johnson outboard rpm's

[bchaffin125]

I own a 1996 hydra sport 175e with a 1992 johnson 120 hp v4 outboard. i am trying to find someone that knows the maxium rpms that is safe for this motor at full throttle. the model number on the motor is j120tletf. I am currently running a 13x19 sst stainless prop. The motor is mounted on a jackplate and it seems it is too low in the water. at full speed and the motor trimmed up it runs at 54 mph @ 5700 rpm's. Is this safe rpm's? Please help with any info of suggestions.

 

[tashasdaddy]

Re: johnson outboard rpm's

just about right, 5500-6000.

 

[bchaffin125]

Re: johnson outboard rpm's

would it be beneficial to install a higher pitch prop and raise the motor on the jackplate in an attempt to maximize performance of the boat and motor.

 

[Tacklewasher]

Re: johnson outboard rpm's

Trying to figure out your prop slip given what you said. Anyone know the gear ratio for the 1992 120hp? I thought it was 2:1 but that gives a negative slip.


I'd play with the jackplate and see how high you can run it without spending money on a prop. Don't be too fussed if you get a bit over 6,000 for a short time, doubt you would hurt the motor. Just see what playing with the jackplate gets you.

Be careful when going too high that the prop doesn't vent and you spin up real fast.

You are better off having your setup run over 5,500 at WOT and going up in pitch may hurt that too much.

 

[tashasdaddy]

Re: johnson outboard rpm's

going up in pitch may hurt you hole shot also. also water pick up, must be watched. you don't want to burn it up.

 

[Dhadley]

Re: johnson outboard rpm's

It's a 1993 and 6000 + won't hurt it in the least. It should be a 2:1 gearcase unless someone has changed it so the numbers don't quite make sense. Is that 54 mph on GPS?

Like Tacklewasher suggested, play with the jackplate & height. You may just find your holeshot improves as well.

 

[bchaffin125]

Re: johnson outboard rpm's

The 54 mph is on the factory speedometer. the motor is a 1992 v4 if this helps anyone. I talked with a prop shop today and he is going to cup my prop friday. he said this would lower the rpms a bit and help the performance. He also gave me a diagram on some measurements to take from the bottom of the boat to the floor when it is level and also pull a measurement from just above the prop at the trim tab to the floor. he said that subtract the lesser measurement from the higher and this is how far the motor needs to be moved up on the jack plate. FYI, i came up with 2 inches difference. I am going to start with a 1 inch raise. What do you guys think of this information i have obtained? Thanks for all the info so far.

 

[Dhadley]

Re: johnson outboard rpm's

Your model number (tlet) indicates the motor is a 93. There are some important changes between a 92 and a 93 especially in the cooling system. Just keep that in mind when ordering parts.

You don't want to get too far below 5700 as the max rpm with an average load. After the cupping is done - and it all depends on how much is added and where - you can play with the jackplate to get the max performance for that prop on that hull with that setback.

 

[kenmyfam]

Re: johnson outboard rpm's

My humble opinion !!!!
Keep the RPM over 5,500 at WOT

 

[bchaffin125]

Re: johnson outboard rpm's

THANKS FOR THE INFO ON THE YEAR OF THE MOTOR. i AM TAKING ALL YOU GUYS ADVICE AND HAVE THE PROP CUPPED BY A PROFESSIONAL WHO HAS BEEN DEALING WITH BOAT PROPS FOR ABOUT 30 THIRTY YEARS. THEN I AM GOING TO RUN IT A DAY. THEN I AM GOING TO RAISE THE MOTOR 1 INCH AND GO FROM THERE. I FISHED YESTERDAY AND WHILE RUNNING AT TOP SPEED I CHECKED THE WATER PRESSURE GAUGE AND IT READ 28. I WAS ADVISED TO KEEP THIS NUMBER ABOVE 15 DURING THE PROCESS OF GETTING THE RIGHT SET-UP. ANY COMMENTS ON THIS PRESSURE AT 5700 RPMS?

 

[Dhadley]

Re: johnson outboard rpm's

There's more to it than just pressure but we don't get concerned unless it goes below 18. The problem is that as you go up the water intakes may start to pull air. Just keep an eye on the gauge and an ear open for the hot horn (make sure it works now).