Re: 1978 Mercury 90hp inline 6 5000 RPM +
Yes the 90's were designed to be more of a heavy boat workhorse than the 115 which is the same engine with some carb, ducting, gearbox was 2.0 rather than the 2.3 the 90 had and maybe timing changes and all. It is rated at 5500 max. I had the later year 115 for some 7 years and sold it to a family member who still has it. Literally a bullet proof engine. I ran mine at 6000 WOT and it still runs there.
My current 3 cylinder 90 runs 5600-5800 being rated for 5500. Course that is where the engine develops it's rated hp, not it's maximum allowable rpm.
Case in point, I had a pre-ignition runaway problem recently that I caused by playing with the carbs and having the linkage disconnected between the carbs and the timing and my engine took off on it's own, turning the ignition key off did nothing, and it pegged the tach at 7000. I don't know what it's terminal rpms were but it ran like that for a couple of minutes before I could get over the shock and figure out how to get it stopped.
I have since installed my new OEM factory linkage (replacing broken existing) and tweaked my carbs properly and the engine runs like nothing ever happened.
I will say that my experience is that any engine, especially 2 cycles run better at the higher rpms where the torque curve has maxed out and has started falling. In that sinario, any engine loading drops the rpms which backs you down the torque curve which increases the torque and helps the engine to sustain the extra load, rather than being on the front side of the torque curve and any loading backs you down the torque curve and the torque falls off, the engine lugs, the rpms drop, ditto, ditto, till the engine just shuts down.
My 2c,
Mark
