OT- Running mower engines full throttle?

MarkL · Aug 1, 2006

Can anyone tell me the rational behind running a small lawnmower engine at full throttle?? Seems everywhere I go I see people running their lawn tractors full throttle when only cutting 1 inch of grass....why?? I'm from the old school and only use as much throttle as I need to get the job done without using extra fuel and putting all that wear and tear on a small air cooled engine. My small 8 hp Wheelhorse thats 20+ years old can mow the same amount of grass with the guy running around his city lot with his 23 hp tractor running at 3800 rpm while he's wasting fuel and such. Seems like a waste to me. Everyones comments are appreciated. Best regards, Mark in hot Buffalo

Jaledu · Aug 1, 2006

Years ago when I worked as a mechanic in a small engine shop we would always instruct the customers to run their equipment at full throttle when mowing. Some engines would simply overheat when put under load at lower RPMs. They need that flywheel spinning faster to cool down the engine block.

This was more of a problem on tractors where the engine was enclosed and unable to radiate much heat other than the airflow thru the shrouding. In fact Cub Cadet's, for a number of years, actually had the throttle marked to only mow at full throttle.

kenh · Aug 1, 2006

These engines are most efficient at full throttle.
Also cooling is a factor.
The higher HP models pull a much larger blade.
Why bother to cut an inch off the grass?
Longer grass survives the heat longer when long.

s7hss · Aug 1, 2006

There is some rationale, yes. Different cooling fans. Until somewhere around 1985, you often saw forward-inclined fans, which worked well, but weren't the most efficient at full-throttle. Backward-inclined fans are common nowadays and they move more air at high engine rpms. I'm not an engineer, but given the difference in fan styles, could be they run a little cooler at WOT than at part throttle, given a constant load.

You're not overstressing a small engine by running at full throttle. They're designed for it. Mowing when you're only taking off 1" is a waste of time, though.

agrip · Aug 1, 2006

There is a difference between full throttle and full speed.

Almost every Genset guru will tell you that an engine running at 1800 will last a lot longer than the same at 3600.

It seems to me that the "sweet spot" is in the zone close to where the torque curve peaks, which is close to minimum brake specific fuel consumption.

Decent cooling air pressure-flow.
Decent load variation tolerance.
Decent mechanical longevity and work done balance.
Decent windage loss.
NO unnecessary racket.
Decent valve train pounding.
Yada Yada

Hth Ag

macona · Aug 1, 2006

Usually a mower deck will rot out before anyone actually wears out one of those engines. Or someone runs it out of oil...

LD · Aug 1, 2006

There probably isn't a good reason to mow an inch of grass at full tlhrottle. Unless you have a powered mower and that lets you get the job done faster so you can get back to the recliner, remote, and cool beverage.

If you let the grass get a few too many inches of top growth, you risk luggin' the motor down. Most have a primitive governor that pulls the throat closed under heavy load to keep from stalling, if enough throttle isn't there to start with you'll restart more than you want.

I've downsized the lawn a lot and use an electric mower now; there's only one speed. The neighbors are grateful on early Sunday mornings.

dsergison · Aug 1, 2006

8 hp Wheelhorse ..... his 23 hp tractor

you 8hp was bhp continuous duty..... his 23hp is "peak/imaginary/marketing bullshat"...

you engine was designed to operate at lower rmp, his at higher.

so, you see. it's all about the same.

John in MA · Aug 1, 2006

Don't forget splash lubrication--lower speed=less splash. The blades also give the best cut at speed. Better lift, etc. They'd run blades even faster than currently done if it wasn't for safety regulations.

When small engines are running at top speed without a load they're actually not using much fuel or generating much heat at all. The thrtottle plate is almost closed. It's only when you load them that they start opening up. Lower speeds frequently aren't even governed, so you can't get much done.

That's talking push mowers. Modern riding mowers are rediculously overpowered--it's like the HP race at the end of the 1960s. Part of it is to make up for bad deck designs, mulching also takes more power. But there's no point to a 23HP riding mower. My father's car several years ago wasn't much mower powerful.

