Stage 1: By simply installing COMP Cam's Pro Magnum 1.52:1 ratio roller rocker arms and Ma
Horsepower and torque are the two common units of measurement to describe the capabilities of an engine. Of course, if a little of anything is good for most hot rodders, a bunch is better, so the larger the horsepower and torque numbers are the more we like them. That, however, usually leads to the debate about which is more important.
Defining The Differences
The first definition we have to get out of the way is work, which is defined as changing the position of an object against an opposing force. As an example, with gravity as the opposing force, if you lift a 20-pound manifold off the floor and put it on a shelf 5 feet off the ground, you've done 100 ft-lb of work (20 x 5 = 100). Now factor in time and you've got horsepower, which is work done over a period of time.
James Watt, he of steam engine fame, came up with the notion of horsepower because he needed a way to sell his new invention as replacements for horses that were the power source of the 18th century. There are a number of versions concerning Watt's method of determining what constituted horsepower. One involved a typical horse walking in a circle while hitched to capstan that turned a mill, pump, or whatever. Somehow, Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with that number is suspect (which we'll get to). Watt observed that the horse traveled at approximately 181 feet per minute; he multiplied that by the 180 pounds of force the horse produced (181 x 180) and came up with 32,580 ft-lb/minute, which he rounded to 33,000. Another study involved ponies used for bringing coal up from underground mines. Through a somewhat convoluted process, Watt determined the average horse could raise 200 pounds 165 feet in one minute, which equals 33,000 ft-lb/minute.
Comp's Pro Magnum rockers feature rollers in the fulcrums as well as the tips.
With all that said, we have to point out that Watt was selling steam engines with a money-back guarantee and he was not about to take a steam engine back or return any money. So it doesn't come as a surprise to find there were those who claimed that Watt's definition of one horsepower was a far greater amount of work than a horse was capable of and that his steam engines were vastly overrated. In fact, in 1843 Frederick Simms prepared a paper stating that a horse that was worked at 33,000 ft-lb/min was likely to drop dead in short order. Nonetheless, Watt's numbers of 33,000 ft-lb/minute are what we work with today when measuring horsepower.
Compared to horsepower, torque is a fairly simple concept-it's twisting or turning effort. However, it may or may not result in motion. At this point, we need to make an important distinction between work and torque. As we said, work implies movement and is measured in ft-lb, while torque is the ability to do work and is measured in lb-ft. Think of torque this way: If you pushed on a crank handle that was 5 feet long with 20 pounds of pressure, you would be applying 100 lb-ft of torque whether or not the crank was turning.
A new ZZ4 was strapped to the dyno and its baseline performance was established before any
Which Is More Important?
From a driver's standpoint, torque is what you feel when you get shoved back in the seat, and a car will accelerate the hardest when the engine is at its torque peak. In most cases, engines develop maximum torque at low to intermediate speeds. This is primarily due to the fact that the cylinders have enough time to fill with a combustible mixture of air and fuel. The more air/fuel mixture there is in the cylinders, the higher the pressure is from combustion. But as the engine speeds up, volumetric efficiency-or how well the cylinders fill on the intake stroke-decreases and consequently torque declines.
So, if torque is what makes things move, why is horsepower important? Think of this the next time you see a big truck take off from a standstill. A large diesel engine may make well over 1,500 lb-ft of torque while only producing 400 or so horsepower; that's because its rpm range is very limited. So, while a big truck engine may be capable of doing a tremendous amount of work, it does it slowly. In simple terms, torque is the ability to do work-the more torque, the more work can potentially be done-but horsepower determines how quickly that work is accomplished.
As most SRM readers know, the device used to determine horsepower and torque is a dynamometer. But what some may not realize is that a dyno only measures torque and speed; horsepower is determined by the formula horsepower = torque x rpm / 5,252. Because of this, you'll see that horsepower and torque lines on a dyno graph always cross at 5,250 rpm (or at least they should).
The supports inside the stock rocker covers require modification due to the width of the P
Dyno results compare the before (broken lines) and after (solid lines) performance. Note w
Stage 2 consists of a new Thumpr camshaft with 227/241 intake/exhaust duration (at .050 li
Comp's beehive springs have a number of advantages, not the least of which is reduced mass
The dyno chart speaks for itself. Stock ZZ4 cams are hydraulic rollers with 208-degrees du
Several things are worth considering on this dyno chart. Note that torque is virtually the
A slightly hotter Mutha' Thumpr cam is used for the Stage 5 combination.
Making Bigger Numbers
There's an old saying that there's no substitute for cubic inches, and increasing displacement is certainly one way to increase performance. However, if that's not an option, there are others. There are a considerable number of modifications that can be made to most engines to enhance performance, but the challenge becomes minimizing the sacrifices that are often involved. Modifications that increase power at one end of the rpm scale often reduce it at the other; in other words, increased high rpm power may come with a reduction on the low end. The trick is to target the engine's intended use, and for street engines it means selecting parts that make a broad, flat torque curve in the rpm range the engine will operate the majority of the time.
