Turbo lag and spool are words that very often get confused. They are similar problems both specific to turbocharged cars, and are both affected by many of the same variables, but differentiating between them is important.
What is turbo lag (and how do I avoid it)?
The majority of turbochargers feature a wastegate—a valve which allows some of the exhaust gas to be directed around the turbine. This allows the turbo’s shaft to spin at a reduced speed, promoting increased turbo life (among other things). Think of it as a ‘stand by’ mode. Since the turbo isn’t needed during relaxed driving anyway, this effect is harmless…
…until you suddenly want to accelerate. Let’s say that you are loafing along, engine spinning 1500 rpm or so. You instantly floor the throttle. The exhaust gas flows through the turbo and cause it to spool (spin up to speed and create boost). However, at this engine speed there isn’t very much exhaust gas coming out. Worse still, the turbo needs to really get spinning to create a lot of boost. (Some turbos will spin at 150,000 rpm and beyond!) So you, the driver, need to wait for engine revs to raise and create enough exhaust gas flow to spool the turbo. This wait time—the period between hitting the throttle at low engine speed and the creation of appreciable boost—is properly called boost response. Many people incorrectly call it lag, which is really something different. Lag actually refers to how long it takes to spool the turbo when you’re already at a sufficient engine speed to create boost. For example, let’s say your engine can make 12 psi at 4000 RPM. You’re cruising along at a steady road speed, engine spinning 4000 RPM, and now you floor it. How long it takes to achieve your usual 12 psi is your turbo’s lag time. Between the two, slow boost response usually causes the most complaints.
There are two aspects to consider when dealing with boost response: engine factors and driver factors. As far as engine factors go, there are many things which affect turbo lag… although most are directly related to the design of the turbo itself. Turbos can be designed to minimize lag but this usually comes at the expense of top-end flow. In other words, you can barter for instant boost response by giving up gobs of horsepower in the upper third of your RPM range. (Behold the catch-22 in designing one turbo for all uses.)
Driver factors are another matter. You basically need to understand how a turbo works and modify your driving style accordingly. To sum it up, don’t get caught with your pants down! If you feel that there may soon be a sudden need for serious thrust, downshift until your engine speed is at least 3000 RPM. This way there will be noticable boost almost as soon as you hit WOT. If you are going up a hill at WOT around, say 1800 RPM and your speed is dropping, you’ll need to downshift just like any other car in the same situation. Remember: turbos need exhaust gas in order to spin. Let them have some when they need it.
See also: spool