Some of this pertains to gassers but alot of it works for us too
from s10planet.com and found on www.coldspeed.net
Horsepower VS. Torque
Engines don’t make horsepower; they convert fuel into torque. Torque is the twisting force imparted to the crank flange and transmitted to the transmission and the rest of the drive train. To some degree torque is the grunt that gets things moving and horsepower is the force that keeps things moving. An engine is most efficient at its torque peak wherever that happens to occur. Below the torque peak engines generally have more than enough time to fill the cylinders while above the torque peak they don’t have enough time to completely fill the cylinders. This is generally beneficial in that it lets engines produce most of the desirable grunt work (torque) at lower engine speeds which means reduced wear and tear and better fuel economy. The ability to extend an engine’s speed range allows it to stretch that torque curve out farther provided the high speed efficiency is there to make horsepower.
Power is torque multiplied by engine speed to produce a measurement of the engine’s ability to do work over a given period of time. The story of its origin is well known, but worth repeating briefly. In the 17th century, steam engine inventor James Watt sought a way to equate the work his steam engine could perform to the number of horses required to perform the same task. Watt performed simple tests with a horse as it operated a gear driven mine pump by pulling a lever connected to the pump. He determined that the horse was capable of traveling 181 feet per minute with 180 pounds of pulling force. This multiplied to out to 32,580 pounds-feet per minute which Watt rounded off to 33,000 pounds-feet per minute. Divided by 60 seconds, this yields 550 pounds-feet per second which became the standard for one horsepower. Thus horsepower is a measure of force in pounds against a distance in feet for a time period of one minute.
By substituting an arbitrary lever length for the crankshaft stroke you can calculate the distance traveled about the crank axis in one minute times engine speed (rpm) and known torque to arrive at the formula for horsepower:
Horsepower = distance x rpm x torque
Horsepower = rpm x torque
Because torque and rpm are divided by 5252, torque and horsepower are always equal at 5252 rpm. If you solve the equation at 5252 rpm, the rpm value cancels out, leaving horsepower equal to torque. If you plot torque and horsepower curves on a graph, the lines will always cross at 5250 rpm (rounded off). If they don’t, the curve is undoubtedly bogus.
So torque is the static measurement of the how much work an engine does while power is a measure of how fast the work is being done. Since horsepower is calculated from torque, what we are all seeking is really the greatest possible torque value over the broadest rpm range we can get. Horsepower will follow, and it will fall in the engine speed range dictated by the many factors that affect the torque curve.
Displacement is the easiest way to achieve torque. Very large cylinders and a long stroke offer the greatest cylinder volume and overall piston area for the fuel charge to push against the crankshaft or lever if you will. Big block engines in the 400 to 500 cubic inch range deliver tremendous torque and they are easier on parts for the same amount of power output. High displacement, stationary industrial engines that produce tremendous amounts of torque are typically quite large. The mass and bulk of one of these engines makes extremely large displacement engines impractical for use in cars, hence we are limited to displacement values that are easily packaged within the confines of your typical automobile engine compartment. The practical limit established itself between 400 and 500 cubic inches for most large automobile engines. Hot rodders have stretched this out to about 800 cubic inches with highly modified cylinder blocks and crankshaft strokes, but these engines are not practical or economical for general high performance applications.
This leaves us searching for ways to increase torque in smaller engines by increasing efficiency through the manipulation of mechanical components, gas dynamics and thermodynamics to increase and harness cylinder pressure. There are many ways to do this, but most involve tradeoffs somewhere in the power curve. So to a great degree we are forced to build engines for greater efficiency within a chosen engine speed range. Some combinations will function very well at low speed, others will be strong in the mid-range and still others will only run hard at high rpm. The key is selecting the combination of components that will stretch and fatten the torque curve (efficiency) as much as possible in the driving range we prefer. Our saving grace is the relatively forgiving nature of internal combustion engines wherein torque dissipates gradually as engine speed increases. As long as the induction system can carry the airflow demand created by the cylinders at high engines speeds the torque curve will remain broad. This allows engine speed and horsepower to carry the engine farther in the rpm range before the net effect of induction restrictions at high engine speeds chokes off efficiency. The following are some basic methods for increasing torque and thus horsepower across the typical range of modern performance engine speeds.
Friction robs a great deal of power from an engine. The greatest friction losses are caused by the pistons and piston rings. We overcome this with meticulous cylinder wall and piston preparation. Cylinder blocks that are bored and honed with torque plates in place always contribute to a reduction in friction. This practice reduces cylinder wall distortion caused by head bolt clamping forces; thus the piston travels in the same properly sized bore throughout its stroke and the piston rings are not subjected to changes in tension due to wall distortion. The piston manufacturer’s recommended skirt clearances should be followed in most cases because they have spent countless hours developing a skirt that stabilizes the piston and the ring pack in the bore with minimal friction.
