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I think that equal length primaries are a LOT more important than most people think.
Shorter primaries push the scavenging effect further up the RPM range than longer tubes. So what happens when you have a short tube on one cylinder and a long tube on the next cylinder? The scavenging pulls more out of the long tube cylinder at low RPM and more out of the short tube cylinder at high RPM. With better scavenging comes better cylinder filling on the intake side... so at low RPM the short tube cylinder is running richer than the average of the two and the long tube cylinder is running leaner than average. At high RPM the situation is reversed.
So with non-equal length headers, some of the cylinders are ALWAYS out of tune. HOWEVER, you CAN NOT see it because your O2 sensor is in the collector and reads the AVERAGE of all the cylinders in that collector. So with unequal length headers, it's quite possible to have an engine as a whole is running a good mixture across the board, but NOT ONE cylinder running the correct mixture through the entire operating range.
Headers work by two mechanisms: "resonant" or "frequency" tuning (determined by primary length) and "momentum" or "velocity" tuning (determined by primary diameter).
Frequency tuning:
As the exhaust valve opens, the cylinder pressure is high. The rush of hot pressurized gas passing the valved creates a high pressure pulse that travels down the primary pipe at the speed of sound. When it arrives at the collector, which is for the purposes of wave mechanics an OPEN pipe end, it is reflected back toward the exhaust valve as a LOW pressure pulse. At the primary tuning RPM, this low pressure pulse arrives back at the exhaust valve just as it is closing and helps to suck the last remaining exhaust gases out of the cylinder right before the exhaust valve closes.
If the engine ALSO has a tuned intake system (like TPI), then the low pressure pulse will enter the cylinder (sucking exhaust out in the process), go through the intake valve and travel up the intake tract, augmenting the engine's natural low pressure pulse produced by the intake valve opening. It reflects off the intake plenum as a high pressure pulse and arrives back at the intake valve as it is closing and pushes the last little bit of mixture into the cylinder.
To select the primary runner length, first decide what you want the engine's torque curve to look like. Specifically, at what RPM do you want peak torque to occur? Select a cam accordingly. IE, if you want peak torque at 4000 RPM, then select a cam for which that RPM is 1/3 to 1/2 the way up the cam's recommended operating RPM range. Example: if you want peak torque at 4000 RPM, then select a cam with an advertised power range from 2,000 to 6,000 or from 3,000 to 6,000. Once you have your cam picked out, look at the duration of the exhaust lobes and calculate how much time the exhaust valve will be open at your chosen RPM.
Example: 4,000 RPM is about 67 RPS; each revolution takes 0.015 seconds. A 225 degree cam keeps the valve open ( 225 / 360 = ) 0.625 revolutions. The valve is open 9.4 milliseconds at 4,000 RPM.
Once you know that, calculate how far sound will travel in that amount of time at your exhaust gas temperature. Divide this length in half (pulse has to travel down the primary AND come back up).
For street engines, this number will be MUCH larger than a practical header tube length. Select an integral fraction of this number (1/2, 1/3, 1/4, etc) that will make a reasonable length for an exhaust primary. If your calculated length is 144 inches, 1/2 is 72 inches, 1/3 is 48 and 1/4 is 36. 36 inches is a reasonable length for an exhaust primary, but the others are not.
The pulse travels MUCH further in the time the valve is open than is reasonable to make a primary pipe. The length can be "folded" into a shorter pipe, but the effect is decreased. In physical terms, when the pulse travels down the pipe, reflects and comes back to the valve, this is the "first reflection". If the pulse goes down a half length pipe, reflects off the collector, comes back to the valve in the middle of the valve event and reflects off the open valve, then goes back to the collector, reflects, then arrives back at the valve as it is closing, the system is using the "second reflection" (from the collector). As the number of reflections increases, the strength of the pulse goes down. Most street systems use the fourth reflection. An engine that turns extremely high RPM (for example Indy and Formula 1 engines) can use fewer reflections for a stronger tuning effect.
Momentum tuning is a little more vague. I haven't heard a good explanation, but it goes something like this: As the gases are pushed out of the chamber by the piston, the speed of the flow and the mass of the gas create a certain momentum in the flow. In order for reversion to occur, this momentum has to be overcome by high pressure at the outer end of the flow and low pressure in the cylinder. In other words, the momentum of the gas can CREATE a low pressure area in the cylinder and keep going, but slows down in the process. As the exhaust valve opens and the cylinder blows down its residual pressure, the pressure wave used in resonant tuning leads the exhaust flow out of the cylinder. As the piston travels up the bore, it pushes gas out, accelerating and then maintaining flow velocity. As the piston crosses the halfway mark and continues upward from mid-stroke, it begins to slow. The piston is moving its fastest between 1/4 the way up the bore and 3/4 the way up the bore, but it begins to slow down at the halfway point. Once it hits the 3/4 point, it slows much more dramatically. At this point, the piston is not pushing very fast on the exhaust gas and it could begin to slow and stagnate, except that the flow that has already been pushed out of the cylinder KEEPS GOING and leaves low pressure behind it. This low pressure helps to pull the last remnant of the exhaust gases out of the chamber to make way for new mixture. It can also augment the low pressure pulse that travels up the runner when the intake valve is opened.
