Is extended WOT bad for the engine?

[KnotConnected]

Just as the title says. I've got a 1994 7.4l with Bravo III that has about 1060 hours on it. Running wonderfully (for the time being), but I just took my first 3 hour trip (The beast crawls at a steady 24 MPH) and was told (After I got back) that running it at 100% throttle (WOT) is hard on the engine. At WOT it was running at 4,000 to 4,100 RPM. Oil pressure a scoshe over 40, temp @ about 150.

Thoughts, ideas, comments?



P.S. a little sample of the weekend!
http://i46.photobucket.com/albums/f117/n915aa/FishCreek-2013/IMAG0605_zps19d1e489.jpg
http://i46.photobucket.com/albums/f117/n915aa/FishCreek-2013/IMAG0579_zpse27b32ad.jpg
http://i46.photobucket.com/albums/f117/n915aa/FishCreek-2013/IMAG0586_zps357a9531.jpg

 

[Fun Times]

Re: Is extended WOT bad for the engine?

I've got a 1994 7.4l with Bravo III that has about At WOT it was running at 4,000 to 4,100

Hi there, just wanted to mention that you are a little low on your wide open throttle RPM.

WOT specification is between 4200-4600 for your engine model. Preferably it would be better to be propped closer to the 4600 rpm range.

Are you trimming your stern drive up a little bit once your on plane to help get the boat and RPM up to recommended speed specifications?

 

[KnotConnected]

Re: Is extended WOT bad for the engine?

Hi there, just wanted to mention that you are a little low on your wide open throttle RPM.

WOT specification is between 4200-4600 for your engine model. Preferably it would be better to be propped closer to the 4600 rpm range.

Are you trimming your stern drive up a little bit once your on plane to help get the boat and RPM up to recommended speed specifications?

Yeah, I trimmed way up. Got me another 2-3 MPH, but i didn't pay too much attention to the RPM. Will have to check again when I go out next time. If i can get it to WOT closer to 4600, is it ill advised to punch it down to WOT for long cruises again?

 

[84EdH]

Re: Is extended WOT bad for the engine?

If you re-prop to get the rpm wot in the 4500 to 4600rpm, you will plane sooner and with less throttle and use less fuel, at cruising rpms around 3300 to 3600 or so. and you wont have to cruise for hours at wot. In my opinion, true wot of 4600 for your engine, is definetly more strain on your engine components. Hours on end of wot is hours on end of heavy stress on your engine components. With oil pressure strong, and engine temp good, you are not looking for an immediate break down, however, you are shortening the life of the engine. 1000 hr engine is no spring chicken!

 

[tpenfield]

Re: Is extended WOT bad for the engine?

As stated the WOT RPM is a bit low . . . is your top speed at WOT 24 mph? or is that a mid-range cruising speed?

I would not run the engine at WOT for extended periods . . . nothing good can happen to it.

 

[Bondo]

Re: Is extended WOT bad for the engine?

Yeah, I trimmed way up. Got me another 2-3 MPH, but i didn't pay too much attention to the RPM. Will have to check again when I go out next time. If i can get it to WOT closer to 4600, is it ill advised to punch it down to WOT for long cruises again?

Nope,.... Runnin' at Wot will no doubt shorten the motor's overall hours of service, 'fore it needs rebuildin' or replacin',...

Generally, Cruise speed is 'bout 75%, or alittle over 3000 rpms to 'bout 3500 rpms for Most hulls,...

Is this a New to You boat,..??
If not, has it ever done Better than 24 mph at 4100 rpms,..??

Also, it's a fair sized ship, is it Trailered, or a Dock-Queen,..??
If it stays in the water, I'd guess the bottom needs Cleanin',...

 

[KnotConnected]

Re: Is extended WOT bad for the engine?

Yeah, its a new-to-me boat, so I'm not sure if it's ever been quicker.

Understood to the no-no on WOT factor. Thanks!

Bond-o, I've got a trailer, but she sits in the water all summer. I did just redo the bottom paint this last winter, but judging by how much junk is on the outdrive, the hull probably has some goobers too.

The idle RMP also likes to back too far down, and needs a turn or two of the screw every now and then, so I'm thinking the carb might be due for a good cleaning or rebuild.

 

[agallant80]

Re: Is extended WOT bad for the engine?

I run pretty close to WOT. I run at 4,000 to 4,200 RPM. Part of this is due to my dumbness when I bought the boat and got only the 5.0 thinking it was enough. Its not. When the motor gives out and I have to replace it I will put a 5.7 in her. With that said the boat is new (2013) so it will be more than a few years before I have to replace the motor in her. I wonder how long she will last? Also I have cats on my boat which I hate so when it comes time to repower I am going to get an ECU for non cats.

