Chevy Header And Exhaust Tech

Chevy Header And Exhaust Tech - CHP Insider
http://images.chevyhiperformance.com...dan_lemons.jpgWe're all for getting freaky. Whether it's a 632 in a Chevy II drag stud, an SB2.2-headed street machine, or an LS1 in a Rustang, freaks add a dash of zest to a hobby prone to vapid parity. The big question is where freaks get their headers. That's where Dan Lemons of Lemons Headers comes in. If you have a combination for which off-the-shelf headers simply don't exist, Lemons will build you a set in any conceivable configuration. A former employee of a major header manufacturer, he founded his own company in 1997. His first crack at building custom pipes was a set of massive 2.5-inch headers for a customer's big-block '68 Camaro. Lemons somehow got them to fit-and almost overnight he had 10 more orders for his now-famous creation. Today the company specializes in hard-core race headers, where surviving inclement wheelstands is as important as making maximum power. Here's what Lemons has to say regarding header design, exhaust theory, installation tips, and how custom headers are built.
Stepped Primaries
With stepped header designs increasing in popularity, what are their benefits over a nonstepped design? According to Lemons, a stepped header scavenges better than a straight-tube header and broadens the torque curve. Furthermore, Lemons' race collectors utilize a transition cone, which also assists with scavenging. Making sure the flange opening is larger than the head-port opening improves flow as well. "When we are determining the proper tube size for an application, we will sometimes use a stepped header as an in-between size," he says. "For instance, if a 21/4-inch tube is too small for a certain application but a 23/8-inch tube is too large, then we'll build a header that's stepped from 21/4 to 23/8 inches."
Primary Diameter
"I don't think there is an easy rule of thumb or formula for determining proper primary diameter, and it more often comes down to what works in the real world," says Lemons. "Engine builders and experienced header builders are usually a good source for input. When sizing a header, there are obvious things to take into consideration such as displacement, compression ratio, and horsepower. However, we must also consider the weight of the car. Heavy cars need more torque, as do cars with big tires. On a car with small tires, we may use a bigger tube to kill some of the torque and help manage the shock to the tires at launch, especially if there is nitrous or a blower. If a car utilizes a full exhaust system, we may lean toward a larger tube or collector size, since even a straight-through muffler will still add some backpressure. Converters with lower stall speeds will require more torque than a high-stall converter-hence smaller tubes-while nitrous and blower motors like bigger tube sizes. To illustrate our point, let's suppose you have a 540ci big-block with 14.0:1 compression making 900 to 1,100 hp on motor plus another 300 to 500 hp on nitrous. In a 3,100-pound car with 10.5-inch tires, a 5,500-stall converter, and mufflers, the motor would typically need a stepped 21/4- to 23/8-inch header with 4- to 41/2-inch collectors."
Fenderwell Headers
Although they require cutting up some sheetmetal, fenderwell headers offer packaging benefits over more conventional headers. "With fenderwell headers, there is lots of spark plug room, you don't have to worry about oil pan or starter clearance, and in most cases they are very easy to install," Lemons explains. "The drawbacks are that they require cutting your inner fenderwells and compromise tire clearance, which can limit a car's turning radius. Also, if the collectors are pointing downward, they'll blow dust on your neighbors in the pits."
Collector Length
Experimenting with collector length is a decades-old trick, but there are some simple facts to remember before tweaking away. "A longer collector will typically make more torque, while a shorter collector will typically make less torque but more top-end power," Lemons says. "If a car has a full exhaust system, it will be less sensitive to the length of the collector. With open headers, sometimes it's pretty common to use a larger collector in addition to collector extensions to fine-tune the torque requirements to launch the vehicle out of the hole."
Raw Materials
The two most common materials used in building headers are mild steel and stainless steel, each with its pros and cons. "We build headers in both materials, and the most significant difference between the two is that stainless costs about twice as much," Lemons says. "Stainless is a stronger material and won't rust, but it discolors and weighs more. Mild-steel headers that are well-coated inside and out make a lot of sense, since they're cheaper, lighter, and very durable. We use 3/8-inch-thick header and collector flanges, 18-gauge tubing, and 16-gauge collectors. Both types of metals are TIG welded."
Primary Length
Primary length definitely has an effect on the power curve, but in stock chassis cars, getting all four tubes from the flange to the collector in a normal location limits how much the length of tube can vary from one design to another. "The difference between a complicated design with many bends and a simple design with fewer bends is only a few inches, which isn't enough to significantly impact the power curve," Lemons explains. "However, having the ability to use different tube sizes allows much greater flexibility in designing a header. Smaller tubes act like longer tubes, and bigger tubes act like shorter tubes."
Bends
While a bunch of fancy bends can boost a header's intimidation factor, they're not necessarily desirable for ultimate performance. Lemons says in a perfect world, a header would only have one bend: out and back. However, the confines of a stock chassis makes this impossible. "To make things even worse, performance headers require putting too big a tube into too small a chassis and trying to get them as close to the same length as possible," he explains. "This is not an easy task. We try to keep our designs as free-flowing as possible by using as large a radius in our bends as we can in addition to keeping the tube square to the exhaust port without back-cutting the bend at the flange."
