We're racing through history!
There has been a lot of chatter lately about the potential of an automotive powered unlimited hydroplane--and for good reason. They are loud. They would clearly be a fan favorite. They attract an entirely different group of potential sponsors such as automotive products, motor oil, spark plugs, etc., and the general public can more easily identify with a Chevy, Ford, or Chrysler engine similar to what they drive at home rather than something powered with a turbine.
In the past, automotive powered Unlimiteds were only marginally successful at best. Today, however, modern engine technology and the adoption of rules involving a higher minimum weight for turbine powered boats with a lower minimum for piston powered boats, it appears that automotive power could come into its own as a competitive option.
A supercharged automotive V-8 with aluminum block as used in high performance cars (e.g., dragsters) weighs approximately 500 pounds, or two of them about 1000 pounds. Today, a turbine boat must weigh a minimum of 6,575 lbs. empty, so the hull and systems must weigh roughly 6,000 lbs. without the engine, although this may include some fixed ballast weight.
However, empty weight of a modern technology hull can be much lighter than what is required to make minimum weight for turbines. A race ready hull without engine could be built for much less, say, 4,000 pounds maybe less with modern construction techniques. I used to do the official weighing of these boats long before the minimum weight is up to what it is today. You would be surprised at how light some of the boats were then, prior to the current minimum weight restrictions (remember the first Pay 'N Pak turbine boat, or the Sutphen Spirit? - VERY light!). With that in mind, a twin supercharged automotive engine powered boat could easily be built to weigh in the neighborhood of 5,000 lbs. dry. So, making the 5,775-pound minimum weight for piston powered Unlimiteds is not a problem today. Further, twin supercharged automotive engines can produce a combined horsepower approaching that of the T55-L7C under the current turbine fuel flow restrictions. So, in theory, the resulting horsepower to weight ratio could produce a very competitive boat.
There are negatives. A turbine engine theoretically operates like a "constant horsepower" engine when held at full throttle. That means, when the output shaft is lugged down, torque increases to keep the horsepower constant. Of course, in reality they really don't operate exactly like that, but close. A piston engine on the other hand will lose horsepower and torque when the output shaft is lugged down to the point where the engine falls of the torque curve. So, the engine, gearbox, and propeller, and the hull design for an automotive powered boat would need to be optimized such that the engine RPM can be maintained at a level to produce the horsepower and torque needed to remain competitive, especially in the corners. Ed Cooper has done this very well with the aircraft piston-powered U-3.
An automotive powered Unlimited would also need to carry a lot of fuel to complete a warm up and five lap heat, perhaps even twice as much as a turbine powered boat. Note that a Top Fuel dragster burns about 20-25 gallons of fuel in the warm up plus a quarter mile run. An automotive powered Unlimited wouldn't burn fuel that fast, of course, but will burn it much faster than a turbine boat. Regardless, even with the extra fuel the "wet" weight of one of these boats could still be considerably less than the turbine boat fueled and ready to race.
The main downside to the automotive technology today is cost; the cost of the engines themselves and the cost of development of the engine-boat combination. I gave a talk on Unlimited Hydroplane Aerodynamics and Hydrodynamics to the Engineering Society of Detroit at the Gold Cup this year, and several people from the automobile manufacturers were present, and showed significant interest.
Now, if the automotive manufacturers got involved, that would really be something.