shorties to full length...
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shorties to full length...
I know switching to full length moves the pwer band to the higher RPMs.Considering theyre both 1 5/8 would peak horspower would still be the same??What is more beneficial,switching to full length with the same diameter(1 5/8)headers or use the same design(shorties but with bigger diameter.
Daz
Daz
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From: Kingwood (just NE of Houston), TX, USA
Full-lengths are an order of magnitude better than shorties. A quality set of full-length headers is tuned to actually suck the exhaust gases out of your engine; shorties just give the exhaust a smooth, easy way to get out to the atmosphere. Thus, full-lengths will give you more power at ALL RPMs. There may be a greater power gain at high RPM, but your low end will remain just as strong as ever, if not stronger. Hence, your opening statement that "switching to full length moves the pwer band to the higher RPMs" is, if not inaccurate, somewhat misleading.
As I said above, shorties just make it easier for the exhaust gas to get out of the engine. Bigger shorties provide a larger path, but they're pretty much useless unless your engine needs that larger path, i. e. your engine is built for high-RPM power (in which case you will lose some low-RPM power), or you're using a high-displacement engine. Because of the tuning effect that good full-lengths have, full-lengths are always a better choice than shorties. In fact, I'm almost certain that 1 5/8 full-lengths will make more horsepower than 1 3/4 shorties (outside of the above circumstances).
Unfortunately, there is very little room for full-length headers in our engine compartments. It's very probable that you'll have some problems with fit, and there's no good way to keep your cats. What's more, I'm not sure that the available full-lengths fall into the "quality" category (I've had bad experiences with both Hooker and Hedman). One day I may try welding some myself; if I succeed, I will share results here.
Finally, I recommend that everyone who is even slightly interested in headers click around http://www.headersbyed.com (there's tons of good info, but the layout's a little confusing).
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Jon
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1988 Trans-Am GTA w/ L98, Futura GSX tires, and a rusted-out no-name cat-back
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Mods: TB coolant bypass...more on the way!
As I said above, shorties just make it easier for the exhaust gas to get out of the engine. Bigger shorties provide a larger path, but they're pretty much useless unless your engine needs that larger path, i. e. your engine is built for high-RPM power (in which case you will lose some low-RPM power), or you're using a high-displacement engine. Because of the tuning effect that good full-lengths have, full-lengths are always a better choice than shorties. In fact, I'm almost certain that 1 5/8 full-lengths will make more horsepower than 1 3/4 shorties (outside of the above circumstances).
Unfortunately, there is very little room for full-length headers in our engine compartments. It's very probable that you'll have some problems with fit, and there's no good way to keep your cats. What's more, I'm not sure that the available full-lengths fall into the "quality" category (I've had bad experiences with both Hooker and Hedman). One day I may try welding some myself; if I succeed, I will share results here.
Finally, I recommend that everyone who is even slightly interested in headers click around http://www.headersbyed.com (there's tons of good info, but the layout's a little confusing).
------------------
Jon
------------------
1988 Trans-Am GTA w/ L98, Futura GSX tires, and a rusted-out no-name cat-back
------------------
Mods: TB coolant bypass...more on the way!
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Joined: Feb 2000
Posts: 592
Likes: 1
This seems to be a subject that causes a lot of confusion and conflicting opinions.
When the exhaust valve opens, the exiting gases move across the valve at the speed of sound. As the exhaust plug moves through the exhaust port it creates a low-pressure area behind itself, which scavenges(sucks) residual gases from the cylinder. This action continues even as the plug moves past the port and into the exhaust manifold(or header). Thus regardless of the type of manifold or header there will be some initial scavenging effect. However there is a greater scavenging effect yet to come.
When the exhaust plug reaches the point where the primary pipes converge(the collector), the low pressure area that is trailing the plug reverses and moves back towards the cylinder. When this scavenging pulse reaches the exhaust valve, the greater scavenging effect begins. Optimized scavenging occurs at an rpm when the scavenging pulse enters the cylinder just as the intake valve begins to open. This optimum scavenging event is referred to as 'resonant-tuning'.
Luckily, through careful design of the header, we can control the rpm at which resonant-tuning occurs. By varying the size of the primary tube(by diameter or length) we can adjust the resonant-tuning rpm. Using a longer pipe will incresase the time the scavenging pulse has to travel from the collector to the cylinder and therefore increases the rpm at which resonant-tuning occurs. Increasing the diameter of the pipe will slow the speed of the scavenging wave and, will also increase the resonant tuning rpm. If we move the resonant-tuning rpm high enough, in some cases the peak horsepower rpm may increase. Keep in mind though, as we move the resonant-tuning rpm higher, scavenging becomes less effective at lower rpms(which is why too large headers CAN decrease lower rpm torque).
Additional considerations:
1. Tube diameter plays a very important role in how much energy is contained in the scavenging pulse. Given two headers which create resonant-tuning at the same rpm(large diameter and short length vs small diameter and long length), the header with the smaller pipe diameter will create more energy in the scavenging pulse. Thus if space is not a consideration, it is best to choose the smallest possible diameter pipe, and then adjust length to control the resonant-tuning rpm. Though 1 5/8" is the most popular tube diameter for 350 cid engines, additional power can be extracted by using pipes as small as 1 1/2"(or even 1 3/8"). Unfortunately, with this small of a diameter, very long lengths are needed to achieve the desired 'tuning' rpm. For instance: if we wanted to use a 1 1/2" diameter pipe(which would increase scavenging over a very wide band) on a 350 cid engine and wanted a tuning rpm of 3500, we would need a primary pipe that was approximately 37" in length. Obviously in the tight confines of a third-gen chassis, this would be impractical. Thus we use larger diameter pipes(1 5/8" and above) and shorter lengths.