 

[HT32BSX115]

Re: Is extended WOT bad for the engine?

Just as the title says. I've got a 1994 7.4l with Bravo III that has about 1060 hours on it. Running wonderfully (for the time being), but I just took my first 3 hour trip (The beast crawls at a steady 24 MPH) and was told (After I got back) that running it at 100% throttle (WOT) is hard on the engine. At WOT it was running at 4,000 to 4,100 RPM. Oil pressure a scoshe over 40, temp @ about 150.

Thoughts, ideas, comments?

Howdy,


Nice pictures!

You need to get yourself a known tach and use a GPS to measure your speed (your drive-pitot is probably plugged) .

You also need to know what your RPM really is.

As everyone said previously, after you trim the boat for best speed, note your WOT RPM and speed.

If it's below 4200 and your engine is otherwise running correctly....... you are over-propped (assuming you're not dragging your anchor or you're seriously overloaded/waterlogged etc)

You want that engine turning 4600 with the boat lightly loaded at full throttle.

That's a big heavy boat (7500lbs +) it's not going to be a fast one.


What props do you have on it? Do you know the drive gear ratio?

Oh, a good cruise RPM might be to run at 3500-4000 RPM. BUT, it shouldn't take WOT to achieve it.
If it does, you either have an engine problem, you're over loaded, or you're over-propped.



Regards,


Rick

 

[KnotConnected]

Re: Is extended WOT bad for the engine?

Howdy,


Nice pictures!

You need to get yourself a known tach and use a GPS to measure your speed (your drive-pitot is probably plugged) .

You also need to know what your RPM really is.

As everyone said previously, after you trim the boat for best speed, note your WOT RPM and speed.

If it's below 4200 and your engine is otherwise running correctly....... you are over-propped (assuming you're not dragging your anchor or you're seriously overloaded/waterlogged etc)

You want that engine turning 4600 with the boat lightly loaded at full throttle.

That's a big heavy boat (7500lbs +) it's not going to be a fast one.


What props do you have on it? Do you know the drive gear ratio?

Oh, a good cruise RPM might be to run at 3500-4000 RPM. BUT, it shouldn't take WOT to achieve it.
If it does, you either have an engine problem, you're over loaded, or you're over-propped.



Regards,


Rick

Hey Rick!

Thanks! I cross-checked the speed with the GPS, and SOG reflects the speedo gauge. The Tach i don't necessarily trust. The boat IS 10,000+ pounds, so I don't expect to get anywhere in a hurry. I also replaced the lower unit with a rebuilt this spring (due to a horrible corrosion issue), so I suppose it's not beyond the possibility that the gear ratios are off. Also, I'm not sure which props are on her, but it sounds like that's a poss ability too.

 

[HT32BSX115]

Re: Is extended WOT bad for the engine?

Hey Rick!

Thanks! I cross-checked the speed with the GPS, and SOG reflects the speedo gauge. The Tach i don't necessarily trust. The boat IS 10,000+ pounds, so I don't expect to get anywhere in a hurry. I also replaced the lower unit with a rebuilt this spring (due to a horrible corrosion issue), so I suppose it's not beyond the possibility that the gear ratios are off. Also, I'm not sure which props are on her, but it sounds like that's a poss ability too.

With that sort of weight, you should have either a 2.20:1 or 2.00:1 ratio drive.


The prop pitch is stamped on the props. I would expect either 24p (which might be a bit too "tall") or 22p props. it'll depend on what ratio drive you have

The ratio may or may not be indicated on the side of the drive on a "sticker".

If it's a rebuilt, whoever did the job may not have put the (right) sticker back on indicating the ratio.

You really need to KNOW the RPM accurately. It's the only way you can determine the health of your engine.

You also need to know your ratio........(with the boat on the trailer, you can place the drive in gear and turn the engine by hand with the plugs out..... at the prop, have a helper count "blades" . 2 engine revs = 1 prop rev [2.00:1] a tad more than 2 engine turns to get 1 prop rev probably indicates a 2.20:1 drive.......slightly less= 1.81:1 drive........You shouldn't have a 1.81:1 drive on a boat that heavy......

Knowing that you can use a prop slip calculator to estimate the prop pitch you *should* have based on approx how fast a 454 should push that boat. (I'm guessing around 35-40 mph)
[understand, this is not an exact science!!]

Prop Slip Calculator


///

 

[Glaspar_Fan]

Re: Is extended WOT bad for the engine?

One other thing if you have low RPM - Make sure the engine is putting out all it was designed to. I noticed on my boat that the previous owner had swapped carburetors. To make a long story shorter, he botched the linkage and cable so full throttle only opened the carb about 80%. It's barely opening the secondaries when the cable comes up hard against the bracket.