What should RPM's be at WOT

[utadventure]

I have a 4 cyl, 120HP Merc engine with pre-alpha outdrive. It's in a 1968, 16.5' HydroSwift V-hull boat. I use it mostly for cruising, tubes with the family and just tried a knee board this past weekend.

I'm curious as to what the ideal RPM should be at WOT. I think my set up is under performing but don't know what it should be.

Enlightenment would be appreciated.


Dave

 

[ryanr623]

Re: What should RPM's be at WOT

4200-4600 is what my year manual says. I have a 18 foot boat with a mercruiser 120, and it does about 4000 rpm with a 19 pitch prop, so im going to an 18 pitch prop to be safe.

 

[QC]

Re: What should RPM's be at WOT

I'm curious as to what the ideal RPM should be at WOT

There really isn't an "ideal" RPM, but the range it needs to fall into is 4200 - 4600 with a typical load.

I think my set up is under performing

In what way is it under performing? If you say that it is sluggish out of the hole, then I would recommend you prop towards the 4600 RPM point. If you want more top speed then I might answer differently. Also, there is no guarantee that your engine is running right. All of our propeller recommendations will assume the the engine is performing at its best

Enlightenment would be appreciated.

Enlightening begun

 

[utadventure]

Re: What should RPM's be at WOT

There really isn't an "ideal" RPM, but the range it needs to fall into is 4200 - 4600 with a typical load.


In what way is it under performing? If you say that it is sluggish out of the hole, then I would recommend you prop towards the 4600 RPM point. If you want more top speed then I might answer differently. Also, there is no guarantee that your engine is running right. All of our propeller recommendations will assume the the engine is performing at its best


Enlightening begun

That's exactly what I'm looking for (4200-4600 rpm).

With just a light load (wife and I) the rpms are topping out at 3750-3800 rpm and 25 mph. Not that good if we add passengers or pull someone.

Now that a have a goal to shot for, I can start checking timing, fuel/air mixture, choke, etc, etc, etc on the motor. Once I feel comfortable with that working properly, I can then look at whether I should change the prop (current: 15 1/4 X 15p Aluminium)

I appreciate your responses and any suggestions of things to check to make sure the motor is running at it best.

Dave

 

[ryanr623]

Re: What should RPM's be at WOT

That's exactly what I'm looking for (4200-4600 rpm).

With just a light load (wife and I) the rpms are topping out at 3750-3800 rpm and 25 mph. Not that good if we add passengers or pull someone.

Now that a have a goal to shot for, I can start checking timing, fuel/air mixture, choke, etc, etc, etc on the motor. Once I feel comfortable with that working properly, I can then look at whether I should change the prop (current: 15 1/4 X 15p Aluminium)

I appreciate your responses and any suggestions of things to check to make sure the motor is running at it best.

Dave

Yeah i'd definitely check the engine if your only running a 15 pitch prop. Boat speedos are horribly inaccurate (use GPS) but your still too low on the RPM side of things.

 

[QC]

Re: What should RPM's be at WOT

Also, verify the tach is accurate. Many aren't

Do you have any data that differs from above? Any past history info?

 

[ryanr623]

Re: What should RPM's be at WOT

Also, verify the tach is accurate. Many aren't

Do you have any data that differs from above? Any past history info?

Any ideas on how to do this? I have wondered this myself, i assumed i was off 300 rpms due to the 19 pitch prop, but maybe its the tach.

 

[QC]

Re: What should RPM's be at WOT

First thing is to check if there is an adjustment on the back of the tach for 4, 6 or 8cyl engines. Many are set wrong. The best way is to get a photo tach. They read a piece of reflective tape that you can place on the crankshaft pulley. Some timing lights have tachs too . . .