Spool & Transient Responce...........
10-20-2010, 03:15 PM
#1
Registered User
Thread Starter
Spool & Transient Responce...........
I have been talking to a lot of members recently about “Spool”. I wanted to clarify a few things that we all need to keep in mind when thinking about the topic. This is the first draft with your input I will expand and contract as needed.
The “Spool”
The spool is defined as the rpm at which the desired boost pressure is reached and has many variables involved in determining it. To obtain a full “Spool” the turbo has to reach the a point at which the exhaust gases accelerate the turbine wheel and increased the velocity of the turbine shaft so that the delivered CFM from the compressor exceeds the needs of the engine and creates positive pressure in the cylinder. This is related to but not the same as “Transient Response”, which I will go into later.
From this definition we can conclude that the turbine design which drives the acceleration, compressor wheel design ie how much CFM it can provide for a given shaft speed, engine displacement, cam shaft design and pressure drop through the system piping to the cylinders (Not just to the throttle body). In addition there are indirect variables such as the pressure differential across the turbine wheel and compressor. On top of these variables there are other variables in the tune which can affect the transient response and spool by burning fuel across the turbine to accelerate the impeller with more direct forces.
Turbine Wheel Design - Basically we can only select the diameter but there are other variables that play a role such as the number of impeller blades, mass of the impeller, impeller inducer and exducer Diameter. As a general rule the larger the impeller diameter the slower the spool and the better the topend due to less exhaust back pressure.
Turbine Housing Design - The most important choice here is the standard that is selected on the flange ie T3, T4….and divided. A larger T# the larger the flange and flow area at the loss of a more focused exhaust pulse on the turbine wheel. Divided housings, otherwise known as a twin scroll, offers advantages in the spool by better focusing the exhaust pulses on the wheel and coupling cylinders to increase the scavenging effect. After this selection has been made the next step is to select the A/R and the larger the A/R the larger the housing. This impacts performance by accelerating exhaust gasses increasing the spool with a small A/R and reduces restriction using a larger A/R yielding a better top end.
Compressor Design - As with the turbine we can only have the ability to select the diameter and larger diameter equal slower spool and better top end. Unlike the turbine if a compressor wheel is selected that is too large you will have issue with surge or run very inefficiently. To determine if you are selecting the correct wheel you should consult the compressor map for the model you are selecting. This map show efficiency and is derived from the pressure ratio (air inlet pressure / boost pressure) and plotted against CFM. Efficiency is a measure of how much heat is added from heat of compression in a ideal world and the actual heat that is imparted to the incoming charge. When reading the map you have to be concerned with the surge line when trying to balance spool and topend.
Compressor Housing - Similar to turbines there is a less talked about compressor housing A/R which also needs to be selected. This does effect the spool but not as dramatically as the turbine. Usually you will want to select the largest cover housing available for your given application which may be restricted by physical dimensions. When it is available, particularly when using a large turbo, an Anti Surge Housing will help to prevent lowend surge however this is at the cost of efficiency when boosting. The last consideration I look at is the inlet Diameter and outlet diameter, remember reducing the pressure drop is key so larger is better in this situation.
Engine Displacement - This is very straight forward, the larger the displacement the higher the CFM across the turbine which increased the ability to accelerate the system. However, increased displacement will also take in more air so the impeller must spin faster to feed the engine with enough CFM to create boost also.
Cam Shaft Design - With our S2000’s this is rarely talked about but cam shaft design can play a large role in spool. First by reducing the overlap you reduce the amount of incoming air that is blown through the engine without being burned which should intern increase the spool. Reduced overlap requires less CFM to be supplied to the engine at a given power level at the expense of reduced exhaust gas expulsion. As with all engines increased lift can also effect the spool by reducing the pressure drop across the valve but this is typically negligible with an efficient dual cam profile such as ours due to the secondary lobe having a very large lift and minimal pressure drop to start with. Unlike N/A cars duration plays a smaller role since the air is being forced into the engine not drawn in.
Pressure Drop to the Engine - Touched on above the loss of pressure from the outlet of the turbo to the cylinder plays a role. The lower the pressure drop the less the turbo has to work to hit the boost set pressure. Many of the FMIC application have anywhere from 3-7 PSI pressure drop as a design and after coolers are usually 2-5PSI lower then FMIC and of course no cooling even less. However since high intake temperature mean less density and mass flow after cooling the supply air is critical even after a few PSI of boost.
