rpm's

[Anonymous Poster]

rpm\'s

This is a general question and may sound a bit stupid, but can anyone tell me what is the optimum RPM one should use when running their tractor. I have a B7300 and use it with a 4' bushog that I use on my two acres (rolling hills and trees)

 

[Bird]

Re: rpm\'s

glharvey, for a brush hog to cut as cleanly and well as it's supposed to, I think you need to run it at the full PTO rpm. If you have a tachometer, that speed will be marked on it, but if the B7300 doesn't have a tachometer, then "full throttle and ease back just a tiny bit" is what most dealers tell you (usually full throttle is only a couple of hundred rpm more than PTO speed). You should never be able to hurt the machine running it a full throttle, but for a lot of work there's nothing wrong with running it slower just to cut down on the noise and perhaps fuel consumption so long as you're not lugging the engine.

Bird

 

[art]

Re: rpm\'s

Bird's right on with what he said. Mowers are designed to mow at full RPM for the best performance but that doesn't mean it needs to be there on all jobs.

 

[bpence]

Re: rpm\'s

glharvey, I have a 1995 B7100 (same basic tractor as the B7300, just a couple of years older without some of the niceties the B7300 has) and I also run a 4' bush hog. I usually run at full throttle when bush hogging as I run up and down some hills and the machine seems to operate better under those conditions. However, when I'm cutting a flat area and the stuff is only 1-1 1/2 ft. tall, I'll usually back down a little off full throttle. Just don't seem to need "full bore" when the machine's not working that hard.

Bob Pence

 

[Anonymous Poster]

Re: rpm\'s

I do have a tach on the B7300 and it is marked with 540 for the PTO. I guess I am still a bit slow on what is full throttle. Run the tack at 1000 rpms? or 3000 rpm? I figured that when I am mowing the PTO will get what it needs when I engage the PTO and the rest is for the engine and as long as it runs smooth (no lugging) I am doing ok.

 

[hayden]

Re: rpm\'s

No, not quite right. If I hear you right, you believe the PTO will always run at 540 RPM regardless of engine speed. This is not true.

The PTO speed is directly proportional to the engine speed. The slower the engine goes, the slower the PTO goes. The mark on your tach indicates the engine speed that corresponds to 540 RPM at the PTO. When using 540 PTO implements, especially mowers, snow blowers, and chippers, you should run the engine at the indicated 540 PTO speed. Things like backhoes and winches are less crucial.

FUll throttle usually runs the engine right at or slightly above the 540 PTO RMP speed. The comments about "backing off a bit" were addressed to tractors that run somewhat faster than 540 PTO RPM at full throttle setting. If that's the case with your machine, then back off the throttle util the tach is at the 540 PTO point (within 100 RPM or so)

Make better sense now?

 

[Bird]

Re: rpm\'s

glharvey, Hayden answered right. I'm not sure what "full throttle" is on your tractor, but probably just under 3000 rpm. For example, on my B2710 the PTO speed of 540 is 2584 engine rpm, engine horsepower is rated at 2600 rpm, but if I pull the throttle wide open I get about 2800 rpm. In other words, very small differences.

Bird

 

[glennmac]

Re: rpm\'s

I, too, go around in circles trying to make sense of rpm's.

My BX had no tach so I just ran everything close to or at full throttle. My 2900 has a tach so I assumed it has some important use. If it does, I don't know what it is. Then again, after 40 years of driving cars, I still haven't figured out what use a tach is on a car. (They even have them on automatics.)

I say this because PTO rpms have no correlation with the engine rpms showing on the tach. So who cares what the engine rpms are. Might as well just have a scale of 1 to 10.
Here are my questions:

1. When a PTO is X rpms, does that mean you should run it at (a) no more than X, (b) at least at X, or (c) at exactly X?

2. What speed do I run my belly mower at, which is connected to a 2500 rpm PTO? The tach does not correlate 2500 pto rpms to the engine rpns at all, as it does for the 540 rpms for the rear pto. My mower manual says to mow at "full throttle". But I dont know whether that means (a) 2500 engine rpms, (b) the unmarked and unidentified 2500 pto rpms, or (c) pin the tach.

2. What speed do I run my brush cutter at? The manual says to run it at 540 pto speed. But that takes me back to question 1 -- exactly pto speed, at least pto speed, or no more than pto speed. Isn't more blade speed better? So why shouldn't I run at "full throttle" like the belly mower.

4. What speed should I run my loader at? My manual says to run it between 1700 and 2200 rpms. Boy, is that counterintuitive. I was running it close to full throttle on the belief that more power is better and that its lift rating would have been measured at full throttle. What's going on here, and when am I supposed to choose between 1700 or 2200 rpms?

5. What rpms do I run at when just pulling something heavy like a ground engaging implement or a log? Dont see anything in the manual.

6. What rpms do I run at when driving down the highway to the gas station to get fuel, Snapple and Cheese-Its?

I find this all tedious and just run everything close to full throttle except when I am just cruising around admiring things.

Glenn

 

[Bird]

Re: rpm\'s

Glenn, my B2710 tach is marked for "PTO" speed which means turning the rear PTO at 540 rpm when the engine speed is 2584 rpm. Now the mid-PTO (which I'll probably never use) is supposed to turn at 2537 rpm at 2600 engine rpm. Now don't ask me why they use those numbers in the manual when the engine rpm is so close to the same and the mid-PTO is supposedly a 2500 rpm PTO anyway (instead of 2537). But the bottom line is that the engine rpm should be the same (or too close to tell the difference) for either PTO. And if anyone doesn't believe the engine rpm and PTO rpm are directly connected, just put an implement on, turn on the PTO, run the engine up to full throttle or PTO speed, watch what's turning on the implement, then idle the engine and see how much the implement also slows down.

Now you don't have to run the engine at the 540 PTO rated speed, but things like mowers and tillers will do the best job at that speed. They'll work at a lower (or faster) speed, but not as efficiently. On the other hand, when using something like a posthole digger, I'll certainly run it slower so it doesn't swing or whip around; easier to control.

On all the tractors I've used, wide open throttle gets approximately a couple of hundred more engine rpm than the rated PTO rpm. In other words, less than 10% faster than the rated PTO rpm. So if you want to run it wide open, not much difference. So that accounts for the fact that dealers and service reps say, if you have no tach, go wide open, then pull back just a little bit.

And yep, you're right. They even put tachometers on cars with automatic tranmissions, and I had to pay extra to get one on my wife's Escort because I like to know just how fast the engine is turning. In the past, I had aftermarket tachs installed on two pickup trucks, and on a motorhome when I was traveling.

As far as the proper rpm when using your loader; suit yourself. I like the 1700-2200 speed; it's quieter, but has enough power in most cases. If I think I need to push harder, or if I want the hydraulics to work faster, i.e., raise or lower the bucket faster, fine, I'll increase the engine rpm.

And if I drive the tractor on the road, I'll just pull the throttle wide open and go.

Don't know if this helps, but gosh, I might catch up with Mark on the word count, too, since I'm told I've passed him number of messages./w3tcompact/icons/wink.gif

Bird

 

[Anonymous Poster]

Re: rpm\'s

Thanks for all the great information. I have always enjoyed this database for the information that I have received. I recently did my 50 hour service because of the information and encouragement of the ones that posted information.. again thanks