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Gravely bug bit.
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Discussion Starter #1
From the 1990/1991 CSU and using an engine speed of 3300 RPM and factory blades.

2-W tractors at 1179 RPM PTO speed.
30 " 16,562.1 FPM
36" 2-blade 18,868.7 FPM
40" kidney 2-blade 15,482.3
40" Comm 15,432
40" 3 blade 18,405.2
50" 3-blade 18,163.5
60" 3-blade 20,598.2

4-W tractors at 946 RPM PTO speed.
40" 3-blade 14,767.9 FPM
50" 3-blade 14,574
60" 3-blade 16,527.5
72" 3-blade 17,324.3
50" wing 14,756.4
40" Comm 12,383.3

Pro Master units
PM-16, PM-50 50" 18,163.5 FPM at 1179 RPM PTO speed
PM PU, 50" 15,928 FPM at 1861 RPM PTO speed
PM PU, 60" 17,368.3 FPM at 1861 RPM PTO speed.

So with those figures, the 35-G turns the decks on the front at:
40" Comm 24,360.8 FPM at 1861 RPM PTO speed
50" 3-blade 28,670.4 FPM at 1861 RPM PTO speed
60" 3-blade 32,513.3 FPM at 1861 RPM PTO speed 6058 RPM spindle speed
No wonder it sounds like a turboprop ( C-130 ) if they are wound up to full engine speed.
 

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Premium Member
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Those figures are about right based on my measurements.

Someone please make this data a sticky.

Thanks Don!
 

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Old Iron Connoisseur
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That is very neat info. Hopefully this will make a sticky as I've often wondered this data. Also explains why some decks work better on a two wheeler than a rider.

Does anyone know if the PTO speed of the L/C tractors comes close the the later Kohler/Briggs two wheel tractors? I know there were different gears used to handle the different input RPM.
 

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Old Iron Connoisseur
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Those figures are about right based on my measurements.

Someone please make this data a sticky.

Thanks Don!
Do you remember what the 50" Deck on a four wheel rider with 40" pulleys generates? Would also be a handy bit of info to add to the list if this makes a sticky. I guess also with that computed, it would be easy enough to get the speed on a two wheel tractor too.
 

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I Love All Color Tractors
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Gravely bug bit.
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Discussion Starter #6 (Edited)
Do you remember what the 50" Deck on a four wheel rider with 40" pulleys generates? Would also be a handy bit of info to add to the list if this makes a sticky. I guess also with that computed, it would be easy enough to get the speed on a two wheel tractor too.
The 40" 3-blade turned 5043.3 RPM at the blade.The 50" turned 4081.2 at the blade. So the 50" with 40" pulleys installed would turn at about 22445.35 FPM at the blade tip.

That is just a little slower than the 35-G running at 2700 RPM with a normal 50" deck on the front. And it will really stand up the grass for cutting. A 50" off of a Pro Master 50 will appear in pictures in a couple months on the 35-G.
 

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Old Iron Connoisseur
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Thanks Don!

You don't happen to know the PTO speed of the L/C tractors vs. Kohler tractors do you? Since I don't have any Kohler two wheelers running right now I'm curious how much, if any, the input speed changes from the two power plants.
 

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Gravely bug bit.
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Discussion Starter #8
Thanks Don!

You don't happen to know the PTO speed of the L/C tractors vs. Kohler tractors do you? Since I don't have any Kohler two wheelers running right now I'm curious how much, if any, the input speed changes from the two power plants.
2838 RPM on an L engine gave a PTO speed of 1023 RPM in Low range. 2728 RPM at the engine gave 1364 RPM at the PTO in High range. Get a good running L/C and you could see 3000 RPM at the engine and 1500 at the PTO in High with no load.
 

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Old Iron Connoisseur
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Thanks Don!

One more thing....

Am I correct in assuming that the info in the first post was at WOT in Hi Range on a Kohler tractor?
 

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Gravely bug bit.
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Discussion Starter #10
Thanks Don!

One more thing....

Am I correct in assuming that the info in the first post was at WOT in Hi Range on a Kohler tractor?
3300 RPM in High range. While Gravely set them at 3300 they could go to the factory spec of 3600 RPM. The speed of the PTO at 3300 RPM is 888 RPM in Low range.
 

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Gravely bug bit.
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Discussion Starter #12
Thanks Don! I don't know where you keep digging up all of this info.. but I am glad you share it with us!
Gravely published most of the info but it is a matter of knowing where to look to find it. The unpublished things, like the 35-G data, can be found thru simple math. (1861/1179)* X FPM = FPM on the 35-G where X = known FPM on the deck on a two wheeler.
 

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As Richard indicated, these numbers are also in line with my own actual readings taken on my 18-GLX.

And as Don indicates, you can up the RPM to 3600, which I have done.

My tractor now has a no load WOT of almost exactly 3600 RPM with the governor spring in the last hole - M18S Kohler twin - 4 wheel tractor.

Engaging the deck lowers the RPM to 3500, engaging the vacuum/bagger lowers it another 30 RPM to 3470.

