|The MGA With An Attitude
WHAT IS POWER? (Engine Horsepower) - PP-106
Power is a unit of work, a rate of energy transfer. In the English system we think in terms of Horsepower. In the Metric system the unit of measure is more commonly Kilowatts. The conversion is roughly (in round numbers) 1-HP = 750-Watts (3/4-KW). Inversely, 1-KW = 1.33-HP (4/3-HP). Since this web site deals with a vintage MG (English heritage from the 50's), we will be discussing calculations in units related to HP, and let anyone else do the 3/4 or 4/3 conversion for Kilowatts. In mechanical terms, 1-HP = 550 ft-lbf/s. Read this as Feet times Pounds Force per Second (commonly stated as Foot Pounds per Second). This amount of work will raise a 550-pound weight 1-foot in 1-second, or a 55-pound weight 10-feet in 1-second. That is, we are commonly speaking in terms of some force applied over some distance in some amount of time.
For the record "Pound" in these calculations is always a unit of force, not weight. An object that weighs 1-pound on the surface of the earth will weigh nothing when left to free fall in space. The Pound Force is a multiple of mass times acceleration. The object that weighs 1-pound on earth is applying a force of 1-lbf on whatever is supporting it due to earth's gravity. Gravity is expressed in terms of acceleration. Earth's gravity is equal to acceleration of 32-feet per second per second, or 32-ft/s^2. If this same object is in free space, and you apply a force sufficient to accelerate it at a rate of 32-ft/s^2, that force would be 1-lbf. For the purpose of engine horsepower discussion, lb is commonly used in place of lbf, considering that we intuitively understand what is a pound weight on earth. All this gibberish is just to clear up the difference between units of weight and force, so hopefully no one will feel the need to argue about the calculations.
The distances for horsepower functions do not have to be either vertical or in a straight line. Power transmission components in motor vehicles are commonly rotating parts. For example, take a wrench 1-foot long, put it on a bolt, and rotate it 360-degrees (one full turn). The handle end will travel 6.28-ft (2 times pi times the radius). If a force of 1-pound is applied to the wrench for this maneuver, the torque applied to the bolt (or shaft) is 1-lb-ft, and the work done would be 6.28-ft-lb. Notice which unit is written first. Torque is traditionally expressed as lb-ft while work is traditionally expressed as ft-lb. If you apply this to a hand crank with 1-ft radius, and you crank it continuously at one turn per second, you get work being done at a rate of 6.28-ft-lb/sec (0.0114-hp).
One Horsepower is otherwise defined as 550 ft-lb per second (units of power), or 33000 ft-lb per minute. For the suggested rotational radius of 1-ft and speed of 1-turn per second, a force of 87.6 pounds (550/6.28) will generate or transfer 1-HP. Getting into larger numbers related to a car engine, 100-hp at 5000-rpm would be 105-lb-ft of torque.
(100 x 550ft-lb/s x 60s) / 5000rev/min / 6.28ft/rev = 105-lb [at 1-ft radius].
An MGA engine might have about 80-BHP (Brake HorsePower). The word "Brake" refers to an engine brake (dynamometer), so BHP is the power measured directly at the crankshaft. 80-hp at 5600-rpm works out to be 75-lb-ft torque. Power is a multiple of torque and rotational speed. The torque can actually be higher at lower speed with more fuel/air and more pressure in the cylinders, although power will be lower with the slower engine.
A race engine may develop 50% to 100% more power (120-160-hp). Some of this can be a result of running faster, but it does not run twice as fast as the street engine. The rest of the increase is a result of increasing torque by ingesting more air (and matching fuel mix) into the cylinders with each stroke of the pistons. Ingesting and burning more fuel/air results in higher combustion pressure on the piston, leading to more torque on the crankshaft. Since it is difficult to make a vintage engine run at extremely high speeds (like 12,000 rpm for instance), the power output will be closely related to how much air can be encouraged to enter the cylinders on each intake stroke. If a race engine might be tweaked to make maximum power of 150-hp at 7000-rpm, it will need to develop 112.5-lb-ft torque at that speed. This would require stuffing 50% more air into the cylinders with each intake stroke.
Okay, bring in the clowns. This is where the discussion turns to better breathing, hot camshaft, bigger valves, more valve lift, larger carburetors, increasing displacement, free-flow exhaust system, porting and polishing the cylinder head, tuning length of intake runners, increasing compression ratio, and maybe mixing special fuel to deal with higher temperature and pressure. Subsequently the engine must also be made more durable so it will not self-destruct under the conditions of increased stress. When you run out of tricks with free flow air being pushed only by atmospheric pressure, you might turn to supercharging (forced air induction). As combustion pressure and temperature go higher the fuel may need to be a more noxious mix. A dragster engine might use nitro-methane in methanol-alcohol, or even nitrous-oxide. For the 1957 and 1959 land speed record runs in MG EX-181, developing over 300-bhp from 1500cc displacement required an enormous supercharger, and the fuel was 86% methanol laced with nitrobenzene, acetone, and sulfuric ether. Don't spill this stuff on your hands or breath the fumes.
Getting back to more reasonable things related to your street machine, just keep in mind that more power involves higher engine speed and/or more torque. Either of these increases stress on the engine parts. Once you get to the output end of the crankshaft you can consider how to handle the increased torque that needs to run through the clutch, gearbox, propshaft, differential, wheels and tires. Also consider what chassis modifications might be reasonable for suspension, steering, wheels, tires and brakes. Most of these topics are covered in more detail in other articles on this web site. I am rather fond of referring to the MGA as a 1950's vintage car that was built with 1920's technology. When you consider how well these cars work in original form you might appreciate the job done by the factory back in the 50's in getting everything right with components matched to work together with the intended engine displacement, contemporary fuel formula, diminutive vehicle size and desired minimal weight.
We may occasionally refer to this as a "compromise", like selecting tires and brakes that are large enough to do the job but not so large as to result in detrimentally excessive unsprung weight, and keeping cost affordable (competitive) for the consumers. With passage of time comes improved technology and more possibilities for "improving" the cars from the original factory specifications. Today we might install electronic ignition, alternator, radial tires and alloy wheels, increase engine power output a little, add a power brake booster or even electric power steering. With more power on tap we might consider upgrading suspension and brakes to go with stickier tires and stronger (heavier) wheels. We might also consider lower final drive ratio, overdrive or 5-speed gearbox to go with modern expressway speeds. Each such change of parts should be followed with due consideration about matching other parts of the car to maintain a well balanced system.
At some point, preferably before you put too much money into mismatched "upgrades", you should ask yourself and demand a reasonable answer as to how far you intend to go toward turning a nice original vintage car into a more "modern" toy. If you really want a modern sports car you could just buy a Miata. If you really want a Miata that looks like an MGA, you might end up spending a lot of time and money rebuilding the car and end up with reduced resale value. It's your toy, and you can do anything you like with it, and lots of people do modify them in sometimes strange ways just because "it feels good". I can only advise that if you increase power significantly you should follow a synergistic approach and try to maintain a fairly well matched set of components.