Header design
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To some, a header is just a bunch of tubes that connect the exhaust port to the rest of the exhaust system. To the more mechanically curious, it is a system of tuned length and diameter tubes connecting to a device which amplifies and optimizes the wavelength of the exhaust, effectively sucking burnt gases out of the engine's cylinders. | To some, a header is just a bunch of tubes that connect the exhaust port to the rest of the exhaust system. To the more mechanically curious, it is a system of tuned length and diameter tubes connecting to a device which amplifies and optimizes the wavelength of the exhaust, effectively sucking burnt gases out of the engine's cylinders. | ||
| − | 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? |
| − | 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? | + | ==A bigger header isn't 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? | ||
| + | ===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. | 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. | ||
| − | What horsepower does your engine ''REALLY'' make? Most | + | 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. |
| − | That brings | + | ===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. | ||
| − | + | Most street engines that are operated in the idle to 5500 range (yes, your 350 may rev 6500, but is it making any power up there?) work very well with 36"-38" primary tubes. This is the length necessary for the shock wave in the tube to reflect back to the exhaust valve and create a vacuum which will suck the burnt gases out of the combustion chamber. This is provided that the primary tube has the proper diameter to keep the velocity of the gases up. | |
| − | + | ====Exhaust pulse==== | |
| + | Velocity is created in the exhaust system from an exhaust pulse traveling through the primary tube and as rear part of the pulse cools, will create a vacuum. This vacuum will help to pull the next exhaust pulse out of the cylinder. This leaves a cleaner cylinder with less spent exhaust fumes and more room for the incoming air/fuel mixture. More air fuel = more power and torque. | ||
| − | + | ===Equal primary tube length=== | |
| + | 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". | ||
| − | + | 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? | |
| − | + | 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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Check out this pair of Big Block Chevy Headers | Check out this pair of Big Block Chevy Headers | ||
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