Header design
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| − | + | To some, a header is junch 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 gasses out of the engine's cylinders. | |
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| − | + | In the street rod world, absolute mechanical efficiency often takes a back seat to appearance, clearence issues and ease of installation. However, most of us overlook the benefits of a properly designed and built header and how it can improve driveability, 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? | |
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| − | [ | + | 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? I had a pair of Hooker headers for a 318 powered Dodge pickup once, nice torque and driveability improvement, but that's the only pair I have ever seen with tubes that small. Pity that. |
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| + | Just like putting a 300 degree duration cam in a 350 inch small block with 8:1 compression will kill any driveability 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 driveablility, not to mention fuel economy. | ||
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| + | What horsepower does your engine ''REALLY'' make? Most guys 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 I 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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| + | That brings me to primary length. Let me begin by saying, those "shortie" headers are not headers, just tubing manifolds designed for clearance and not horsepower or torque. I know they look like they would flow better than manifolds and probably do in many instances, but 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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| + | I know a lot of you guys are using them and are happy with them, but Hot Rods are about making things better, faster and more efficient aren't they? Just giving some food for thought. | ||
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| + | Anyway, back to primary tube length - 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 gasses out of the Combustion chamber. This is provided that the primary tube has the proper diameter to keep the velocity of the gasses up. | ||
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| + | Another thought - 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 availble 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. I have measured a 16" variance from longest to shortest tube on these units. | ||
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| + | Check out this pair of Big Block Chevy Headers | ||
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| + | [[Image:unequal.jpg]] | ||
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| + | the driver's side rear tube (in yellow) must be about 10"-12" shorter than the next tube (in red). | ||
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| + | This would mean that the short primary tube would scavenge at a higher RPM and the long primary tube would scavenge at a lower RPM for the respecive cylinder. Therefore the clyinder with the short tube will be running lean at low RPM and the long tube clyinder will be running lean at the high RPM and would require different jetting and timing than the others as well. How do you do that with a standard kettering distrubutor and a simple carburetor?. Thats why equal length is important, so you can tune your car. | ||
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| + | This has been demonstrated on a '69 corvette with Headman side exhaust. The engine did not respond to idle screw adjustments at all. There was an 18" difference in primary tube lengths. Switching to an equal length header made adjusting the carb easy and idle vacuum went up 2 inches of vacuum. | ||
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| + | Equalness is usually defined as being within 2 inches between the lonest tube and the shortest, 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 do to different clyinders performing better at different RPM. It is left to the reader to decide if the flattening to torque curve is a good thing to be doing with headers. | ||
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| + | I guess all we can do is create our headers properly or reconfigure existing units to work as they should and not give ourselves tuning headaches. When big name header manufacturers say equal length, they must be talking out their collectors. | ||
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| + | Now we get to the collector - ever see a header with 1 5/8" primaries that had a 2 1/2" collector? No, me neither. But that is the optimum diameter for most street headers. Run that right into a 2 1/2" exhaust and you have a sweet system, lots of torque where you need it and better fuel economy. The collector should have a gentle smooth shape from the four tube area down to the final diameter as well to keep things moving smoothly. | ||
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| + | While we are on the subject, I see the proliferation of 3" exhaust systems out there. If you have very healthy big block,running a blower,some turbo, or have even a ton of nitrous, you go boyee! About the only street application of the monster 3" diameter pipe is when you count on having a engine with large gobs of top-end power. Otherwise, velocity is king in exhaust and 3" is probably too big to keep the speed of the gasses up in the exhaust, and there goes that bottom end torque again! Many reccomend either 2 1/4" or 2 1/2" diameter pipe for street V-8's. | ||
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| + | Summary: | ||
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| + | 1. "shortie" or a "block hugger" - is ''not'' a header | ||
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| + | 2. Bigger diameter primary tubes are NOT better in most street applications | ||
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| + | 3. Longer tubes are better for torque | ||
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| + | 4. Equal primary tube length is very important for tuning and power output | ||
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| + | 5. A long and smooth collector is a good thing | ||
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| + | 6. Just like header tube size, be reasonable with the size of your exhaust system | ||
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