1983 7.5 vs 1984 6 and 8
O.k. here it goes? Johnson/Evinrude made a 7.5hp from 1980 to 1983, and in 1984 instead made a 6hp and 8hp. These three engines are of the same family and share most of the same parts. The 6hp and 8hp models continued for many years slowly evolving and changing. For the sake of clarity, and because I just purchased a 1983 7.5, I am going to compare to just the 83 model to the 84 models. The big difference in 84 between the 6 and 8 were the reed plate (the 8 had plate with 3 reeds per cylinder, while the 6 had a plate with 2 reeds per cylinder), the intake manifold (re-designed port lines up to the reeds), and the size of the carb jet- or ?high speed orifice?. Extra reeds, better air flow and more fuel explain the 2 hp difference in the 6 and 8 hp models. Makes lots of sense.
With this information, one might assume that the 7.5 is very close to the 8, I mean ? horse? really? Was it just marketing? Did OMC think a 7.5 sounded nice in 1980? After all, my Alumacraft T-12 is from the 80's too, and the capacity plate calls for no more than 7.5hp. The part number for the motor is J8RCTA. The ?8? in the part number seems to help prove this case.
Well? No. The 7.5 seems to have more in common with the 6! how can this be? If you study the parts diagrams on line you find that the 7.5 utilizes the smaller reed plate and intake manifold style (2 reeds per cylinder). It does have a jet size between the 6 and 8, but this alone is the only thing I can find. It doesn't make sense to me that a jet change makes a 1.5 hp increase, while it takes an entirely better intake system to give the extra ?.
I noticed that the exhaust tube on the 7.5 is not the same as the one on either of the newer engines. Theoretically this could have a lot to do with it. What I don't get though is why omc would drop a better exhaust pipe. If it is the exhaust pipe, people would have figured this out by now and combined the better pipe with the better intake system to exceed 8 hp. Since no one is bragging about having this mod it probably doesn't work.
Can anyone shed some light on the 1.5 hp difference between the 6 from 84 and the 7.5 from 83? I took my 7.5 out on my T-12 and it topped out at 18mph (200 lb user, 5gal gas, a pole and a tackle box). Seems like the ?7.5? is honest, but how?
Any insight appreciated.
-Thanks!
O.k. here it goes? Johnson/Evinrude made a 7.5hp from 1980 to 1983, and in 1984 instead made a 6hp and 8hp. These three engines are of the same family and share most of the same parts. The 6hp and 8hp models continued for many years slowly evolving and changing. For the sake of clarity, and because I just purchased a 1983 7.5, I am going to compare to just the 83 model to the 84 models. The big difference in 84 between the 6 and 8 were the reed plate (the 8 had plate with 3 reeds per cylinder, while the 6 had a plate with 2 reeds per cylinder), the intake manifold (re-designed port lines up to the reeds), and the size of the carb jet- or ?high speed orifice?. Extra reeds, better air flow and more fuel explain the 2 hp difference in the 6 and 8 hp models. Makes lots of sense.
With this information, one might assume that the 7.5 is very close to the 8, I mean ? horse? really? Was it just marketing? Did OMC think a 7.5 sounded nice in 1980? After all, my Alumacraft T-12 is from the 80's too, and the capacity plate calls for no more than 7.5hp. The part number for the motor is J8RCTA. The ?8? in the part number seems to help prove this case.
Well? No. The 7.5 seems to have more in common with the 6! how can this be? If you study the parts diagrams on line you find that the 7.5 utilizes the smaller reed plate and intake manifold style (2 reeds per cylinder). It does have a jet size between the 6 and 8, but this alone is the only thing I can find. It doesn't make sense to me that a jet change makes a 1.5 hp increase, while it takes an entirely better intake system to give the extra ?.
I noticed that the exhaust tube on the 7.5 is not the same as the one on either of the newer engines. Theoretically this could have a lot to do with it. What I don't get though is why omc would drop a better exhaust pipe. If it is the exhaust pipe, people would have figured this out by now and combined the better pipe with the better intake system to exceed 8 hp. Since no one is bragging about having this mod it probably doesn't work.
Can anyone shed some light on the 1.5 hp difference between the 6 from 84 and the 7.5 from 83? I took my 7.5 out on my T-12 and it topped out at 18mph (200 lb user, 5gal gas, a pole and a tackle box). Seems like the ?7.5? is honest, but how?
Any insight appreciated.
-Thanks!
