In a common internal combustion engine: 3/4 throttle vs. full throttle

Factual Questions
1

Prefer that WAG’s be avoided here.

Take a common internal combustion gas engine (typical boat or car engine).

Are there specific reasons that operation at wide open throttle (WOT) would cause any realistically measurable or considerable engine wear over and above running the same engine at 3/4 or 4/5 throttle?

These debates rage in any forums discussing car or boat engines: WOT is bad. Run at 3/4 or 4/5 throttle = good/better!

Forget fuel economy. The issue at hand is about engine wear/damage. What is bad about running at 5000 RPM (wide open throttle in an example engine) that goes away when running at 4000 RPM?

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2

It’s a continuum. Running at wide-open throttle is harder on an engine than running at 3/4 throttle; that doesn’t mean that an engine operated at 3/4-throttle will last indefinitely.

Similarly, running at 5000 RPM is harder on the engine than 4000 RPM, but 4000 RPM isn’t problem-free.

Running at higher loads:

-obviously puts more stress on parts. Generally not a big deal, as the parts are designed for this; mechanical failure of major engine parts (piston, conrod, crankshaft, etc.) is pretty rare.

-causes higher peak combustion temperatures, so everything in there is running hotter. Again, the major engine parts don’t care much, but the oil viscosity goes down when it gets hot, and if it gets really hot, then it starts to break down.

-causes more blow-by of combustion gases past the piston rings due to higher pressures in the combustion chamber. This stuff gets into the crankcase oil. The oil has additives in it to deal with this stuff, but it won’t last forever.

Having said all that, engines are designed to do what they do. In my car or on my motorcycle, once the engine is fully warmed up, I won’t shy away from WOT. My last bike went 135,000 miles before I sold it, never had a bit of trouble from the engine despite a lot of sporty riding. Car is at 100,000 miles right now, no engine issues.

Engine oil is also designed to do what it does, and engine designers are very conservative when it comes to specifying oil change intervals. Some friends of mine sent samples of used crankcase oil from their bikes to oil testing labs, and the results indicate that they could safely double the oil change interval to 12,000 miles without any detriment. If you use decent quality oil and change it at the specified interval, then go ahead, use your engines to have as much fun as you can.

3

In a motorcycle group that I frequently ride with, there is this woman who had a - I forget the size exactly - roughly 200 cc Suzuki dirt bike, and she liked to ride with us.

Now, most of us are riding sport touring or dual sport bikes with a fair mixture of touring and cruiser style bikes mixed in. Everyone else is riding a bike with a MUCH larger engine than she has.

So, one day last fall, a group of us are riding near the Mexican border and we’re doing - oh, I don’t know - 80-85 MPH pretty consistently. And she’s keeping up, just as she always did, though she’s riding at full throttle all the time. I am riding behind her…waaaaayyy behind her, actually, because the loud buzzing noise that bike made was very irritating. Suddenly, there’s a puff of smoke out her exhaust, a loud BANG, and the bike stopped running.

There was oil all over the outside of the motor. No obvious holes, and it would crank over, but it wouldn’t start. Busted cam, I think. Maybe a hole in the piston; sounded like maybe it was sucking when it shouldn’t have been… Not sure, didn’t take it apart.

The moral: Running wide open is the most stressful way to run the motor. Highest RPM, highest pressures. As Machine Elf commented, it is a continuum and when running wide open you are running clear at one end of that continuum.

Your engine WILL run full throttle for an extended period, but if you find yourself often doing that, then you need a bigger motor if you want it to last.

4

To my knowledge, it’s more the RPMs that kill you, not the throttle position. You can be at 2000 RPM at WOT in a car and your engine will be perfectly happy. Or you can redline your engine at 20% throttle for hours and hours and expect a greatly reduced life.

Boats are a bit different because there are no gears, and so there’s a more direct connection between throttle and RPM.

General aviation aircraft are generally rented in “hours”, where an hour is actually the number of revolutions the engine would make in one hour at a standard speed. So they certainly think that the important factor is number of revolutions, not the wall time, or throttle position, or anything else.

5

I think that people here are using throttle position as a surrogate for RPM. I know I was. But you are correct; it is not the same thing at all.

6

It’s not a bad surrogate most of the time, even with cars–with an automatic transmission, most cars will downshift and run at a higher RPM if you floor it. But if you select a specific gear (especially with a manual transmission), you can have WOT at a comfortably low RPM.

I know that when climbing hills, I generally select the highest gear I can, even if it means that the accelerator is on the floor. I could be at half-throttle at a higher RPM, but my general impression is that I would have more wear in that case.

