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Jimmbo I suggest you go and learn a bit more about outboards made by omc before making such comments.
porting is done on the engine sleeves, venting is done at the prop. 75 stingers were not a bottom end motor, thats was due to being a higher top end motor
also the cylinderhead was a higher compression head on the 75's to help compensate for the higher porting..
The johnson and evinrude 75hp stingers made over 80hp at the flywheel at 7200 rpm. (about 68hp at the propeller). mercury 70hp motors realy made about 50/55 at the prop. unless you found a 650xs, totally different to normal motor.
why do you think they were so successful in their racing boats ? I am just talking the fishing motors not the mod 50 that produced over 105hp like the 31m or the
triple that had the exhaust on the other side that produced 115hp !!!!
Most 75hp motors did not have a rev limiter and could infact climb to around 8,000 rpm with small props. this obviously will make the life expectancy drop[ if anything should go wrong.
on the 75 hp motors the 15inch models came with what was known as the nitro foot gearcase,
nitro foot is the small gearcase found on most 15 inch motors of 75hp running at 1.86 to 1 ratio and using props around 12x22 pitch...
The longshaft (20 inch mids) came in 2.42 ratio and used what became known as the clubfoot gearcase.
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At 7,000 rpm the life expectancy is not hindered at all on the 75hp stingers, those that did have limiters were often 7200 rpm not 6700 like the later 56 cu in ignition.
Being as the OMC 75hp motor was very well designed it was the reason that mercury could not keep up with the omc triples .they became known as the mercury killers.
if you look at the much later 56 cu in omc SST60 race motors and see them race against the old 49cu in 75hp stingers you willl see there is hardly any difference in top end speeds attained. maybe 10mph unless the sst60 has been modified..
What you seem to be forgetting is that the stinger 75hp was a very high performance motor, it came with its different carbs, higher exhaust ports, and the ports were chamfered at the tops, unlike the previous 55,60,65 and 70hp variants.
maybe this will help you understand a bit about the 75's.
these boats below are run with total stock motors, NOT the omc race motors that are over 100hp, these are stock motors you could just walk into a dealer and buy.
engine pictured is a stock Evinrude Hustler 75 short shaft, identical apart from colours to the johnson 75 stinger.