Automotive engines use four strokes of the piston (intake, compression, power, and exhaust) during two revolutions of the crankshaft to produce power. For applications like drag racing or chasing Bonneville records, where wringing the absolute maximum potential from an engine is required, there are a multitude of internal modifications that can be made to address those four strokes. But, for the most part, street engines often receive "bolt-on" improvements. As an example, the intake function is often addressed by changing the intake manifold and carburetor. To take full advantage of that change, the cfm of the carburetor, the capacity of the intake manifold runners, must be compatible with the ports in the heads, valve sizes, compression ratio, and cam specifications. Everything about the engine's operation is related and components must be compatible.
The ZZ4 aluminum heads are replaced for even more of a power increase. COMP's Stage 4 pack
Another sure method of increasing power is raising the compression ratio. At one time, "shaving" heads to reduce the size of the combustion chamber, the installation of thin head gaskets, pop-up pistons, and other tricks were used to raise compression. And while the rewards in performance were great, so was the potential for engine-damaging detonation if high-octane fuel wasn't used-a problem that became even more pronounced with the advent of catalytic converters and unleaded gas. While we all know the octane rating of gasoline isn't nearly what it used to be-thanks to the more precise control of air/fuel ratios afforded by electronic fuel injection and computer-controlled ignition curve-compression ratios of contemporary OEM engines are again on the rise. But there are practical limits to compression ratios for non-computerized applications.
Changing cam profiles also have a huge impact on performance. For low-end power, closing the intake valve early on the intake stroke helps build cylinder pressure early, but the trade-off is poorer filling of the cylinders at high speeds. On the other hand, increasing duration, or the length of time the intake valves are open, also increases top-end performance. While it seems odd, leaving the intake valve open even after the piston is on the rise on the intake stroke helps the cylinder fill at higher engine speeds. The reason is the columns of air in the manifold runners have enough inertia to keep the mixture flowing into the cylinders even as the piston is beginning the compression stroke. Delaying the opening of the exhaust valve to get every bit of energy from the expanding gasses also helps. Of course, one of the secrets to making this work is an intake manifold with runner lengths and capacity that support the increased duration. Then there's the downside-at low engine speeds, the mixture is literally blown out of the cylinders (which helps to cause that lumpy idle we all love) while also bleeding off cylinder pressure, lowering the engine's effective compression ratio. The upside is that longer-duration cams also make it possible to increase the static compression ratio, which then helps to salvage lost low-end power. There are certainly other factors involving cam timing that come into play, including valve lift, exhaust valve timing, overlap (the period of time both valves are off their seats), and lobe separation (the number of degrees between the intake and exhaust cam lobes). We'll look at the impact of these changes shortly.
The heads used in Stage 4 are 23-degree Pro Action heads from Racing Head Service.
When it comes to the power stroke, there are more significant areas of concern than you might imagine. For many street engines, common modifications are aftermarket heads and improved ignition, but there certainly are other changes that can yield positive results. Improved sealing of the combustion chamber, which means everything from the design of the pistons and rings to the cylinder wall finish, various coatings to confine heat and reduce friction, and the design of the heads' combustion chambers all impact burning of the fuel, which produces power.
Of course another important component in the power-producing process is the ignition system. Usually, changing ignition systems by itself won't noticeably increase power (unless it's replacing one that is malfunctioning), but for optimum performance the spark has to be adequate to ignite the mixture in the cylinders reliably, and initial timing as well as advance curves must be correct not only to take advantage of the engine's modifications, but to complement the fuel being used.
Exhaust is the final stroke of the process of power production. The fact is, if you can't completely clear the cylinders of spent gasses, there's less room for fresh, combustible mixture and that would obviously have a detrimental effect on performance.
Valves in the RHS heads are 2.02-inch intake and 1.60-inch exhaust compared to the ZZ4's 1
Headers are generally preferred on street rods over factory-type exhaust manifolds for a variety of reasons-including looks, fit, and performance-but like everything else we've discussed, they must be matched to the other engine modifications. The basic style of header, primary tube diameter and length, collector type and size, exhaust pipe diameter, and muffler design all have to be considered.
Pulling It All Together
While we've talked about the four strokes of an engine from an individual standpoint, we've also tried to show the relationship between them and make the point that engine modifications must all be compatible with one another. A big cam with a small carburetor and a restrictive intake manifold, or a big carburetor, ported heads, and a stock cam aren't good combinations. But we can show some combinations that work great, thanks to COMP Cams.
Recently, the crew at COMP performed a variety of dyno tests on GM's most popular crate motor, the ZZ4. These 350ci small-blocks are factory-rated at 355 hp @ 5,250 rpm with 405 lb-ft torque at 3,000 rpm. While those are healthy numbers, as a result of the company's research, COMP Cams has come up with five individual coordinated kits to boost the performance of these engines while taking the guesswork out of picking parts. Each kit has been painstakingly studied to make it easy to pick the level of performance that's just right for your application. Here's a look at them all.
The Stage 4 combination provided a hefty increase in torque and horsepower.
Stage 5 uses the same heads as Stage 4 with the Mutha' Thumpr cam from Stage 3.
With this combination, torque was off very slightly-just 2 lb-ft-and horsepower was up 9.