idea? why not a very long plunger and 3-4 coils down it. each coil could be progressively programmed voltage/amperage and timing between them, where voltage/current is lower on 1st coil, higher to each coil after, as the timing for them gets shorter and shorter.

something like that could allow it to be tweeked for different car weight too maybe, or controlled by a "gas pedal" (kinda faking RPM's) and launched with "dropping the clutch" pedal or slapping the shifter back like a typical econo-rail.

another concept, use 1/8 scale RC car wheel (motor spun) that comes down to contact the hotwheel, same "gas pedal" to spin up motor RPM's, and cable "shifter" to pull the spinning wheel down for the launch.. over rev thrown off track = wreck or blown engine.

1/8 scale foam (on road) front wheels about 3" diameter 1-1/4" wide. with identical motors/gears, it'd only need a simple voltage display "tach". much simpler setup..
rev to desired voltage and drop the hammer.

if the cars are in a "launch box" (lightly pulled back into position by spring or rubberband)
the wheel coming down onto the top of the box to kick it (and car) forward, theyed launch about any car you put in it.. launch box could be 6" long or more to make advantage of the motor+wheel spinning arrangement. car being in rear of box with back wall giving the shove, when the box hits end of throw, it still has the 4 sides acting like a gun barrel to help guide the car straight.

look again.. if the car's are in a 6-7" long track width box, the wheel kicks the box forward with the box's back wall putting the shove to the car.. box being on slides.

if it were made 2 sides, top, and back, to fit over a section of track.. sides having the slides at bottom edges is a fairly simple arrangement, like "hat section" stock ( |-|_ ) -end view-.

the foam rubber wheel-gear-motor arrangement could even come from a regular hotwheels battery operated speed booster, just no gear/wheel to side B. they never worked with open wheel cars very well (if at all) anyway.