If the primary diameter is too small, the gas speed is very high and the momentum effect is very strong at low RPM. However, with the increased speed comes increased parasitic loss on the engine. The piston pushing the exhaust gas out has to supply the energy of the flow (residual combustion energy goes into the pulse that creates the resonant effect), so as the flow speed increases, the piston has to supply more energy to the flow. Energy increases as the SQUARE of speed, so the energy requirement goes up much faster than the velocity.
With the primary smaller than optimum, velocity will be high and the momentum effect will be strong at low RPM, increasing low RPM torque. At high RPM, the engine will use a LOT of energy pushing the gas out through the small tube and may not be able to get all of it out by the time the exhaust valve closes.
With the primary larger than optimum, momentum effect will be very weak at low RPM and low RPM torque will suffer. The engine will not achieve a good exhaust speed until AFTER peak torque RPM and thus will not be able to utilize the momentum effect for as much of its RPM range as it should. With optimally tuned primary diameter, the engine gets best benefit from the momentum effect at the peak torque RPM. This involves chosing primary diameter so as to achieve an OPTIMUM (NOT maximum) velocity at peak torque RPM. I do not know what this velocity should be or how to determine it. Headers by Ed has a publicly available table that gives guidelines for primary diameter based on HP/cylinder.
As if that weren't enough complexity, resonant and momentum tuning interact with each other to a limited extent. A 10" primary 1 5/8" inches in diameter, such as might be found in a "shorty" header set, does NOT have the same momentum tuning effect that a 30" primary 1 5/8" in diameter has.
Stepped primaries: Some builders advocate stepping the primary diameter up as distance from the exhaust port increases. This is to compensate for the loss of exhaust heat and energy through the walls of the header pipe as the gas travels down the pipe. This is a small effect. Heat loss (and the accompanying need to step pipe diameter) can be minimized through the use of ceramic coatings and header wrap, as advocated by Smokey Yunick.
Collectors: Collector design is CRITICAL to getting the most out of a set of headers. The collector works on all the primaries the way the primaries work on the exhaust ports. I don't know a whole lot about specifying collector attributes either.
Shorter primaries push the scavenging effect further up the RPM range than longer tubes. So what happens when you have a short tube on one cylinder and a long tube on the next cylinder? The scavenging pulls more out of the long tube cylinder at low RPM and more out of the short tube cylinder at high RPM. With better scavenging comes better cylinder filling on the intake side... so at low RPM the short tube cylinder is running richer than the average of the two and the long tube cylinder is running leaner than average. At high RPM the situation is reversed.
So with non-equal length headers, some of the cylinders are ALWAYS out of tune. HOWEVER, you CAN NOT see it because your O2 sensor is in the collector and reads the AVERAGE of all the cylinders in that collector. So with unequal length headers, it's quite possible to have an engine as a whole is running a good mixture across the board, but NOT ONE cylinder running the correct mixture through the entire operating range.
Headers work by two mechanisms: "resonant" or "frequency" tuning (determined by primary length) and "momentum" or "velocity" tuning (determined by primary diameter).
Frequency tuning:
As the exhaust valve opens, the cylinder pressure is high. The rush of hot pressurized gas passing the valved creates a high pressure pulse that travels down the primary pipe at the speed of sound. When it arrives at the collector, which is for the purposes of wave mechanics an OPEN pipe end, it is reflected back toward the exhaust valve as a LOW pressure pulse. At the primary tuning RPM, this low pressure pulse arrives back at the exhaust valve just as it is closing and helps to suck the last remaining exhaust gases out of the cylinder right before the exhaust valve closes.
If the engine ALSO has a tuned intake system (like TPI), then the low pressure pulse will enter the cylinder (sucking exhaust out in the process), go through the intake valve and travel up the intake tract, augmenting the engine's natural low pressure pulse produced by the intake valve opening. It reflects off the intake plenum as a high pressure pulse and arrives back at the intake valve as it is closing and pushes the last little bit of mixture into the cylinder.
To select the primary runner length, first decide what you want the engine's torque curve to look like. Specifically, at what RPM do you want peak torque to occur? Select a cam accordingly. IE, if you want peak torque at 4000 RPM, then select a cam for which that RPM is 1/3 to 1/2 the way up the cam's recommended operating RPM range. Example: if you want peak torque at 4000 RPM, then select a cam with an advertised power range from 2,000 to 6,000 or from 3,000 to 6,000. Once you have your cam picked out, look at the duration of the exhaust lobes and calculate how much time the exhaust valve will be open at your chosen RPM.
