Engines don't make horsepower; they convert fuel into torque. Torque is the twisting force imparted to the crank flange and then transmitted to the transmission and the rest of the drivetrain. 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; 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 that 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 18th 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 out to 32,580 lbs-ft per minute, which Watt rounded off to 33,000 lbs-ft per minute. Divided by 60 seconds, this yields 550 lbs-ft per second, which became the standard for 1 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 around the crank axis in one minute multiplied by engine speed (rpm) and known torque to arrive at the formula for horsepower:

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.

Torque is the static measurement of 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 the greatest-possible torque value over the broadest-possible rpm range. Horsepower will follow suit, and it will fall in the engine speed range dictated by the many factors that affect the torque curve.

Increased displacement is the easiest way to achieve increased 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. 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 is between 400-500 cubic inches for most large automobile engines. Big-block engines in this range deliver tremendous torque, and they are easier on parts for the same amount of power output. Car crafters have stretched displacement out as far as 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 some sort of tradeoff somewhere in the power curve. 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 speeds, others will be strong in the mid-range, and still others will only run hard at a high rpm. The key is selecting the combination of components that will stretch and fatten the torque curve (improve 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 engine 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.

Mechanical Efficiency

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 a torque plate 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-fit 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.

One way to improve mechanical efficiency that most people ignore is through the use of special antifriction 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, the coating 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. Altogether, 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 on which to run. 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. Small-block Chevy builders should avoid the temptation to use a 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 a high rpm. Excessive oil pressure or a bigger pump with taller gears robs power throughout the entire rpm range. 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. See the accompanying sections for further discussion of these subjects.

Thermodynamic Efficiency

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. 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 them 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, in which 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 cylinder-head chamber roof. When the piston approaches the cylinder head at high speed, this area squashes 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. 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 0.030 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. A higher ring package will also reduce 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 directions, the piston is generally machined smaller or tapered above the top ring land to keep it from hitting the cylinder wall during this rocking. The space created here is very tight and can collect unburned or partially burned gases; these intermittently mix with the fresh, incoming charge and 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 you add up all the small amounts of torque that you gain from these details, you'll be surprised at how much total power you have really gained.

Compression Ratio

Much like increased engine displacement, higher compression ratios are a sure path to increased torque. The overriding factor is, of course, fuel quality and detonation. There are numerous factors to consider here. Finer atomization of the fuel and more precise control of air/fuel ratios via electronic fuel injection has allowed O.E.M. manufacturers to increase compression ratios above 10:1 in some late-model, high-performance cars. The very latest LT4 Corvette engines are actually sneaking up on 11:1 compression ratios again because of the inherent efficiency of electronic controls and the combustion-efficiency gains made in the cylinder heads and induction system. Carburetors are less precise, but there are other ways to increase torque with higher compression in carbureted engines running 92-octane gasoline. Many street engine combinations running a big cam for top-end power experience a significant loss of low-end torque. This occurs because the intake valves close much later when the piston is farther up the bore. Thus, the dynamic compression ratio is less than the theoretical compression ratio that assumes full-stroke piston travel. If you are going to run a big cam, one of the bonuses is that you can increase the compression ratio slightly without incurring a detonation penalty. The increased compression will boost the low-end torque and extend the top-end power range. Experienced engine builders have found that 9:1 compression engines require at least a 270-degree (advertised duration) cam. On the other hand, 10:1 engines are happy with a 280-degree cam, and a 290-degree cam will allow you to run nearly 11:1 compression. Depending on other engine variables, such as combustion-chamber shape, bore diameter and ignition timing, some engines will detonate under these conditions. In these cases you need to go to a smaller cam or run slightly less total timing. In any event, the idea is to use as much compression as possible relative to the cam profile in order to gain low-end torque without detonating.