Coatings
"Heat retention does make horsepower, according to the dyno guys, but I don't know that there is any scavenging benefit," Lemons says. "I do feel there is some benefit to a header that is well-coated on the inside to help control rust and carbon buildup, which could disturb the gas flow. When installing ceramic-coated headers, they should first be wiped down with rubbing alcohol. Use a clean white cloth to apply the alcohol, then a clean dry white cloth to remove the residue. After the headers have been installed, repeat the alcohol cleaning process to any accessible areas. This will remove the fingerprints, grease, excess antiseize, and smudges. Next, start your engine and let it idle for about five minutes, then shut it off and let it completely cool down. Run the engine again for about 10 minutes, then let it cool down. This process will help bake the new finish on, making it tougher, and will greatly reduce the chances of dulling."
Custom Headers
With a plethora of off-the-shelf headers on the market, most hot rodders have the luxury of buying a set that suits their application right out of a catalog. However, uncommon combinations, such as an 18-degree-headed small-block in a third-gen Camaro, will probably require a set of custom headers. According to Lemons, customers can expect to pay roughly $1,200 for a set of mild-steel custom headers. "In our jigs, we have the ability to build a header for any cylinder head as well as different deck heights," he explains. "We can make it in the tube size the customer needs and in a stepped or nonstepped configuration, but the engine needs to stay in the stock location. It takes us about two days to build a header once we start on it, but because we custom-make every header, we always have a lead time. If you need your headers by a certain date, be sure to give your builder plenty of advance notice."
Tri-Y Headers
Four-into-one headers are the most popular design on the market, but tri-Y (four-into-two-into-one) headers are another alternative. Tri-Ys join two pairs of primary pipes together in a Y-shape ahead of the collector. This essentially shortens up the primaries and merges them into a secondary pipe before reaching the collector. "They tend to scavenge well and can broaden the powerband, but they can be finicky and you may spend a lot of time on the dyno finding the combination that is perfect for your setup," Lemons explains. "Some of the race teams that were using them have gone back to a four-into-one style header because tri-Y headers are too sensitive to minute changes in design."
Surviving Wheelstands
"We started this business building drag race headers, where there are two high priorities: spark plug room and ground clearance," Lemons says. "It's a constant challenge for us. Most of our new Pro Touring and drag racing designs have the primary tubes actually higher than the bottom of the oil pan. So if your wheelstand got to the headers, it got the oil pan too. We put a lot of effort into improving and redesigning our headers. If we feel we can pick up a little more ground clearance, increase space around the starter and oil pan, or improve access to the spark plugs, then we'll redesign a header. On our big-block headers, we TIG-weld silicon bronze on the outside of the flange to keep the tube from cracking, but keeping them off the ground is the key."
Gaskets
Since most enthusiasts have better things to do with their time than fix header leaks, using the right gasket and installing it properly is critical. Ideally, a header-flange gasket should have openings larger than the exhaust ports to ensure the gasket will not obstruct exhaust flow. "We have had great success with copper gaskets, since they're extremely flat, which promotes an excellent seal, and do not expand or contract as much as fiber gaskets," Lemon says. "Copper can't crush like a fiber gasket, so we recommend using a thin layer of copper silicone on each side of the port opening."
Dimples
When in doubt, frustrated wrenchers turn to their trusty hammers. Fortunately, Lemons says there's nothing wrong with dimpling a header to simplify installation. "Making a small flat spot in a tube probably has a minimal effect on performance, but I wouldn't do it if you don't have to," he advises. "If your header is touching the steering column, for instance, there may be a way to adjust the column outward. If it's the starter that's in the way, you may have to use a mini starter. If you are trying to use a header made for a stock configuration on a tall-deck block or a raised-port head, then take a hammer with you."
Design Parameters
"Designing a header that installs easily is a blessing, but we won't sacrifice performance or fitment to make a header easier to install," Lemons promises. "Some of our headers are a pain to install, but once they're in they fit well, have good spark plug access, and have plenty of ground clearance and room around the oil pan. All our race headers are made with individual tubes and slip-on collectors, so you install one tube at a time and the collectors slip on and bolt in place. This makes all the difference in the world. Many of these headers can be installed in 15 minutes with the engine and starter in place."
Bolts
Everyone seems to have a different theory on how to prevent header bolts from backing out, but here are some suggestions straight from an expert. "There are header bolts with locking washers that work well if you need them, and if your headers have extra-thick flanges, be sure to use bolts that are extra long," says Lemons. "The combination of heavy-duty header flanges and flat copper gaskets doesn't expand and contract like thin flanges and fiber gaskets, which virtually eliminates the potential for bolts to loosen over time. Once the headers are installed and the engine is started, always retighten the header bolts after the engine is warmed up."
Heat
Ceramic coatings do a great job of keeping underhood heat down. Header wrap can work very effectively as well. One fact that is often overlooked, however, is that a properly sized header can make a big difference. "If your header is too small for your engine, then the header and the engine can overheat," Lemons says. "This is especially true with blower motors."

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