7

I have been told not to let my RPM drop too low when under heavy load since this puts a lot of stress on bearings.

8

Seems odd. Regardless of what gear/RPM/throttle/etc. you’re at, the engine has a maximum torque, and the bearings are going to be designed to withstand this. Of course, the bearings are designed to handle a particular maximum RPM as well, but not necessarily for extended periods–heat is always a problem.

Besides, the bearings shouldn’t really be “seeing” most of that torque–it’s the driveshafts and so on that see torque.

Older engines could have a problem in that low RPM/high load could cause pinging, but that’s not a problem with most new engines.

9

Flow through a butterfly valve is pretty nonlinear with respect to the valve position anyway. The difference in flow between 80% to 100% isn’t nearly the same proportion as the difference between 5% and 25%. If you are running at 80% throttle, that’s almost the same as 100%. What kills the engine is other factors such as bearing selection. If an engine was designed for it, running at full open throttle would not be a factor.

10

That was kind of my point upthread. The 200-cc dirt bike described by Bemused almost certainly wasn’t designed to run at WOT for minutes on end. Boat engines, OTOH, are expected to do so, and are designed accordingly, with adequate cooling mechanisms, oil viscosity and circulation, and so on. So go ahead, stomp on the family sedan when you get on the highway on-ramp; it’ll be fine for that. It will probably not be fine running the Daytona 500. OTOH, a recreational boat engine should be able to tolerate WOT all day long, if you bring along a suitcase full of gas money.

11

A 2 stroke engine can have some unusual characteristics. Running at high RPM with low load can cause excessive wear. If the lubricant is mixed in the fuel (as in most 2 strokes), not enough lubricant is mixed in the fuel to handle the amount of friction in the crankcase and piston walls. In general 2 strokes have a narrow optiomal operating range and the mix of oil in the fuel is designed for optimal operation in that range. This can happen in a motorcycle running at high RPM in a low gear for extended periods.

12

Don’t most lawnmowers recommend WOT? I know that my current and past riding mowers have.

13

My current tractor recommends full throttle all the time. My previous tractor only specified full throttle while mowing. Probably the max blade RPMs were matched to full throttle so they couldn’t be overspun.

14

I should have specified that I was talking about the bearings where the piston rod connects to the crankshaft. Under high load low RPM circumstances the piston rod to crankshaft interface will experience high stress.

See here: http://forum.miata.net/vb/archive/index.php/t-376591.html

15

Can we take tractors out of the discussion, please? They have simple induction systems and are just best running at max rpm/throttle, because they run cleanest, smoothest and happiest there. They aren’t exactly screaming like some high-RPM boat engines or car engines.

Boat engines: Many are just marinized car engines from GM converted my Mercruiser (Mercury Marine) or Volvo.

Not, many outboards are purpose built for high-rpm running, but there still remains a manufacture recommendation to not run at WOT for extended periods; rather, one should pull back to 4/5 or 3/4 throttle.
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16

The control lever on your lawn tractor/mower/snowblower selects an RPM, not a throttle setting. That lever changes the setpoint for the The governor, which manages the throttle plate so as to approximately maintain that desired RPM. Some governors are centrifugal, others (as on my 4-hp Briggs & Stratton walk-behind mower) use a paddle that reacts to the breeze coming off of the flywheel.

Higher RPM is desirable because the engine can produce more power there. If you get into thick grass with the engine only turning 1800 RPM, it’s more likely to bog/stall than if you get into it with the engine turning 3600 RPM.

17

Bristol in the UK sell a top of the range sports car withan engine hours gauge. Top Gear mentioned a US firm (Chrysler?) who wanted to go one further and have an RPM gauge that showed total cumulative engine revolutions, that would quickly reach silly figures.

18

That’s like the opposite of the situation I mentioned with 2 strokes. The engine speed will affect the amount of lubrication. For a 2 stroke, low load and high RPM is the problem. It’s probably not an issue at high load and low RPM for a 2 stroke because they don’t operate very well that way at all.

19

One thing to note about modern automobiles is that the advanced fuel injection systems today have different fuel mappings for wide-open throttle compared to partial-throttle acceleration. Under normal acceleration, emissions is the main concern so you’re not getting quite the maximum power you could if a different fuel map was used. At WOT, emissions is a secondary concern to power.

That’s not necessarily saying WOT is that much worse though. To produce this extra power, the engine is run richer which causes the temperature of combustion to be lower. So basically you have a bit of a surge in power which causes more engine wear versus lower temperatures which reduce engine wear. I’m not sure which has a larger impact.