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.