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GLOSSARY:
1 - Tuning theory
2 - Jetting size chart for basic modifications
3 - Plug chopping (Is my engine rich or lean?)
4 - Changing jets (Which jet does what?)
1 - Tuning theory
This section deals primarily with what intake and exhaust modifications do to the tuning of our engine, and how we can modify that state of tune to take advantage of the modifications. I've split it up into a few sections; Intake and Exhaust. If you're after the quick jetting size chart, see the next section.
Nearly all bikes prior to the mid 90's used carburettors to meter a fuel and air mix into the engine, and some modern bikes (the Suzuki GS500 is one example) are still equipped with them. It is the carburettor that I will concentrate tuning methods around, as fuel injection is beyond the scope of my knowledge. Besides, the first FI setup on a production bike only came along in 1982 on the Honda CX500 Turbo, and most of the bikes we mess around with here predate.
The carburettor is tuned at the factory to run well in cooperation with the stock engine setup. This means that it is tuned to the flow characteristics provided by your stock engine, exhaust, and intake. Any changes to those will effect the carburettion, thus you will need to tune the carburettor for the new setting.
Intake
The intake for your bike (working back from the combustion chamber) consists of your intake valves and ports, carburettor manifolds, carburettor(s), filter element(s) and airbox (if any). These work together to meter both fuel and air to your engine in a controlled fashion, so your engine runs cleanly and smoothly whilst providing usable power. The most important part of this delivery system is the carburettor, as it is the item that mixes the fuel/air mix and sends it on its merry way to be turned into power.
When we talk about 'flow characteristics', we are really talking about restrictions in the intake and exhaust. Such restrictions come in the form of air boxes and filters, the diameter of the manifolds, ports and valves (both intake and exhaust) and the type of silencing gear the bike is equipped with. A change to any of those items will generally require a change in your carburettor tuning to compensate for the change in flow charactieristics.
The largest change in flow on most bikes can be acheived by modifying the intake. This can be done in several ways, each with their own advantages and disadvantages. They are each discussed, along with their crude jetting requirements, below. Remember, for all intake modifications any shortening of the intake tract will raise power up the rev range.
Stage 1 - High-flow filter
The first , and easiest, step is to replace your stock filter media with an aftermarket one. A well-known company is K&N. This item simply replaces the stock air filter with one that provides more airflow. The advantages of this are that it is easy to do, will not affect the look of your bike, and will often give a modest boost in power (1-2hp). Many aftermarket filters are also washable, and thus reusable. The disadvantages are that it can be costly for a good brand-name performance filter(s), it will not compensate for poor airbox design, and that you may still need to change your carburettor tuning to compensate - no matter what it says on the box! Overall it is a good step for the novice tuner, and those wishing to retain the stock look of their bike but still have a bit extra kick.
Stage 2 - High flow filter and modified airbox
The second stage includes the stage above, but also some modifications to the stock airbox to provide better flow characteristics. This can be achieved by simply drilling a few more holes in the airbox, but is best done by duplicating the type of intake already there. These can vary from simple drilled holes, to intricate ram-air setups. A good system of induction that is easy and effective is a reverse-taper inlet. This is simply a tapering tube that directs air into the airbox. The end closest to the airbox is widest, and tapers down slightly as it gets further away from the box. The reason for this is that turbulent air is slowed and calmed by the widening of the intake duct, thus providing denser air to your airbox. In laymans terms, it gives you more power!
The advantages of this are that you can generally order or make these items fairly cheaply, or scavenge them from a wrecking yard. They are easy to attach with silicone or araldite, and won't affect the stock look of your bike too much. The disadvantages are that you need to have space to fit them, they should point in the direction of flow, and that it isn't simply a 'bolt on' part. You will also need to change your carburettor tuning to compensate for the increased air flow.
