rpm range for 79 johnson 100hp?

[as2222]

I have a 79 100hp johnson javelin. I have a 17'' prop on it now running 4600rpm wot. I wanted to know what is the rpm range soppose to be on this motor? Thanks

 

[Dhadley]

Re: rpm range for 79 johnson 100hp?

You're about 1000 rpm short of your goal. Unless it's on a very heavy boat you might have an engine issue rather than a prop issue. It's important to verify the motor is running correctly when establishing a baseline.

 

[as2222]

Re: rpm range for 79 johnson 100hp?

The engine was just checked over so it is not an engine problem but its on a 17 bass boat. The boat it self weighs about 1200 dry.

 

[Chinewalker]

Re: rpm range for 79 johnson 100hp?

A 100hp ought to be able to spin a 17 on a bass boat. A friend of mine had to run a 21 with his Evinrude 100 on a 17-foot Whaler Montauk...

 

[Haffiman]

Re: rpm range for 79 johnson 100hp?

You are about 400 short of obtaining max power from the engine and 900 below maximum allowd.
On the other hand you are 100 above minimum recommended.
It might be a question of engine installation height and trim.
The higher you get within the allowed, the better hole shot.
The closer you get to 5000, the better power and the closer you are to the lower usually gives the best CRUISING economy.

 

[emdsapmgr]

Re: rpm range for 79 johnson 100hp?

The original factory recommended operating range was from 4500 to 5500. More updated thinking is to prop the engine to run at the top of this range, or 5500. The engine will still make good, reliable hp up to 5700. Running the engine top speed at only 4600 is now considered "lugging" the engine-you need to go down in prop pitch for better performance and fuel economy. Target 5500-5700 with a light load in the boat.

 

[Haffiman]

Re: rpm range for 79 johnson 100hp?

The original factory recommended operating range was from 4500 to 5500. More updated thinking is to prop the engine to run at the top of this range, or 5500. The engine will still make good, reliable hp up to 5700. Running the engine top speed at only 4600 is now considered "lugging" the engine-you need to go down in prop pitch for better performance and fuel economy. Target 5500-5700 with a light load in the boat.

A slightly too simple way of putting it.
Where the engine is most 'happy' in the RPM range may be read out of the condition of the spark plugs.
We are dealing with a rather old design, specifications are given for fuel with quite a bit different density than is used today, engine has probably quite some running hours and wear and even improved oil quality may come into play. Prop efficiency at different rpm should not be forgotten as well.
If one wants to 'play' in the upper designed limits, read the plugs and they will tell. Perhaps a re-jet might be needed.

 

[as2222]

Re: rpm range for 79 johnson 100hp?

thanks for the advice. I talked to a place that will let me try a 15in pitch Ill try this first. thanks