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Turbo Lag? 
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LQFP144 - On Top Of The Game
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Joined: Sun Jul 18, 2010 2:35 pm
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What is turbo lag or what do you consider turbo lag. I would think its the time it takes to make the turbo output the most possible at the set boost.
Any thoughts on this. I also believe turbo lag is more of a problem in lower gears, esp cars gear short from the factory that didn't come with a turbo.


Thu Aug 08, 2013 2:40 am
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At a fixed RPM lag is the time between a change in throttle and the steady state boost matching that throttle arriving. It's the time it takes the turbo to go from a lower speed to a higher speed in order to build pressure in the intake manifold, at a fixed RPM, with a change in load/required output.

Not to be confused with boost response curve AKA boost threshold which is the fact that you get 1psi at 3k, 3psi at 3500, 10psi at 4k, 18psi at 5k+. This is what idiots refer to as lag. This is not lag.

In my truck, at 5k, trailing throttle, NO boost, big turbo barely spinning, I stand on it, the lag is very short indeed. On the other hand, if you stand on it at 1500 RPM you can listen to the turbo wind its way up to the 1kPa you might make (if you're lucky), and it takes some seconds to do this (if held at 1500). On anything OEM the lag is almost imperceptibly short.

On the other hand, the figures I quoted above, which are from my truck, mean that if you are accelerating through a gear, and you start at 3k, you feel like the engine's power "lags" your command, because it takes N seconds to get to 5k where full torque is available.

Like your comments on AFR and gears, gears have nothing to do with it, just ramp rate of RPM. It is due to actual lag that the boost response curve varies with RPM ramp rate.

I bet someone in particular jumps in here to argue about this anyway, though.

Why such an elementary question?

Fred.

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Thu Aug 08, 2013 11:33 am
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