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Most of the time when I am mowing, I run my 448 (60" deck) at about 80% throttle. It seems to cut fine and I'd like to save a little gas.

However, I got to thinking - was the machine (Onan B48M) designed to be run a WOT when mowing? Is mowing at 80% throttle over-taxing the engine in some way (even though the engine sounds fine and doesn't bog down)?
 

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A bit of My experiance; I allways like to say that because It seems Overy one will dissagree; My dad bought 2 new tractors Back in the 1950, His idea was to open the throttle wide open then back it off maybee about 3or 4 degrees well within 2 years both of the engines broke there crankshafts; Many years later I built high performance engines, Balanced, and printed, them; now when you do that it must be done an a spicific RPM You balance an engine at 4500 rpm and run it at 5200 rpm it will build up what is called Harmonic viberation and will try to desstroie its self from within. that being said when the factory tests an engine for horsepower, harmonic,torque, that is at wide open throttle; That is were the engine is designed to run. I just sold a second mower to the same buyer. He said the only complaint he had was after 3 or 4 cuttings he had to charge the battery, His wife was there , she done must of the mowing She said she only cuts at a bit more then half throttle. Heres the problem. Lets say the charging system puts out 9 amps at full governed RPM. but the Electric Blade Engagement clutch Draws 9 amps It is not a demishing draw, after a while the battery will go dead.. If it has a throttle run it wide open or pay the repair bill.
 

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Small motors are designed to run wide open, and I believe they will cool better when operated in that manner. It is real bad to run a hydro at less than wide open.
 

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Lessee, the engine is seeing the same load as before but now it gets only 80% of the cooling air.

Every manual for these tractors that I have ever seen says WOT when running powered attachments.

They are also governed, by the manufacturer, for whatever the design speed is. It's not like a car where WOT is all she can wind before coimng apart. Used to be that lots of the muscle cars, if they were geared deep, would be turning well over 3,000 RPM cruising down the highway for hours at a time and nobody ever worried about that.
 

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The deck puts a tremendous load on an engine and the engine needs to run at full throttle for maximum effeciency and longevity.
 

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One thing that is being overlooked here is this.

If you buy an Ag tractor that has one or more mechanical PTO shafts sticking out from the rear axle, you will also find that the dash-mounted tachometer has markings on it to show you the speed the engine must run at to achieve a true 540 rpm output speed or a true 1000 rpm output speed at either of those PTO's.

The engine in a lawn, yard or garden tractor is no different except that most do not come with a tachometer that shows rpm. Instead the tractor manufacturer relies upon the engine maker to supply him with engines that are GOVERNED to run at 3600 rpm, when the throttle control is pushed to wide open. Essentially, the throttle is NOT the same as a gas pedal in a car or truck that used to be mechanically connected to the throttle plate in the carb before computerized engine management came into being.

Outdoor power equipment engines have carbs with the throttle plate connected to the governor and the governor is constantly sensing the true engine rpm and then opening and closing the throttle plate to try and maintain the governed speed or a lesser speed if you back off on the dash lever.

Farm machinery such has haybines, hay balers, powered rakes, powered tedders, corn cutters and other such equipment are designed to run with an input speed of either 540 rpm or 1000 rpm. The same holds true of finishing mowers, bush hog mowers, sicklebar mowers, rear mounted snow blowers, powered manure spreaders, liquid manure spreaders and so forth.

The engineers who design the mower decks and snow blowers for lawn, yard and garden tractors expect YOU to deliver the 3600 rpm needed to make these attachments run at peak efficiency. Engines need to spin at a certain speed in order to develop their rated horsepower and torque so that when you hit an area with thick, long grass, the tractor will power right on through it l

Another issue is what's called "tip speed" and that term applies to mower decks and to snowblowers. Deck blades not only chop off the tops of the grass blades but they also are shaped to create a huge amount of suction to make the blades of grass stand up straight so that they are all cut off at the same height as the blades elsewhere on the lawn. If you don't run the deck at WOT, then the deck blades can't do their job properly. Snowblowers are no different. If you don't want chute plugging and other issues, then WOT is the way to go.
 

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I agree with all the opinions regarding WOT and PTO speeds. The engine needs proper cooling when it is air cooled and manufacturer recommendations are lab tested and proven. A liquid cooled engine is different and does not rely on RPM for max cooling. That said, I run my engines at 75% to 80% throttle and have never had a failure. There is just something about WOT that bothers me :praying:. Keep the blades sharp and the deck clean.

:trink39:
 

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^ Same here, all the previous explanations are valid, however if conditions don`t require full throttle, I tend to pull back a wee bit. Think of high summer when you`re really only cutting the buckhorn and other tall weeds. Not thick, lush cool weather grasses.
I would also add that the "fan" air flow is also somewhat proportional with the heat generated.
Fewer rpm`s equals fewer combustion events and therefore less heat generated, therefore the diminished airflow is adequate. Many of us pull a heavy trailer at less than W.O.T when possible, and suffer no overheat effects.
NONE of this is meant to refute the concept of turning a deck at design speed, which I do when necessary. If the engine isn`t tipping into the governor (i.e. laboring) I tend to throttle back to "max sustained cruise speed"
J.M.O.
 

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The Lawn mowers with decks attached and run with a belt should run about WOT, usually in the throttle notch, remember that the speed of the deck needs to be fast. You control the speed and don't let the engine lug down for any amount of time by setting the drive speed with the transmission or the automatic tranny. A tractor with a PTO and one that uses attachments should mow within the PTO RPM range.
 

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Yep good thread, learned alot on this one.

