Turbo lag

In an episode of Top Gear, Jeremy Clarkson explains that the biggest problem about the Mitsubishi Evolution VIII MR FQ-400, is the turbo lag. To illustrate his point, a race was arranged between the car and some station wagon in top gear. The Evo was getting left behind and never caught up because it ran out of runway.

And so now the millions of people believe that lag is a problem inherent in all turbo cars, especially in any Evo.

Thanks for mis-educating the public, Jeremy.

The fact is that JC was NOT demonstrating turbo lag, but boost threshold. A rolling start at low RPMs in the wrong gear is a test that's calculated to make turbo cars look bad because a turbo car typically has lower static compression ratios as well. That is to say, if you were to take an equivalent normally aspirated car with the exact same gear ratios, same weight etc, put it in top gear from a slow roll, it will only be marginally faster than the turbocharged version until the boost comes in.

Modern day turbo cars have practically no turbo lag. A turbo has an efficiency range at which it's designed to start producing power, and this is dependent on RPM, A/R, center housing rotating assembly construction, compressor/turbine design/material, and other factors.

Here's the datalog from my car.

I'm near 2000 RPM, with the throttle at around 52%, and the car is already starting to make boost. This is in 4th gear. Max boost in this segment is 1.2 bar; observe how manifold pressure smoothly tracks throttle position.