Turbo Lag
Dec 18, 2005, 02:33 PM
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Turbo Lag
Found this post on a 350Z forum I'm on as well.
Originally Posted by ayman350z
What Is Turbo Lag?
Turbo Lag is a term used to describe the time that it takes from when you push the accelerator to full throttle to when the turbocharger comes onto boost. Since turbochargers are designed and sized to operate at specific rpm ranges and air flow conditions, some turbo lag is inherent in the system. Turbos can be sized for specific applications to perform best at low, mid, or high rpm ranges. However, just like any performance add-ons such as cams, manifolds, heads, etc., you can't have peak performance at all rpm ranges.
To fully understand turbo lag, you must first understand the operation principles behind what causes a turbo to make boost. Like most forced induction systems, boost is not made by rpm alone. There is a big difference in the airflow through an engine at 6000 rpm in neutral, 6000 rpm in first gear at part throttle, and 6000 rpm in high gear at full throttle. Turbochargers are even more effected by these airflow differences and especially load differences than most other devices. This is why you may be able to see a small amount of boost with a positive displacement supercharger at WOT in neutral but you will most likely not even see zero inches of vacuum at WOT in neutral with a turbocharger.
Being that turbochargers are so sensitive to air flow and especially loads, you will get very different turbo lag characteristics on the same vehicle during different conditions. For example, the loads placed on the system in lower gears are not as high as the loads placed on the system in the higher gears. This applies to 1st, 2nd, 3rd gear etc., as well as to rearend gears and tire sizes. Short tires with low gears such as 4.88's will not produce engine loading like a taller tire with 3.08 gears. This situation also applies to the weight of the vehicle, including any trailers that the vehicle may be pulling.
A given turbocharged vehicle may produce full boost in first gear by 3500 rpm. This same vehicle may produce full boost in 4th gear by 2300 rpm. The same vehicle, when pulling a heavy trailer, may produce full boost by 1800 rpm even in 1st gear. These conditions are more specific to manual transmission vehicles as the boost has to build in each gear after the throttle is closed during the shift. Automatic transmission vehicles operate differently as they can be "loaded" by stalling up the converter and placing enough load to build boost before the vehicle even moves. Also, with an automatic transmission, the throttle is not shut during shifts which causes the boost to "pop up" because the turbo is supplying enough air flow to feed the boosted engine at high rpms and suddenly the rpms are drastically cut down by the shift as well as increased load has been placed on the system at the same time. This can cause an engine that is accelerating in 1st gear with low boost to instantly go to full boost upon shifting into 2nd gear.
With this understanding of the conditions that cause a turbo to come onto boost, lets look at a real life example of the boost conditions related to rpms and gear selections. The following example is from an LS1 Camaro with a 6 speed transmission and stock 3.42 gears. The turbocharger system is the basic STS turbo kit with a standard Garrett journal bearing turbo with 60-1 compressor. This chart shows a worst case example of how long this particular system takes to come up on full boost. Driver skills can effect some of these numbers dramatically, so this test was conducted under the following guidelines:
The "Gear" column is which gear the transmission was in for the test. The "Starting RPM" is the rpm the engine was at during a closed throttle position when the throttle was snapped to the floor. This was performed by taking the engine RPM to about 200 rpm over the starting RPM, decelerating at closed throttle until the starting RPM was reached, and then opening the throttle to WOT quickly. "Full Boost RPM" is the engine RPM when full boost was reached and the wastegate opened up. "ET to Full Boost" is the time in seconds that it took to go from the Starting RPM to Full Boost RPM. Some of these numbers may seem high upon first observation, but accelerating from 1000 RPM to 3000 RPM in 4th gear can cover over 50 mph of acceleration. You can see by the chart that when the turbo is in the rpm range which it was designed to operate, that full boost can can be reached from a closed throttle to full boost in less than 1 second and in as little as 100 RPM's. There are obviously many variables that can effect these readings, such as turbo temperatures, uphill or downhill grades, using the clutch or an automatic transmission, tuning issues, etc. Once your vehicle is set up correctly and you familiarize yourself with how to operate it most efficiently to keep the turbo in the RPM range it was designed to operate, launching and shifting in ways that promote boost buildup, etc., you will be amazed at the performance and quick spool times that you will achieve.
source:
http://www.ststurbo.com/turbolag
Turbo Lag is a term used to describe the time that it takes from when you push the accelerator to full throttle to when the turbocharger comes onto boost. Since turbochargers are designed and sized to operate at specific rpm ranges and air flow conditions, some turbo lag is inherent in the system. Turbos can be sized for specific applications to perform best at low, mid, or high rpm ranges. However, just like any performance add-ons such as cams, manifolds, heads, etc., you can't have peak performance at all rpm ranges.