Example: 4,000 RPM is about 67 RPS; each revolution takes 0.015 seconds. A 225 degree cam keeps the valve open ( 225 / 360 = ) 0.625 revolutions. The valve is open 9.4 milliseconds at 4,000 RPM.
Once you know that, calculate how far sound will travel in that amount of time at your exhaust gas temperature. Divide this length in half (pulse has to travel down the primary AND come back up).
For street engines, this number will be MUCH larger than a practical header tube length. Select an integral fraction of this number (1/2, 1/3, 1/4, etc) that will make a reasonable length for an exhaust primary. If your calculated length is 144 inches, 1/2 is 72 inches, 1/3 is 48 and 1/4 is 36. 36 inches is a reasonable length for an exhaust primary, but the others are not.
The pulse travels MUCH further in the time the valve is open than is reasonable to make a primary pipe. The length can be "folded" into a shorter pipe, but the effect is decreased. In physical terms, when the pulse travels down the pipe, reflects and comes back to the valve, this is the "first reflection". If the pulse goes down a half length pipe, reflects off the collector, comes back to the valve in the middle of the valve event and reflects off the open valve, then goes back to the collector, reflects, then arrives back at the valve as it is closing, the system is using the "second reflection" (from the collector). As the number of reflections increases, the strength of the pulse goes down. Most street systems use the fourth reflection. An engine that turns extremely high RPM (for example Indy and Formula 1 engines) can use fewer reflections for a stronger tuning effect.
Momentum tuning is a little more vague. I haven't heard a good explanation, but it goes something like this: As the gases are pushed out of the chamber by the piston, the speed of the flow and the mass of the gas create a certain momentum in the flow. In order for reversion to occur, this momentum has to be overcome by high pressure at the outer end of the flow and low pressure in the cylinder. In other words, the momentum of the gas can CREATE a low pressure area in the cylinder and keep going, but slows down in the process. As the exhaust valve opens and the cylinder blows down its residual pressure, the pressure wave used in resonant tuning leads the exhaust flow out of the cylinder. As the piston travels up the bore, it pushes gas out, accelerating and then maintaining flow velocity. As the piston crosses the halfway mark and continues upward from mid-stroke, it begins to slow. The piston is moving its fastest between 1/4 the way up the bore and 3/4 the way up the bore, but it begins to slow down at the halfway point. Once it hits the 3/4 point, it slows much more dramatically. At this point, the piston is not pushing very fast on the exhaust gas and it could begin to slow and stagnate, except that the flow that has already been pushed out of the cylinder KEEPS GOING and leaves low pressure behind it. This low pressure helps to pull the last remnant of the exhaust gases out of the chamber to make way for new mixture. It can also augment the low pressure pulse that travels up the runner when the intake valve is opened.
If the primary diameter is too small, the gas speed is very high and the momentum effect is very strong at low RPM. However, with the increased speed comes increased parasitic loss on the engine. The piston pushing the exhaust gas out has to supply the energy of the flow (residual combustion energy goes into the pulse that creates the resonant effect), so as the flow speed increases, the piston has to supply more energy to the flow. Energy increases as the SQUARE of speed, so the energy requirement goes up much faster than the velocity.
With the primary smaller than optimum, velocity will be high and the momentum effect will be strong at low RPM, increasing low RPM torque. At high RPM, the engine will use a LOT of energy pushing the gas out through the small tube and may not be able to get all of it out by the time the exhaust valve closes.
With the primary larger than optimum, momentum effect will be very weak at low RPM and low RPM torque will suffer. The engine will not achieve a good exhaust speed until AFTER peak torque RPM and thus will not be able to utilize the momentum effect for as much of its RPM range as it should. With optimally tuned primary diameter, the engine gets best benefit from the momentum effect at the peak torque RPM. This involves chosing primary diameter so as to achieve an OPTIMUM (NOT maximum) velocity at peak torque RPM. I do not know what this velocity should be or how to determine it. Headers by Ed has a publicly available table that gives guidelines for primary diameter based on HP/cylinder.
As if that weren't enough complexity, resonant and momentum tuning interact with each other to a limited extent. A 10" primary 1 5/8" inches in diameter, such as might be found in a "shorty" header set, does NOT have the same momentum tuning effect that a 30" primary 1 5/8" in diameter has.
Stepped primaries: Some builders advocate stepping the primary diameter up as distance from the exhaust port increases. This is to compensate for the loss of exhaust heat and energy through the walls of the header pipe as the gas travels down the pipe. This is a small effect. Heat loss (and the accompanying need to step pipe diameter) can be minimized through the use of ceramic coatings and header wrap, as advocated by Smokey Yunick.
Collectors: Collector design is CRITICAL to getting the most out of a set of headers. The collector works on all the primaries the way the primaries work on the exhaust ports. I don't know a whole lot about specifying collector attributes either.