question on high speed (40D)

Hi people.
Am very amateur and am doing photos of finish line of horse races in Mexico, but this photos need to be very exact on the finish line, with in an inch or two is my margin, and I have created some extra atachments for the camera but for it to work properly exact I need to set the camera to the most fastest shooting mode and most manual I do not want the camera to do any work I need my camera to just be ready to receive the signal from this atachment that conects to the cameras shooter release jack on the side of it.
I only have one shot on each race. I have been using it on TV shooer mode at 1/8000 stabilizer off, M/F but is still not working fast enough and am getting diferent distances on the horses so this tells me that the camera is still doing some of the work.
I have gone reading the manual very many times but I have not been able to figure it out also called cust. serv. 5 times, as you probably notice am very amateur but am trying to learn, Thank you in advance.

Set camera on tripod. Turn off Autofocus (on the lens) and preset focus manually. Turn your dial to M, where you then set the shutter speed, the aperture, the iso.

If the dial on the upper left is anywhere other than "M", then the camera will do some thinking for you.