|THE READ FOR SPEED by Ned Erkman
It is often considered that the performance potential for a high performance vehicle is primarily determined by engine power. Although HORSEPOWER and TORQUE are key ingredients, the power of an engine is not the only requirement for improving vehicle performance. The ACCELERATION RATE of an engine also contributes to performance by increasing the engine RPM at a faster rate of speed thereby reaching maximum power at faster period of time. In effect, more engine power is delivered at a faster rate of time.
The engine acceleration rate is determined by the speed in which the crankshaft rotates. The faster the engine rotates or spins the crankshaft, the faster the vehicle can accelerate. Although engine power creates energy to turn the crankshaft, the acceleration rate is what determines how fast the crankshaft rotates. A faster rotation of the crankshaft helps to turn the drive train components faster, which then rotate the drive wheels faster, thereby creating quicker acceleration for a high performance vehicle.
To better understand the difference between HORSEPOWER, TORQUE and ACCELERATION, consider that a Diesel engine can produce a lot of power, but diesel engines are not used for high performance vehicles because they do not create rapid acceleration
Power alone is not enough to achieve superior performance. HORSEPOWER, TORQUE and a faster ENGINE ACCELERATION RATE are required to increase vehicle acceleration.
To better understand the phenomenon of Engine Acceleration, a comprehensive understanding of Torque and Horsepower are required.
In reference to an internal combustion engine, torque is the force required to create a rotation of the crankshaft and the unit used to measure the amount of rotational force is referred to as foot-pounds. The foot pounds of torque that an engine generates are measured in dyno testing to evaluate engine power output through the rpm range.
Torque Examples: To better understand torque, let’s use an example that most people involved with racing can relate to such as tightening bolts with a torque wrench. When a torque wrench is being used to tighten a bolt the force that is being applied is the torque. It is important to consider that time is not associated with the application of torque, so we can apply torque to tighten a bolt in 2 seconds or 4 seconds or 6 seconds.
Since torque is rotational power, it is also important to consider that the torque of an internal combustion engine is measured in rpm (revolutions per minute). So torque is measured at a specific rate of revolutions per minute and not measured in time.
Horsepower is different than torque in that horsepower involves the time to do work. The definition of mechanical horsepower is the ability to generate 550 foot pounds of work or energy in 1 second or 33,000 ft lbs per minute.
To better understand the energy required to generate 1 HP, look at these examples:
To lift 330 lbs a distance of 100 feet in one minute is
1 HP. (330 x100 = 33,000)
To lift 1000 lbs a distance of 33 feet in one minute is
1 HP (1000 x 33 = 33,000)
To lift 10 lbs a distance of 3300 feet in one minute is
1 HP (10 x 3300 = 33,000)
One Horsepower is basically the work or energy required to lift 33,000 ft lbs per minute and as noted in the above formulas there are many different ways to achieve the equation required to generate 1 Horsepower.
The technical definition of acceleration is defined in Sir Issac Newton’s 3 Laws of Motion. Newton’s Second Law of Motion describes the relationship between force, mass, and acceleration; where acceleration is produced when a force acts on a mass. The basics of Newton’s 2nd law of motion explains that an object will change velocity if it is pushed or pulled with a force. This law states that if you apply a force to an object, it will accelerate, or change its velocity, and it will change its velocity in the direction of the force. The greater the mass of the object being accelerated the greater the amount of force needed to accelerate the object. So Force = Mass x Acceleration.
The product of the mass and velocity is the momentum of the object, which Newton refered to as the "quantity of motion". Since acceleration involves the momentum of the object that force is being applied to, the acceleration of an internal combustion engine is primarily determined by the momentum of the piston, which is the object that force is being applied to by the process of internal combustion. Simply stated the speed of a full 360 degree rotation of the piston affects the speed of crankshaft rotation and the speed of crankshaft rotation is what determines how fast an engine can accelerate through the rpm range. Technically, an engine that accelerates faster will deliver more power in time since the engine is able to reach maximum power at a faster rate of speed.
Acceleration Examples: To better explain the importance of engine acceleration rates, take 2 engines that both make 900 horsepower at 8500 rpm and monitor the time it takes for both engines to reach 8500 rpm on the tachometer. The engine that gets to 8500 rpm faster can deliver more power in time as it is able to process the 900 horsepower faster. Furthermore, if one engine reaches 8500 rpm in 2 seconds and generates the maximum 900 horsepower and the other engine is at 7500 rpm in 2 seconds, where it makes 800 horsepower the slower accelerating engine is delivering 100 less horsepower at the same time. The faster an engine reaches the RPM that maximum power is developed, the more engine power (torque & horsepower) can be delivered.
In the wonderful world of high performance racing engines, an engine that has a fast acceleration rate is able to process air & fuel quickly through the 4 cycles of the internal combustion process, thereby generating more power at a faster rate in time.
Engine acceleration rates are primarily dependent on the function of the cylinder head which is the most important engine component for delivering horsepower, torque and a rapid rate of engine acceleration.
It is the “Acceleration Rate of the Engine” that is most important for maximizing the full potential of a high performance racing engine. This is why the most technologically advanced racing engines such NHRA Pro Stock Engines, NASCAR Engines and Formula 1 Engines have very fast engine acceleration rates.
Dynamometer Testing Horsepower & Torque
To better understand the phenomenon of engine acceleration rates consider that horsepower and torque testing procedures on a rear wheel chassis dynamometer are performed in final drive, where there is a 1:1 transmission gear ratio. Testing in 3rd gear with an automatic transmission or 4th gear in a manual transmission requires a very slow rate of engine acceleration which makes it easier for the engine to cycle air, fuel and exhaust to produce power. The problem with this form of power testing is that the real world of engine dynamics are quite different as the required rate of engine acceleration is much faster in 1st gear and 2nd gear which makes it more difficult for the engine to process air, fuel and exhaust. In effect, engines with slow acceleration rates make less power in 1st and 2nd gear do to difficulties associated with processing air and fuel at a faster rate of engine acceleration. This is also why these engines are poor performers on the track.
The problems that people refer to with respect to developing good power on the dynamometer and having poor performance at the track is usually do to a slow rate of engine acceleration, which is usually a problem with the function of the cylinder head.
Rolling Thunderz specializes in developing sophisticated cylinder head technologies that deliver fast rates of engine acceleration as proven by the performance of our customer’s cars that are equipped with Rolling Thunderz cylinder heads.
Author of The Read for Speed
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