You want your classic car to look factory original, but you want the performance of a more modern car; then consider short stroke engines have more power than big bores.
The shorter the stroke, the more an engine can rev. And the more it can rev, the more power it can make. The more power it makes the bigger the smile on your face while driving your classic car.
Back in the V-10 days of Formula 1, it was not uncommon to see an engine rev to nearly 20,000 rpm—a number you would never see on a classic car than normally red lines at about 6,500 rpm. It was only possible thanks to the engine’s extremely short stroke and wide bore.
An engine’s bore is the diameter of each cylinder, while the stroke is the distance within the cylinder the piston travels. Basically, an engine’s maximum power depends on how many rpm it can produce. The more rpm, the more power strokes, the more power it produces overall. So, it makes sense that the most powerful engines also rev the highest. Because a piston with a short stroke does not have to travel as far each cycle, it can cover a greater distance in the same amount of time versus an engine with a longer stroke and smaller bore. That means more rpm. Similarly, a bigger bore means bigger valves, which means it can take in and push out more air in each cycle. And more air means more power.
It works in the opposite direction, too. Let us say your aim is efficiency rather than power. The best engine to have, then, would be one with a small bore and a long stroke. Why? Well, it is a bit more complicated than the power equation, but it involves surface area. Basically, the more surface area a cylinder has during its combustion, the less energy is lost to heat, resulting in a more efficient cycle.
The short answer is that a bigger bore is generally the best way to get more power. It creates more space, allowing for bigger valve openings, which in turn can bring more fuel and air into the cylinder. This does not work well at low rpm but does at high rpm. That plays well into the other factor. A bigger bore with a shorter stroke also allows an engine to rev higher, which creates more horsepower.
Conversely, a long stroke is generally better for fuel efficiency because it reduces surface area during combustion. With less surface area, there is less room for heat to escape, ensuring more of the energy from combustion is turned into useful work to push down the piston.
A small-bore, long-stroke engine also requires the flame to travel less distance during combustion, which means the burn duration is shorter. That allows combustion to do once again do more work and make the engine more efficient.
However, these are just generalizations. It is possible for big-bore engines to be efficient, or for long-stroke engines to be powerful. But without looking at any other variables, there is a correlation between bore size and power, and between stroke length and efficiency.
Bore and stroke are not the only factors that go into the design of an engine, and that is why these are not hard and fast rules. The mass of rotating parts and the use of turbocharging or supercharging can impact power output and efficiency.
Talking about an engine in isolation also only provides part of the picture. An engine’s performance is ultimately determined by the car it is used in. The choice of transmission and the vehicle’s weight and aerodynamics also affect efficiency. At the same time, a powerful engine is pointless if that power cannot be put to the pavement.
There are only two ways to increase an engine’s displacement: You can bore it (engine boring increases the cylinder diameters), or you can stroke it (engine stroking increases the crankshaft stroke).
Engine Stroking offers the potential for significantly larger displacement increases than those obtained from typical engine boring, but it also requires greater sophistication when selecting and integrating components.
Info can be applied to get more cubic inches from any brand of engine
Rod length or piston compression height (piston-pin location) do not affect the bore size or stroke length, and hence do not alter displacement. However, rod length and/or piston-pin location changes may be required after a change in stroke to properly locate the piston top in the block for a desired deck height clearance at Top Dead Center (TDC).
You only use half the stroke because the crank spins in an arc, and half of the increase in total piston travel occurs at the bottom of the cylinder. The required change in the combined connecting rod center-to-center length, piston compression height, and piston deck height is inversely proportional to one-half of any change in stroke. You can make up the difference by changing any combination of the three variables (rod length, piston compression height, or deck height) the required amount to maintain dimension “H”-whichever is cheapest or easiest for the application.
Even though the same piston and deck height is retained, the overall static compression ratio goes up due to the increase in swept volume that is compressed to the same preexisting clearance volume at TDC. That is also why even though a short-rod (5.565-inch center-to-center length) Chevy 383 small-block can use 350 pistons; the compression ratio ends up higher compared to the ratio obtained with the same piston and 5.7-inch rod in a standard 350.
Offset Grinding
Our hypothetical Ford engine builder installed a longer-stroke crank and shorter rods from the same engine family. But the big displacement increases that stroking makes possible are more often accomplished using modified and/or nonproduction parts. There are several ways to change the stroke of an existing crank. Traditionally, the simplest and easiest method is a process known as “offset grinding.” Normally, when a rod journal is reground to compensate for wear, the machinist reduces its diameter to the next standard available undersize bearing while maintaining the journal’s existing centerline location. But when a crankshaft is offset ground, the rod journal centerlines are moved farther away from the main-bearing-journal centerlines.
Rather than minimally offset grinding the journal down to the next standard bearing undersize (which yields only a small stroke change), performance offset grinding typically reduces the final journal diameter to a smaller size used by a different (but still commonly available) connecting rod.