Camshaft Timing

When you consider valve-timing events, you also have to consider all the other elements acting on the fuel charge and combustion gases in the cylinders. An earlier-closing intake valve starts building cylinder pressure sooner. This increases low-speed torque due to greater cylinder pressures, but it means that the engine is having to work harder to compress the charge. As previously explained, a later-closing intake can enhance top-end torque at the expense of low-end torque, but you can get most of the torque back on the low end with an increased compression ratio. What you look for is a cam profile that promotes increased cylinder filling with earlier intake opening so that the valve is farther off the seat during the early portion of the intake stroke. Then you want to delay the exhaust-valve opening as much as possible to take advantage of all the energy you can from the combustion process before you blow down the cylinder. A quick-opening exhaust valve is helpful here, but, again, there are trade-offs.

This combination builds good torque but tends to increase valve overlap at TDC. This is where the cam lobe separation angle takes control. The lobe separation angle is the angle between the peak of the intake lobe and the peak of the exhaust lobe expressed in cam degrees. Tighter lobe separation angles (less than 110 degrees) make more torque and horsepower, but, with more overlap, the engine experiences poor idle quality and high fuel consumption. Opening up the lobe separation angles (more than 110 degrees) broadens the power band while improving idle and part throttle characteristics. With these wider lobe separation angles, peak torque and power are generally reduced, but the engine becomes very smooth and drivable.

Most street and high-performance engines will perform best when overlap is between 35 and 70 degrees (measured from intake-valve opening to exhaust-valve closing) with the duration as short as possible within the overlap guidelines. If you choose 50 degrees as a middle-of-the-road overlap figure for a pretty hot street machine, the shortest possible duration with this overlap will produce the most torque. You could make more torque with a bigger cam--but only at the expense of driveability and economy.

Cylinder Head Selection

Cylinder heads are where the power is, but there are limitations. You are generally limited to what's available, and, for most people, porting is a luxury. Increased airflow always means more top end power. For the most part, it is better to run a larger valve, if possible, and a shorter camshaft. This allows the larger valve opening to do the work of filling the cylinder while the cam remains relatively mild. Torque will be increased. Bigger valve heads may give you more overall torque than a simple cam swap. If your heads have stock-sized valves and you put in a larger cam, you will have to spin the engine faster to make the same torque and power.

That's the simplified version, but there are other considerations. The length, area and volume of the intake system all affect the engine's output. Most hot street engines will benefit from bowl porting and a good valve job, but you should avoid significantly enlarging the ports. The minute you start enlarging the port, you are bleeding off potential torque. Unless your engine will spend a lot of time at elevated engine speeds, don't start hogging out those ports.

If you have the ability to modify heads, you can extract more torque and horsepower by porting for efficiency, but the process is tedious at best. Street enthusiasts aren't generally in a position to flow heads and check port dimensions. If you are, the intake port area should be about 80 percent of the valve area, and the port should enlarge at a 2- to 4-degree taper out to the plenum. This is pretty standard on most available heads.

Exhaust ports should not be enlarged significantly unless you're running nitrous oxide, which produces a greater exhaust requirement. Most good aftermarket headers have been sized and built to create a negative pressure at the exhaust valve during overlap. This ensures good cylinder scavenging and reduces the potential for exhaust reversion: Exhaust gas speed remains high, and the pulse waves are tuned to aid the exiting exhaust charge.

Exhaust Systems

Much of your cylinder head work is diminished if you are running stock exhaust manifolds and mufflers. Exhaust headers are louder and require more attention than cast iron manifolds, but they offer substantial power advantages. While most aftermarket performance headers are of the standard four-into-one collector design, many street applications could make better use of the old four-into-two-into-one Tri-Y design, which broadens the torque curve and is still capable of making power up to about 6000 rpm. These headers are more expensive and time consuming to produce; hence, they are only available from a few manufacturers.

One of the biggest mistakes made in exhaust-header application is the selection of primary tubes that are too large. Big primary tubes are only necessary to carry the gas volume generated at high engine speeds. Most headers with 1-1/2-inch primary tubes will carry an engine well into the 300hp range, while 1-5/8-inch headers can support up to 400 horsepower, and a little beyond in some cases. This depends a great deal on displacement and engine speed. We have seen 1-3/4 headers support up to 550 horsepower without affecting power on a single four-barrel 350 Chevy running at 7500 rpm. Meanwhile, a 480hp, twin carburetor 302 Ford running at 8000 rpm gained 13 horsepower by switching to 1-7/8-inch primaries. It is usually better to err on the small side for a street engine so that torque remains strong. Pipes that are too large generally hurt the bottom end more than small pipes hurt the top end.

