what is going to happen when the ejection stage hits and those hot gasses go all over inside your car? you can buy motors that don't have that stage (unless you've got them already)
Yes...most model rocket engines have a delay before backfireing to release the parachute (in a actual model rocket). So, it'll either cause the truck to slow down a little bit, or seeing as nitro is flammable and the engine can get too hot...