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.

+

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 gas's out of the engine's cylinders.

−

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 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?

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.

Line 15:

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.

−

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.

+

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 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. I have measured a 16" variance from longest to shortest tube on these units.

Check out this pair of Big Block Chevy Headers

Line 23:

the driver's side rear tube (in yellow) must be about 10"-12" shorter than the next tube (in red).

−

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.

+

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 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 as well. How do you do that with a standard kettering distributor and a simple carburetor?. Thats why equal length is important, so you can tune your car.

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.

−

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.

+

Equalness is usually defined as being within 2 inches between the longest tube and the shortest, about as close as you can measure with a tape measure at the swap meet.

−

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.

+

There have been claims by some manufacturers that unequal lengths broaden the torque curve do to different cylinders 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.

Line 36:

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.

−

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.

+

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 boy! About the only street application of the monster 3" diameter pipe is when you count on having an 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 recommend either 2 1/4" or 2 1/2" diameter pipe for street V-8's.

Summary:

Header design

Revision as of 10:54, 26 November 2007

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@@ Line 1: / Line 1: @@
-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.
+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 gas's out of the engine's cylinders.
-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 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?
 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.
@@ Line 15: / Line 15: @@
 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.
-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.
+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 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. I have measured a 16" variance from longest to shortest tube on these units.
 Check out this pair of Big Block Chevy Headers
@@ Line 23: / Line 23: @@
 the driver's side rear tube (in yellow) must be about 10"-12" shorter than the next tube (in red).
-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.
+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 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 as well. How do you do that with a standard kettering distributor and a simple carburetor?. Thats why equal length is important, so you can tune your car.
 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.
-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.
+Equalness is usually defined as being within 2 inches between the longest tube and the shortest, about as close as you can measure with a tape measure at the swap meet.
-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.
+There have been claims by some manufacturers that unequal lengths broaden the torque curve do to different cylinders 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.
@@ Line 36: / Line 36: @@
 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.
-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.
+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 boy! About the only street application of the monster 3" diameter pipe is when you count on having an 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 recommend either 2 1/4" or 2 1/2" diameter pipe for street V-8's.
 Summary: