To extract maximum horsepower from Chevy W engines, the exhaust system must be properly tuned to the cam and intake setup. Like other Chevrolet engines from the era, the Ws were equipped with cast-iron exhaust manifolds. From the start, it was important for GM to have an exhaust system in place that would keep up with the biggest engine Chevrolet had at the time. And when it comes to race and hot rod W engines, engine builders often spend hours working on the heads and intake to make more horsepower. But if those spent gases cannot get out of the engine and proceed out the exhaust pipe in a quick and efficient manner, it’s money wasted. The good news is in all the aftermarket parts for W engines, the exhaust has not been forgotten.
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Chevrolet’s insistence that the 348 fit the small-block Chevy foot- print meant, in part, that the tight, almost 180-degree route of the spent gases clearly needed a better way to get out of W engines. That move opened the doors for the new part being perfected by racers at the time called headers, and better yet, tuned headers.
Ws were there when header history was made. Jere Stahl designed and built the Chevrolet Z11 headers for Bill Jenkins and Dave Strickler in 1963 for the famous Old Reliable Chevy. They were the first independent, four-tube headers and replaced the header industry standard Tri-Y headers. Stahl went on to develop adjustable primary and stepped headers, among his other header products.
When Chevrolet built the Z11 engines and raced them, headers evolved from just a set of open pipes to a tuned piece of the engine that actually contributed to the power of an engine. Headers started out as just open tubing welded to flat steel flanges that mimicked the bolt pat- tern of the exhaust manifold. The early idea was to just get the exhaust gases out of the engine as quick as possible. Many times, those pipes were larger, much larger, than the openings on the exhaust manifolds. That was often a case of too much as one of the factors of managing exhaust is the delicate balance of back pressure and vacuum. The vacuum was created when the gases left the engine. Once balanced, air rushed in to fill the void of the vacuum.
So starting with that principle, we know that an engine pushes spent gases out via the exhaust valves of the cylinders. If that flow meets resistance, it is known as back pres- sure. The idea of headers is to reduce back pressure, so the engine can better use its power to make rear wheel horsepower. Ideally, headers create an environment that draws or pulls the exhaust gases out of the cylinders as fast as can be accomplished.
Stock, cast-iron exhaust manifolds create back pressure as do exhaust system components, such as the pipes themselves, mufflers, resonators, and tail pipes. As this is a book on W engines, we’ll limit this chapter to just headers and leave the rest of the exhaust system for other discussions.
Fortunately, you can find headers in short-tube and long-tube designs for most Chevrolet chassis equipped with a W engine and headers for hot rod and street rod applications. However, you don’t have nearly as many options as a Mark IV Chevy big-block. Having said that, Hooker, Doug’s, Sanderson, and Edelbrock are some of the manufacturers that make high-performance exhaust for the W engine. When selecting a header, you need to consider the diameter of the primary tube and this largely determines the engine speed in which maximum torque occurs. The exhaust gases travel faster through a smaller tube and slower through a larger-diameter tube. A smaller tube is often the best choice for street applications because maximum torque is placed lower in the power band. Conversely, if you’re searching for ultimate performance, a large diameter race header places the torque peak high in the power band. The length of the primary largely affects the profile of the torque curve. Thus, it determines how narrow or large the useable torque the header produces. As a general guideline, long- tube headers produce more useable power below the maximum torque rating and short-tube headers provide more power after the torque peak. For a street engine, you want to make most of your power from low to mid-range, because a street 409 is not going to be revved up high like a race engine. Therefore, you want to make the most amount of power from 1,500 to 3,500 rpm. In addition, the length of the primary tubes should be realtively equal so cylinder pressure and scavenging is about equal for each bore.
For a street engine, many owners opt for a 13⁄4-inch primary tube, as well as a 3-inch collector. If you’re building a street/strip or full race engine, you often opt for a 23⁄8-inch- diameter primary tube and a shorter-length collector. When speccing your headers for your engine, you should consult a knowledgable engine builder and discuss the appropriate set of headers for your engine. And/or you should talk to a tech representative and tell him your engine’s specific setup. With this information in hand, you can select the correct header for your particular engine.
When selecting headers for a full- size Chevy passenger car or truck, you need to consider the clearance of the tubes. In the case of the Impala, you often are required to cut the inner fenders or leave them off if you want to use a larger set of 4-into-1 headers. If you want to retain your stock sheetmetal, often times you have to use a Tri-Y header.
Sanderson offers shorty headers to fit most chassis, roadsters, street rods, and lake-style headers. Hooker, a renowned name in headers, offers a wide range of Super Competition headers to fit many models of American V-8s, including the W engine. The Super Comp headers have tuned primary tubes measuring 17⁄8-inch in diameter and are 36 inches in length. In addition, the collector measures 3 inches in diameter. Two models are available–one in flat black and other with silver metallic ceramic coating. These headers are for 1958–1964 full- size cars and station wagons.
