Header design
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In the street rod world, absolute mechanical efficiency often takes a back seat to appearance, clearance issues, and ease of installation. However, most of us overlook the benefits of a properly designed and built header and how it can improve drivability, power output and fuel economy. If you are building headers or modifying existing headers, why not try to keep the physical operation of a header in mind while working on it? | In the street rod world, absolute mechanical efficiency often takes a back seat to appearance, clearance issues, and ease of installation. However, most of us overlook the benefits of a properly designed and built header and how it can improve drivability, power output and fuel economy. If you are building headers or modifying existing headers, why not try to keep the physical operation of a header in mind while working on it? | ||
| − | + | ==A bigger header isn't necessarily a better header== | |
| − | + | The two most important aspects of header design are tubing diameter and primary tube length. This is definitely one area where the "Bigger is Better" philosophy doesn't cut it. Most very mild small blocks out there would perform better with 1 1/2" primary tube headers on them. Ever try to find primary tubes that small? | |
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| − | + | ===Tubing diameter=== | |
| − | + | Just like putting a 300 degree duration cam in a 350 inch small block with 8:1 compression will kill any drivability and torque (but the idle sounds neat - until you hear a high compression big cam motor), putting a set of 1 3/4" headers on a mild small block will kill torque and drivablility, not to mention fuel economy. | |
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| − | + | What horsepower does your engine ''REALLY'' make? Most people overestimate horsepower, RPM range, etc. of the motor in their ride. Consider that the GM ZZ4 crate motor makes 355 hp and the Mopar Performance 5.7 Hemi crate motor makes 360 horsepower with great heads (as for the Hemi, excellent heads), roller cams and brand new everything. How much power is your 350 with 50,000 miles, stock iron heads, 268 degree cam and 8:1 compression going to make? The two engines mentioned above would be ideal candidates for headers with 1 5/8" primary tube headers at 36" long with a 2 1/2" collector and exhaust system. | |
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| − | + | ===Primary tube length=== | |
| − | + | That brings us to primary length. First of all, those "shortie" headers are not headers, just tubing manifolds designed for clearance -- not horsepower or torque. Although they look like they would flow better than manifolds (and probably do in many instances), unless you are running a supercharger, you need more than flow out of a header. The bothersome part of the "shortie" (other than length) is that the collector is so short and causes a lot of turbulence right where the flow needs to be smoothed out. | |
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| + | Exhaust headers (and intake runners for that matter) can be tuned to length to give a power boost at a given RPM. Tuned length is a function of the speed at which the boost is desired and has nothing to do with diameter of the tubes. When the exhaust valve cracks open, there is a strong pressure pulse sent down the tube at the speed of sound in the high temperature gas. When this pressure wave reaches a larger diameter such as in a header collector, there is a low pressure (slight vacuum) wave reflected back up the pipe at the speed of sound. The goal is to have this low pressure wave reach the exhaust valve just as it closes which scavenges the cylinder causing a better charge of clean air and fuel. Since it is a function of fixed velocity of sound in the exhaust gas, the slower the desired tuned speed, the longer the pipe needs to be. Interestingly, the shape of the pipe (turns) isn't critical so a basket of snakes header is just as effective as straight tubes. The diameter of the tube should be as small as possible which strengthens the magnitude of the pressure pulses. Velocity of the gas really has nothing to do with the tuned speed available in header design but is a negative when it comes to friction losses. | ||
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| + | The main advantage gained in equal length, independent primary header tubes is from the strong negative pressure pulse that is reflected from the tube end when the strong positive pressure pulse form the exhaust valve reaches the collector. Other pulses from other header tubes are of much smaller magnitude in the tube of interest and can be ignored. Thus tuning length is very easy to determine once you have an estimate of the speed of sound in the hot gasses. A useful equation is | ||
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| + | L = (120*V)/N | ||
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| + | where | ||
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| + | L is the pipe length, less port length in head, in inches; | ||
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| + | V is the velocity of sound in hot gasses, in feet per second (ft/sec). Values of 1300 ft/sec to 1700 ft/sec are common; | ||
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| + | and, | ||
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| + | N is the engine rotational speed, in rotations per minute (rpm). | ||
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| + | Using V = 1700 ft/sec the equations simplifies to | ||
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| + | L = 204,000/N. | ||
