I’m sure some of you have heard of anti-lag systems or sometimes known as misfiring systems, thanks to anime like Initial D. You might have noticed that most of the cars have flames coming out of their exhaust when they shift gears. If you’ve watched Top Gear’s review on the Prodrive P2, you will see that the P2 has it too. That’s a misfiring system in action. Let’s just call it anti-lag from now on. So what does anti-lag do?
First of all, we have to have a quick re-cap on how turbocharging works. Essentially, exhaust gas is used to spin a turbine which compresses intake air. But what happens when you shift gears? You have to lift your foot off the throttle. Exhaust gas velocity drops, and the turbine blades slow down. Intake boost pressure drops. For the next gear, you have to wait until the turbine spins up again for the boost to kick in. This is what you call turbo lag. However, turbo lag should not be confused with boost threshold, which is the minimum engine RPM which is required for the turbine to physically supply boost.
A typical antilag system works when the throttle is closed, which is what happens when you take your foot off the accelerator pedal. An antilag system introduces some unburnt petrol into the exhaust manifold. The heat of the manifold causes the unburnt petrol to combust, and the exhaust gas produced by this combustion keeps the turbine spinning, preventing the blades from slowing down – effectively reducing or eliminating turbo lag.
There are two ways to go about this. An aftermarket anti-lag system like those that come with aftermarket ECUs like MoTeC will adjust ignition timing during closed throttle situations to a point where the exhaust stream becomes really rich with unburnt petrol. This is done by retarding the timing to about 40 degrees after top dead center. Top dead center is the point where the piston is furthest from the crankshaft. Rally cars use a different way of introducing petrol into the exhaust manifold. There are separate injectors that introduce fuel directly into the exhaust manifold. Some of this mixture will continue burning in the turbine and down the exhaust system, resulting in the flames and explosions you hear coming out of the exhaust muffler.
The use of anti-lag greatly reduces the life of a turbocharger and engine because of the extreme heat from the combustion in the exhaust manifolds. This is why it is normally only used in races and rallies and not suitable for street use, as the lifespans will become too impractical. It also increases emmissions and is generally too loud for street use, making it illegal on the road.
Video: Subaru Imprezas showing off their Anti-Lag Systems