Eli5: how do some cars with turbos max out their torque and horsepower at a low rpm?


My car has a turbo, and it makes max torque and horsepower at 2k rpm and drops off from there. In theory, shouldn’t the higher the rpm’s are mean more torque and horsepower with more air going through the turbo?

In: 4

What kind of car is that?

Do you mean it shifts gears automatically when you hit 2k rpm?

Higher RPMs does mean more air going through the turbo but doesn’t necessarily mean more boost (which would give you more torque, all other things being equal). Going higher RPM than that means more pumping loses (work to move air through the system), less time for combustion, more friction losses, etc. This is why the power always rolls off eventually, the gain in RPM isn’t enough to offset the loss in torque (true for turbos or non).

2k RPM seems low unless you’re talking about a big diesel though.

Old turbos were set from the factory, the vanes were designed to sit at one point and couldn’t move. It either spun up quickly and didn’t provide much boost, or provided a lot of boost but had bad turbo lag

Modern turbos are far far less rigid. Plenty of modern cars come with turbos that have some ability to change their internal airflow. They range from secondary valves/vanes that gives a boost at lower RPMs to spool it up quicker, or variable geometry turbos where the angles of the vanes can be changed on the fly as it spools up so it spins up quickly at low RPMs and can still push a lot of air at high RPMs.

The secondary optimization for low RPMs boost engine efficiency, which is generally right around peak torque to give better fuel economy, but its still a turbo and sucking down extra air at higher RPMs so it can also give you more power. Pretty much everything generates peak power in the 5-7k RPM range because torque trends down slower than the RPMs ramp up so the math says higher RPMs = more power