How does horsepower affect performance?

CT_Tree_Guy · Dec 19, 2005

It is truly amazing how much stronger the vortex over your wallet becomes as the Horsepower increases. 

I don't know if this is just one of those "tall tales" or not, but I heard that one time when Daryl pulled out his wallet it caused a twister that wrecked a trailer park two counties over! /forums/images/graemlins/laugh.gif

nomad · Dec 26, 2005

( 3320 John Deere (32.5 hp; 25.5 PTO hp and 62 lb-ft/torque, non-turbo) VS a 3720 JD (44 hp; 35.5 PTO hp and 84 lb.ft torque); would they pull a box blade the same, or one better because it has more power. )

If you are going to use your tractor only for the box blade, then, forget about PTO because box blade does not use PTO. So, it is meaningless to compare their PTO powers/torques. For box blade, pulling/tracking/dragging forces will be important. And, these forces will be dependent on the gearbox gear ratios, tyres, speed of tyres, weight of tractor, etc. Too many factors. If we assume all of these factors are same for two tractors under comparison, then, the one with more engine power will produce more pulling (linear) force that will pull the box blade.

If your implement was another one, like tiller/mower/etc, that would be powered by PTO, then, your need would be higher torque at PTO. But, in this case, your tractor will move slower if the torque is increased at the PTO. So, an optimization calculation is needed according to your all implements you will use.

Usually, people are confused about torque and horsepower/power. Simple and general rule is that:

Power = Force x Velocity

If it is a rotational motion (like at PTO), then,
Power = Force x Distance / Time = Torque x 1/Time = Torque x Angular Speed. - Here, Angular Speed can be expressed in terms of RPM.

So,
POWER = TORQUE x RPM
(if you want to express Power in HP, then, this equation will be Power = a Coefficient x Torque x RPM)

It is the Torque (or Force in it) that DOES the Work. Remember your car spends its energy to increase RPM, hence its Torque of cars is smaller. So, if you want your tractor to do more work, your should increase Torque. You can do this by decreasing RPM for the same tractor. But, then, your tractor will move slower. So, optimization calculation has to be done here. If you use more POWERfull tractor, then, look at its gearbox ratios, in other words, look at "how much of engine power is converted to Speed (rpm) and how much of it is converted to Torque" at the PTO. Chose higher PTO torque if you are going to do heavier works. If you want to do a (lighter) work FASTER, then, chose a tractor with less PTO torque / higher PTO rpm.

tglass · Dec 28, 2005

From what I've seen horsepower, torque, and pto horsepower always have alot to do with the perfomance of a tractor, but I think alot of people are overlooking the type of transmission with the same comparisons. My 3320 with motion match, load match e-hydro etc. really doesn't not put much of a load on the engine, while the wheels are pulling all they can or spinning. Could have bought the next size bigger, but understand the only difference was the blower (5-h.p. more). For the diffence in cost I couldn't justify it. Just my thoughts and like to hear some other opinions.

daTeacha · Dec 28, 2005

Right. The latest electronic wizardry from Deere lets the computer decide how much fuel, what hydro setting, etc. etc. will provide the ability to get the work done as efficiently as possible. About all you have to do is steer and run the loader. That's probably going to be computerized next, then you can stay inside and watch TV while the tractor does all the work.