Some of the HP is a little overrated, but not by much. It's not like electric motors. Check out the size of the engines they put into riding mowers now. An old 8HP Briggs/Kohler could fit _inside_ some of the new ones.

bcstractor · Aug 1, 2006

Another reason to run full throttle is that the cutting action works on an impact basis. If you think about - at some point when you lower the speed it will not cut at all - the grass will just bend out of the way.

bluchip · Aug 1, 2006

You mentioned lawn tractors....if hydrostatic they need pretty good speed for the pump. For good life as well as higher ground speed. The zero turns are commonly labeled for max speed.

chuckey · Aug 1, 2006

OT, you small engine gurus can probabley advise me:- A mate of mine wants me to fix his Clarks 5 kw generator, the engine is a 4 stroke and runs roughly, spitting sparks out of the exhaust. I think its the exhaust valve is shot. Any advice (other then run).
regards frank

Carl Darnell · Aug 1, 2006

I owned and operated a small engine shop for several years. The engines use a govenor to regulate rpms. The engines are designed to operate at that speed and do best there. There is also the aspect of blade tip speed. The blade is designed to cut best at full rated speed. It is the same with riding mowers you should run full throttle to get the tip speed needed and use the gears to get the ground speed for the height of the grass to be cut. I suggest you never cut wet grass. One of the things to check at tuneups was gov. speed. Many shops never do that. I always checked and set the gov. speed.

Carl Darnell · Aug 1, 2006

chucky, I will take a guess that it may be carboned up or running hot or running lean or all of the affore mentioned. Compression test will tell leaking valve. Check comp. Then squirt oil in spark plug hole, spin engine several times and retest. If comp is higher the rings are worn. If it still has low comp. the valves are most likely leaking. It will take 50 psi or better to run. Lower than that and it may not start. There are exceptions to that rule.

s7hss · Aug 1, 2006

I sometimes wonder if these big motors are used to compensate for dull blades, or, rather, people who don't change blades. I get the impression a lot of folks are oblivious to this factor, though certainly not all.

I used to have a 4-hp Wheel Horse rider with a 32" deck. That was a little cast-iron Kohler motor. Some of you may remember the mid-engine jobs, with the motor right between your legs, transverse crank, exhaust piped out to the operator's right side. The operator sat more or less over the transmission.

From the crankshaft, the deck mainshaft was driven by a V-belt. Both spindles were geared to that mainshaft via two pairs of bevel gears. Even with everything well greased, you had to run that motor at full throttle to get the deck spun up. Once it was running, though, it would mow 6" tall grass without bogging down. It wouldn't go fast, but it did work well and consistently.

Wish I still had that old-timer, too. If I was making a point (not sure I was), it's that you can do it with less than today's specs.

kenh · Aug 1, 2006

HP not impportant?
I have a nice hill to mow.
Mine old MF/12HP Kohler/manual tranny/44" deck was in grunt mode mowing up.
New Husky/18HP/hydrstatic/48" deck goes up w/o much effort.

s7hss · Aug 1, 2006

Didn't say power's not important. Merely stating that you can get by with less than what's available today, that's all.

The stuff you can run today is nice, no doubt.

kartracer55 · Aug 2, 2006

Chuckey, Instead of a compression test do a leakdown test, or both. You may not have a guage but if you have any freinds/neighboors who are big into working on cars they might have one. Tell them you need a "leakdown tester or Cylinder leakage gauge"

Jim

Carl Darnell · Aug 2, 2006

A comp test is much easier. To do a leak down you have to have an air regulator and know what a new engine will blow by and then you have to get the piston on top center and keep it there with the pressure on. That's where it gets dangerous with 100psi on a piston at TDC. It's not to bad in multicylinder engines but a single cyl. is tricky. A comp test will tell everything you need to know.

macona · Aug 2, 2006

Dont forget a lot of the decent riders have attchments such as tillers. This is where all that extra HP comes in handy. We had a couple of riders with 9-12hp engines and tlling could be a chore with them in hard soil.

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