View Full Version : torque or hp????

08-09-2004, 06:35 PM
I am wondering if a 21 hp diesel motor would cut as well as a....say 25hp gas engine due to more torque??? Anyone know this answer? Also what is the difference in 3:73 and 4:11 rear end?

08-09-2004, 07:36 PM
I know that torque is better for like brush cutting.

08-09-2004, 07:42 PM
New blade, to answer your questions in order.. 1 torque is the ability to do work, horsepower is the ability to accelerate ( sorta, I am grossly over simplifying this to put it in layman's terms ).... 2 The difference beween 3.73 and 4.11 is the number of turns the drive shaft makes in relation to the axles... In other words 3.73 turns of the drive shaft to 1 turn of the axle shafts. The 4.11 will pull better, but not get as good of fuel economy all other things being equal, generally..

08-09-2004, 07:42 PM
when the growing gets heavy its the torque that counts

08-09-2004, 10:58 PM
If you have torque and horsepower you have something to brag about.

08-10-2004, 12:21 AM
Along with that torque you need blade speed.
A diesel will develope it's torque at a lower rpm. In order to get the blade speed up you have to use a smaller pully on the spindle. To use the smaller pully you need more hp. It's all interconnected. If it wasn't, all cars woud have 25 speed transmissions.

08-10-2004, 01:06 AM
when i read this it made me instantly bust out the December 2000 issue of petersons 4-wheel and offroad magazine and turn to page 100 (yeah, i sit in my "office" a lot and i have plenty of time to read). it explains torque and horsepower for trucks, but gives up interesting formulas.

Work = Force x Distance

Horsepower = lb.ft. per minute/ 33,000
Horsepower = Torque x rpm/ 5,250

Torque = Horsepower x 5,250/ rpm

New Gear Ratio = New Tire Dia./ Old Tire Dia. x old gear ratio

i'm sure these can be converted to lawnmowers, the gear one i'm not sure what you would use for other then the guys that drive lifted trucks with out making some mods, you'll see what gears you should be running.

08-10-2004, 04:51 AM
Speaking from experience i can tell you this. Torque is the measure of twisting force, and in this case it would be the crankshaft. Horsepower however is a measure for how long it takes to create torque. Horespower will be smaller or greater depending on the torque. DALM you are correct in you're findings as they come from a reputable source, however, the equation you provided is used to determine the gear ratio needed to retain stock Speedo accuracy when making extreme changes to you're tire sizes on you're truck. For example, switching from a size 275/75/R16 to a 35"12.5"R15. As for desiels, they DO develop torque at a much lower RPM. Anyone with a Desiel truck will notice this looking at the RPM gague. I think most will redline anywhere from 3500 to 4000 RPM as where you're gasoline truck will redline at about 6000-6500 RPM. Secondly, gear ratios are for example, 3.73 gears would mean that you tires will rotate 3.73 times for every 1 rotation of the driveshaft. The HIGHER the gear ratio equals the slower top speed and the worst gas mileage, but it will get there much quicker. That is why drag cars have gear ratios that are astronomical. They need to get to 300 MPH in 3.5 seconds...See what I'm saying??...SO, you decide what's best for you're situation with the info we provide.. GOOD LUCK TO YOU. -Forest

08-10-2004, 07:41 AM
Originally posted by Forest
3.73 gears would mean that you tires will rotate 3.73 times for every 1 rotation of the driveshaft. The HIGHER the gear ratio equals the slower top speed and the worst gas mileage, but it will get there much quicker.

I think that it's actually 3.73:1. That's the gear ratio which states size not rotation. It's inversely related to rotation. So, it takes 3.73 rotations of the drive shaft to rotate the wheels once. The second statement is right. If the first statement was right, 4.11 would give you a higher top speed.

08-10-2004, 10:04 AM
Oh forest, BTW top fuel dragsters, the ones you speak of that go 300 mph in a 1/4 mile, Use in the neighborhood of 2.90 gears,Have no transmission other than a reverser to back up after the burnout, and are direct drive after the cluch fully locks up.

08-10-2004, 11:33 AM
I guess I don't understand the formulas listed above.

If Torque = Horsepower x 5,250/ rpm, that would say that a 21 horsepower gas would have the same torque as a 21 horsepower diesel.

It also says that torque goes down as RPM's go up. I always thought it was just the opposite, a motor running at 2000 RPM would have more torque than one at idle.

I haven't seen the article, but I think they must be referring to something else? Maybe the torque applied to the ground by the wheels?

I know the wheels of a dragster change diameter as the speed increases. This maximizes the torque at low speeds and increases the circumference at high speed to effectively change the final drive ratio.

Not trying to start something here, just confused ...:confused:

08-10-2004, 11:37 AM
Forest has text book correct answers re torque and horsepower and probably wants to rethink gear ratios.BTW its real hard to beat a steam engine for low rpm torque

08-10-2004, 11:44 AM
One of the many subjects I know nothing about - steam engines.
Why is their torque at low RPM's?
Are they still used for anything?

08-10-2004, 12:30 PM
A 21 hp gas engine and a 21 hp diesel engine do have the same torque IF they produce that 21 hp at the same RPM. Diesels typically produce their peak hp at a lower RPM than gassers, so they produce more torque. As for engines producing less torque as RPM goes up, that is true only at RPMs higher than the point at which the engine produces maximum torque. Small car engines like Hondas produce maximum torque at speeds up around 5,500- 6,000 RPM. Since they produce maximum torque above 5252 RPM, their maximum torque is less than their maximum hp. A big truck engine like a Detroit Series 60 actually has a flat torque curve from 1200-2100 RPM. For the 470 hp model, I think it is 1450 ft./lbs of torque.

08-10-2004, 12:37 PM
I guess i should have stated my gear ratio as i meant it....3.73:1 instead of just 3.73(thought yall would catch on). But any how....3.73:1 is the gear ratio in the carrier(pumpkin) of you're rearend. And sh!t, i DO stand corrected. After thinking about my 2:30a.m. answer for a second i caught what yall are saying. The correct answer is that the tires spin ONCE for every 3.73 rotations of the driveshaft, and YES you are right about the top fuel dragsters in the sense that they have a simple gear box(in out box) rather than a transmission. Thanks for the correction.

08-11-2004, 01:58 AM
i never really understood the formulas much, i just listed them hoping one of you know more about it than me, and could then in turn apply those to lawn mowers.

lets see if i can elaborate more on one those formulas:

it says for example that if your peak torque of 300lb-ft occurs at 3,000rpm, to get horsepower numbers, the answer is 171.3hp at 3,000rpm (hp = 3,000 rpm x 300 lb-ft/ 5252) - which is not the engines peak horsepower.

i guess you would need to run your truck or mower... through a dyno test to see these things, but not everyone can. so maybe figure out you peak torque on a diesel and gas engine is, or peak hp, and then you can figure out the amount of hp is being used to produce the torque and what rpm is ideal or what ever. like i said, i guess you would have to be pretty smart to figure some of this out when it comes to lawn mower engines.

08-11-2004, 02:07 AM
actually 3.73.1 or 4.11.1 refers to the fact it takes 3.73 revolutins of the pinion gear to make one revlution of the ring gear if I am not terribly mistaken. And gear ratios are inversely proportional, ie a higher numerical ratio is actualy a lower ratio. hence 4.11.1 gears are lower than 3.73.1 gears as far as gear ratios go. sorry I'm a truck freak. four wheeler mag is my obsession