Turbine Differential Pressure - To increase the total energy of the emerging exhaust gas pulses you want to lower the backpressure from the exhaust system so that the differential pressure across the turbine wheel is reduced. To accomplish this you need to first remove restriction in the manifold and turbine housing so that very little energy is lost from the exhaust pulse. Then you need to remove all the back pressure you can from the downpipe and exhaust system.
Inlet Air Pressure Drop - I added this to reinforce the fact that the lower the pressure drop is across your filter, screen…. The better the response. Though a turbo is not effected as much as a positive displacement supercharger gains can be seen with reductions in this pressure drop. Similarly reducing the incoming turbulence can also help.
So now you know what effects the spool and you have your car built how do you actually measure it. The best and only way I know of is to data log the engine rpm and boost pressure. This however is often done incorrectly and transient response skews the results by delaying the response. In my car I cannot hit full boost until 7000RPM unless I brake boost but I can hit my desired boost level at lower levels in higher gears. So the best way to measure spool is to put the car in a high gear 4th or above usually and go wide open throttle at a very low RPM and wait until the boost set pressure is reached. In addition to this notes should be made on the barometric pressure and or temperature and altitude. The denser the air the easier it is for the compressor to pressurize the intake. This is often exemplified by much higher boost pressure in the winter versus the summer or the reduction of power at high altitudes.
Transient Response
Now to talk about the often miss represented Transient response or time it takes to pressurize the intake with sufficient RPM to create boost. Rather than being measured by RPM this is measured in time. To measure transient response all you need to do is data log time and pressure and take the car above a RPM that is higher than the point you reach full spool at little to no throttle input then go full throttle until you reach full boost.
Many of the factors that affect the spool however we are focused on acceleration so the turbine and intake impeller mass, differential pressures and flow restriction are the more important keys in transient response. As a general rule the smaller the turbo the better the response, but this is due to having the turbo either closer to the full spool RPM or being well above it, not focusing on the ability to free flow.
Not listed above is the volume of the intercooler system, which play’s a larger role in transient response as the turbo must pressurize the intake system then the motor and the added volume can make a big difference.
Probably the most often discussed is the bearing type, by reducing the force of friction the impellers can accelerate much faster. This is why we see ball bearing turbos being sold over journal bearing.
Please feel free to add anything I may have left out that you think it is important.
The “Spool”
The spool is defined as the rpm at which the desired boost pressure is reached and has many variables involved in determining it. To obtain a full “Spool” the turbo has to reach the a point at which the exhaust gases accelerate the turbine wheel and increased the velocity of the turbine shaft so that the delivered CFM from the compressor exceeds the needs of the engine and creates positive pressure in the cylinder. This is related to but not the same as “Transient Response”, which I will go into later.
From this definition we can conclude that the turbine design which drives the acceleration, compressor wheel design ie how much CFM it can provide for a given shaft speed, engine displacement, cam shaft design and pressure drop through the system piping to the cylinders (Not just to the throttle body). In addition there are indirect variables such as the pressure differential across the turbine wheel and compressor. On top of these variables there are other variables in the tune which can affect the transient response and spool by burning fuel across the turbine to accelerate the impeller with more direct forces.
Turbine Wheel Design - Basically we can only select the diameter but there are other variables that play a role such as the number of impeller blades, mass of the impeller, impeller inducer and exducer Diameter. As a general rule the larger the impeller diameter the slower the spool and the better the topend due to less exhaust back pressure.
Turbine Housing Design - The most important choice here is the standard that is selected on the flange ie T3, T4….and divided. A larger T# the larger the flange and flow area at the loss of a more focused exhaust pulse on the turbine wheel. Divided housings, otherwise known as a twin scroll, offers advantages in the spool by better focusing the exhaust pulses on the wheel and coupling cylinders to increase the scavenging effect. After this selection has been made the next step is to select the A/R and the larger the A/R the larger the housing. This impacts performance by accelerating exhaust gasses increasing the spool with a small A/R and reduces restriction using a larger A/R yielding a better top end.
