Repeatable times would be easy but you could still loose it on reaction time, and even with the slightest bit of wind can change your ET enough to make you loose a race so you would have to know if to apply more throttle or back off to get your time back in line. Some races are won and lost by as little as 0.001 of a second. all you have to do is go 0.001 of a second quicker than your dial in and youv lost.
Say for example you get within 0.5 of a second of your dial in time but your reaction time is 1.2 seconds and your opponents runs within 1 second of his dial in but gets a reaction of 0.5 seconds, They have won the race by 0.2 seconds.

You have to start thinking about things that you wouldnt think about usually.
Wind direction, this could speed you up or slow you down easily.
Tyre pressure, slightly less pressure in your tyres causes rolling resistance and more inflated tyres effectively changes your gearing.
Wear on the engine/motor, Quite obvious really
Even the slightest bit of weight. Somthing as simple as having lunch, and amount of fuel in the tank can make a differance.
It sounds stupid, but if you get a few of these things happen at once it can make a big difference, Lower tyre pressure, more weight and a head wind, That might be half a second lost on your run. half a second is a huge amount.

As soon as you get a few guys running consistant times it really gets interesting.