How Long Is Too Long At WOT?

[racerone]

So there is no clear warning about sustained ----WOT---- for hrs at a time shall we say.

 

[HT32BSX115]

And the 5.7 4bbl I have now gets pretty thirsty at WOT so I don't go there very often.

This is probably the "key" here.

As long as you're "propped" correctly, you're not really going to hurt anything except your wallet.

 

[puddle jumper]

My two cents. A wile back I was reading a comment that stated that if you don't run at wot your leg would fly apart. Now saying that I run my boat at 3200 rpm on plain for hours with no problem. I think if your boat is in good condition wot is not going to hurt it if run at for long periods of time. Wot on my 4.3 is 4800 and I only go there to get up on plain.

 

[82rude]

Mercury marine went approx. 50,000 miles all out on lake x in the late 50,s?Even filled up on the run!75 HP 2 STROKE https://vimeo.com/5370345

 

[harringtondav]

I've got a question on this topic. I've noticed OH cam engines I've owned, L4 and V6, red line in the low 6Ks, while my push rod Dodge 5.2 is at 4600 rpm. Similar to my push rod Merc 4.3.

So is the push rod valve train the weak link?

My design know-how is in gear design, not engines. So I've assumed a stern drive's WOT limit was as much to protect the high pitch-line velocity, high tooth bending load input spiral bevel set.

 

[Scott Danforth]

the weak link in most engines is the crappy spec balance, and to some extent the stamped steel rockers. A proper internal balance of the rotating assembly and some decent parts and you can spin a pushrod V8 at higher RPM's. Many 10,000 RPM push-rod motors out there.

oiling in most outdrives is the limiting factor, as they were designed for the 5000 RPM limit of most motors

 

[TyeeMan]

oiling in most outdrives is the limiting factor, as they were designed for the 5000 RPM limit of most motors

That is a very interesting comment that I never would have given a thought to. So if you want to turn an outdrive faster how does one do that? Oil pump, pressure lubrication?

 

[jimmbo]

There is more valvetrain mass to move in a Pushrod engine than in an Overhead Valve Engine, so that can have an effect on Max RPM. And while what Scott said about 10,000 rpm pushrod engines existing, it gets very expensive in a hurry to attain those RPMs and the cam life is short due to the very stiff springs needed to keep the lifters against the cam lobes, and the very rapid lift/closing rates

 

[Scott Danforth]

the volvo AQ series has a really nice oil pump in them. so much so, that if you grenade the gears on the bottom, it will pump the debris to the top.

in addition to proper oiling, you also need to look at the bearings themselves, shimming, and cooling

 

[jimmbo]

to add to Silvertip comment.... more engines die of improper maintenance and neglect than they do when you run em like you stole em

If I were to drive anything stolen, I would be the most conservative driver ever, easy on the gas, stop at all Stop Signs, do every signal and shoulder check. No hot rodding, or jack rabbit starts, nothing to attract Henry Handcuffs attention

 

[jimmbo]

Mercury marine went approx. 50,000 miles all out on lake x in the late 50,s?Even filled up on the run!75 HP 2 STROKE https://vimeo.com/5370345

Those were 2 strokes. and those engines needed lots of pampering, and some outright cheating to make it looked like they made the 50,000. Oh 50,000 miles was run, but the original powerheads that started the run, weren't the ones crossing the finish line. Complete sets of pistons, crankshafts, even complete powerheads were changed on the sly.

the book Iron Fist has a chapter devoted to Operation Atlas, and tells of the shenanigans that happened

 

[Scott Danforth]

OK, then drive it like you rent it.

 

[Maclin]

oiling in most outdrives is the limiting factor, as they were designed for the 5000 RPM limit of most motors

That is a very interesting comment that I never would have given a thought to. So if you want to turn an outdrive faster how does one do that? Oil pump, pressure lubrication?

Add a drive shower first, just for cooling. But you really need a Merc Racing drive of some ilk for sustained 5000+ rpm.

 

[HT32BSX115]

Crap , I thought you were supposed to run at WOT as often as possible.

I have ran mine for probably an hour straight in the low 4s .

I boat at a huge lake (Powell) and when we get glass I can make some good distance in one run . Of course keeping an eye on temp and listening for any changes. Looks like I will have to change that strategy and do some putting along the way.

