Drag Race 101: The benefits of better breathing—again. Choosing the right headers
In our last installment, we discussed how to get more air and fuel into your engine. While getting more in will definitely help power, if you can’t get that extra combusted material out of the engine, it’s all for naught.
Choosing the correct headers is more than just opening a catalog, and looking for the right fitment. If you properly choose your headers, the power gains can be extensive. But, how do you choose the correct ones? First you need to understand the different types of headers, and the pros and cons of each. Next, you need to categorize those pros and cons based on which ones are the most important to you.
During the exhaust stroke of an engine, exhaust gases are forced out of the combustion chamber through the open exhaust valve. The movement of the escaping exhaust creates energy pulses that form a “vacuum.” This vacuum pulls additional exhaust gasses out of the combustion chamber so that the maximum amount of fresh fuel and air can be drawn in. This process is called scavenging.
The process of choosing the right headers has been debated for years: does an engine needs a long-tube or short-tube, equal-length or conventional length, and finally, what size primary tubes?
We can start a huge debate, and offer advice on how to choose a great header but, we are sure the debate can and would get overly heated because there are so many different designs. However, if you follow a few basic fundamentals when choosing headers, you can make it simple to select the right headers for your car.
Choosing a race header for a street car can render less-than-stellar results. Likewise, putting a street header on a race car will definitely hurt power. For the purpose of this article, we’ll focus on race headers.
Header type is a huge determining factor of how torque and horsepower work together. Primary tube length and size, will hugely impact your torque curve. Add to that a secondary primary tube (step header), and things change even more. When running a full exhaust system, the entire exhaust needs to be considered when choosing headers, but we’re talking race cars, and very few run an exhaust.
First and foremost, whatever header you choose, they have to fit your vehicle. Let’s face it, if they don’t, what good are they? After that, you will want to pay attention to the size of the primary tubes. If your primary tubes are too large, this will cost you torque and horsepower by slowing down the rate (velocity) at which the exhaust gases travel through the system.
Smaller diameter pipes will flow a lesser volume of exhaust gas than larger ones, but the exhaust gas in the smaller pipe will actually flow faster. One caveat is that this only occurs until you reach the engine rpm where the volume of exhaust gas requires a bigger primary-tube diameter. Since a smaller primary tube will flow faster, that smaller tube will scavenge the cylinders more efficiently. Be warned, if the tube is too small, proper scavenging will not occur, because the backpressure created cannot be overcome. If running a header with a primary tube that is too small, you also run the risk of excessive heat building-up. The undersized exhaust tube means the engine is struggling to scavenge hot gases from the chamber. Choosing the correct primary tube-size comes down to engine displacement, and how much horsepower your engine will make. If the primary tubes are too large, you will lose torque, because the larger tube causes a loss of velocity coming out of the chamber. This affect—to an extent, is also determined by valve overlap. This velocity creates a draw or scavenging effect, where the hot gases leaving the engine help draw a cooler air and fuel mixture into the chambers.
When trying to decide on the right primary tube diameter, here is a formula that can get you in the ballpark.
Primary Pipe Area (PPA) = (peak torque rpm ÷ 88,200) × 1 cylinder cubic inch
To find the PPA, we use A=πr²
For a few common header sizes, this is:
1-5/8 inch = 2.07
1-3/4 inch = 2.19
1-7/8 inch = 2.53
For example, let’s say that you are building a 350 engine with a peak torque goal of 5,000 rpm, your (PPA) Primary Pipe Area = (5,000 ÷ 88,200) × 43.75 cubic inches. Your PPA = 2.48. Theoretically, a header primary tube between 1-3/4, and 1-7/8 would be perfect. Since there is no “in between”, the 1-7/8-inch primary tube header would be the better choice.
When it comes to racing, things are never that simple. To fully understand which length and diameter for a custom set of headers, you need to consider every aspect of the engine’s combination from camshaft, cylinder heads, fuel type, power adder if any, and, of course, the engine’s RPM range. For example, cars running nitrous need to move more volume than naturally aspirated engines. For that reason, it sometimes pays to contact the manufacturer, and get their recommendations for your car.
The collector is not only connected to the primary tubes to merge them into one, it can greatly affect the power output of the engine. The collector must control both the exhaust and pressure waves that converge within it. Collector diameter is critical to power production, and the diameter should be large enough to support the amount of exhaust gas entering it. Each cylinder sends a burst of exhaust gas through the primary tubes and into the collector at around 350 feet-per-second. If you multiply that by the four primary tubes, that is a lot of air to allow through. Some headers have each primary tube end in the collector with a simple cutoff of the tube. These abruptly-ended tubes create a swirling affect inside the collector which actually slows their ability flow. This makes proper collector diameter even more crucial.