The Block Hugger header is a popular and relatively new design. These are primarily used in street rods in which clearance issues are more important than performance. Block Huggers are used to fit bigger, wider Ws that are mounted in an older, more narrow chassis. Block Hugger headers provide the biggest tube diameter possible while keeping the pipes closer to the block to clear the frame and any other obstruction, such as the steering gear. They are short-tube headers that have their collectors basically pointing straight down to connect with 90-degree tubes that start the exhaust pipe system. Flange connections join the two pieces. Patriot offers block headers for the W, which feature mandrel-
bent tubes. The 16-gauge cold-rolled- steel tubes are finished in ceramic or Ti-tech. A few unique features include a conical collector sealed and flared extension pipe so the pipe can be easily fabricated. High-quality flanges and gaskets provide a precise fit to prevent leaking and loss of horsepower.
Tri-Y headers are still being offered to W-engine fans looking for more performance. Their design uses smaller, shorter pipes to cre- ate a header that promotes vacuum when used with a megaphone col- lector. They also enjoy one of the other unspoken benefits of headers— they’re just plain cool. Doug’s Headers offers a Tri-Y set with a mandrel bend for the W engine that is made of 16-gauge steel primary tube and a beefy 3/8-inch flange attachment. These feature a silver ceramic coating and include premium gaskets.
When the 1958 Chevy 348 was released, its system consisted of a cast-iron exhaust manifold flowing into 2-inch diameter, steel tube exhaust pipes. Then a type of H-pipe connected the dual exhaust pipes that flowed into separate mufflers and then 2-inch tailpipes. The next year, the exhaust on the 1959 models was upgraded to 2½-inch outlets on the manifold to 2½-inch exhaust pipes going into the muffler then going to 2-inch pipes and resonators. The use of the manifold with the 2½-inch outlet continued through the end of the run for W engines. Later, when the 409 and its increased power became available, the exhaust was changed, first with new, better flowing 2½-inch outlet manifolds and then bigger 409 dedicated, header-like manifolds that still used a 2½-inch outlet.
Exhaust Port Pattern
The exhaust-port pattern on a 348 and 409 is the same. Even the rarefied Z11 uses the same exhaust manifolds and gaskets, and there- fore, the bolt pattern for the exhaust manifolds is the same. That means any improvements to help open up the exhaust for a better flow works across the three W engines and all their heads.
There is, however, a big difference between the exhaust ports on a W. Like the valves inside W-engine heads, the exhaust ports are staggered. When looking from the exhaust side of a head, the staggered openings of the four exhaust ports can easily be seen. On a centerline along the head and between the exhaust ports, the center two ports are slightly below the centerline of the outside ports. Again, among all W engines, the 348, 409, and even Z11 heads use this pattern.
Unlike small-block Chevys, W spark plugs are located above the exhaust ports, and as a result, it’s easier to remove and replace spark plugs in a W engine. If you closely examine the exhaust ports, you notice that the staggered exhaust ports of the W head on top. A closer look shows the spark-plug holes and their location above the exhaust ports on the same heads. On the W, the bolt holes are at an angle across the outer ports. And finally, there is a difference on the bolt holes of the two, center ports. In addition, the W uses three in a triangular pattern on those center ports, and these are required for the bigger ports of the W engine. In the case of the mounting holes for the manifold, it is more likely a matter of where there was enough metal to locate the bosses for the holes on both the heads and the manifolds.
Single- and Dual-Exhaust Configuration
With Chevrolet’s 265 and 283 small-blocks, a 2-barrel engine had a single exhaust system while engines equipped with 4-barrel carburetors got dual exhausts. With the more powerful 348 ci, a single exhaust system would choke those additional 65ci. So, from the start, dual exhaust was the only one available on W-engine cars. Chevrolet never used a single, 2-barrel carburetor on any W engine.
In a dual-exhaust system, the exhaust is run in two lines, and then combined into one pipe later in the system. Back pressure is created at the junction. Each bank of the engine has its own dedicated exhaust line that generally does not meet the other and create unwanted back pressure. In the case of an X or H pipe installed to connect the two lines, dyno tests have consistently shown improvements in power gains. Knowing the minimum back pressure, engineers adjusted the exhaust flow with the outlets of the exhaust manifolds.
