How long at WOT?

[dingbat]

Re: How long at WOT?

thanks--I guess--I have a carbed 2 stroke!

when i hit that dead area I back down to the live area. I don't run WOT except a short distance coming home at the end of the day to burn off carbon.

Any easy fix to the dead zone? Does it indicate an air leak in the fuel line somewhere?

It could be your prop for cavitation "burn". I had the same "dead zone" until I had my prop reworked.

 

[truckermatt]

Re: How long at WOT?

i run WOT for a few minutes, but generally the lake isn't glass and running that fast rattles your fillings right out sometimes.

plus i am more into water sports and WOT is too fast for that, 25-30 mph is my cruise that i like, and i can still have a conversation on board.

 

[MH Hawker]

Re: How long at WOT?

And another reason I will never buy a 4 stroke.

 

[Captain Caveman]

Re: How long at WOT?

I'll play the devil's advocate for a minute...

On my last new truck, the dealership scolded me a few times for not driving it hard enough. They said that the engine should make it up into the higher RPM bands more. To make them happy, I started making my wife drive my truck every month or so. Then it got the motor humming and probably rubbed some tread of the tires!

Of course, that's not on a marine engine but it's a four-stroke comparison. It's good to get them up to WOT every once in a while. In fact, when I'm doing the 35-mile trip across my lake I make sure to hit multiple throttle positions throughout the cruise. Usually waves and turns dictate that but I try not to always cruise for long periods at the same speed/throttle position.

 

[Water logged]

Re: How long at WOT?

Met a guy down at the river, had occasion to hang out with him over the last few years. He has a Merc 115 a couple years older than mine. About 3 years ago his engine started giving him a lot of problems. It needed a total overhaul. When he picked it up they told him the computer showed he never had it over 75 percent rpm, and mostly under 50 percent. He was told to run at WOT for several minutes every time he took the boat out. The mechanic said Wot is better for the engine than long low rpm runs. Just passing on what a Mercury tech told him for what it's worth.

Glenn

 

[Philster]

Re: How long at WOT?

Mercury Optimax 2-strokes from the early years are literally not designed to run at low RPM. They run out of tune by nature and load up with carbon deposits.

The newer ones are tuned and constructed/coated differently to counter the problems associated with low-RPM running.

 

[jkust]

Re: How long at WOT?

At most I'll go WOT for maybe 20 minutes. It's not that I'm afraid anything will break but I'm always leary about going wide open on anything for too long. Whether it's a dremel or a boat engine. Besides rarely is there a time where the chop doesn't dictate I pull back from WOT.

 

[haulnazz15]

Re: How long at WOT?

I'll put in my .02 as well. On the 2-stroke O/B or the jet ski (2-stroke as well) I'll run them flat out much more willingly than I will the push-rod 351w in the runabout. I don't worry about spinning the 2-strokes up and leaving them at or near redline for 8-10 minutes, but I don't run the V8 WOT for more than a minute or two. I do routinely get the boat to WOT/top speed for at least 20-30 seconds about once every trip out at the lake just to "blow the cobwebs out" and stretch her legs a bit. No empirical evidence that it does anything good/bad, but it's more of a verification that everything is still in good running order if I can hit max operating rpms. I also don't get much satisfaction in getting across the lake @ 50mph than I do @ 35-40mph. I suppose if I had a couple of blown big blocks strapped into an offshore hull I'd be a little more willing to drop the hammer and scream across the lake. I don't have the bank role to support the fuel usage of that kind of beast though, lol.

 

[90stingray]

Re: How long at WOT?

don't get much satisfaction in getting across the lake @ 50mph than I do @ 35-40mph.

I don't either... i just love being on the water. Wether it's WOT, cruising, or anchored out... it's always a good time!

I get that 2 stroke guys need to be at max rpm's during operation... they do get fouled up when ran at half speed. Seems like most people don't like running their 4 stroke WOT for too long. That's cool. Thats how i feel too. Just wish i had more room in our lakes to leave her maxed out for a bit longer. But like i said, i am having fun either way.

 

[bruceb58]

Re: How long at WOT?

Home Cookin': If your engine acts like that (4/5 vs full = same power), you could be allowing more air into the engine like that, but the fuel flow can't keep up. That's runnin' lean.

It would really be most serious on a carbed 2-stroke engine.

If the engine doesn't increase in RPM between 4/5 and full throttle position, you aren't pumping through any more air so fuel mixture going lean is not an issue.

I never run my engine at WOT for more than a few minutes just to make sure it can. WOT will have more wear on an engine depending on load than an engine that is not at WOT.

