Evinrude 115 vs. 140 - Prop pitch & Speed

[Bryant D]

I've got a 24' pontoon that I rebuilt into a work barge a few months ago. I currently have it rigged with a '82 115 Evinrude that I rebuilt and propped with a 13 3/4" X 13 pitch stainless prop. I'm running right at 30mph WOT at 5,600 rpm with motor trimmed up.

I also have an early 80's 140 motor that I have considered going through and trying on the boat since the weight of both engines is esentially the same.

My question is this...since the pitch of a prop is the distance that prop will travel forward (in inches) per each revolution, is it safe to say that no matter the horsepower of the engine, that the same prop being turned the same RPM will result in the same top end speed of the boat? In other words, if I go to the trouble of changing out my engine (115) with the other (140) and both turn the same RPM at full throttle will the top end speed be the same?

I know theoretically the 140 should be able to turn the same prop at a higher RPM, but my question is if both engines were run at the same RPM (say 5,500) would top end speed be identical?

I'm just trying to figure out if it's really worth changing out the 115 with the 140.

Thanks!

 

[flyingscott]

All motors have an RPM range to work in. If both your motors have the same RPM range the bigger motor should be able to turn a higher pitch prop to stay in the rpm range. Your 115/140 motors may be the same motor depending on the years of them. In the early 80s OMC went to a prop rating so the 140 powerhead rated motor became the prop rated 115 hp. If they are the same yrs I seriously doubt you will see enough of a performance gain to warrant the change. Pontoon boats do not respond to hp increases like a regular boat will.

 

[jimmbo]

An 82 115hp is a crank rated engine. Pre 1985 140hp were also crank rated. Post 1984 140hp were entirely different engines, and were prop rated.
If your 140 is pre 1985, it is basically the same engine as your 115, with higher compression cylinder heads, and a megaphone tuner in the exhaust. The peak of the torque curve occurs at a higher rpm, thus the higher HP. This enables higher pitch prop to be utilized for a given rpm. You are already over revving your engine, so a 140 would need a higher pitch prop to keep the revs down, but that will slow your hole shot, for two reasons. First, the higher pitch prop is a bigger load on the engine. Second, by raising the rpm where peak torque occurs, it reduces the torque at lower rpms. That is why lighter boats respond better to this engine than heavy boats. For heavy boats during that era, the next engine up was a V6 150hp. Only rated 10hp more, but a 50% increase in displacement, it had a huge difference in load capacity.

 

[interalian]

An 82 115hp is a crank rated engine. Pre 1985 140hp were also crank rated. Post 1984 140hp were entirely different engines, and were prop rated.
If your 140 is pre 1985, it is basically the same engine as your 115, with higher compression cylinder heads, and a megaphone tuner in the exhaust. The peak of the torque curve occurs at a higher rpm, thus the higher HP. This enables higher pitch prop to be utilized for a given rpm. You are already over revving your engine, so a 140 would need a higher pitch prop to keep the revs down, but that will slow your hole shot, for two reasons. First, the higher pitch prop is a bigger load on the engine. Second, by raising the rpm where peak torque occurs, it reduces the torque at lower rpms. That is why lighter boats respond better to this engine than heavy boats. For heavy boats during that era, the next engine up was a V6 150hp. Only rated 10hp more, but a 50% increase in displacement, it had a huge difference in load capacity.

I was sad to find my '79 donor 140hp had the same exhaust as my '82 90hp, no megamaphone. Center section and powerhead on the '79 were original, but it had a replaement LU (which I kept just in (gear)case).

To the OP question: If the 115 is running well and in good shape, I'd just leave it be. 30mph from a work barge is a good clip in my books. Save the 140 for if the 115 grenades. Use the same 13" pitch prop but stay away from full throttle, keep it to 5500.

 

[jimmbo]

I was sad to find my '79 donor 140hp had the same exhaust as my '82 90hp, no megamaphone. Center section and powerhead on the '79 were original, but it had a replaement LU (which I kept just in (gear)case).

I was referring to the Bubbleback as the megaphones

 

[interalian]

I was referring to the Bubbleback as the megaphones

Ah.

 

[Faztbullet]

If your 1982 115 is a flat back its about 90hp at prop....The 140 should have the bubble exhaust and its 115hp at prop. This will allow you to go up 1 pitch size in prop....