Swap a 1975 70 hp for a 1975 115 hp

[jdaghir]

I have a 1975 Evinrude 70 hp (70573E, long shaft) mounted on a 24' pontoon. It reaches a top speed of 20 mph @ 5000 rpm at WOT. It's a great running engine, but I have always felt it was a little under-powered for the boat, and since I boat on the Ohio River, power tilt/trim would be nice (lots of rocks & snags in the shallows).

I have run across a Craig's list ad from someone who has a 1975 Evinrude 115 hp that they want to sell for $800 or trade for a lower hp motor. It looks like it has power trim/tilt, they claim it runs well and the skeg & cavitation plate appear to be in good shape.

Any thoughts on swapping my 70 hp for the 115? I would of course get compression readings and see it run before swapping.

The 70 hp is a great motor - very reliable and easy to work on. How does the reliability and serviceability of the 115 compare?

At the same cruising speeds, would the 115 hp have better, worse or about the same fuel economy as the 70?

The 70 hp is an inline 3 cylinder, while the 115 hp is a v-4. Would the 115 run any smoother/quieter than the 70?

Since they are from the same year, will the controls & control cables be plug & play? Can we each keep the controls & cables we have or will we have to swap? My control box doesn't have a provision for a tilt/trim switch, is that included in the control box for the 115 or is it a separate switch?

My 70 hp is the long shaft version, which I believe equates to a 20". It appears the 1975 115 hp was only made in one shaft length. Is that correct, and does anyone know what that shaft length is?

Thanks for sharing your knowledge & experience!

 

[emdsapmgr]

First of all, you are running the 70 with the wrong prop. The correct prop on that engine should let the engine achieve at least 5500 at WOT. So, when you run the current prop, you are missing out on some performance and economy of your current engine. Keep in mind, that 70 looper only has 49 cubic inches. The 115 has 99 cubes. Almost double. You won't believe the difference the 115 will make. Lots more torque. You will find the 115 will burn a lot more fuel, however. Both control boxes should work between engines. Power trim is a really great feature-esp if the trim switch is on the control box throttle handle-so I'd swap out the control boxes for that reason only. {The 115 box would not have had the switch in the handle, unless it was a newer box on an older engine.} I never did like the early crossflows that had the trim switch on the dash. It works, but it's inconvenient.

 

[F_R]

115 long shaft is also 20". It was a number of years before 25" extra long shaft became available.

 

[jdaghir]

Thanks for the info, I appreciate it.

I know I'm a little over-propped. According to the manual WOT should be 4500-5500. I had a 9" pitch Hydrus on it and it did 16 mph @ 6100 rpm WOT. I guessed and switched to a 13” pitch Solas Amita. Should have gone with an 11". I usually go through one prop a season so I'll most likely need a new one next season.