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on this matter, I have been experiencing what is probably lag. If I am "on cam" and "drop the hammer" it picks up a second or 2 later with a real thud. This is recent and hasn't been behaving like this before, it so sudden and aggresive like someone has booted the car forward. It used to be more progressive and probably faster, is this a side effect of the hot weather or something i need to be watching??
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Turbo lag is basically the time it takes for your turbo to spin upto speed from the moment you press the throttle, if there is a 2sec delay before the power comes in, then I should get it checked, shouldn't be more than 1 sec delay, unless you are pulling 1k revs of course. I had the 2 second delay problem a while back, before I got my car remapped, turned out to be a knackered MAF, so maybe worth a look?
Hot weather usually reduces performance a little due to heatsoak, and sometimes smoother power delivery because of it.