AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 256)

Page 256 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
Anti-Lag
Turbo lag is defined as the time delay before the turbocharger has produced boost pressure.
Since turbochargers are designed and sized to operate at specific rpm ranges and air flow
conditions, some turbo lag is inherent in the system. To fully understand turbo lag, you must first
understand the operation principles behind what causes a turbo to make boost.
Boost is not made by rpm alone. There is a big difference in the airflow through an engine at
6,000 rpm in neutral, 6,000 rpm in first gear at part throttle, and 6,000 rpm in high gear at full
throttle. Turbochargers are even more affected 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 any boost at WOT in neutral with a turbocharger.
A given turbocharged vehicle may produce full boost in first gear by 3,500 rpm. This same
vehicle may produce full boost in 4th gear by 2,300 rpm. The same vehicle, when pulling a
heavy trailer, may produce full boost by 1,800 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.
Anti-lag is when a turbocharger is subjected to a very late burn causing the exhaust to expand
in the turbine housing. This energy is concentrated at the turbocharger causing a very quick
turbo spool up. The EMS supports two different types, including a rally style and drag race anti-
lag.