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| + | There have been various permutations on this basic design like tri-Y headers, stepped tubing size, etc. Each takes advantage of modifying the pressure pulse arrival time at the instant the exhaust valve closes to achieve a scavenging/ higher volumetric efficiency/ more torque result. The good is that you can achieve a very significant torque increase at the design rpm. The bad is that you likely will also achieve less torque at other RPMs. | ||
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| + | There are other design theories like the Helmholtz resonator which are useful in designing systems with more than one degree of freedom than a single pipe/cylinder, i.e., Tri-Y. | ||
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| + | As you can see from this discussion, most popular aftermarket headers are poorly designed for any performance purpose. Tubes are too short, too big, and all different lengths. Most "street rod" headers are not designed for performance, rather to fit inside the typical smoothie envelope. "Performance" headers are designed to look zoomie I guess because I can't figure out any other design criteria when I study them. | ||
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| + | Incidentally, this is the principle that Chrysler used on the cross ram intake manifolds they put on big block passenger car engines in the late 50s. The velocity of sound in the cold intake gasses is much slower than that in hot exhaust so tuned length for a street intake is much shorter @ about 18" from the valve seat to the plenum. Thus they put a 4bbl carb on either side of the engine and crossed over long ports. Looked and performed great! | ||
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| + | ====Equal primary tube length==== | ||
| + | =====Argument for equal primary tube lengths===== | ||
| + | If the length of the primary is part of the tuning equation, how well does an engine run with different primary tube lengths? Try and jet that carburetor without pulling your hair out! Most of the commercially available headers out there have a large variance in tube length. Check out a set for a big block mopar in a B or E body for an example. The variance between longest and shortest tubes on these units can be as much as 16". | ||
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| + | [[Image:unequal.jpg|right|frame|Big block Chevy headers. Note how the driver's side rear tube (yellow) must be about 10"-12" shorter than the next tube (in red).]] | ||
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| + | In the header photograph to the right, the short primary tube would scavenge at a higher RPM and the long primary tube would scavenge at a lower RPM for the respective cylinder. Therefore the cylinder with the short tube will be running lean at low RPM and the long tube cylinder will be running lean at the high RPM and would require different jetting and timing than the others. How do you do that with a standard kettering distributor and a simple carburetor? That's why equal length is important: so you can tune your car. Not only do equal length tubes make the engine tunable, but make more torque in the RPM range for which they were intended. | ||
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| + | This is particularly true for V6 engines. In the case of the 173ci 3.1L or the 3.4L pushrod engines, the pulse of the air pressure wave, as well as the sound wave, need to dissipate at a particular length so that a proper negative wave travels back AT THE RIGHT TIME to the exhaust valve, aiding the exhaust flow. In the case of the GM pushrod v6's the firing order is sequential and therefore cylinders fire on alternate sides of the block on each compression stroke. Generally, those v6 engines operate best with 1.5" pipe and a primary length of 32" to 36" depending on the compression and torque curve desired. It should be noted that most headers used for the FWD and Mid-Engine v6's are not optimum in performance. | ||
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| + | =====Argument against equal primary tube lengths===== | ||
| + | Equal length headers are good for a certain part of the RPM range of a typical engine. However, when buying an equal length header you are left with the length the manufacturer wanted to use, NOT the correct length for the engine you are building. How do you know if its the right length? | ||
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| + | Different primary tube lengths are not nearly as hard to tune. This type of header shown has proven itself for decades to be a well designed, good flowing header that will free up a good amount of horsepower compared to stock manifolds. | ||
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| + | "Equal length" is usually defined as the longest and shortest tubes being within 2 inches of each other (about as close as you can measure with a tape measure at the swap meet). | ||
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| + | There have been claims by some manufacturers that unequal lengths broaden the torque curve due to different cylinders performing better at different RPM. It is left to the reader to decide if flattening the torque curve is a good thing to be doing with headers. However, a flat broad torque curve makes better drivability, and a smoother power band than a peaky engine. | ||
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| + | How does one unequal length tube make for a broader power band? This sounds like someone trying to explain away poor design. So does a 180 degree intake manifold have just one shorter smaller runner? | ||
==Collector== | ==Collector== | ||
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