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Old 01-03-2003, 01:49 PM's Avatar is offline Moderator
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'Horsepower' for an internal combustion engine is calculated by

(Torque, ft-lb) X (RPM) / (5252).

At very low rpm, it is difficult to generate much horsepower even with high torque as the equation shows. As the RPM increases power goes up proportionltely. These engines all have the characteristic that they produce torque poorly at low speed but build torque as speed increases. This torque increase is a function of everything discussed on this board; compression, cam timing, ignition, induction design, exhaust design, component strength, etc. etc. Any engine has a peak horsepower -- an rpm value at which the power available from the engine is at its maximum. An engine also has a peak torque (not necessarily at the peak power point, strange as it might seem) at a specific rpm due to the above described design considerations. You will often see this expressed in a brochure or a review in a magazine as "320 HP @ 6500 rpm, 290 lb-ft torque @ 5000 rpm" (the figures for the 1999 Shelby Series 1). When people say an engine has "lots of low-end torque," what they mean is that the peak torque occurs at a fairly low rpm value, like 2,000 or 3,000 rpm. You might think that if an engine produces 100hp @ 5,000rpm, it should produce at least 200hp @ 10,000rpm and 400hp @ 20,000rpm, and so on indefinietly. Unfortunately, as speed increases, engines begin to fail at making torque due to the physical limitations of moving the valves and pistons, getting the gasses in and out, igniting and completely burning the charge, etc. Thus at some point the torque curve begins to fall off and the engine produces less power at increasing speed.

Another thing you can see from a car's horsepower curve is the place where the engine has maximum power. When you are trying to accelerate quickly, you want to try to keep the engine close to its maximum horsepower point on the curve. That is why you often downshift to accelerate -- by downshifting, you increase engine rpm, which typically moves you closer to the peak horsepower point on the curve.

The 4 cylinder and V-8 you mention both suffer from the above limitations.

The more cylinders, and the bigger the bore, the more torque can be genterated at a given speed. The rule of thumb is 'There is no substitute for cubic inches!'

The smaller engine can create only about half the torque of the engine with twice the cylinders so can only generate about half the power. Other than that, there isn't any difference in how they work.

[ January 03, 2003: Message edited by: ]</p>
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