Compressor Design - As with the turbine we can only have the ability to select the diameter and larger diameter equal slower spool and better top end. Unlike the turbine if a compressor wheel is selected that is too large you will have issue with surge or run very inefficiently. To determine if you are selecting the correct wheel you should consult the compressor map for the model you are selecting. This map show efficiency and is derived from the pressure ratio (air inlet pressure / boost pressure) and plotted against CFM. Efficiency is a measure of how much heat is added from heat of compression in a ideal world and the actual heat that is imparted to the incoming charge. When reading the map you have to be concerned with the surge line when trying to balance spool and topend.
Compressor Housing - Similar to turbines there is a less talked about compressor housing A/R which also needs to be selected. This does effect the spool but not as dramatically as the turbine. Usually you will want to select the largest cover housing available for your given application which may be restricted by physical dimensions. When it is available, particularly when using a large turbo, an Anti Surge Housing will help to prevent lowend surge however this is at the cost of efficiency when boosting. The last consideration I look at is the inlet Diameter and outlet diameter, remember reducing the pressure drop is key so larger is better in this situation.
Engine Displacement - This is very straight forward, the larger the displacement the higher the CFM across the turbine which increased the ability to accelerate the system. However, increased displacement will also take in more air so the impeller must spin faster to feed the engine with enough CFM to create boost also.
Cam Shaft Design - With our S2000’s this is rarely talked about but cam shaft design can play a large role in spool. First by reducing the overlap you reduce the amount of incoming air that is blown through the engine without being burned which should intern increase the spool. Reduced overlap requires less CFM to be supplied to the engine at a given power level at the expense of reduced exhaust gas expulsion. As with all engines increased lift can also effect the spool by reducing the pressure drop across the valve but this is typically negligible with an efficient dual cam profile such as ours due to the secondary lobe having a very large lift and minimal pressure drop to start with. Unlike N/A cars duration plays a smaller role since the air is being forced into the engine not drawn in.
Pressure Drop to the Engine - Touched on above the loss of pressure from the outlet of the turbo to the cylinder plays a role. The lower the pressure drop the less the turbo has to work to hit the boost set pressure. Many of the FMIC application have anywhere from 3-7 PSI pressure drop as a design and after coolers are usually 2-5PSI lower then FMIC and of course no cooling even less. However since high intake temperature mean less density and mass flow after cooling the supply air is critical even after a few PSI of boost.
Turbine Differential Pressure - To increase the total energy of the emerging exhaust gas pulses you want to lower the backpressure from the exhaust system so that the differential pressure across the turbine wheel is reduced. To accomplish this you need to first remove restriction in the manifold and turbine housing so that very little energy is lost from the exhaust pulse. Then you need to remove all the back pressure you can from the downpipe and exhaust system.
Inlet Air Pressure Drop - I added this to reinforce the fact that the lower the pressure drop is across your filter, screen…. The better the response. Though a turbo is not effected as much as a positive displacement supercharger gains can be seen with reductions in this pressure drop. Similarly reducing the incoming turbulence can also help.
So now you know what effects the spool and you have your car built how do you actually measure it. The best and only way I know of is to data log the engine rpm and boost pressure. This however is often done incorrectly and transient response skews the results by delaying the response. In my car I cannot hit full boost until 7000RPM unless I brake boost but I can hit my desired boost level at lower levels in higher gears. So the best way to measure spool is to put the car in a high gear 4th or above usually and go wide open throttle at a very low RPM and wait until the boost set pressure is reached. In addition to this notes should be made on the barometric pressure and or temperature and altitude. The denser the air the easier it is for the compressor to pressurize the intake. This is often exemplified by much higher boost pressure in the winter versus the summer or the reduction of power at high altitudes.
Transient Response
Now to talk about the often miss represented Transient response or time it takes to pressurize the intake with sufficient RPM to create boost. Rather than being measured by RPM this is measured in time. To measure transient response all you need to do is data log time and pressure and take the car above a RPM that is higher than the point you reach full spool at little to no throttle input then go full throttle until you reach full boost.
Many of the factors that affect the spool however we are focused on acceleration so the turbine and intake impeller mass, differential pressures and flow restriction are the more important keys in transient response. As a general rule the smaller the turbo the better the response, but this is due to having the turbo either closer to the full spool RPM or being well above it, not focusing on the ability to free flow.
