In our two previous Motor Series installments, Vol. 38, No. 4, and Vol. 38, No. 5, we looked at some critical engine dimensions and key terminology that will help you avoid making costly mistakes when buying or building an engine. We also touched upon several relationships influential for achieving component synergy. Having these essentials will help you build an engine with maximized power and performance for your application. As a refresher, you might want to dig out those installments from your performance library. In the following text, we’ll deal with two interrelated topics: torque and horsepower. The relationship between torque and horsepower is one of the most important concepts to understand when designing and building an engine that will satisfy predefined performance objectives.
Much is written about dyno testing and engine performance. Peak torque and horsepower numbers are usually pointed out, as is other information. Yet there is more to performance than just peak power numbers, because peak numbers may not indicate an engine’s true temperament. In fact, many have asked what exactly the difference is between torque and horsepower, and whether an engine can be built favoring one or the other. By understanding the relationship between torque and horsepower, you’ll be well on your way toward maximizing power and performance.
Simply put, torque is a twisting or turning force that engineers measure in “pounds” and “feet,” or lb-ft, in the English system. One lb-ft of torque is equal to a 1-lb force applied at the end of a 1-ft lever. Engine torque is typically measured on a dynamometer and can also be defined as the potential to do work. Unlike horsepower, torque does not take into consideration the element of time, which gauges the rate at which an engine can perform work. An engine’s power is actually established by first measuring torque at a given rpm and then calculating horsepower.
Basically, horsepower is equal to torque multiplied by rpm. Therefore, any increase in torque increases horsepower at a given rpm. That is why it’s better to concentrate on improving torque than horsepower to achieve the best engine performance. Competition engine builders always concentrate on improving torque within the rpm range their engine needs to operate.
Engine torque is determined by the percentage a cylinder is filled at a given rpm. The greater the cylinder fill, the greater the torque will be. If power is to be increased, it is crucial to improve the engine’s ability to breathe. Peak torque is reached when the engine runs out of air or loses its ability to breathe better. That is the point of maximum cylinder fill. An engine can continue to make more horsepower even when torque is falling, as long as rpm is increasing faster than torque is falling. Therefore, if maximum torque is the point of maximum cylinder fill, then maximum horsepower is the point where torque is falling off faster than rpm is increasing.
Displacement, intake and exhaust tracts, cylinder heads, cam timing, stroke, and rod length are among the factors that control the amount of torque and the torque peak for a given engine. Race engines are normally designed to turn a higher rpm and have greater cylinder fill and torque at higher rpm levels. However, this often results in a very peaky or narrow power curve with reduced power at lower rpm. A street-driven engine is usually best designed to have a flat or broad torque curve at low and middle speeds, although it may sacrifice some torque at the top end. High-performance engine modifications generally shift an engine’s maximum torque to a higher rpm, although this can result in the loss of low-end power. The key to building a “happy” engine is to maximize torque in the rpm range-the most important item for meeting one’s objectives and riding style.
Even though all engines produce torque, an engine that produces peak torque at low rpm is generally referred to as a torque engine, while one that produces peak torque at high rpm is referred to as a horsepower engine. Since torque and horsepower curves always cross at 5,252 rpm, an engine will always produce more lb-ft of torque than hp below 5,252 rpm, and more hp than lb-ft of torque above 5,252 rpm; this is the reason a four-cylinder import engine that revs to 13,000 rpm can produce higher horsepower than a V-Twin while yielding less torque at the crankshaft. Long-stroke, small-bore (under-square) engines such as the V-Twin are typically lower-revving, with high torque below 5,252 rpm. On the other hand, big-bore, short-stroke (over-square) engines are normally higher-revving and produce high horsepower at high rpm from a relatively low crankshaft torque reading.
Most riders, whether into touring, hot street, or racing, generally prefer a torque engine with a wide, flat torque band over a peaky high-horsepower engine producing power in a narrow rpm range. For a street rider, a torque engine makes riding easier and more fun because you don’t have to shift as much when trying to keep the engine rpm in the powerband. Racers usually build an engine that maximizes torque over its entire working rpm range instead of just a small portion. In fact, if you were to dyno-test some of the fastest drag-race engines, you would probably discover that most, if not all, do not produce as high a maximum horsepower as some dyno shoot-out engines. Instead, they tend to have a flatter, less peaky power curve that produces more power over the entire working rpm range. To achieve maximum performance, a torque motor should be cammed, geared, and ridden differently from a horsepower motor. For example, a torque motor benefits from different valve timing and lower gear ratios (higher numerically), along with a lower rpm redline.
