1989 OMC 5.7 Cobra WOT rpm

The manual for my boat and OMC 5.7 Cobra call for a WOT rpm of 4200 to 4600 rpm.
The max WOT rpm I can get is in the 4300 rpm range at about 37 mph gps. This is after I trim the drive to get max wot rpm without going too far and causing the prop to start slipping.
This is a Donzi GT250 25 ft cabin cruiser with a new Michigan Wheel 15x17 aluminum prop.

How does this sound?
Should I be expecting to get closer to 4600 rpm?

Re: 1989 OMC 5.7 Cobra WOT rpm

Donzi GT250 said:

The manual for my boat and OMC 5.7 Cobra call for a WOT rpm of 4200 to 4600 rpm.
The max WOT rpm I can get is in the 4300 rpm range at about 37 mph gps. This is after I trim the drive to get max wot rpm without going too far and causing the prop to start slipping.
This is a Donzi GT250 25 ft cabin cruiser with a new Michigan Wheel 15x17 aluminum prop.

How does this sound?
Should I be expecting to get closer to 4600 rpm?

Click to expand...

Howdy,


Aside from the fact that a small block is not a lot of engine for a 25ft boat, check the following link.
Sticky: ------------ADULTs ONLY -------------------------No Exceptions-----


It's at the top of this forum.

Scroll down to #13 and you find most of the reasons an engine doesn't make correct WOT RPM.

Regards,


Rick

Re: 1989 OMC 5.7 Cobra WOT rpm

Thanks for the quick reply.
Looking at the list I can safely eliminate several of the listed possible causes.
But I will be checking the items I can't eliminate at this time.
But my question still remains.
The book said that WOT rpm should be between 4200 to 4600 rpm.
Would 4300 rpm be considered acceptable given these parameters listed in the manual?

Re: 1989 OMC 5.7 Cobra WOT rpm

The book said that WOT rpm should be between 4200 to 4600 rpm.
Would 4300 rpm be considered acceptable given these parameters listed in the manual?

Click to expand...

Ayuh,... If I'm not mistaken, 4300 Is in fact between 4200, 'n 4600....

Re: 1989 OMC 5.7 Cobra WOT rpm

Donzi GT250 said:

The book said that WOT rpm should be between 4200 to 4600 rpm.
Would 4300 rpm be considered acceptable given these parameters listed in the manual?

Click to expand...

Well, yes...........maybe.....

The engine usually makes maximum HP at it's max RPM. GM (OMC, Merc, Volvo) generally suggested that you keep your WOT RPM in that range......... but most of them also suggest selecting a prop that will give you an RPM near the top of the range when you are operating lightly loaded AT the lake you will normally use the boat(elevation above sea-level)

Doing the above will help ensure that when you DO pile in a few extra people+ dogs, etc, or operate at that mountain lake, you'll still be well above the minimum recommended RPM.

I propped my boat (below) to give me 4600 at WOT....... it required 26p props. The recommended range for my 1997 GEN VI 7.4L engine is ( I think) 4200-4600. The rev limiter is set at 4700 RPM.

I also have 24p props but I could tell that I was "getting into" the rev limiter.

A couple of years ago I took the boat to (6000ft elevation) Lake Tahoe in Ca/NV. I changed to the 24p and piled 8 people and 3 big dogs inside!

I was not confident it would even plane! I think I turned around 4200 RPM WOT and it did plane but it was pretty slow out of the hole.


It probably would have been better with 22p props.


Cheers,


Rick