2.As we have established that diameter is important for energy, and length can adjust timing, it is very important to have all tubes as nearly equal as possible. This ensures that tuning rpm and scavenging energy is equal in all cylinders. However we can also use this to compensate for cylinder to cylinder imbalances created by other components(such as the intake manifold or cylinder heads).
3. If space considerations preclude the use of equal length tubes, we can vary diameter to achieve equal resonant tuning in all cylinders( a la "Edelbrock" headers), at the risk of variances in scavenging energy.
4. As the resonant-tuning rpm is dependent on the intake valve-opening(IVO) event, some consideration should be given on matching the exhaust header to the camshaft. For instance: with a camshaft with short intake duration(which lowers the peak hp rpm), it would be foolish to use a header tuned for very high rpm. If this camshaft make abundant low-end torque at the cost of higher rpm horsepower, then it would be wise to choose a header tuned for an rpm above peak torque. This can balance the output for an extended rpm powerband. As always balancing the pro's and cons of each component of the engine, will lead to best overall result.
------------------
He who hesitates,... is lost!
[This message has been edited by 88IROCs (edited December 01, 2000).]
When the exhaust valve opens, the exiting gases move across the valve at the speed of sound. As the exhaust plug moves through the exhaust port it creates a low-pressure area behind itself, which scavenges(sucks) residual gases from the cylinder. This action continues even as the plug moves past the port and into the exhaust manifold(or header). Thus regardless of the type of manifold or header there will be some initial scavenging effect. However there is a greater scavenging effect yet to come.
When the exhaust plug reaches the point where the primary pipes converge(the collector), the low pressure area that is trailing the plug reverses and moves back towards the cylinder. When this scavenging pulse reaches the exhaust valve, the greater scavenging effect begins. Optimized scavenging occurs at an rpm when the scavenging pulse enters the cylinder just as the intake valve begins to open. This optimum scavenging event is referred to as 'resonant-tuning'.
Luckily, through careful design of the header, we can control the rpm at which resonant-tuning occurs. By varying the size of the primary tube(by diameter or length) we can adjust the resonant-tuning rpm. Using a longer pipe will incresase the time the scavenging pulse has to travel from the collector to the cylinder and therefore increases the rpm at which resonant-tuning occurs. Increasing the diameter of the pipe will slow the speed of the scavenging wave and, will also increase the resonant tuning rpm. If we move the resonant-tuning rpm high enough, in some cases the peak horsepower rpm may increase. Keep in mind though, as we move the resonant-tuning rpm higher, scavenging becomes less effective at lower rpms(which is why too large headers CAN decrease lower rpm torque).
Additional considerations:
1. Tube diameter plays a very important role in how much energy is contained in the scavenging pulse. Given two headers which create resonant-tuning at the same rpm(large diameter and short length vs small diameter and long length), the header with the smaller pipe diameter will create more energy in the scavenging pulse. Thus if space is not a consideration, it is best to choose the smallest possible diameter pipe, and then adjust length to control the resonant-tuning rpm. Though 1 5/8" is the most popular tube diameter for 350 cid engines, additional power can be extracted by using pipes as small as 1 1/2"(or even 1 3/8"). Unfortunately, with this small of a diameter, very long lengths are needed to achieve the desired 'tuning' rpm. For instance: if we wanted to use a 1 1/2" diameter pipe(which would increase scavenging over a very wide band) on a 350 cid engine and wanted a tuning rpm of 3500, we would need a primary pipe that was approximately 37" in length. Obviously in the tight confines of a third-gen chassis, this would be impractical. Thus we use larger diameter pipes(1 5/8" and above) and shorter lengths.
2.As we have established that diameter is important for energy, and length can adjust timing, it is very important to have all tubes as nearly equal as possible. This ensures that tuning rpm and scavenging energy is equal in all cylinders. However we can also use this to compensate for cylinder to cylinder imbalances created by other components(such as the intake manifold or cylinder heads).
3. If space considerations preclude the use of equal length tubes, we can vary diameter to achieve equal resonant tuning in all cylinders( a la "Edelbrock" headers), at the risk of variances in scavenging energy.
4. As the resonant-tuning rpm is dependent on the intake valve-opening(IVO) event, some consideration should be given on matching the exhaust header to the camshaft. For instance: with a camshaft with short intake duration(which lowers the peak hp rpm), it would be foolish to use a header tuned for very high rpm. If this camshaft make abundant low-end torque at the cost of higher rpm horsepower, then it would be wise to choose a header tuned for an rpm above peak torque. This can balance the output for an extended rpm powerband. As always balancing the pro's and cons of each component of the engine, will lead to best overall result.
------------------
He who hesitates,... is lost!
[This message has been edited by 88IROCs (edited December 01, 2000).]
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