Launch Control: Ignition Retard (Building Boost)
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Ignition Retard (Building Boost)
05.28
| 00:00 | The techniques we have just discussed will work well for naturally aspirated engines or factory turbocharged engines that offer a low boost threshold. |
| 00:08 | If you have a modified turbo car with a large turbo, though, launching using just a rev limit on its own won't provide enough energy to spool the turbo, and you can find that you can't create enough power to effectively launch the car. |
| 00:22 | In this case, you may need to use some ignition retard to help the boost response. |
| 00:28 | Retarding the ignition timing is only effective if you're using an ignition cut type rev limit, and it works by retarding the ignition event on the cylinders that are still firing. |
| 00:38 | Depending on the size of the turbo and how much boost we need to create, it's not uncommon to retard the timing all the way back to TDC or even as much as 10 to 20 degrees after TDC. |
| 00:50 | The more retard we use, the more aggressively the turbo will be driven. |
| 00:56 | This ignition retard works by continuing the combustion into the exhaust manifold, as well as causing the unburnt fuel from the other cylinders to combust. |
| 01:06 | The result is explosions in the exhaust manifold that can be very effective at spooling the turbo. |
| 01:12 | There are a few cautions that need to be discussed here. |
| 01:16 | Firstly, ignition retard, or even an ignition cut type of rev limit, shouldn't be used if there's a catalytic converter fitted in the exhaust. |
| 01:25 | The pressure oscillations can quickly destroy the cat, turning it into a cloud of expensive ceramic dust. |
| 01:32 | Secondly, large amounts of ignition retard aren't suitable if your engine is fitted with hydraulic lifters in the valve train. |
| 01:40 | The large pressure spikes in the exhaust manifold can actually cause the exhaust valves to pop back off their seats briefly. |
| 01:47 | With a hydraulic lifter, this will let the lifter expand or pump up, and then the valve will be held open. |
| 01:54 | The result is that the engine can drop one or two cylinders while on the two step and run poorly until the lifters bleed back down, which may take 30 seconds or more. |
| 02:04 | Lastly, if you have a turbocharger that's fitted with a ceramic turbine wheel, you should never use ignition retard during launch control. |
| 02:12 | The pressure spikes and explosions in the exhaust manifold will quickly destroy a ceramic turbine wheel. |
| 02:19 | Provided your engine and turbo are up to the task, ignition retard can be very effective, creating almost as much boost as you want on a turbocharger that otherwise may barely produce positive boost. |
| 02:32 | Tuning the ignition retard introduces another element as we can now vary the amount of engine power with the engine RPM or the ignition retard, and it may not always be completely obvious which we should change. |
| 02:46 | The first point to keep in mind is that we want to be operating the engine at an RPM where it is actually capable of making boost naturally. |
| 02:55 | A good starting point would be the engine's peak torque RPM. |
| 03:00 | Using a lot of retard, it may be possible to make the turbo produce boost at an RPM where it normally wouldn't if we just accelerated down the road or performed a dyno pull. |
| 03:11 | This can lead to problems when the launch control is disabled, and there may not be sufficient energy to maintain boost and hence engine power. |
| 03:20 | Once you've settled on an RPM limit, you can start retarding the ignition timing. |
| 03:24 | As always, the way to tune the system is to use data logging, but at least to start with, we don't want to actually launch the car. |
| 03:32 | We just want to bring the engine onto the launch limit and log what boost the engine produces. |
| 03:38 | Start by retarding the timing five degrees at a time and watch the effect of this retard on the boost. |
| 03:45 | The further the ignition is retarded, the more energy is available to drive the turbo, and hence the more boost we can expect. |
| 03:53 | With very large turbos on small-capacity engines, it isn't uncommon to end up retarding the timing to as much as 10 to 20 degrees after TDC. |
| 04:04 | With this amount of retard, as well as the boost pressure that accompanies it, you'll be putting a lot of heat stress into the turbo and exhaust manifold. |
| 04:13 | For this reason, it's important to keep your tests as short as possible, and allow plenty of time for everything to cool down between tests. |
| 04:23 | Once the ignition retard is dialed in and producing the sort of boost you want, I generally make small changes to the launch control by varying the engine RPM. |
| 04:32 | I find this is the most effective way of adjusting the engine's power based on track conditions and grip levels. |
| 04:40 | If you're using a lot of ignition retard, then this will naturally affect the engine torque, making the engine RPM drop when the launch control switch is active, and also making the engine very slow to build RPM. |
| 04:54 | Many ECUs allow the tuner to define when the ignition retard will become active. |
| 05:00 | Perhaps above a specific engine speed or throttle position. |
| 05:04 | This allows the engine to respond normally as you're approaching the start line and also quickly achieve our target launch RPM before the retard is applied. |
| 05:14 | If these parameters are available to you, I'd normally try and bring the ignition retard in above 90% throttle, and around 500 RPM below your launch RPM target. |