The actual measured spindle speed on the 50" deck is 3380 RPM at the 3470 RPM egine speed, providing a cutting rate of 15,035 FPM.

While I did not take detailed measurements with the governor set at 3300, similar losses are to be expected and actual speeds are likely less than the numbers GRAVELY published.

The increased engine speed did noticeably increase cut quality and vacuum/bagger performance. Not only is the cut quality better, the bagger is much more clog resistant in heavy grass.

Since many other applications of these engines run them at 3600 RPM, I believe the engine design is still well within its design limits at 3600 RPM, and doubt any noticeable difference in service life is likely to result from the higher RPM. Additionally, my experiance with all types of engiens suggests that the more easily the work is done, the longer the engine is likey to last. My governor has been set at 3600 RPM for two seasons now with no problems. Not bad for a "free" upgrade.

Sheldon
 

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Gravely bug bit.
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Discussion Starter #14
As Richard indicated, these numbers are also in line with my own actual readings taken on my 18-GLX.

And as Don indicates, you can up the RPM to 3600, which I have done.

My tractor now has a no load WOT of almost exactly 3600 RPM with the governor spring in the last hole - M18S Kohler twin - 4 wheel tractor.

Engaging the deck lowers the RPM to 3500, engaging the vacuum/bagger lowers it another 30 RPM to 3470.Running at 3600 gives less governor control than 3300 RPM. Lower setting will take up the load without running out of carb as fast as it would at the higher speed.

The actual measured spindle speed on the 50" deck is 3380 RPM at the 3470 RPM egine speed, providing a cutting rate of 15,035 FPM.Gravely figures would put the RPM at 3443 and a cutting speed of 15,324 FPM. Less than 2% difference in the RPM and FPM.

While I did not take detailed measurements with the governor set at 3300, similar losses are to be expected and actual speeds are likely less than the numbers GRAVELY published.

The increased engine speed did noticeably increase cut quality and vacuum/bagger performance. Not only is the cut quality better, the bagger is much more clog resistant in heavy grass.

Since many other applications of these engines run them at 3600 RPM, I believe the engine design is still well within its design limits at 3600 RPM, and doubt any noticeable difference in service life is likely to result from the higher RPM. Additionally, my experiance with all types of engiens suggests that the more easily the work is done, the longer the engine is likey to last. My governor has been set at 3600 RPM for two seasons now with no problems. Not bad for a "free" upgrade. The Kohler Command Pro 27 on my Rapid XZ is set at 3750 no load. It drops to 3600 under full load. For long life, load should fall in the 50 to 75% of rating of HP.

Sheldon
Red comments are mine.
 

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Don, completely agreed.

Some others have commented in the past that they got excessive hunting with the govenor set at higher speeds, I have not experianced any such problems.

I don't know Kohlers HP/torque curve, but based on my results 3300 RPM is likely the slowest setting that gets you into the engines rated power range, with the true 18 HP being much closer to the 3600 RPM speed, likely somewhere between the two - right where I am now operating.

I do completely understand the "running out of carb" issue and it is likely that the RPM drop under load at the 3300 setting is not as much, but 3470 is still faster than any such number, resulting in a 500 to 1000 FPM increase in cutting speed, which in actual use has resulted in improved performance with no ill effects.

Sheldon
 

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I have played with governors and RPMs on all manner of engines and not once did hunting ever occur after raising the maximum RPM. I have seen hunting on many engines and either the governor/carb linkage is not set correctly or the fuel mixture is not correct. In any case small adjustments of the governor/carb linkage can make a huge difference. A good example of governor hunting can be seen on a diesel engine with worn governor linkage.

Putting 40" deck pulleys on a 50" deck will raise the blade tip speeds to about 20,000 SFPM (Surface Feet Per Minute) and while that is a huge win in terms of quality of cut, it comes at a price. A deck modified in that fashion will consume about 12 hp just spinning while the tractor is stationary. Attaining and maintaining a 3300 RPM engine speed while cutting is quite impossible with a 12 hp engine. I have held the throttle plate wide open on a good 12 hp engine and the RPMS never went over 3300 RPM. That means that when cutting grass, that engine will bog down to below 3000 RPM with the resultant drop in blade tip velocity. On a 5665 that means that one will have to use low range to actually cut grass which defeats the whole purpose behind raising the spindle speeds. I would not raise the spindle speed on a 50" deck unless the tractor had at least 16hp on tap.

Governors aren't perfect and there will be a loss in RPM when a load is placed on the engine. The exception is an electronic governor but not many small engines have that feature.

As far as engine maximum RPMs are concerned, not many engines will last when spun faster than designed. Conversely if the RPMs are lower, the engine will last longer between rebuilds. I am a big proponent of keeping the speed of the engine down to something reasonable. 3000 RPM is plenty, 3300 is max and 3600 is just too high for me. The way I see it, if higher mower deck speeds or blower speeds are wanted, then a pulley change is in order but also realize that with the higher speeds, more power is needed. For more info regarding this, google for "Affinity Law". Application of the Affinity laws is what saves me over 80% on my pool pump power costs.
 
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