Matt
 

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I agree with the advice given above. The loads on internal engine parts imparted by centrifugal and inertial forces increase at the square of the RPM so higher RPM does equate to higher component loads. That said, damaging forces can also be encountered due to lugging the engine - I.E. running it under it's intended optimal power band. As stated above, engineers had an optimal RPM range in mind and this should be obtained and maintained by the governor controlled throttle at WOT (wide open throttle). In other words, excessively high RPM can be detrimental to engine longevity, but running at too low an RPM can cause problems as well.

In reference to cooling, if you don't already do it I recommend that you let your engine idle for approximately half a minute before you shut it down after full throttle use. The reason for this is that the engine becomes a giant heat sink after shut-down because you are shutting off the cooling air flow. If not allowed to cool down and normalize certain components of the engine may actually rise in temperature temporarily causing the weakening of parts and carbonizing of oil internally. As with RPM, you also need to operate your engine within a good temperature range. I.E. let it warm up before exerting full throttle.

JN
 

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IMHO the air cooled engine has a "powerband" or "performance curve" that it should be operated in without any negative results. This 'band" is not confined to "WOT" .... The # 1 factor linked to longevity or premature failure of any rotating piece of equipment is vibration. The closer to 0 mils of vibration the OEM or Rebuilder balances the equipment, the longer it will last. We had 3,000 HP. centrifugal air compressors turning 20,000 to 55,000 rpm. wheel speed and the vibration limit was set at .5 mils shutdown. They ran 24/7 - 360 for 40+ years. If they were balanced like most of the junk nowadays, they wouldn't last a week. I think the average GT operator can hear & feel the sweet spot the tractor is running at and that will make it last a longtime with proper maintenance.

:trink39:
 

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^ + 100 The elusive sweet spot, some folks have what I call "mechanical sympathy"
some do not.. the equipment of the former typically outlasts that of the latter..
JMO
 

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All of my machines, a 1985 448/P218 repower, a 1990 4018/18 Vanguard repower, and a 2008 Scag Tiger Cub 26 B&S ELS all run at 3,600 RPM at WOT and that is where I run them at while mowing. With good lube oil, a relatively clean air filter and 89-91 octane fuel you should have no problems with one major exception which is carbon buildup. Carbon buildup can be an engine killer especially with Onans and other L head engines.
If I had a 60 inch mower deck I would never run below WOT while mowing. All of my machines have 48 inch decks and collection systems.
Bob MacGregor in CT:thThumbsU
 

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Discussion Starter #17
All of my machines, a 1985 448/P218 repower, a 1990 4018/18 Vanguard repower, and a 2008 Scag Tiger Cub 26 B&S ELS all run at 3,600 RPM at WOT and that is where I run them at while mowing. With good lube oil, a relatively clean air filter and 89-91 octane fuel you should have no problems with one major exception which is carbon buildup. Carbon buildup can be an engine killer especially with Onans and other L head engines.
If I had a 60 inch mower deck I would never run below WOT while mowing. All of my machines have 48 inch decks and collection systems.
Bob MacGregor in CT:thThumbsU
I was with you until you mentioned the 89-91 octane fuel. Higher octanes aren't recommended for small engines, are they?
 

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Well, there we go. For mowing, the consensus is WOT.

Now I know... thanks!
Awareness of operating parameters is important to get the best performance and longevity from equipment. Tasks like mowing and blowing require sustained periods of high demand on the engine.

3600 RPM seems to be a common reference point for most air-cooled gasoline powered outdoor equipment. For example, an Onan P218 produces 18hp at 3600 RPM. OnanParts.com posted a long thread on rebuilding Onan engines -http://www.mytractorforum.com/showthread.php?p=958527. Post#68 shows the performance chart for a P218.

While published maximum rated hp is measured at 3600, max torque occurs at a lower RPM. So, why 3600? I am not a student of the industry but I assume manufacturers want a common form factor to reduce costs and maximize their market. In the US, a 2-pole generator head must spin at 3600 RPM to produce 60 hz (cycle) Alternating Current (AC). 3600 RPM ÷ 60 cycles = 60 revolutions per second. Next time a big storm blows through and you have an extended power outage, go outside and you will hear the soothing sounds of portable emergency power generators, screaming along at 3600 RPM.

If 3600-RPM is the max-sustained target, then it makes sense that the engine is tuned and balanced for this target RPM. Engineers design pulley, gear, and mower deck ratios to achieve the optimal blade tip speed.

For other uses, the rules change. My Ingersoll 6018 Loader Operators Manual shows the idle speed of 1200 RPM, and max no-load speed of 3600 RPM. This computes to 2400-RPM range between idle and full throttle. The manual recommends ¾ throttle for normal loader use, and reduced throttle to conserve fuel. ¾ throttle (75%) computes to a 3000-RPM target (or less) while operating the loader.

Side note: For backhoe equipped 6018 units, the throttle is set between ¾ and WOT when using the backhoe. The 6018 has a 2-section pump. (9 GPM for PTO/Travel, lift) and 5 GPM for power steering, loader, and backhoe). While using the backhoe, only the 5 GPM section is actively working. I would be curious to know the recommended setting for a Case 648 when using the backhoe.

Back to mower stuff - I also have an Ingersoll HRM48H 3-point finish mower, and the manual says to operate the engine at WOT (the PTO on the 6018 is powered by the 9 GPM pump section). WOT should give the flow/pressure necessary to achieve optimum blade tip speed.

Assuming you are still reading, here is a thought. If you buy into the 3600 RPM WOT concept, do yourself a favor. Check the WOT, no-load speed of your tractor. Chances are, it is not set for 3600 RPM. A discussion a few months ago on the yahoo forum focused on tachometers and on how many Case/Ingersoll tractors were not set up or operating at 3600 RPM WOT. Many were set much lower.
 
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