There are numerous methods for increasing engine torque. The most obvious is described by the axiom “there’s no substitute for cubic inches.” More cubic inches results in more torque, especially if the displacement is added by using a longer stroke. And large engines require fewer compromises to achieve a wide powerband. In general, increasing stroke length shifts the torque peak to a lower rpm. This is especially true for a long-stroke engine built with stock-flowing cylinder heads. On the other hand, improving an engine’s ability to breathe moves the torque peak to a higher rpm.
Carburetion should be based on matching the carb’s cfm airflow to the engine’s displacement and rpm limit. Too large a carb reduces air velocity through the venturi and reduces the signal to the jets, resulting in poor atomization and less metering accuracy. The consequence is less torque, along with poor throttle response and ridability.
Installing a set of high-flowing cylinder heads will increase cylinder fill and torque, but torque will be improved mostly at midrange and high rpm. To maximize horsepower and still maintain torque at the bottom end, consider the following: Basically, airflow should be matched to engine displacement and rpm. For a given cfm airflow, keep valves and ports as small as possible, because mean airflow velocity has a significant effect on where torque peaks and the amount of torque at low rpm. As rpm increases, an engine pumps air until intake velocity increases to the point where port friction loss stops the increase in air intake. Mean flow velocity is dependent upon intake-runner or exhaust-pipe cross-sectional area, not volume. Therefore, peak torque rpm can be controlled by selecting parts with specific cross-sectional areas.
Smaller valves and ports maximize velocity for improved cylinder fill and exhaust scavenging at low rpm. The ratio of the valve-seat I.D. to the valve-head diameter is critical for flow and torque. Generally, for best performance with the Harley two-valve V-Twin engine, intake valve-seat I.D. will range between .88-.93 of the intake valve-head diameter, while the exhaust seat I.D. will fall between .86-.88 of the exhaust valve-head diameter. Changing the ratio will change high- and low-lift flow numbers. The lower the ratio, the larger the seat radius becomes and the smaller the throat diameter. A smaller throat diameter tends to reduce high-lift flow. The optimum ratio usually gets smaller as the valve diameter gets smaller. Port and valve shapes, along with bore and stroke, are other factors that contribute to the ideal ratio.
Another important cylinder-head consideration affecting volumetric efficiency is combustion-chamber design. Increasing combustion efficiency through optimized combustion-chamber design and fuel distribution will improve not only volumetric efficiency but also thermal efficiency, thus resulting in increased torque.
Increasing the compression ratio to the maximum allowable by the gasoline octane will add torque to any engine. However, when a long-duration cam is also installed, the effect of a compression increase is of much greater magnitude, especially at low rpm. Cam timing (especially the intake valve closing) must be matched to the compression ratio to achieve an optimum engine. Cam manufacturers deliberately keep the intake valve open for many degrees after the piston reaches bottom dead center to improve cylinder fill and torque at high rpm. However, at low rpm, a late-closing intake reduces cylinder fill and torque. Torque lost at low rpm can be regained by increasing the engine’s mechanical compression ratio, so the corrected compression ratio maintains a predetermined level.
For example, let’s say we have an engine with a mechanical compression ratio of 10.5:1 and a corrected ratio of 9.3:1. Closing the intake valve later will reduce the engine’s corrected ratio, but increasing the mechanical ratio beyond 10.5:1 can bring the corrected ratio back to 9.3:1. A street engine with a good combustion-chamber setup and run on pump gas can handle approximately 9:1-9.5:1 corrected compression ratio without suffering detonation, while an engine fed race gas can handle between 11:1-13:1 or even higher.
Of the four valve-timing events, the closing of the intake and opening of the exhaust have the greatest effect on torque output. Closing the intake valve late increases high-rpm horsepower at the expense of low-end torque. Nevertheless, remember that increasing the mechanical compression ratio recovers much of the lost low-end torque. Closing the intake valve early does just the opposite of closing it late. Be careful, because closing the intake valve too early with high compression can cause the engine to become detonation-prone.
On the exhaust side, opening the exhaust valve later extracts the maximum amount of energy from the expanding gasses and increases bottom-end torque, but opening it too late creates pumping losses and severely hurts top-end power. On the other hand, opening the exhaust valve early improves top-end torque at the expense of the bottom end. An early-opening exhaust is best suited to high engine speeds and high compression. Obviously, there are trade-offs. To maximize bottom and midrange torque, select a cam that closes the intake valve early and opens the exhaust valve late.
Recognizing the relationship between torque and horsepower is crucial to building an engine that develops power where your riding style demands it. And once you thoroughly understand that relationship, you’ll be well on your way to selecting the best product combination for your riding style.