jcmseven · Dec 28, 2005

This has proven to be a nice discussion. The opinions of all are appreciated. This serves to address an issue that has confused me about tractors of all sizes. The motif that a higher horsepower tractor is always better is a little foggy. I have been one of those who always tries to get the highest horsepower machine that I can afford, but interestingly, I am not always sure this is the correct logic. Most tractor companies push the "PTO hp" marketing tool, which to me only applies to those that use PTO driven attachments. Even then the numbers are RPM related and do not necessarily apply. With regard to ground engagement, torque seems to be the predominating factor. The more I think about it, the less difference I think more hp would make, especially in tractors with similar weights, tires and gearing. This counterposes my original thought. With regard to John Deere machines in particular, the point of LoadMatch was brought up. To me, LM is an excellent feature, but I wonder with regard to pulling heavy objects (a box blade for example) how much difference it really makes. For example, before my current tractor, I had a JD 3720 (like the one my friend is looking at) and before that a 4310 (which I do not THINK is quite as poweful as a 3320). Aside from some minor tread difference, they had the same tires, were roughly the same size and both were hydrostatics. I had a 72" boom mounted blade that I used occasionally for snow removal. I used the blade with each machine as I kept it during the sale of the 4310. On flat ground, I felt both plowed equally, but on some of our steep hills, the 4310 would slow down under the blade (snow) load and start to bog a little. To combat this, I would flip on the LoadMatch which would immediately bring the RPM up. The tractor ground speed though would slow to such a point to do this that the lack of momentum caused it to slip, which prevented it from pushing much more. One hill in particular I could never climb with the 4310 with any real load on the blade. The 3720 on the other hand, would maintain its speed, with a load, pretty easily. I could move snow and climb up hills without stopping mainly I think due to the extra torque. I rarely had to engage LoadMatch and when I did, it seemed the tractor did not have to slow as much to maintain RPM. Therefore (and realizing these are two different models, may not apply to other brands, and has a ton of variables) I feel the 3720 was the better pusher of the two. I also found sometimes LM can be slightly detrimental to performance (not often, though). I do not however, think there was much difference on flat ground, and I could tell little difference with the box blade, but I did not use it too much. I suppose this brings up the point that not much of this would be noticed with regard to ground engaging (non PTO) activities, unless the situation or terrain were very demanding. In retrospect this may be the point: some of us might pay 15-20% extra to gain the 5% that we may never use, but that might be the 5% that matters when the going gets tough. The big question is how much is that worth? Everyone is different there

John M

flwiii · Dec 29, 2005

Yes, I agree, it has been a nice discussion on the subject. I still don't see the fascination with HP. Isn't it best to look at the ingredients instead of the mixture as it pertains to performance? You can't have HP without Torque and RPM. These are the base elements of HP. It like looking at two glasses of water, one is heavy water and one is drinking water. Without looking at the base ingredients, it going to be difficult to decide which is best to relieve your thirst.

My interest has always been to look at both torque and RPM. If I plan on heavy pulling power, I want the maximum torque to be as low in the RPM range as possible. This can at times be an engine whose HP is lower. If I want to travel long distances at an extreme high rate of speed, I want the maximum torque higher in the RPM range.

3RRL · Dec 29, 2005

Very good post John,
I posted earlier how I would rather have the added horsepower and torque and you hit the nail on the head with your scenario of how much it's worth to the individual. In my case, I chose the larger, heavier, more hp and torque model because the price difference was only $1,000. Plus I got a 4 cylinder instead of the 3 cylinder and more cubic inches. One other reason was the smaller model (I heard) was going to be discontinued. These factors (to me) were worth the extra money.
I feel with that if I have the extra horsepower and torque, getting it the ground is another obstacle to overcome. But if you do overcome that problem (which is possible), you have the added power at your disposal.
Thanks,

Posterboy · Jan 7, 2006

Calculating HP:
Engine #1 -- (50 * 2500) / 5252 = 23.80 HP

Engine #2 -- (50 * 8000) / 5252 = 76.16 HP


Hey, can I get engine #2 for my weed-wacker?

Hi, My Name is Andy and I'm a Horsepower Junkie! /forums/images/graemlins/grin.gif

willfick · Jan 7, 2006

I agree, John. A good discussion wherein I picked up useful thoughts on hp, torque, ballast, etiquette, etc.
As something of an unrequited horsepower junkie myself, I've often looked at that line of twenty-five hp lawnriders chained to each other in front of Lowes and wondered could the whole line-up together outpull my 25hp Farmall H.
Wm