A smooth bore generally improves ring seal and reduces friction. The best honing finish depends on the type of rings and the final application. The piston ring manufacturer’s recommendations are your best bet. Rings should be hand fitted with ring gaps set to the minimum recommended clearance. Piston rings should also be very carefully checked in each individual piston to ensure the minimum recommended side clearance. If a ring is sticking due to too little side clearance, friction will soar. If a ring is too loose, it may flutter and drag intermittently while bleeding off precious cylinder pressure. Second rings should be fitted with slightly larger gaps than top rings to ensure the ability of trapped gases to escape rather than cause the top ring to flutter and loose seal.
Mechanical efficiency may also be improved with proper bearing clearances. Set your bearings to the loose side of recommended clearances and run a low-viscosity synthetic oil. This is worth some torque and almost always shows at least a 10 horsepower gain on the dyno. One way to improve mechanical efficiency that most people ignore is through the use of special anti-friction coatings for pistons, rings and bearings. These coatings are available in do-it-yourself kits from mail-order houses such as Summit Racing. When properly applied, they can get you another 10 horsepower or so. The ideal application would use coated components with optimized clearances and a good synthetic oil for maximum friction reduction. All together there may be as much as 20 horsepower available with the right combination of friction reducing ingredients.
Another component of friction reduction is the preparation of the cylinder block bearing saddles and the crankshaft. Cylinder blocks should be align honed to minimize frictional losses. This gives the crankshaft a straight set of bearings to run on. Likewise, the crankshaft must be straightened to eliminate runout and the entire reciprocating assembly must be properly balanced to minimize drag created by uneven forces.
More torque may be gained if you use a well designed oil pan with an effective oil scraper and aerodynamic shaping of the crank throw leading edges. Chevy builders should avoid the temptation to use a high volume or big block style oil pump. Use a properly clearanced small block pump and set it to deliver only the pressure necessary to provide optimum lubrication. Most small blocks never need more than about 60 psi, even at high rpm. Excessive oil pressure or a bigger pump with taller gears robs power throughout the entire rpm range. Moreover, excessive oil pressure with a hydraulic camshaft can also force lifter pumpup and initiate intermittent valve float. Also consider the pumping losses caused by the induction and exhaust system. This should lead you to careful consideration of each system because the engine’s ability to work efficiently is largely controlled by these systems.
This is really combustion efficiency and it all has to do with getting the correct air fuel mixture in a well sealed, active combustion chamber with a properly timed high energy spark. Spark timing and chamber shape influence this tremendously, but most engines make optimum power at wide open throttle with a 13.1:1 air fuel ratio give or take a couple tenths depending on the engine’s overall efficiency. You want your carburetor or fuel injection system to optimize this air fuel ratio as fast as possible when you go WOT, and you want to maintain that fuel curve throughout the rpm range. This can be no small trick with a carburetor and is certainly easier with electronic fuel injection where oxygen sensor monitoring of the exhaust gas allows the computer to continuously adjust the fuel ratio.
Engines with a large quench area and a smaller combustion chamber are generally more combustion efficient. The quench area is the flat top portion of the piston adjacent to the valve reliefs. The flat portion of the piston deck corresponds to the flat portion of the chamber roof. When the piston approaches the cylinder head at high speed, this area squishes the charge toward the ignition source or spark plug to promote turbulence and a faster burn. Some studies suggest that you can have too much quench, but most engine builders feel that optimizing combustion chamber quench is a proven path to power. This is best accomplished on engines with steel connecting rods where rod stretch is not a factor. Aluminum rods can stretch and may cause the piston to contact the head. On many steel rod engines you can juggle the head gasket thickness and the piston deck height to maximize quench. Steel rods allow the quench clearance to be set as tight as .030-inch or slightly less in some cases. This promotes maximum charge activity to increase combustion efficiency.
If you have the luxury of custom pistons, your piston manufacturer can also move the ring package higher on the piston to provide greater piston stability and minimize crevice volume. This is the very small area between the piston and the cylinder wall above the top ring. Because all pistons experience some small degree of rocking as they reverse direction, the piston is generally machined smaller or tapered above the top ring land to keep it from hitting the cylinder wall on rock-over. The space created here is very tight and can tend to collect unburned or partially burned gases that intermittently mix with the fresh incoming charge to contaminate the mixture or alter the air fuel ratio ever so slightly. Paying close attention to these kinds of details can add up to a significant torque bonus. When we add up all the small amounts of torque we recover from these details you’ll be surprised how much total power you have really gained.