The small-block Chevy responds well to exhaust system modifications, but that doesn’t necessarily mean tube headers are required. You may have heard it said that Chevrolet never made an efficient exhaust manifold for the small-block and that tubular headers are essential for optimum performance. That isn’t entirely true and there are many applications where a factory style exhaust system is much more desirable. The fact is that small-blocks can run reasonably well with castiron exhaust manifolds. The trick is to have a well-designed and integrated system. If your application requires quiet efficiency and/or the need to fit into a cramped engine compartment, then cast-iron factory exhaust manifoldsare your most affordable choice.
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The most obvious application for cast-iron manifolds is a street machine that is being built on a strict budget. This is particularly true when the application involves an engine swap where a specific set of headers is just not available. In some cases, an existing header can be slightly modified or rebuilt to fit the engine/chassis combination, but it’s more likely that one of the many types of factory manifolds will fit with very little difficulty. They are all designed to hug the side of the engine closely, and they have either center exit or rear exit head pipe flanges. For engine swappers who desire a clean, quiet installation, factory manifolds are the key. And in many cases, you can boost performance by using the old Corvette exhaust manifolds with larger 21⁄2-inch openings. They are still available through GM (PN 3797901 lefthand w/generator mount on side; PN 3846563 left-hand w/generator mount on end; PN 3814970 right-hand w/generator mount on side) and some of the specialty Corvette restoration houses.
Today’s performance applications can take advantage of emission certified tubular factory exhaust headers designed for the Corvette and for 1986 and later Camaros and Firebirds. Camaro/Firebird headers are manufactured by SLP Engineering and sold by the GM Performance Parts network. Complete exhaust systems are available for these F-body cars, and in many cases a talented fabricator could modify these headers to work in almost any swap. They are a good choice, because they are built from heavy-wall stainless steel.
Standard tubular exhaust headers are a different story entirely, and it’s worth taking a quick look at their development. Engineers have long understood that a major portion of an engine’s energy passes out through the tailpipe in the form of heat. They also realized that while the most effective way of utilizing this energy is through turbocharging, other techniques can be employed to take advantage of the inertia of these spent gasses to help draw in the fresh intake charge. Much of the theory we discussed in the camshaft chapter can be put to good use with the addition of tube headers. The scavenging effect of a properly designed header can assist induction and boost power. The main thing to remember is that although headers do much more than simply reduce backpressure across the engine, there is no guarantee they will make more power. Since the primary tubes separate each of the engine’s cylinders from the others, it offers the opportunity to let each cylinder help the others to draw in a combustible mixture and exhaust spent gasses. During the early search for the benefits of this so-called scavenging effect, header designers discovered that the length, diameter, and shape of both the primary pipes and the collectors had a great deal to do with the header’s ability to boost power. Typically there are two things happening inside a header when the engine is running. As each exhaust valve opens, a pressure wave is created that travels down the length of the primary pipe at approximately 1,700 feet per second. When the pressure wave reaches the end of the tube and exits into the collector, the sudden release of pressure creates a slight suction pulse that travels back up the header tube to the exhaust port where it actually helps to draw out residual gasses and draw in the incoming fuel charge. The suction pulse is further strengthened by the inertia of the exhaust gasses that are moving through the pipe away from the exhaust port at about 300 feet per second.
Naturally, it is difficult to get all of these elements timed exactly right, and that is the chief reason for making all the primary pipes equal in length. Interestingly, this important point is much lauded but seldom practiced. In most cases it just isn’t practical to make the primary pipes precisely equal length. Some headers get really close, while others miss by a mile without even apologizing. As it turns out, the actual benefits are sometimes difficult to discern unless you have access to a dyno or lots of track testing time. In a race motor, optimum header tuning is extremely important because the engine was built with the intention of equalizing and optimizing power output from each cylinder. When a header system is designed and built with this in mind it can operate very effectively. On the other hand, street motors must be built with many other considerations, like sound deadening and fuel economy, so much of the scavenging effect is lost. For that very reason it is less important to have equal length tubes on a street header.
In fact, it’s pointless to look for a street header that has equal length tubes, especially if you’re going to route them through some sort of muffled exhaust system. (What we find is that the reduction in back pressure increases engine efficiency without regard to header pipe length.) Regardless of the claims, it would prove difficult for one manufacturer to clearly demonstrate that his header design is superior to another in a street-oriented application. It is true that street header design is important in terms of pipe diameter and collector length and diameter, but most manufacturers are pretty well dialed in on this stuff and the important differences center more around quality of construction, ease of installation, spark plug access, and the useful life of the header.
This still doesn’t mean that every header on the market is going to be a top performer. The major manufacturers all exercise pretty much the same approach. They know that a good street header is going to use small diameter primary pipes to help keep exhaust velocity high at slow engine speeds. The collectors will generally be longer than those on a race header since the longer collector helps to promote low-speed torque. The reasons for this are complicated, but it revolves around the header’s ability to scavenge the cylinder despite the remainder of the exhaust system.
Race motors follow a completely different formula. While primary pipe size has decreased on street headers, race headers continue to get larger; at least that is the case with top running comp cars where the engines are putting out about 1,500 hp. When the engine is capable of making that much horsepower, it can effectively use the 21⁄8- and 21⁄4-inch headers that some of the pros are running. But you have to remember that these are very high-speed headers that don’t really become effective until the engine is running above 7,000 rpm.