The first sets of 348 exhaust manifolds were almost anti-performance because the exhaust gases travel out of a W’s combustion chamber upward, and then make a turn that is almost 180 degrees to go down and out the manifold. As the W engines are almost 3 inches wider at their widest point, which is the outer part of the head, that means the gases almost have to double back in and down toward the block to meet the same opening of the exhaust pipes that meet the ends of the exhaust manifolds. In fact, one look at the first 348 manifolds show a notice- able difference in how each side’s manifold exits to meet the exhaust pipe. The driver’s side has that offset for the engine’s cylinders being more forward on that side. The passenger side has the outlet almost point- ing straight down. A more detailed inspection reveals the driver’s side manifold outlet is centered under the back edge of the fifth cylinder (third one from the front). The passenger-side manifold outlet is almost perfectly centered under the fourth cylinder, which is the second one from the front. Because of this positioning, the path of the gases from their cylinders is unequal in length. Not only are the outer tubes of the manifold of unequal length, but between the inner ports being lower than the outers and almost directly over the manifold’s outlet, the center tubes are close to half as long as the outers. As written in the aftermarket section of this chapter, the length of the tubes from the cylinder heads to the single exhaust pipe, or collector, is an important factor in the performance of a header or manifold. Chevrolet would later confirm that with the newer manifolds for the 409.
Chevrolet did make some more efficient exhaust manifolds. When the standard manifold outlet was 2 inches in diameter, a better-flowing 2½-inch version became available on the high- performance versions of the 348. Clearly illustrating how the same pat- tern for manifolds was used through the entire run of W engines, that 348 high-performance manifold was used on the 340 hp versions of the 409 as well as on most truck engines. And speaking of trucks, there is one simple way to tell car exhaust manifolds from those used on trucks. Car manifolds had heat tubes for the choke while truck manifolds did not.
The exhaust manifold that allowed for more power was the one that came out with the revised version of the 409 released in late 1962. The very first 409s used a manifold very similar to the ones used on the 348s in as much as there were no out- ward provisions for a more balanced or tuned flow. When Chevrolet over- hauled the 409, part of that make- over included an entirely new set of exhaust manifolds. Looking very much like a cast-iron version of tube headers, that was exactly how they were designed. The tubes were of a more equal length and any curves or changes of direction in their routing were smoother transitions instead of hard angles used in previous models.
The two center ports were still lower than the outers but the header style of the new manifold allowed for them to pass behind the outers and join in at the outlet or collector of the manifold. For racing classes that didn’t allow tube headers, the new manifolds were the way to get more performance out of the new engine. There were two versions of this new header/manifold—standard and high performance.
It is not known how much Pontiac influenced the development of these manifolds as similar Pontiac versions were actually casaluminum, header-based manifolds for racing only applications. The new 409 manifolds allowed the engine to take advantage of all the high- performance changes made to create more power. They were even used on the rare Z11 engines. And that is surprising as the rare Z11 didn’t have a special exhaust system from the factory, yet had special heads and intake. The Z11’s exhaust system came with the same 409, 425-hp version cast-iron exhaust manifolds and a full exhaust of pipes, mufflers, and resonators. It didn’t take racers long to remove most of that and replace it with a set of custom made tube headers to go racing.
Ideally, a set of headers has the best routing, the best length of pipes, and the best diameter of those pipes flowing into the best diameter of col- lector that has the best length. But the headers have to fit in a chassis and therefore, some design compromises are made. One is the angle of how the exhaust ports exit the block. The W-engine headers are offered in a limited selection, which is unlike other engine platforms that offer multiple pipe and collector diameters. While there may not be a tremendous variety of headers for the 348 and 409, there are many fine sets that are suitable for many models and chassis types.
Most header manufacturers start with the horsepower and target RPM of the engine that needs the headers. They factor in the lowest RPM it will run as well as the highest. When the engine is bolted to a manual trans- mission, any drop in RPM is factored in as well as how long the engine will be in a particular RPM range. With those numbers in mind, shorter primary tubes (the ones from the flange to the collector) make more power in the upper RPM ranges while longer primary tubes make more at the lower end of the range. Along those lines, the vehicle’s weight and type of racing factors in as well. Another way is to calculate the number of horse- power per cylinder and prorate that for primary size and header builders have and guard their own formulae for using that method. Primary tube size can range from 1½ to 23⁄8 inches outside diameter.
The length of the primary tubes is a factor, too. Longer tubes, or equal length primaries, can deliver more torque while shorter tube headers can make more power at the lower end of the RPM range. An added bonus of shorter tube headers is a little easier installation as their physical size is smaller. Generally, though, it is widely accepted that longer-tube headers make better racing headers.
Collector size is another factor. Measuring that in both diameter and length, collectors are often used in racing for tuning an engine to a particular track. The short version of collector sizing is the diameter of a collector should match the power of the engine, and the diameter of the tube should match the vehicle’s exhaust system.
From there, it’s all about the chassis and fitting the headers to it. When chassis limitations must be met, along with the location of the exit of the headers, areas such as steering, crossmember, clutch linkage, frame rails, ground clearance, and the engine itself must be addressed.
Written by John Carollo and Posted with Permission of CarTechBooks