The small-block Chevy guys are fortunate that their favorite engine design has two different rod journal sizes available. That is not the case for many other engines. In fact, most engines built with offset stroker cranks use rods from a different model engine or aftermarket specialty rods. This may require the use of pistons with a different than stock (for the original engine) pin diameter.
Welding
Engines with large rod journals (such as Buick, Oldsmobile, and Ford 351 Windsor) can achieve decent stroke increases via offset grinding and nonstock rods. But in most cases, radical stroke increases require other solutions. Welding is a traditional procedure for obtaining big stroke increases. This process involves adding material to the top side of the rod journal, then regrinding to the original rod journal size but with the journal centerline moved outward in relation to the main-bearing centerline, thereby increasing the stroke. Forged cranks are more suitable welding candidates than cast cranks, but in any case, traditional welding induces tremendous heat which adversely affects the crank’s metallurgical strength. In recent years sophisticated submerged arc-welding and reheat-treating processes have been developed that address these issues, but the cost of a welded stroker has correspondingly increased to the point that for popular engines like the small-block Chevy, a custom-ground stroker crank made from a universal raw forging is cheaper in many cases. Still, welding remains a viable alternative on those off-brand engines for which universal raw forgings are not available.
Custom Cranks
Once reserved for professional racers, custom forged or billet stroker cranks are now increasingly common in higher-end street/strip cars. Universal “econo” raw forgings are available for the more popular engine families. Their quality and metallurgy are acceptable for most uses. Typically, a universal raw forging is made with an elliptical rod journal so almost any stroke can be ground into it. The drawback is that grinding an elliptical journal into a finished round journal interrupts the continuity of the forging’s grain structure, effectively negating its supposed advantage over a billet crank. Assuming you can afford it, you big-arm boys may as well buy a custom billet crank.
Limiting Factors
So many possibilities, so little room. With all the stroker options for these days, you would think the sky’s the limit when it comes to building giant engines. But real-world stroke increases are limited by the physical constraints of the cylinder block. We have already discussed the problems with reciprocating assembly stack-up, but there are also other clearance issues: Big stroker cranks may hit the oil pan rails, and the rails can sometimes be trimmed, but there’s danger of breaking into an oil passage or water jacket. Clearance problems with the bottom of the cylinder bores or with the camshaft are also common.
There is also the problem of the overlap between the main and rod journals. As stroke increases, the overlap in the areas of circles defined by the main and rod journal diameters decreases. Less overlap reduces crank strength and rigidity. The amount of acceptable overlap is determined by the strength of the crank material, the engine’s power output, and its intended use.
When regrinding a finished crank into a stroker, care must be taken not to run into the internal oil passages. Attempts at welding the original passage shut and drilling a new passage usually prove unsuccessful, eventually the crank cracks in the fillet area.
Piston Problems
As we have seen, increasing the stroke, and making no other changes usually causes the piston to stick out the top of the block. Shorter connecting rods are usually not the best solution; rather, pistons with raised pin heights help move the top of the piston back down below the deck. The pistons can be made shorter but only to a point-there must be room for the ring package above the piston pin. Even using thin rings (1/16 -1/16 -1/8 inch or metric equivalents), figure on a minimum total piston deck-to-pinhole top dimension of about 0.750-inch (more if a valve relief extends below the piston deck, as is the case on big-block Chevys). Various devices allow running the oil ring through the pinhole area, and there are even two-ring pistons; while acceptable for regularly torn-down race engines, these solutions are not recommended for long-term street use. You could also go to a smaller diameter pin and bush the rod-but “a smaller stick breaks easier.”
Also, the bottom of the piston must clear the crank throw at BDC. If a clearance problem exists, either the crank counterweight or the piston can be re-machined. Do not re-machine the counterweight in a circular arc or there will be insufficient weight left for balancing purposes. Instead, the counterweight must be cam-ground for clearance by an experienced custom crank shop.
Connecting Rod Challenges
Engines that operate over a broad rpm band (like acceleration engines or street engines) perform best with rod/stroke ratios in the 1.7:1 range. Engines that operate within a narrow rpm band (such as superspeedway, oval-track, or offshore racing-boat engines) like even higher rod/stroke ratios. As the stroke increases, the rod needs to get correspondingly longer to maintain the optimum rod ratio-but the longer the stroke, the less room there is to fit a longer rod. Too-short rods increase cylinder wall thrust-loading and restrict maximum rpm potential. Bottom line: On a big stroker, use as long a rod as you can get away with, based on the smallest practical piston.
Finally
Because of these complexities, leading aftermarket suppliers have developed integrated stroker kits. They have the experience to know what is or is not practical. Unless you are experienced in this area, it pays to consult a recognized expert.
So now you know that a stroker engine is just an engine that has had the stroke increased to more than was originally from the factory thus increasing the stroke of the engine (and also the bore, too) you can increase the size of the engine.
The payoff in added performance is worth the hassle-you can make the engine look stock on the outside but pack it discreetly with additional displacement on the inside…and no one’s the wiser until you blow their doors off. All engines can be modified to achieve greater horsepower and performance with the correct machining and properly matching parts.
Who cares about torque.
Anyone who knows anything about making power.