mechanic recommends less than wot? why?

I am curious as to why a mechanic would recommend a less than WoT usage on a 1986/7ish mercruiser 140. The reason he gave was that the engine has blow-by that covers the engine in oil or some such thing and that running it around 3000-4000 rpm is recommended and to not hit the WoT of 4900 which is the recommended rpm for this engine I think. I've always been under the assumption that running at anything LESS than wot will actually hurt the engine more than running it at WoT. Why am I hearing something different? What is blow-by anyway? And why would it cause the need to run at less than optimal rpms?

It should be noted that if there is a problem that is causing this, I would want it fixed. I wouldnt want to ignore it and hope it goes away. Is this something he should be fixing rather than telling me to run at less than WoT. If there is something to fix, what would it be? I wouldnt even know what to ask him to fix.

Re: mechanic recommends less than wot? why?

Howdy,


If he's telling you to NOT run at WOT ALL THE TIME, he's right!

You shouldn't run ANY engine at WOT continuously.

There's nothing wrong with occasionally running at WOT for short runs.....

It will burn more fuel and wear out quicker @WOT.


regards,


Rick

Re: mechanic recommends less than wot? why?

First of all your engine has a max WOT of 4200 to 4600. Not 4900.
Your engine will run at WOT for as long as it lives. Running any engine at full speed and heavy load is going to wear it out faster.
If your engine is wearing out and the rings have a lot of wear, you will get blow by. The blow by normally is piped through a fitting in the valve cover through a hose to the flame arrestor. Where the oily fumes are burned and sent out the exhaust. If there is too much blowby, it will cover the engine compartment with an oily film over time.

Your mechanic is just trying to get you to take it easy on your engine so it will last longer. If you don't care, then run it all the time at WOT, it's your engine.

It should be noted that if there is a problem that is causing this, I would want it fixed.

Click to expand...

Nothing that a complete engine overhaul won't cure.

Re: mechanic recommends less than wot? why?

Ok, so then this is what I would call a "Picnic" problem (I'm a programmer, not a mechanic or boat guru) lol. Problem in Chair, not in Compu.... err Engine. hehehe. Picnie problem lol. PATH problem. Problem At The Helm. So it sounds to me like this is the correct recommendations then. For some reason I was under the assumption that anything less than WoT was bad when it's not. How about trolling speeds? I've always avoided trolling with the main engine because I thought it was harder on the engine than WoT. I have an electric motor I use for trolling but under heavy load (4 people), the battery dies quickly and we dont get much time in. Could I instead troll with the main engine when the battery dies? It does go very slow at slow speeds, slow enough to troll (1.5-1.9 GPS) which is about the correct speed I troll at with the electric anyways.

So I guess the next question would be, just what is the recommended WoT RPM designation for anyway? If it's NOT recommended, why recommend it? hehe. I've always run at around near the 5 marker on the RPM gauge, its like a little bit less than 5. I dont know if that would be 4900, 4600 or anything like that, there arent any real indicators as to the increments other than 1,2,3,4,5,6. I guess I am confused as to why it would be called a recommended RPM.

Re: mechanic recommends less than wot? why?

I would say that unless you have a carb issue, idling, or whatever you wanna call it, is not bad. I had another boat that used to load up with fuel if I ran it at idle too long, but as soon as I throttled up it blew it out again. WOT is like overclocking your computer without proper ventelation. It will work breifly, but if you keep it up you will burn something up.

Most boats have a "cruising" speed or RPM. WOT spec is just that, a spec. It doesnt mean to run it that way all the time.

Blowby means you have bad rings. Engine probably needs a refresh, or rebuild. If you take it easy, it will last longer and you will have less to fix when you do rebuild it. If you continue to run at WOT all the time with blowby, you will likely throw a rod or spin a bearing etc etc. Then you could be looking at a much more expensive rebuild.

HTH.

Re: mechanic recommends less than wot? why?

You should also verify the RPM at the tach is correct. If it is they I would prop up to get the WOT RPM down in the proper range.

Re: mechanic recommends less than wot? why?

However going up in prop pitch will put more load on engine at any RPM, so it's about 50/50. Just back off a little and save your engine.

Trolling is OK if you can do it off idle. At idle with an improperly adjusted or needing repair carb you can build up carbon. Carbon that can mess up your rings and valves.

Re: mechanic recommends less than wot? why?

There is a recommended WOT to help determine the pitch you should use. If you can not achieve the WOT recommended by your manufacturer then you may be over propped.