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I do believe as most turbo timers bypass the thatcham devices it invalidates your insurance, I also believe it is illegal to leave a car with the engine running (keys in or not)
impreza turbos are water cooled but the timers do have a use.
on cold mornings I use mine to defrost the car with engine running without keys in ignition
Start car, drive off, warm up for a few miles.
Stay off boost for a mile or so before destination and switch off.
Ditto, my rule of thumb is no more than 3k revs and very considerate throttle until 2x the time it takes the water to get up to temp. if yours is like mine it will let you know when its cold!