I guess I should preface this to say, the OP wasn't talking about racing engines or drives.......


There's probably a couple of things to keep in mind when running any stock I/O at WOT

At Lake Powell for example, you're at approx 4000ft MSL AND if it's 100ºF your density altitude is about 7000ft. (and it's frequently hotter)

You lose approx 3.5% power per 1000ft of density altitude, so your 175hp 302 is only really (if in perfect trim, RPM, tune etc) capable of producing somewhere around 130hp.

Even if you "prop" for maximum recommended RPM, (and you're not turbo or super-charged) you're not even capable of producing 100% sea level HP

So, running it at "WOT" all day is no problem. If you haven't jetted the carb for the higher altitude, it'll run a little rich and burn a little more fuel and might even foul the plugs, but it won't hurt to do it ALL DAY LONG.

Aircraft engines (like marine engines:high cu-in, low compression, slow turning) do this all the time......... once you get to around 4000 MSL or so (and you have an adjustable pitch propeller) the throttle is likely already at WOT and will stay there until TOD (top of descent) (hours)

Most marine engines are not configured to produce the kind of maximum power (their automotive counterparts are capable of) The engines *could* do it if allowed to turn faster and Most SBC's and SBF's can produce a LOT more power but are compression & RPM limited (for "regular" I/O operation like indicated above) So the engines by design are already "de-tuned" (due to low compression, cam design and max RPM etc)

Running them at WOT isn't going to "hurt" them. You'll still use more fuel than you will at "cruising" RPM and speeds, and you'll wear out the engine a little quicker than if you never ran it at WOT but I don't think anyone will get there.

This is sort of like the oil argument. You're not going to damage most marine engines from running at WOT (or using the "wrong" oil) before they fail for some other far more common reason like freeze damage, submersion, exhaust reversion, or neglect.


Cheers,


Rick

 

[Redrig]

I guess I should preface this to say, the OP wasn't talking about racing engines or drives.......


There's probably a couple of things to keep in mind when running any stock I/O at WOT

At Lake Powell for example, you're at approx 4000ft MSL AND if it's 100ºF your density altitude is about 7000ft. (and it's frequently hotter)

You lose approx 3.5% power per 1000ft of density altitude, so your 175hp 302 is only really (if in perfect trim, RPM, tune etc) capable of producing somewhere around 130hp.

Even if you "prop" for maximum recommended RPM, (and you're not turbo or super-charged) you're not even capable of producing 100% sea level HP

So, running it at "WOT" all day is no problem. If you haven't jetted the carb for the higher altitude, it'll run a little rich and burn a little more fuel and might even foul the plugs, but it won't hurt to do it ALL DAY LONG.

Aircraft engines (like marine engines:high cu-in, low compression, slow turning) do this all the time......... once you get to around 4000 MSL or so (and you have an adjustable pitch propeller) the throttle is likely already at WOT and will stay there until TOD (top of descent) (hours)

Most marine engines are not configured to produce the kind of maximum power (their automotive counterparts are capable of) The engines *could* do it if allowed to turn faster and Most SBC's and SBF's can produce a LOT more power but are compression & RPM limited (for "regular" I/O operation like indicated above) So the engines by design are already "de-tuned" (due to low compression, cam design and max RPM etc)

Running them at WOT isn't going to "hurt" them. You'll still use more fuel than you will at "cruising" RPM and speeds, and you'll wear out the engine a little quicker than if you never ran it at WOT but I don't think anyone will get there.

This is sort of like the oil argument. You're not going to damage most marine engines from running at WOT (or using the "wrong" oil) before they fail for some other far more common reason like freeze damage, submersion, exhaust reversion, or neglect.


Cheers,


Rick

IDK how I missed this originally , but that was great education for me there .

I knew that elevation had a lowering effect on HP , but I didnt realize that 4k in elevation and a hot summer day turned my 302 into a 4 banger in essence. Also good to know that long runs at WOT wont kill my engine.

thanks for information Rick!