Stage 3 - Individual ('Pod') filters
Pod filters are a very popular item, and are essentially a circular piece of filter media that is attached directly to your carburettor(s). This provides much better throttle response and a higher maximum airflow rate compared to many stock setups. The downside is that you may experience a loss of your midrange power in exchange for better topend characteristics. You will need to alter your carburettor tuning, perhaps down to float height and needle settings - be prepared to take your float bowls off more than once, there is no spot-on tuning guide that will tell you which jets and settings will be perfect. With patience, you will be able to get good jetting again and hopefully enjoy the change from stock.
Stage 4 - Open velocity stacks
Velocity stacks work by providing a smoothed, linear airflow to your carburettors. It should be noted that the difference in total airflow between a good set of stacks and a simple open tube is 1% or less. If you want that 1%, then good for you - as a bonus they also look pretty cool and have a great induction sound, much like pod filters...but rawer and louder. The advantages are killer throttle response and better topend because you have maximised total flow and negated any restrictions imposed by filters or airboxes. The bad news is that you will need to rejet your carby, and unfiltered engines should be serviced and inspected much more regularly than engines with an airfilter.
The reason for this is that small particles of dust and dirt can enter your engine (if you do not use a filter) and cause accelerated/excessive wear. A good set of velocity stacks is also generally fairly expensive, compared to pod filters. If you insist on using velocity stacks, I would recommend you use a clip-on filter media as well. These are normally an aftermarket option and can be bought with the stacks, or made yourself with fine-gauge wire and filter foam.
Exhaust
The exhaust tract of your bike consists of everything from the exhaust valves back, including your ports, manifold and silencer. You may also have some sort of emission device, such as an Exhaust Gas Recirculation system (EGR) or exhaust sparking system, both designed to reduce emissions from your engine. This section deals with modifying your exhaust system - mainly your manifold and silencer setup. Remember, any shortening of the exhaust tract will tend to move power further up the rev range.
Stage 1 - Debaffled/unpacked silencer.
This simply modification removes the baffle from your silencer, or the packing, or both. The baffle is generally a simple perforated tube running the length of the silencer. Packing, usually glass-fibre, is wrapped around this to help muffle sound and provide a longer tract for the exhaust gasses to pass through and transfer their energy to by way of heat and vibration, thus quieting your exhaust note down. These items also reduce total flow, thus robbing your engine of potential horsepower, so removing the packing and/or baffle can help give you some more kick...after a carburettor retune of course! The upside is that its free, or only the cost of jets. The bad news is that it may draw unwanted attention to your bike via the loud exhaust note, and potentially be illegal.
Stage 2 - Slipon system/aftermarket silencer
It is what it sounds like - an aftermarket silencer(s) for your bike. Some of these are simple replacements, but you can get performance items from companies such as Two Brothers, Tyga Performance and Akrapovic just to name a few. These items are generally well designed and come in such nice materials as lightweight aluminum, carbon fibre, or even titanium - all desiged to save weight. The inner baffle is often readily removable for tuning or track purposes, too. I would bet on them providing no more than a few horsepower, but the weight savings and exhaust note may make it worth it. I would recommend a plugchop (See Section 4) to see if you need to retune your carby.
Stage 3 - Full system
A full system is everything, the headers, manifold, exhaust outlet(s) and silencer(s). A popular tuning item, the best are tailored to your bike and will be of a tuned length so as to maximise performance via efficient exhaust scavenging, where resonance from the exhaust system helps pull burnt gasses out of the combustion chamber(s) and ready it for a fresh intake charge. You can gain a fair bit of power by fitting one of these systems, although that will depend on other modifications done to the bike, and how bad your setup was before. Carburettor tuning is a given, but you will benefit from a shiny new exhaust and better performance, possibly even fuel economy and weight savings.
For the record, a tuned-length 2-1 or 4-1 pipe is designed to help the exhaust pulses flow cleanly after one another, using the low-pressure area in their wake to help pull the next one along. This provides better scavenging and overall flow characteristics. In other words, more power.
