. Overbore the cylinders .030 inch and you have 383 ci. Push the bore to 4.060 inches and have 388 ci. You also have more torque.
In addition to the advantages of a stroker, there are disadvantages, especially if you’re bent on pumping the most displacement possible into a 351C or 400 engine. When you stroke a 351C or 400 to its limits, you lose piston skirt, which hurts stability. You also push the piston pin into the piston ring land area, which weakens piston design. It also puts the pin close to the piston dome, which exerts too much heat on the pin and boss. These are disadvantages that shorten engine life.
Another factor with stroking is rod length. When you haul that piston deep into the cylinder bore, you are also bringing it closer to the crankshaft counterweights, which creates conflict. This means you need a longer connecting rod to get the piston down there without interference with the counterweights. Sometimes you can find off-the-shelf connecting rods to complete your stroker. Other times you are forced to custom make connecting rods that work. More expensive stroker kits have custom parts such as rods and pistons. More affordable kits have off-the-shelf parts.
Stroker kits often mandate custom pistons to keep things friendly at the top of the bore. A 408 or 427-ci stroker, for example, has custom pistons with pin bosses pushed way up into the ring lands. This drives the cost up. It also shortens engine life.
Stroker Power Facts
There are plenty of myths about making power, especially in the Ford camp. Folklore tells us it’s easier to make power with a Chevrolet than a Ford. But this is pure nonsense. You can make just as much power with a Ford for the same amount of money you can with a Chevrolet. What gives the Chevrolet an advantage is numbers—shear volume. Chevys are simply more commonplace than Fords. But even this is changing because Ford’s popularity has grown dramatically in recent years. When it comes to seat-of-the-pants performance, there’s no black magic here, just the simple physics of taking thermal expansion and turning it into rotary motion that makes you feel good about your engine.
To learn how to make power, you first have to understand how power is made inside an engine. The amount of power an engine makes depends on how much air and fuel you can pump through the engine, plus what you do with that fuel/air mixture during the split second it lives and dies in the combustion chambers.
You have to think of an internal combustion engine as an air pump. The more air and fuel you can “pump” through the cylinders, the more power you’re going to make. This is why racers use big carburetors, manifolds, heads, superchargers, turbochargers, and nitrous oxide. Racers understand this air pump theory and practice it with reckless abandon; sometimes with catastrophic results. But good racers also understand the “too much of a good thing” theory. Sometimes it can cost you a race. It can sometimes cost you an engine.
Getting power from the “air pump” takes getting liberal amounts of air and fuel into the chambers, then squeezing the mixture as hard as you can without damaging the engine. When you raise compression, you increase the power the mixture yields. It is the intense heat of compression coupled with the ignition system that sparks the yield of energy from a mixture. The more compression you have, the greater the heat you have to ignite the mixture.
Problem is, when there’s too much compression, and the resulting heat, the air/fuel mixture can ignite prematurely resulting in preignition and detonation. So you have to achieve the right compression ratio to get the most from the fuel you have. Today’s street fuels don’t tolerate much more than 10.5:1 compression. This means you have to look elsewhere for answers in the power equation, such as more aggressive camshaft profiles, better heads, better port work, hotter ignition systems, exhaust headers that breathe better, state-of-the-art intake manifolds and carburetors, even electronic fuel injection where you never thought of using it before.
The thing to remember about gasoline engines is this: The air/fuel mixture does not explode in the combustion chambers; it “lights off” just like a gas furnace or water heater. Because the mixture is compressed and ignited, it lights off more rapidly. Combustion in a piston engine is a “quick fire” that sends a flame front across the top of the piston. Under ideal circumstances, the flame front travels smoothly across the piston dome, yielding heat and pressure that act on the piston and rod uniformly to create rotary motion at the crankshaft.
A bad light-off that originates at two opposing points in the chamber is the pre-ignition or detonation factor. The opposing light-offs collide creating a shock that hammers the piston dome, which is the pinging or spark knock you hear under acceleration. The objective is to get a smooth, quick fire, with the flame front traveling in one, smooth direction for maximum power. An abnormal lightoff can also happen prematurely from advanced ignition timing or red-hot carbon in the chamber.
Power management is having the right balance of ignition timing, fuel mixture, compression ratio, valve timing events, and even external forces such as blower boost or nitrous input. All of these elements have to work together if you’re to make productive power. Let’s talk about some of the elements you need to make power.
For one thing, the science of making power must tie in with how you intend to use the engine. And that’s where most of us get it wrong all too often. In our quest for stroker torque, we sometimes forget how the vehicle is going to be driven and used. If you are building a stroker to go drag racing, the way you build your engine is going to be different than the guy who builds one for trailer towing. By the same token, road racing engines should be executed differently than drag racing powerplants.
So how do you approach each engine’s game plan? Street engines for the daily commute need to be approached for good low- and mid-range torque. Drag racing engines need to make power at high-RPM ranges. Road racing engines need to be able to do it all; down low, in the middle, and at high RPM because they’re going to live in all of these ranges while racing. Engines scheduled for trailer towing need plenty of low-end torque. They also need to be able to live comfortably at mid range when you’re going to be pulling a grade.
Proper Assembly Technique
When it comes to assembly practices, engine professionals stress two main areas: cleanliness and double-checking your work. Never assemble an engine in the same area where it is torn down. Even minute amounts of dirt, dust, or grit can stop an engine cold.
I stress double-checking your work because this approach actually saves time. If you think it’s inconvenient to check your work two and three times, consider the inconvenience involved in a nuisance tear down because there’s high oil consumption or having to collect the pieces of a scattered engine because something critical was missed during the assembly process. Even check it thrice and sleep better.
Engine building is an exacting exercise in physics where every detail must be covered to ensure success. Power comes from taking all of that thermal energy and harnessing it above the piston during light-off. Fuel and air don’t explode in a combustion chamber as you have been led to believe. Combustion is a quick-fi re and the smooth application of heat energy. In theory, it should be a smooth quick-fi re with a nice flame front across the tops of the pistons converted to rotary motion with an attitude. You want to make the most of a brief moment of heat energy times eight.
The easiest way to make power is to raise compression. However, you don’t want too much compression. Compression ratio depends upon the plan and the fuel available. The other quickest way to unleash power is less internal friction. Internal friction is reduced with obvious means such as roller tappets, doubleroller timing set, and lightweight, full roller rocker arms. However, there are other ways, such as more liberal clearances and lightweight components, which is a balancing act in itself because you also want durability, good oil pressure, and low oil consumption.
Even with all details covered, it is no guarantee an engine will stay together or make the power expected. It is those troublesome areas you cannot foresee material weaknesses and defects, which can fail when least expected. This means you must be attentive to everything you have control over in the build process. When in doubt, check it out.
The most savvy engine builders begin assembly with a mock-up phase where the bottom end is assembled and lubed up without piston rings and checked for proper clearancing, especially if they’re building a stroker. A mock-up allows you to check critical clearances all around. This means rods and journals have to be checked to make sure they’re going to clear the bottoms of the bores. You must have .060-inch minimum clearance between rod and block. Be careful how much iron is ground away because you risk going all the way through. The most you’re going to be able to get into a 351C is 408 ci with a 4.040-inch bore. Again, a 4.060- inch bore is discouraged unless you are very confident of a sonic check.
Written by George Reid and Republished with Permission of CarTech Inc