There has been an improvement in primary-tube ending, and these improvements range from cones that are welded to the primary tubes, to slash-cut pipes, and even multi-stage 4-2-1 collectors. Finally, many racers adjust the length of the collector, to “tune” the exhaust, but this is definitely a “trial and error” approach.
4-into-1 Long-Tube: This is the classic-looking header that has been around for decades. This is the header that everyone recognizes, and makes up the biggest majority of the aftermarket offerings for street and race applications. Long-tube headers offer the most significant decrease in backpressure on the chamber, due to the separation of exhaust gases. In other words, each chamber flows into their respective individual pipe. Long-tube headers are also the most versatile headers, because they can be tuned differently. By altering the primary-tube diameter and length, as well as the collector diameter and shape, manufacturers can account for different horsepower and operating ranges. These are the go-to header design for most street and race cars.
Tri-Y Long-Tube: Tri-Y headers combine two, short primary-tubes that are connected via a two-into-one collector. That collector then steps into an even larger-diameter secondary tube. Finally, the two secondary tubes merge into a two-into-one collector. The complete header creates an overall length that is similar to a long-tube design. Tri-ys works well at building more torque from idle to around 4,500-5,000 rpm. This makes them very capable for street use.
Shorty headers came into existence because of the need for a header to fit late-model vehicles that must conform to emissions standards. Late-model exhaust systems posses catalytic converters and oxygen sensors that are close to the exhaust ports. Shorty headers allow the retention of the catalytic converters, and are better than most manifolds, but the short tubes are also too short to maximize any potential gains. Some performance-oriented cars (like Mustangs and SRT Dodges), actually come from the factory with a shorty-style header. These factory headers work well considering the confines, and are rarely bested by shorty headers.
These headers were brought about as an alternative to shorty headers. They are slightly longer than a shorty, but not as long as a long tube. Therefore, they don’t suffer from ground clearance issues like a full length header. Mid-length headers keep the tubes and collector above the chassis, but by design, do offer a longer primary tube. Mid-length headers will generally make more usable power on the street than a shorty header, but not as much as a long-tube or tri-Y design.
When you finally decide which header is best for your car, you have another decision to make. Do you want your headers raw or coated? A ceramic-coated header provides an added thermal barrier that keeps heat inside the primary tubes, and helps keep ambient temperatures low in the engine compartment. That means cooler, denser intake charges and increased horsepower. Coated headers will maintain their “clean” finish, even in extreme conditions. Headers are also available with a painted finish. While these finishes are a more affordable alternative to coated headers, they don’t usually provide the performance or durability benefits.
A new, major design change in exhaust technology comes from Flowtech. They tell us that their header’s Afterburner Turbo Chamber design combines a race-proven four-into-one design with the torque managing capabilities of a Tri-Y header. The turbo chamber helps smooth out exhaust turbulence for increased performance and a broad power curve.
For a Tri-Y header to have the proper effect on performance, the cylinder firing order must be properly paired so that the next firing cylinder is separated. For example, an LT-1’s firing order is 1-8-7-2-6-5-4-3 so pairing of the 1 and 5 and 3 and 7 cylinders on the passenger side and the 2 and 4 and 6 an 8 cylinders on the driver’s side would be the proper sequence. You also need to have the proper primary-tube size for the engine displacement and application. The collectors also need specific convergent angles of 7-20 degrees for optimum efficiency. We have seen horsepower and torque numbers dramatically affected by these angles in Tri-Y headers. Testing has shown that the primary head pipes (first section of primary tubes) must be between 17 and 21-inches long for proper pulse scavenging.
Long tube headers are best at building horsepower and torque from mid-to-high rpm range (i.e. 3,000 rpm and up). These are great for high-revving machines, but they do sacrifice some low-rpm power. Hooker Competition headers have been around forever. They are an economical, high performance header that is designed for the car guy that is running a stock or mildly-modified engine. The Hooker Competition header utilizes a long-transition collector to reduce back pressure and increase exhaust flow. Hooker also has what they call a Super Competition header which is a more race-oriented header.
There is not much call for a mid-length header in a racing application. Mid-length headers will generally make more usable power on the street than a shorty header will, but rarely will they make as much as a long-tube or tri-Y design.
A Shorty header will deliver more power horsepower and torque from idle to the mid-rpm range than a long-tube header. They are typically CARB-Approved, as they retain the factory cats on late-model muscle cars. This makes shorty headers perfect for your daily commute or hauling your boat up a hill, but not so much for a race car.
A spike is a common choice to smooth flow as it transitions into the collector from the primary pipes. This version features four flat sides, and we have seen some with concave sides that match the primary tube’s walls.
If you’re looking for the ultimate in a race header, these “U-weld it” drag racing headers are perfect for getting the most out of your high performance engine. When you by a weld-it header kit, you will have more work to do for assembly, but unlike a universal-fit header, they are welded together by you to fit your car. They don’t usually come with a header collector, but you buy that separately, so you can make it the length you need to improve your engine power.