top speed issue with 4.3L OMC Cobra

[goobtroob10]

I just bought a 17.5 ft four winn runabout with open bow with the 4.3L omc cobra it usually runs at about 4700 rpms and the most speed i can get is about 35-37 mph, the specs say that it can go 45-48mph. I'm at about 5000 ft above sea level so this obviously affects it some but that much? it has a 15/15 pitch prop, how can i get more speed out of it?? Is it the right pitch?

 

[a70eliminator]

Re: top speed issue with 4.3L OMC Cobra

It seems to me that engine behind a 17' boat should be able to turn a 19P at least if not a 21, with a 15p I would think it would rev to the moon if it was in good tune but the cam probably is holding back.

 

[wca_tim]

Re: top speed issue with 4.3L OMC Cobra

Sounds like something else is wrong... There's a sticky in the i/o forums below about diagnosing this sort of thing.. just walk through and check everything... often time is engine not running right, etc...

 

[goobtroob10]

Re: top speed issue with 4.3L OMC Cobra

thanx for the help. i'll check with the dealer on props.

 

[mthieme]

Re: top speed issue with 4.3L OMC Cobra

Agree with A70...if not a 23P.
I have a 19' -4.3 - running a 21P. Tried a 19P last summer and lost 4 knots.
There was a thread last fall about 4.3's and top end. You might do a search for it. There were a lot of replies with basic data that you might benefit from.

 

[SteveDavis]

Re: top speed issue with 4.3L OMC Cobra

Say, I have an '86 Crestliner that is 17.5'. I also have a 4.3L OMC Cobra. The boat has been in storage for the winter, but I believe I am running a 21 pitch. I hit 55mph pretty easily, at somewhere near 45-46,000RPM. My problem is I need faster planing. I am going to try a 19 or even a 17 pitch. It would seem that your 15 pitch is way to low for what you are looking for.

 

[steelespike]

Re: top speed issue with 4.3L OMC Cobra

Be sure it is in excellent tune,has good compression and throttle is opening all the way.Your speed is slow but about right for a 15X15 prop.Is the prop in excellent shape?Even allowing for the elevation I think you should be overreving with that prop.