Long-track oval cars are still running headers between 13⁄4- and 2-inch diameter, even though they are pulling amazing horsepower levels. They need the broader torque capabilities inherent in a smaller-tube header. It’s plain fact that just about every bracket small-block will be perfectly happy with 13⁄4-inch headers. While some of the really nasty ones may derive some benefit from 17⁄8- inch pipes, a 2-inch header is out of the question unless you’re making over 650 hp and running over 8,000 rpm—so don’t kid yourself. Stick with what works!
Whatever your application, there are a number of disadvantages related to tube headers that you should consider before making your selection. There are also some tuning procedures that will make or break your installation, so don’t ignore them. The major problems with tube headers are not so much disadvantages as they are things you may wish to consider. They are, of course, noisier, but not much if the exhaust system is designed properly. Many people feel that they run hotter, and in fact they radiate heat from a greater surface area, but on the other hand what could be hotter than a cast-iron manifold two hours after you shut off the motor. With headers you can grab the pipes in less than an hour. The chief problem with headers is usually leakage, breakage, rust, or a combination of the three. These are all problems that can be controlled with a little maintenance and making sure you shop for the best quality when you put down cash for your new pipes. In most cases, a header that has a thicker port flange will last longer and leak less. Some makes offer thick-wall pipe for improved noise and corrosion resistance, and paramount in any street header selection is the ease of installation and access to spark plugs.
Other Considerations Whenyouinstallyourheaders,you should at least paint themwith a hightemperature paint. Many headers are already paintedwhenthey come out of the box, but don’t let their shiny new appearance fool you. It is usually plain lacquer anditwillburnrightoff as soon asyoustart theengine.Modernheaders are available with special coatings that resist almost everything. They have a ceramic coating that makes them nearly bulletproof and they look great. The best thing about these coatings is that they don’t wear off, burn off, or substantially discolor with use. If you have ever had a set of chromed headers youknowhowdisappointing it is tosee them blue once they get hot. After an experience like that, you’ll appreciate theversatilityof these coatings.Mostof the major header manufacturers offer coated headers out of the box, and you can have your own headers coated by Hooker,HPC(High-performanceCoatings), and Jet Hot. This adds to the expense, but it ismore thanworthit. In fact, many enthusiasts have begun coating their entire exhaust systems.
One of the biggest complaints about headers is that they don’t seem to work after they are installed on an engine. This is often because the installer did nothing to retune the engine to complement the headers. Headers may require a jetting change (slightly richer) and the improved scavenging often requires a little less total timing, due to better combustion efficiency. It’s helpful to install a new set of spark plugs with the headers and run them for a few days before checking their color. You can’t really tell much from plug color on a street engine, since there are too many variables, but they will give a general indication of the engine’s fuel mixture. If your old plugs were burning a medium tan color, the engine was probably running at close to the optimum air/fuel ratio. The header installation may lean the mixture enough to turn the plugs almost white.
The Complete System
A fully integrated exhaust system continues beyond the header or manifold collector. It doesn’t matter whether you’re running cast iron factory manifolds or tube headers, performance will be improved if you keep back pressure in the exhaust system to a minimum. The very least you should do is equip the car with a dual exhaust system— but there is a right way and a wrong way to go about it. There are specific points to consider when constructing a truly effective dual exhaust system. The first thing you need is larger-than-stock exhaust pipes that are routed with a minimum of restrictive bends. Two-inch pipe is considered a good size, but if your engine is really healthy and you want to achieve maximum performance, you should use 21⁄4- or 21⁄2- inch pipes. This is especially true if you are running a large-displacement engine like the 400-ci.
There are also a couple types of bends to choose from. Common dualexhaust systems made at local shops are often press bent, while performance pipes are available with mandrel bends. Press-bent pipes shrink in size at the bends, but higher-flowing mandrel-bent pipes have a consistent diameter throughout, even during tricky over-the-axle bends.
You’ll want to augment the largediameter exhaust pipes with good free flowing mufflers. If you really want the truth, this excludes side-pipe mufflers and glass packs. They simply are not an effective alternative to a welldesigned muffler. The variety of available mufflers is considerable, but in reality there are a couple of commonly available units that really work, so why look for anything else? Most of the available “turbo” type mufflers, like DynoMax Super Turbos, will work. Turbo mufflers are similar to production mufflers but they offer freer flowing insides with larger passages. Chambered Flowmaster and straight-through Magnaflow and DynoMax Ultraflow mufflers have also become enormously popular in recent years. The different types of mufflers have unique sounds, so listen around before you choose.
If you want a quiet system, add some full-length tailpipes behind the mufflers. If you have built an effec- tive system, tailpipes of the same diameter will not hurt performance, but they will definitely help keep the car quieter. It’s important that you use a crossover pipe between the two sides of the dual exhaust system. This helps reduce pressure peaks, improves performance, and makes the exhaust note quieter. The tube should be the same size as the exhaust pipes or, if space permits, it can be even larger for more effective dampening. The location is not critical, but you should strive to keep it within at least 18 inches of the collector or at least ahead of the mufflers (it serves no purpose after the mufflers, where system pressure is much lower). If you’re having a custom exhaust system fabricated, ask the fabricator to use a thicker gauge tubing, as opposed to standard exhaust tubing. Tubing is available in several different wall thicknesses and you’ll appreciate the difference once you hear it.
Written by John Baechtel and Posted with Permission of CarTechBooks