Quote:
Originally Posted by ovekvam
The turbo lag is present at all engine speeds. The turbo is driven by exhaust energy, and when you go off the throttle, the intake valves are closed. This causes the exhaust flow to stop, and the turbo will spool down. Excess pressure is let out by a valve. Eventually the turbo will be spinning very slowly. Once you go back on the throttle, it has to spin itself up again, and this takes time. At high RPM, you have more exhaust flow, so it will spin itself up faster. The engine will also need more air from the turbo at high RPM, so it needs to spin up to a very high speed to produce boost.
To test turbo lag, go off the throttle a few seconds, and then floor it. Notice how long time it takes from you hit the throttle to the engine is at full power for the given RPM.
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A lot of people who drive ATs mistake this for turbo lag. What it is is the gear box upshifting and not down shifting again quickly enough in response to full throttle
What happens is that when you take your foot off the gas, the gearbox upshifts and you are essentially cruising in a very high gear. Floor the accelerator and it opens the inlet tract fully, reducing inlet air/fuel velocity, so there's very little engine pick up. This is not turbo lag. Also when you floor the gas, the gearbox takes a second or two to downshift, also not turbo lag.... Taken together its simply an engine out of its power band and a gearbox that's slow to downshift.
Try this. Take your foot off the gas (fuel feed is shut off) but instead of just stamping on the gas, manually downshift 1 or 2 gears to increase revs to say 3000 then stand on the gas. Turbo lag is almost zero....because air/fuel velocity is maintained, the car is within its power band and the turbo is able to spool up extremely quickly once fuel and therefore exhaust flow is resumed.
If you try what you suggest in a MT car, while maintaining appropriate revs, you'll find almost no turbo lag....the car responding almost instantly. Given that both AT and MT cars have the same turbo, the difference is the fact that AT upshift when throttle is closed and is then slow to downshift at full throttle