To fully understand turbo lag, you must first understand the operation principles behind what causes a turbo to make boost. Like most forced induction systems, boost is not made by rpm alone. There is a big difference in the airflow through an engine at 6000 rpm in neutral, 6000 rpm in first gear at part throttle, and 6000 rpm in high gear at full throttle. Turbochargers are even more effected by these airflow differences and especially load differences than most other devices. This is why you may be able to see a small amount of boost with a positive displacement supercharger at WOT in neutral but you will most likely not even see zero inches of vacuum at WOT in neutral with a turbocharger.
Being that turbochargers are so sensitive to air flow and especially loads, you will get very different turbo lag characteristics on the same vehicle during different conditions. For example, the loads placed on the system in lower gears are not as high as the loads placed on the system in the higher gears. This applies to 1st, 2nd, 3rd gear etc., as well as to rearend gears and tire sizes. Short tires with low gears such as 4.88's will not produce engine loading like a taller tire with 3.08 gears. This situation also applies to the weight of the vehicle, including any trailers that the vehicle may be pulling.
A given turbocharged vehicle may produce full boost in first gear by 3500 rpm. This same vehicle may produce full boost in 4th gear by 2300 rpm. The same vehicle, when pulling a heavy trailer, may produce full boost by 1800 rpm even in 1st gear. These conditions are more specific to manual transmission vehicles as the boost has to build in each gear after the throttle is closed during the shift. Automatic transmission vehicles operate differently as they can be "loaded" by stalling up the converter and placing enough load to build boost before the vehicle even moves. Also, with an automatic transmission, the throttle is not shut during shifts which causes the boost to "pop up" because the turbo is supplying enough air flow to feed the boosted engine at high rpms and suddenly the rpms are drastically cut down by the shift as well as increased load has been placed on the system at the same time. This can cause an engine that is accelerating in 1st gear with low boost to instantly go to full boost upon shifting into 2nd gear.
With this understanding of the conditions that cause a turbo to come onto boost, lets look at a real life example of the boost conditions related to rpms and gear selections. The following example is from an LS1 Camaro with a 6 speed transmission and stock 3.42 gears. The turbocharger system is the basic STS turbo kit with a standard Garrett journal bearing turbo with 60-1 compressor. This chart shows a worst case example of how long this particular system takes to come up on full boost. Driver skills can effect some of these numbers dramatically, so this test was conducted under the following guidelines:
The "Gear" column is which gear the transmission was in for the test. The "Starting RPM" is the rpm the engine was at during a closed throttle position when the throttle was snapped to the floor. This was performed by taking the engine RPM to about 200 rpm over the starting RPM, decelerating at closed throttle until the starting RPM was reached, and then opening the throttle to WOT quickly. "Full Boost RPM" is the engine RPM when full boost was reached and the wastegate opened up. "ET to Full Boost" is the time in seconds that it took to go from the Starting RPM to Full Boost RPM. Some of these numbers may seem high upon first observation, but accelerating from 1000 RPM to 3000 RPM in 4th gear can cover over 50 mph of acceleration. You can see by the chart that when the turbo is in the rpm range which it was designed to operate, that full boost can can be reached from a closed throttle to full boost in less than 1 second and in as little as 100 RPM's. There are obviously many variables that can effect these readings, such as turbo temperatures, uphill or downhill grades, using the clutch or an automatic transmission, tuning issues, etc. Once your vehicle is set up correctly and you familiarize yourself with how to operate it most efficiently to keep the turbo in the RPM range it was designed to operate, launching and shifting in ways that promote boost buildup, etc., you will be amazed at the performance and quick spool times that you will achieve.
source:
http://www.ststurbo.com/turbolag
