I've seen them made for engines as small as .45's but theyv work best with the large displacement engines. Nitro is a slow burning fuel and the more "spark" that you can get to it, the better otherwise you won't have a complete burn. When nitro is used in Top Fuel dragsters or Funny Cars, by the time that they're off the starting line the plugs are already gone and the engines are dieseling, this is why they have to shut off the fuel to stop the engines..