Exhaust-system backpressure--as a result of restrictive mufflers, catalytic converters and multiple sharp bends in the exhaust system--can be severely detrimental to good torque and power. Exhaust-pumping losses caused by restrictive exhaust backpressure can be substantial in some applications, and the problem increases dramatically with engine speed. Performance camshafts are also rendered less effective because backpressure typically negates any improved cylinder scavenging during the overlap period. The Catch 22 with exhaust systems is your own personal comfort with the sound level of the mufflers. You can run mufflers with virtually no restriction, but the drone may drive you crazy the first time you take a 100-mile trip. The best approach for most street engines is to complement all the other torque-building efforts you have applied by using a Tri-Y header with at least 2.5-inch diameter exhaust pipes and the least restrictive muffler you can stand relative to sound levels. A crossover tube to balance the pulses from each cylinder bank can help smooth the sound a bit, and it may add a very slight amount of torque depending on the rest of the application. It is usually worthwhile.

Ignition Timing

Incorrect ignition timing has the potential to stall most of your efforts to improve torque and horsepower. Cylinder pressure or best combustion pressure provides its maximum effect at about 12 to 18 degrees after the piston has passed TDC (top dead center). A faster burning charge will require less timing, while a slower burning charge needs more timing. If you have concentrated all your engine building and tuning efforts toward building maximum cylinder pressure (relative to fuel quality and detonation resistance), at the end of the compression stroke you will have a fast-moving flame front that needs less timing. If you have compromised cylinder pressure in some way, the charge will burn more slowly and require more timing. If you have done your job well by increasing breathing efficiency and the compression ratio, you will need less overall timing.

In most cases you will have selected a big cam to complement your desired power combination. This usually reduces breathing efficiency at low engine speeds, while enhancing it at high engine speeds. To make this work to its best advantage, you should alter spark advance to fire the plug sooner at low engine speeds. A competent distributor shop or tune-up shop can set your advance curve to take maximum advantage of your combination. This is absolutely critical to taking full advantage of all your other modifications.

Carburetor Sizing

Carburetor selection is frequently an afterthought based on what a friend is running or what is available for the cheapest price. Carburetors are typically chosen according to an engine's displacement and rpm range. To some degree, this has made 750-cfm Holleys the default carburetor for all applications. This only works because the carburetor has the ability to meter fuel over a broad range, but carburetor sizing plays an important roll in building optimum torque and horsepower. Smaller carburetors are commonly suggested for building torque, because their smaller venturis keep air velocity high to promote good fuel atomization. If you want to broaden the power band to retain good torque at the low end and extend power at the top end, you can make a case for a larger carburetor if it is teamed with the appropriate mix of components. The primary reason for keeping venturi size small is to maintain air speed through the boosters. This is especially critical with single-plane manifolds and larger cams, which generate weak booster signals at low rpm and the resulting loss of atomization quality and metering accuracy. This results in reduced torque output and poor driveability, but correcting it with smaller high-speed venturis may limit power at the high end.

Holley's annular discharge boosters offer the increased sensitivity to deliver low-speed booster sensitivity in a larger venturi bore while allowing greater airflow at high engine speeds. Different variations of these boosters must be properly applied to get the greatest gain, so the carburetor has to be custom-built in the aftermarket to match your application.

Cold Air Efficiency

Finally, anything you can do to enhance cool air flow into the engine will be good for torque and horsepower across the entire rpm band. Remotely sourced inlet air is almost always cleaner and cooler than engine compartment air. Use an aluminum intake manifold with the carburetor exhaust heat passage blocked off. Manifolds with the runners separated from the valley keep the charge cooler. Duct your inlet air from outside the car and keep the ducting insulated from engine compartment heat. Make your inlet ducting at least 4 inches or larger in diameter, and keep the path as short and unrestricted as possible. Be sure to duct the air through a high-flow air filter prior to entering the carburetor or throttle body. These simple modifications can increase torque from 3 to 5 percent, and they will also increase power at high engine speeds due to unrestricted airflow and a cooler charge.