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Autronic SM4: Launch control
Autronic SM4: Launch Control
First, what is Launch Control? Launch control is basically a lower RPM rev limiter allowing driver to hold the engine at full throttle and maintain RPM’s at Launch RPM. This is done by retarding the ignition or/and cutting the fuel. Turbocharged cars see an additional benefit in the ability to build boost. This gives you more consistent and reliable launches with maximum power. As an example...
CAUTION: Make sure your valve train in strong enough when using ignition retard. Valves might stay open in some cases so be careful. Also, take care if your car is equipped with catalytic converter.
Hardware installation: Installation is quite simple and can be done with one switch. In my case I have 2 switches connected to I/P SW1. One at clutch pedal and other in series used to deactivate the launch mode when needed. Same input is also used for WOT Shift.
Launch Parameters, programmed using Ecucal:
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1. Launch cut mode: In this case revs are limited by retarding ignition and cutting fuel. Other option is Fuel Cut. Ignition cut is more responsive but be careful as explained earlier… 2. Launch rpm trig: This parameter is used to activate the launch. In order for launch mode to activate, switch has to be on and RPM has to be less than this parameter, 3000rpm in my case. 3. Launch TPS: Switch has to be on and TPS below this limit to enter the Launch mode. 4. Launch KPH: Switch has to be on and the speed below this limit to enter the Launch mode. 5. Launch RPM Source: Can either be the single Launch RPM value in the line below or variable Launch RPM by using the GPC12 table. 6. Launch RPM: RPM used in launch mode. This is the lower RPM limiter and RPM is limited to this value when in Launch mode. 7. Launch RPM Offset: The Launch RPM Offset value is more for turbo engines to build boost but is the amount of RPM below the Launch limit that the ignition retard is fully applied. 8. Launch Rate: Rate the RPM limit increases once launch is cancelled either by the switch being released or the Launch ends speed being reached. 9. Launch ends: Speed at which launch is terminated. The Launch ends speed is Vehicle speed, not Driven wheel speed. 10. Ign retard source: Can either be from GPC 13 table or the single value in the Launch retard line. 11. Launch retard: Amount how much the ignition is retarded during launch mode. This parameter is not available if GPC13 is selected as the Launch retard source. In my case, ignition is retarded by 10 degrees. So, if advance in Main ignition table is 24 degrees, ignition is retarded by 10 degrees to 14 degrees. |
Launch activation and lower RPM limiter:
As described earlier, launch control system is basically a lower RPM limiter. Following picture shows a graph from the activation phase, parameters shown in this log:
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(Double-click the picture to open the complete log, new tab is opened)
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Engine is idling at 478.268 sec and clutch is pressed in as seen from the I/P SW1. In order to activate the launch mode, following must be true:
- RPM has to be below 3000rpm
- Throttle must be below 101%
- Speed has to be below 9kph
- I/P SW1 must be high
As seen from the log, all conditions are met. Launch mode is activated and Launch revlimit is set to 5000rpm as defined in the settings.
At 479 throttle is pressed in further and 5000rpm limit is reached. At this point, fuel injection is cut and no fuel is injected. Ignition is also retarded by 10 degrees as defined by the Launch retard parameter. See orange and brown traces.
No fuel = No combustion, air alone can’t explode as engine is just pumping air.
Note from Autronic, Ian Hamwood: Lean conditions can be caused on other cylinders with fuel cut because of reversion pulses. The cylinder that you fuel and spark cut may have been getting some fuel from another cylinder's reversion pulse so there is no problem there (wiil either be too lean to ignite or spark cut will inhibit combustion), but the cylinder being cut may be providing some fuel to another cylinder via reversion. That is what can cause the lean conditions on non-cut cylinders. In an engine with individual throttle bodies and no plenum you can get a slight lean out after a fuel cut if the runner was wet with fuel and the engine sucked the runner dry during the cut. With a v1.09 CPU if the transient fuel is enabled it can compensate for the runner drying effect after a fuel cut.
Roughly 250mS later fuel is injected again briefly to keep the engine running and then injection cut again.
At 479.376, clutch is released and car starts moving.
Below is a new graph, take a look at the Launch revlimit value. It is actually a revlimit calculated from Launch rate parameter and increases the Launch revlimit value until Launch ends value is reached or switch is released. If this “moving” revlimit is reached, power is reduced according to Launch Cut Mode parameter. An example of this can be seen at 480.125sec. This feature can be effectively used as a Traction control to limit the wheel spin. But, if you cut too much, your acceleration will be slower…
Launch continues and at 480.951 launch is terminated since Launch ends speed is reached. At the same time Launch revlimit value increases to 32767 and normal engine revlimit will be used from this point on. That’s pretty much all there is to it.
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(Double-click the picture to open the complete log, new tab is opened)
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How to find the optimum launch: Best parameter for this is the Vehicle speed (Non-Driven wheel speed) and how quickly the wheel speed increases. Take a log and select the point where the launch ends. Press T to enter the Tagged mode and select the time period you are interested in. Then press S to open the Statistics view. Note the Rate/Sec column and the value. Aim for the highest value here.
Here's the end result:
Next question is what is faster way to accelerate, keep wheels spinning or not? Read about it next week…