- boingk
1 - Tuning theory
2 - Jetting size chart for basic modifications
3 - Plug chopping (Is my engine rich or lean?)
4 - Changing jets (Which jet does what?)
1 - Tuning theory
This section deals primarily with what intake and exhaust modifications do to the tuning of our engine, and how we can modify that state of tune to take advantage of the modifications. I've split it up into a few sections; Intake and Exhaust. If you're after the quick jetting size chart, see the next section.
Nearly all bikes prior to the mid 90's used carburettors to meter a fuel and air mix into the engine, and some modern bikes (the Suzuki GS500 is one example) are still equipped with them. It is the carburettor that I will concentrate tuning methods around, as fuel injection is beyond the scope of my knowledge. Besides, the first FI setup on a production bike only came along in 1982 on the Honda CX500 Turbo, and most of the bikes we mess around with here predate.
The carburettor is tuned at the factory to run well in cooperation with the stock engine setup. This means that it is tuned to the flow characteristics provided by your stock engine, exhaust, and intake. Any changes to those will effect the carburettion, thus you will need to tune the carburettor for the new setting.
Intake
The intake for your bike (working back from the combustion chamber) consists of your intake valves and ports, carburettor manifolds, carburettor(s), filter element(s) and airbox (if any). These work together to meter both fuel and air to your engine in a controlled fashion, so your engine runs cleanly and smoothly whilst providing usable power. The most important part of this delivery system is the carburettor, as it is the item that mixes the fuel/air mix and sends it on its merry way to be turned into power.
When we talk about 'flow characteristics', we are really talking about restrictions in the intake and exhaust. Such restrictions come in the form of air boxes and filters, the diameter of the manifolds, ports and valves (both intake and exhaust) and the type of silencing gear the bike is equipped with. A change to any of those items will generally require a change in your carburettor tuning to compensate for the change in flow charactieristics.
The largest change in flow on most bikes can be acheived by modifying the intake. This can be done in several ways, each with their own advantages and disadvantages. They are each discussed, along with their crude jetting requirements, below. Remember, for all intake modifications any shortening of the intake tract will raise power up the rev range.
Stage 1 - High-flow filter
The first , and easiest, step is to replace your stock filter media with an aftermarket one. A well-known company is K&N. This item simply replaces the stock air filter with one that provides more airflow. The advantages of this are that it is easy to do, will not affect the look of your bike, and will often give a modest boost in power (1-2hp). Many aftermarket filters are also washable, and thus reusable. The disadvantages are that it can be costly for a good brand-name performance filter(s), it will not compensate for poor airbox design, and that you may still need to change your carburettor tuning to compensate - no matter what it says on the box! Overall it is a good step for the novice tuner, and those wishing to retain the stock look of their bike but still have a bit extra kick.
Stage 2 - High flow filter and modified airbox
The second stage includes the stage above, but also some modifications to the stock airbox to provide better flow characteristics. This can be achieved by simply drilling a few more holes in the airbox, but is best done by duplicating the type of intake already there. These can vary from simple drilled holes, to intricate ram-air setups. A good system of induction that is easy and effective is a reverse-taper inlet. This is simply a tapering tube that directs air into the airbox. The end closest to the airbox is widest, and tapers down slightly as it gets further away from the box. The reason for this is that turbulent air is slowed and calmed by the widening of the intake duct, thus providing denser air to your airbox. In laymans terms, it gives you more power!
The advantages of this are that you can generally order or make these items fairly cheaply, or scavenge them from a wrecking yard. They are easy to attach with silicone or araldite, and won't affect the stock look of your bike too much. The disadvantages are that you need to have space to fit them, they should point in the direction of flow, and that it isn't simply a 'bolt on' part. You will also need to change your carburettor tuning to compensate for the increased air flow.
