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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. | + | {{youcanedit}} |
| | + | ==Introduction== |
| | + | 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. |
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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? | + | 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? |
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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.
| + | ==Collector size== |
| | + | ===Argument for smaller collectors=== |
| | + | Ever see a car header with 1 5/8" primaries that had a 2 1/2" collector? Hooker makes them for trucks, but you don't see them for cars. But that is the optimum diameter for many street headers. Run that right into a 2 1/2" exhaust and you have a sweet system with lots of torque where you need it, and better fuel economy to boot. The collector should have a smooth, gentle shape from the four tube area down to the final diameter to keep things moving smoothly. |
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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.
| + | If you have very healthy big block, are running a blower, turbo, or nitrous, a 3" header may be appropriate. However, about the only street application of the monster 3" diameter pipe is when you count on having an engine with a lot of top-end power. Otherwise, velocity is king in exhaust and 3" is probably too big to keep the speed of the gases up in the exhaust, and there goes that bottom end torque again! Many recommend either 2-1/4" or 2-1/2" diameter pipe for street V-8's. |
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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.
| + | ===Argument for larger collectors=== |
| | + | A larger-than-needed exhaust system won't necessarily "kill" your low end torque. Any engine that makes respectable low end power will continue to make low end power regardless of a sewer pipe sized exhaust. SOME power may be lost, yes. Usually not enough to make a difference on MOST V8 engines that are even close to being tuned right. Even a stock big block can make complete use of a 3" single exhaust system. Some even come with that size STOCK. |
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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.
| + | The majority of torque "lost" from the bottom end of the power band has actually been moved upwards in the powerband, into the midrange. This can help performance during highway driving and towing, and may provide better passing power when needed. A more powerful midrange tends to make the low end seem less powerful, simply because the engine pulls better in a different part of the RPM range. Most people that think they lost low end power are simply noticing that the engine pulls better in a different way, even if NO low end power was ever lost. |
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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.
| + | ==180 degree exhaust system== |
| | + | One thing that has not been discussed is the unequal firing order from one bank to the other. One college research paper on a Jaguar racing engine having a 105% volumetric efficiency, the torque peaks vs rpm looked like the Grand Tetons. |
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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.
| + | The 1963 Ford Indy engine had the "bundle of snakes" exhaust system on the top with pipes crossing over to have 4 equally spaced exhaust charges in each exhaust header, some race car headers had pipes crossing under the trans to get equal charging in each header. This system is called a 180 degree exhaust. It has some advantages in racing situations, but for the most part it's a very difficult and costly system to manufacture, package and install/service in a production vehicle. |
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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.
| + | ==Gaskets== |
| | + | There are many different types and sizes of header gaskets on the market today. Composite are common, as is fiber types. The composite gaskets seem to hold up a little better, all else being equal, but the main thing is to retighten the header bolts often- like every time the engine is heat cycled- until the fasteners take a set. |
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| − | Check out this pair of Big Block Chevy Headers
| + | [[File:Foil exh gasket sbc1.jpg]] |
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| − | [[Image:unequal.jpg]]
| + | {{!}}Remember- keeping the bolts tight is very important to keeping the gaskets from burning out. |
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| − | the driver's side rear tube (in yellow) must be about 10"-12" shorter than the next tube (in red). | + | For the SBC and any other engine that uses these gaskets as a stock replacement: If/when the gaskets take a dump, try a set of the foil backed composite gaskets that come in most rebuild gasket sets. If they'll fit the head and header ports, they will work as good as most any more expensive composite gasket if the bolts are kept tight. |
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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.
| + | Regardless of what gasket that is used, trim the ID to fit the largest port, be it the header or the head port so there's no overhang into the port. After trimming, start all the bolts and just drop the gaskets into place. Other header gaskets can have the bolt holes slotted in the same way as the foil backed gaskets shown above, for easier installation. |
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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.
| + | ==See also== |
| | + | *Wikipedia article on [http://en.wikipedia.org/wiki/Manifold_%28automotive_engineering%29 exhaust manifolds] |
| | + | *[[Headers]] |
| | + | * http://www.wallaceracing.com/header_length.php |
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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.
| + | [[Category:Engine]] |
| − | | + | [[Category:Exhaust]] |
| − | 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.
| + | [[Category:Undeveloped articles]] |
| − | | + | [[Category:Undeveloped Engine articles]] |
| − | | + | [[Category:Undeveloped Exhaust articles]] |
| − | 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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