Not listed above is the volume of the intercooler system, which play’s a larger role in transient response as the turbo must pressurize the intake system then the motor and the added volume can make a big difference.
Probably the most often discussed is the bearing type, by reducing the force of friction the impellers can accelerate much faster. This is why we see ball bearing turbos being sold over journal bearing.
Please feel free to add anything I may have left out that you think it is important.
10-21-2010, 10:04 AM
#4
In my effort to add more clarity:
Often times turbo's that "spool" well can have terrible "transient response." Usually turbos with mismatched compressor and turbine wheels.
They can show really good spool on a dyno for how much peak hp they produce but in reality have terrible transient response. I personally would rather have something with better transient response then initial spool, but this is hardly ever talked about.
Transient response can be quantified on a dyno but hardly ever is, and it takes a load based dyno with proper loads to get data that will be similar to how that turbo acts whilest the car is on the road.
An example is I have had both a B5 audi S4 and a 2005 legacy gt turbo. Although the Audi S4 spooled at very low rpms...practically right after idle, when coming on and off the throttle it had worse transient response then the legacy GT. The legacy GT spooled at a higher rpm however but had much better boost response to throttle inputs when in an rpm range past spool, hence better transient response.
An s2000 with GT3076r seems to have excellent spool and transient response to my tastes. I would like to a drive a gt35r divided (twin scroll) setup and see how that is.
Often times turbo's that "spool" well can have terrible "transient response." Usually turbos with mismatched compressor and turbine wheels.
They can show really good spool on a dyno for how much peak hp they produce but in reality have terrible transient response. I personally would rather have something with better transient response then initial spool, but this is hardly ever talked about.
Transient response can be quantified on a dyno but hardly ever is, and it takes a load based dyno with proper loads to get data that will be similar to how that turbo acts whilest the car is on the road.
An example is I have had both a B5 audi S4 and a 2005 legacy gt turbo. Although the Audi S4 spooled at very low rpms...practically right after idle, when coming on and off the throttle it had worse transient response then the legacy GT. The legacy GT spooled at a higher rpm however but had much better boost response to throttle inputs when in an rpm range past spool, hence better transient response.
An s2000 with GT3076r seems to have excellent spool and transient response to my tastes. I would like to a drive a gt35r divided (twin scroll) setup and see how that is.
10-21-2010, 10:45 AM
#5
Registered User
Thread Starter
Originally Posted by copec,Oct 21 2010, 01:04 PM
In my effort to add more clarity:
Often times turbo's that "spool" well can have terrible "transient response." Usually turbos with mismatched compressor and turbine wheels.
They can show really good spool on a dyno for how much peak hp they produce but in reality have terrible transient response. I personally would rather have something with better transient response then initial spool, but this is hardly ever talked about.
Transient response can be quantified on a dyno but hardly ever is, and it takes a load based dyno with proper loads to get data that will be similar to how that turbo acts whilest the car is on the road.
An example is I have had both a B5 audi S4 and a 2005 legacy gt turbo. Although the Audi S4 spooled at very low rpms...practically right after idle, when coming on and off the throttle it had worse transient response then the legacy GT. The legacy GT spooled at a higher rpm however but had much better boost response to throttle inputs when in an rpm range past spool, hence better transient response.
An s2000 with GT3076r seems to have excellent spool and transient response to my tastes. I would like to a drive a gt35r divided (twin scroll) setup and see how that is.
Often times turbo's that "spool" well can have terrible "transient response." Usually turbos with mismatched compressor and turbine wheels.
They can show really good spool on a dyno for how much peak hp they produce but in reality have terrible transient response. I personally would rather have something with better transient response then initial spool, but this is hardly ever talked about.
Transient response can be quantified on a dyno but hardly ever is, and it takes a load based dyno with proper loads to get data that will be similar to how that turbo acts whilest the car is on the road.
An example is I have had both a B5 audi S4 and a 2005 legacy gt turbo. Although the Audi S4 spooled at very low rpms...practically right after idle, when coming on and off the throttle it had worse transient response then the legacy GT. The legacy GT spooled at a higher rpm however but had much better boost response to throttle inputs when in an rpm range past spool, hence better transient response.
An s2000 with GT3076r seems to have excellent spool and transient response to my tastes. I would like to a drive a gt35r divided (twin scroll) setup and see how that is.
Good points
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