Can't say I have done any actual scientific testing. It is something that I have been making observations of for quite a few years.
With sound stringers, I was suprised at the lack of visible distortion with the application of about 800 horsepower. These observations made while making casual passes with gradual application of the throttle while not unloading the pump. I would not use this as any kind of gauge for horsepower limits because this isolated case is not truly representative of our demographic.
From the perspective of the engine itself, a friend of mine, while dyno testing with David Vizard, recorded approximately 1/8" of twist at the pan rail of a 750 horsepower BBC. I have also heard many times that using the block side-mounts distorts the the cylinders etc. while under load, but I don't remember at what performance level it is recommended to step up to engine plates.
Moving to a 4-point or rail style engine mounting system relieves some of the stress on the block, but more importantly relocates that stress to the strongest parts of the block. A 3-point mount does not exploit the strength of the bellhousing area. IMHO, the bellhousing does nothing more than support the weight of the back of the block and maintain alignment between the crankshaft and pump shaft. All of the load is localized at one point on each stringer.
The application is what actually concerns me in regards to a mounting system. Typically, adding horsepower implies spirited driving and greater speeds and therefore greater stress loads on the stringers. Getting caught in white caps absolutely hammers the entire boat. I am nervous thinking about 850lbs of cast iron flopping around, supported by only four 3/8" bolts (each side) through a little bit of wood and fiberglass, spaced no more than 4" apart.
A 4-point mount goes a long way to spreading the loads out across a greater length of the stringers. A rail system not only spreads the load out, but spreads the load across 3' of the stringers. With backing plates, the entire stringer would need to fail before the engine could come loose. For this reason, I prefer backing plates. A pretty good argument for how solid rail kits are, one boat that had blown over at 145 MPH, hit transom first. Granted, this boat had an aluminum block and heads, there was absolutely no damage to the stringers or rail kit.
A few years , there was an article about an engine coming loose. The LEO stated, it was the worst boating accident he had seen. There was some discussion about too much horsepower for a 3-point mount. It is possible that there is more to the story than simply a design limitation. Condition and quality of the stringers and glass work will undermine the strength of any mounting system.
