I searched and found a app for my phone that works great.
It uses the accelerometer on your phone to register a vibration as the RC passing by.
It was made for micros...put it on a ramp and every time the micro goes up the ramp it triggers the phone which records the lap # and the time.

But I put a 1/2'' thick board across my backyard track, with some of it sticking beyond the track border and put my phone on the board.(outside the track border) sure enough, it registered every pass and recorded each time. Shows your fastest and slowest lap when you hit finish as well. I was using a 1/12 scale eagle 3, which isn't heavy but made plenty of vibration for the phone to pick up the pass.

Its a free app. Wont work for racing head to head, but you can compete with your buddies for best lap or best overall time. Im sure success will vary by phone, but it worked great for me.

Its called "RC Race Timer".