 

[Redrig]

Most of the time when my boat gets wet it's at Powell , this summer I am taking it to a mountain lake at roughly 7400 feet . That will be interesting to see the impact of that much more elevation

 

[HT32BSX115]

Most of the time when my boat gets wet it's at Powell , this summer I am taking it to a mountain lake at roughly 7400 feet . That will be interesting to see the impact of that much more elevation

Most of the info that I have found refers to normally aspirated (NON-turbo/supercharged) gasoline engines......The approx loss of 3.5% HP per 1000ft is a general "rule of thumb"

Unless you have an EFI engine (which does compensate a little bit for altitude but doesn't give back much of the losses) your losses might be more since you don't normally re-jet a carb......and a little more if you don't re-prop.

In many aircraft with fixed-pitch propellers, you can operate at somewhat higher RPM and get a few more HP but there's a redline to contend with. In an aircraft with a constant-speed propeller, you can just set the RPM at the most efficient point and climbing through somewhere around 4000-5000 ft, the throttle will end up at WOT. Then the only thing left to do is manually adjust the fuel mixture for either max HP or MAX economy etc.

In a boat, with EFI, the mixture should be taken care of so the only other thing you can do is select a prop that provides that MAX WOT @ max rated RPM. (assuming you load to the same weight and trim for max speed/RPM etc)

Also if for example, you do this at your favorite 7400' lake on a cool/cold morning for some fishing and then later in the day you want pull the kids skiing at say 90 degrees F, you may need swap to a slightly lower pitch prop.

Put a belt driven super charger on your engine and you solve most of these problems!!

 

[H20Rat]

In many aircraft with fixed-pitch propellers, you can operate at somewhat higher RPM and get a few more HP but there's a redline to contend with. In an aircraft with a constant-speed propeller, you can just set the RPM at the most efficient point and climbing through somewhere around 4000-5000 ft, the throttle will end up at WOT. Then the only thing left to do is manually adjust the fuel mixture for either max HP or MAX economy etc.
...
Put a belt driven super charger on your engine and you solve most of these problems!!

Aircraft also have to deal with lower air density, so the prop becomes easier to swing the higher you go. Boats don't get that advantage, water doesn't exactly change density much. (unless you are going sea level-ocean to high altitude fresh water)

Turbochargers work great at altitude also!

 

[HT32BSX115]

Aircraft also have to deal with lower air density, so the prop becomes easier to swing the higher you go. Boats don't get that advantage, water doesn't exactly change density much. (unless you are going sea level-ocean to high altitude fresh water)

Turbochargers work great at altitude also!

Maybe you missed the point on my "dissertation" on this . REDRIG mentioned operating at Lake Powell (roughly 3600ft MSL) and operating later this summer at a 7400' MSL lake......

Right this minute, the PAGE, AZ airport ASOS (Automated Surface Observing Systems) reports temp of 16°C , alt setting of 30.04" hg and a density altitude of 5300'

Using https://www.weather.gov/epz/wxcalc_densityaltitude And https://www.weather.gov/epz/wxcalc_stationpressure
(you have to convert actual reported altimeter setting to station pressure to use the density ALT calculator)

So If it were 95°F (35°C ) at Lake Powell, the density altitude would be approx 6800' you could run all day every day at WOT and you wouldn't actually be running at WOT!!

And if he was at that 7400' lake, and it was even just 70°F (with the same typical altimeter setting)

The density altitude would be close to 10,000' (9800) ....................the boat might not even plane unless very lightly loaded!!!


You're absolutely right, Your point of less dense air (also) affecting the aircraft prop and wings is of course "figured" into the performance loss of aircraft that wouldn't be the same for a boat

The propeller isn't affected (like with an airplane) but the engine (which is actually just an air compressor) at even medium altitudes is simply not capable of producing anywhere near as much HP as it could at SL......

AND by the way, MANY (not all by any stretch) aircraft engines are rated for MAX rated power for only 5min and then must be "pulled-back" to "CLB" power or other power setting for which it is rated "continuous"


I know we frequently have thread "creep!" and I like to try to bring some discussions back to the OP's original intent and questions/subject etc...........so except for fuel consumption issues, running any recreational NON-turbo/supercharged boat engine at WOT is for the most part, a non issue.......(the point of the OP's original question)

AND if you operate at higher altitude lakes, it's even less of an issue!

Now, I'm going flying!

Cheers,

Rick