92 OMC Cobra 5.0 WOT?

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thewooddogg

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Jul 13, 2007
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I've got a 92 Four Winns 190 Horizon with an OMC Cobra 5.0. The prop looks like an aluminum 15 x 17. The top of the engine has a label that says WOT should be 4000 - 4400 (or mid 4000's.. can't remember the exact range). However, I take the boat out and the RPM's can easily get up in the 5000's easy and can get close to the max on the gauge which is 6000! The boat seems to get out of the hole at about 3250 or so.

As far as I can tell, I get great out-of-the-hole ability and top speed. Although my speedo doesn't work so well and I don't have GPS to tell how fast I'm going... but I seem to keep up with other boats on the lake just fine.

So, my questions are... do I have the wrong prop for this boat? or do you think my RPM gauge is reading incorrectly?
 
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95yj

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Re: 92 OMC Cobra 5.0 WOT?

This probably belongs in the prop forum.

Prop "looks like"? There should be numbers on the prop to tell you what it is.

That sounds like a light boat to be running a 15x17. Tach is probably right and you are under propped. Only way to know is to get a shop tach and a GPS and start taking readings.

Redlining a stock boat motor at 1500 RPMs over WOT is definately not good for it.
 
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thewooddogg

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Re: 92 OMC Cobra 5.0 WOT?

OK thank you... I've reposted in the prop forum! That sounds like bad news..
 
Gary H NC

Gary H NC

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Re: 92 OMC Cobra 5.0 WOT?

If your tach is even close i bet you could run a 19 to 21 pitch and get you closer to the right rpm range.
 
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thewooddogg

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Re: 92 OMC Cobra 5.0 WOT?

Hmmm...

What type of problems should I watch out for that could have already been caused by the engine going over WOT?

The lakes here are busy and it's not like I'm over WOT for more then a very short period.. most the time I have it wround 3600 pulling my kids on a tube. It likes to hop in and out of the hole at 3500 and under.
 
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CharlesW

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Re: 92 OMC Cobra 5.0 WOT?

What problems you may have caused, I can't help you with.
The 5.0 OMC is probably a Ford engine and unless you float the valves, you aren't likely to have damaged anything.
The outdrive is very likely the limiting factor in high RPM situations. There again, the short duration has probably not done any damage, but.....?

FWIW, we ran a 5 liter GM OMC stringer under propped for 18 of the 19 years we owned it. It was supposed to be limited to 4200 RPM and the prop we ran would let it turn 4800 RPM at WOT. Once I knew the WOT range, I just made sure not to run it that fast. The smaller prop made a world of difference in getting two slalom skiers up when we had a boat load of people
While I wouldn't necessarily recommend it, it worked for us.
 
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tmh

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Re: 92 OMC Cobra 5.0 WOT?

Something isn't quite right here....i think you should be on plane well below 3500 rpm with your boat/prop. Maybe the tach is off a bit on the high side.

Regardless, you need to get proper info on: Actual RPM (but don't test up to 6000 or so!) GPS speed. What prop is it (no guesses)?

Most likely you need a 19" or 21" pitch prop. You should see better performance once you get it nailed down.
 
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95yj

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Re: 92 OMC Cobra 5.0 WOT?

Running a motor over it's redline doesn't necessarily do damage except maybe wear a little faster than normal. The problem is that when you exceed the revs a motor is built for, it has a tendancy to come apart in the worst possible way all at once. Since that didn't happen, you're OK there. The outdrive sure isn't rated to be spun at 6 grand but again if it's still in one piece, you're probably good.

Running at 4800 vs. WOT rating of 4100 isn't going to do a thing since the motor and drive can both handle them. It just wont be very efficient since the motor was set up to get max HP near its WOT.

I wouldn't worry too much about any damage you've done, I would be more worried about doing it again and having the motor come apart.

Once again, find the numbers on your current prop to see what you have. Get a shop tach so you know your actual RPMs. Get a GPS so you know your max speed. Once you have these, you can dial in the correct prop pretty quickly especially if you know people you can borrow props from or have a place to rent them from for testing.
 
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thewooddogg

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Re: 92 OMC Cobra 5.0 WOT?

Thanks for all the great helpful replies! Based off your suggestions, just in case, I'll make sure to run the boat under the WOT according to the tach until I can test the tach and get a gps.

I know for a fact the prop is 15 x 17 (stamped on the back)... I was just assuming it was aluminum because it was painted black.

Also, I did move the switch from 8 cyl and to 6 and back to 8 on the back of the tach yesterday, just in case it might be that simple. I should be taking it out on the weekend so we'll see.

Only 3 things wrong with it that I can see... 1.) It doesn't like to idle, especially when cold (it'll die but start right up again) 2.) it has a bit of oil on the top of the engine block under the carb (not much, but after a day of running it'll halfway fill the little divits) 3.) Last time I took it out it let off a big cloud of white smoke when backing up off the trailer, but didn't do it the rest of the day at all. smelled like exhaust.

Anyone know where I can pick up a cheap shop tach?
 
180shabah

180shabah

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Re: 92 OMC Cobra 5.0 WOT?

tmh said:
Something isn't quite right here....i think you should be on plane well below 3500 rpm with your boat/prop. Maybe the tach is off a bit on the high side....
Click to expand...

He didn't say that this was his minimum planing speed, so 3500RPM isn't out of line if he is underpropped by 4"
 
bruceb58

bruceb58

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Re: 92 OMC Cobra 5.0 WOT?

15x17 is a perfect prop for that drive/engine/boat combo. I had the exact prop with a very similar boat and that exact drive and engine.

I suspect the tach in not indicating correctly. Is the switch at the back of the tach pointing at 4 stroke V8? What does the engine idle at according to the tach?
 
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thewooddogg

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Re: 92 OMC Cobra 5.0 WOT?

Once again... i really appreciate all the help!

Bruceb58,
The switch on the back of the tach looks to have 5 positions.. 4 cyl, 6 cyl, 8 cyl, and two alternator settings of some sort which I don't understand. I checked it out yesterday, it's set to the 3rd setting which matches up to 8 cylinders. I took a screwdriver and moved to 6, then back to 8 just in case (you never know right?!)
 
bruceb58

bruceb58

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Re: 92 OMC Cobra 5.0 WOT?

thewooddogg said:
I took a screwdriver and moved to 6, then back to 8 just in case (you never know right?!)
Click to expand...

That was good to do. I have seen those switches vibrate off their positions before.
 
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thewooddogg

Seaman
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Jul 13, 2007
Messages
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Re: 92 OMC Cobra 5.0 WOT?

Hey everyone,

Thanks for all the advice and help! Believe it or not, I fixed it! I went under the dash before my trip Saturday... the switch was set correctly to 8 cylinders. Regardless, I took a screw driver, moved it to 6 cylinders, then back to 8. Amazingly, it worked perfect! I had a blast on the lake with my family not worrying that the engine was gonna blow up at any second! Thanks for all the advice, I hope this helps some other newbie boat owner someday!
 
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