 

[Philster]

Re: How long at WOT?

Well, two different throttle positions (being that throttle invites more air and fuel) can equal different am'ts of air, but engine RPM might not rise visibly because the air creates a better burn (which can accomplish little in some 2-strokes) or accomplishes nothing, because this isn't precision combustion, but more RPM is stymied by a lack of fuel.

Of course, the extra throttle may provide nothing in terms of power, so the engine is merely under a higher load but unable to spin faster, so the lower throttle setting is still preferred.

 

[QC]

Re: How long at WOT?

WOT will have more wear on an engine depending on load than an engine that is not at WOT.

True that. But the ultimate question is "how much?" I would submit this is a very variable discussion, and I would also submit that the wear increase is lower with more conservatively rated engines. I also don't believe there is a whole lot in this thread other than opinion. Even RPM comments are a question of degree. Here is a question to ponder: Forget load for a second. Why is any RPM point at less than valve float detrimental at all? My guess is that the answer is mostly because we think it is, not because we know it is . . .

 

[QC]

Re: How long at WOT?

Well, two different throttle positions can equal different am'ts of air, but engine RPM might not rise visibly because the air creates a better burn (which can accomplish little in some 2-strokes), but more RPM is stymied by a lack of fuel.

At the same RPM, a wider open throttle always introduces more air. Tough discussion without a complete understanding of the variables. Too many to consider. Also we need to be clear between "throttle" and fuel demand/delivery. They are different

 

[Philster]

Re: How long at WOT?

Well, I'll bet the optimal RPM for an engine is the lowest # of revs curved against the least am't of load (as far from lugging as possible), and it probably coincides somewhat with the point in the power band that results is the highest efficiency, while maintaining proper heat, but not too much.

Not that I can back that up with anything but hooey.

 

[Frank Acampora]

Re: How long at WOT?

Ahhh! But QC: You can not disregard load because unless you are running the engine in neutral to the redline, load will increase significantly with RPM. Remember---at higher RPM we are generating more horsepower and pushing the boat faster--against more drag.

However, I submit that the piston doesn't care if it is traveling 2000 feet per minute or 4000. Similarly it doesn't care if it has 200 PSI against it or 300. All parts of the engine have been designed to withstand normal forces encountered at maximum operating range.

Yes, there will be some wear with age and yes, there is usually a ridge at the top of the cylinder of high mileage engines--however high RPM (Given some parameters) will not accelerate this wear. If you get wear on the components, it happens faster because you have turned more revolutions in the same amount of time. Look at newer cars which count revolutions, time at given rpm, and conditions to determine oil change intervals. Running the engine slowly (at idle) will actually increase the frequency of necessary oil changes over an engine run at high RPM.

On the other hand, let's not forget that the service life of a drag engine turning 10,000 RPM and generating 5000 horsepower or so is only 5 - 10 seconds. They are essentially rebuilt between each run. But, of course, here we are looking at the extremes.

 

[bruceb58]

Re: How long at WOT?

Well, two different throttle positions (being that throttle invites more air and fuel) can equal different am'ts of air, but engine RPM might not rise visibly because the air creates a better burn (which can accomplish little in some 2-strokes) or accomplishes nothing, because this isn't precision combustion, but more RPM is stymied by a lack of fuel.

Of course, the extra throttle may provide nothing in terms of power, so the engine is merely under a higher load but unable to spin faster, so the lower throttle setting is still preferred.

If the RPM doesn't go up, the amount of air consumed also doesn't go up unless you are somehow increasing compression on the fly.

 

[Philster]

Re: How long at WOT?

Two stroke engines are tuned dynamically as they run. Fuel/air mixture is at the mercy of many variables, and if fuel isn't coming to the party, more air is.

 

[bruceb58]

Re: How long at WOT?

Two stroke engines are tuned dynamically as they run. Fuel/air mixture is at the mercy of many variables, and if fuel isn't coming to the party, more air is.

Yes, if its a closed loop fuel injection system. Otherwise, fuel is delivered as a function of the amount of air.

 

[QC]

Re: How long at WOT?

You and I are in agreement, Frank. Load is the variable, but valve float doesn't care . . .

I have no idea what my point is other than most of this stuff is speculation. The impact of load on engine life is variable, the impact of RPM is variable, so what do we have to base any of our positions on? Nuttin' . . .

 

[bruceb58]

Re: How long at WOT?

As far as running at WOT, call Mercruiser or Volvo and ask them how long you can run their engines at WOT and see what they tell you!

I can tell you that piston airplane engines are not rated to run at continuous WOT. They aren't designed that much differently than automotive engines.