Stage 3 - Individual ('Pod') filters
Pod filters are a very popular item, and are essentially a circular piece of filter media that is attached directly to your carburettor(s). This provides much better throttle response and a higher maximum airflow rate compared to many stock setups. The downside is that you may experience a loss of your midrange power in exchange for better topend characteristics. You will need to alter your carburettor tuning, perhaps down to float height and needle settings - be prepared to take your float bowls off more than once, there is no spot-on tuning guide that will tell you which jets and settings will be perfect. With patience, you will be able to get good jetting again and hopefully enjoy the change from stock.
Stage 4 - Open velocity stacks
Velocity stacks work by providing a smoothed, linear airflow to your carburettors. It should be noted that the difference in total airflow between a good set of stacks and a simple open tube is 1% or less. If you want that 1%, then good for you - as a bonus they also look pretty cool and have a great induction sound, much like pod filters...but rawer and louder. The advantages are killer throttle response and better topend because you have maximised total flow and negated any restrictions imposed by filters or airboxes. The bad news is that you will need to rejet your carby, and unfiltered engines should be serviced and inspected much more regularly than engines with an airfilter.
The reason for this is that small particles of dust and dirt can enter your engine (if you do not use a filter) and cause accelerated/excessive wear. A good set of velocity stacks is also generally fairly expensive, compared to pod filters. If you insist on using velocity stacks, I would recommend you use a clip-on filter media as well. These are normally an aftermarket option and can be bought with the stacks, or made yourself with fine-gauge wire and filter foam.
Exhaust
The exhaust tract of your bike consists of everything from the exhaust valves back, including your ports, manifold and silencer. You may also have some sort of emission device, such as an Exhaust Gas Recirculation system (EGR) or exhaust sparking system, both designed to reduce emissions from your engine. This section deals with modifying your exhaust system - mainly your manifold and silencer setup. Remember, any shortening of the exhaust tract will tend to move power further up the rev range.
Stage 1 - Debaffled/unpacked silencer.
This simply modification removes the baffle from your silencer, or the packing, or both. The baffle is generally a simple perforated tube running the length of the silencer. Packing, usually glass-fibre, is wrapped around this to help muffle sound and provide a longer tract for the exhaust gasses to pass through and transfer their energy to by way of heat and vibration, thus quieting your exhaust note down. These items also reduce total flow, thus robbing your engine of potential horsepower, so removing the packing and/or baffle can help give you some more kick...after a carburettor retune of course! The upside is that its free, or only the cost of jets. The bad news is that it may draw unwanted attention to your bike via the loud exhaust note, and potentially be illegal.
Stage 2 - Slipon system/aftermarket silencer
It is what it sounds like - an aftermarket silencer(s) for your bike. Some of these are simple replacements, but you can get performance items from companies such as Two Brothers, Tyga Performance and Akrapovic just to name a few. These items are generally well designed and come in such nice materials as lightweight aluminum, carbon fibre, or even titanium - all desiged to save weight. The inner baffle is often readily removable for tuning or track purposes, too. I would bet on them providing no more than a few horsepower, but the weight savings and exhaust note may make it worth it. I would recommend a plugchop (See Section 4) to see if you need to retune your carby.
Stage 3 - Full system
A full system is everything, the headers, manifold, exhaust outlet(s) and silencer(s). A popular tuning item, the best are tailored to your bike and will be of a tuned length so as to maximise performance via efficient exhaust scavenging, where resonance from the exhaust system helps pull burnt gasses out of the combustion chamber(s) and ready it for a fresh intake charge. You can gain a fair bit of power by fitting one of these systems, although that will depend on other modifications done to the bike, and how bad your setup was before. Carburettor tuning is a given, but you will benefit from a shiny new exhaust and better performance, possibly even fuel economy and weight savings.
For the record, a tuned-length 2-1 or 4-1 pipe is designed to help the exhaust pulses flow cleanly after one another, using the low-pressure area in their wake to help pull the next one along. This provides better scavenging and overall flow characteristics. In other words, more power.
- boingk
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