Acceleration and power vs torque vs "punch in the kidneys"

[scooby dooby doo]

the peak torque figure from the engine doesn't really mean anything since you've got a gearbox strapped on the back that changes the rotational speed and torque (cos that's what gearboxes do).

 

compare a BMW 320d and 320i. the acceleration and peak power figures are very similar but the diesel has about twice the peak torque figure at about half the revs. It'll also have about half the amount of gearing down in the gearbox and diff so the result at the wheels is the same.

 

the diesel in this case has a few bhp more that more than make up for the extra weight. That explains about half the difference in 0-60 times (a few tenths). the other few tenths come from the diesel having a better spread of power so when it's nearer to peak output more of the time as you go through the gears.

 

HOOPY R706KGU Hoopylight R

[rossybee]

 

How's married life treating you?

 

Sssssscottish SsssuperSsssnot!

Ssssneakypeek! whooooaaaa!

 

Edited by - rossybee on 30 Sep 2005 16:45:21

[Farmer_Terry]

I have never really been sure what the torque figure shows. BHP = torque x revs so twice the torque at half the revs = exactly the same BHP. Hence the gearbox will change the revs and the torque in unison so that the power in = power out (or very nearly).

 

My Massey Ferguson tractor has HUGE torque – it will pull anything up any hill with ease IN THE RIGHT GEAR and with those vast knobbly tires to make sure that the torque is put into moving the object not just spinning the wheels.

 

But it is only 90 bhp and doesn’t accelerate terribly well after about 1mph.

 

Because acceleration = force/mass, my take is that bhp/tonne tells you the acceleration you will get. BHP alone tells you the maximum speed you will get, and the torque figure tells you whether you are driving a tractor or not.

 

How does that sound? Just my 2p worth.

 

 

Farmer Terry

[Heres Jonny]

 

Little late in the conversation, but my 2.0 zetec weighs 530kilos!!!

 

Was weighed by Dave K and also weighed in Holland 530kilos road going is this abnormal????

 

Dave K was impressed with its weight, would appreciate anyones comments?

 

Its a 1990 Live Axle, LSD 220bhp zetec for the record

 

 

Cheers,

 

Jonny

[Nifty]

Farmer Terry,

 

My tractor has more torque than yours ! *tongue*

 

Keep off the straight and narrow 😬

 

Edited by - Nifty on 30 Sep 2005 22:46:44

[Slipper man]

I used to have an SLR and coincidentally a Scoob STI V5 at the same time. Peter's point is spot on I think. If all this logic doesn't convince your mate though, how about the facts:

 

SLR 0-60 3.8 secs, 0-100 8.5 secs

STI 0-60 4.7 secs, 0-100 12.2 secs

 

If I remember Autocar right (I may be out by a couple of 10ths, but no more than that). The Scooby is a very fast car - on bumpy 'B' roads faster than a Seven, especially in the wet. On a circuit though it wouldn't see which way the seven went!

[blade_runner]

I have never owned or driven a scooby, quite fancy one. I know on the road they seem like a really fast car, But it gives me such pleasure seeing them dissapear in the rear view mirror when in a caterham on a trackday tho 😬

 

'Pinky Pics' here

[se7enmad]

I think I agree very much with Doh nut and his squat theory here.

The punch in the back may not be related to mathematical advantage in power torque etc but just the physics of the seating and your position in the car. A car that squats will give a different impression than a seven that doesnt hesitate and throws you forwards. Reminds me of when I was a little child and I used to literally sit on the rear axle (no seat) in my dad's GTA. The car used to squat really heavily and stay there nose up for all of the run. 😬

Have you ever been passenger in a racing car without a seat? the feeling is very different to a passenger car and you enter the 'I'm feeling queasy and my head feels funny' territory.

I believe it all relates to the passenger's centre of gravity with relation to the car.

 

 

Antonella

1998 Caterham Classic

my NEW site here

[Farmer_Terry]

Nifty

 

"My tractor has more torque than yours !" - it almost certainly does, but I dont see you driving it. Mine is mine and paid for!! (Scroll down to the bottom) *tongue*

 

 

Farmer Terry

[Nifty]

No I wasn't driving it, one employs others to do that ! *tongue*

 

I was driving the harvester on the field of parsley next door.

 

Keep off the straight and narrow 😬

 

 

Edited by - Nifty on 1 Oct 2005 22:16:05

[PaulD]

How on earth do you get that much parsley to germinate? Presumably not with kettles of boiling water!

 

Paul

[Nifty]

Paul,

 

Collectively we grow about 600 acres of parsley, though that is likely to change dramatically next season since the drying costs have shot through the roof with the price of gas (it is now cheaper for the drying plant to buy in pre-dried from abroad and just package it up - the way most things are going in this country *thumbdown* ).

 

The Breck sands lend themselves to the growing of such crops (some fields are almost like beaches). The important part of establishment is rapid germination and providing we have rain or the land is irrigated germination is relatively easy, due to the sandy seedbeds giving good seed to soil contact and being relatively warm. The other benefit of sand is the lack of slugs which would be a severe nuisance on heavier land.

Also the free draining land allows the harvesters to run over the field without doing too much damage to the plants, which one hopes to keep going for at least 3 cuts and sometimes 4 in a year. This year we are only going to get 3 cuts due to the cold spring slowing the growth earlier. We will still cut about 1 ton of dried product per acre, equating to about £1,000/acre - sounds great, but the costs involved are high, especially with the price of fuel running the trailers to the drier. It should return a profit at this price but next season's forecast price offer of 75p/kg may kill it on the land further from the drier (Garboldisham is roughly 10 miles away whereas the field in the picture at Oakley is a stones throw away).

 

 

Ooops, have I digressed from talking Power and Torque 🤔 ❗ ❗ ..... *tongue*

 

Keep off the straight and narrow 😬

 

Edited by - Nifty on 2 Oct 2005 11:38:37

[Farmer_Terry]

Nifty

 

Looks like you are serious and I may need a red face, 😳 and a slice of humble pie (no emoticon for humble pie I see). No doubt your 7 is faster than mine too.

 

We just run 200 sheep over here in Derbyshire - very small beer by comparison. Thanks for the photos though.

 

Farmer Terry

[Nifty]

Terry,

 

No you don't need to eat humble pie, the pictures are from a friend/customer's farm, not mine - I was just having a bit of fun.

My salaried day job is as an agronomist but I earn pocket money to pay for the 7 etc by assisting with harvesting, usually on the night shifts.

 

And your 7 will be considerably quicker, at present, since mine is a 1400 Supersport, but I am working on that one *thumbup*

 

Editted to acknowledge that 200 sheep on the Derbyshire peaks is a lot harder work than sitting in an air conditioned cab trundling up and down a flat field

 

Keep off the straight and narrow 😬

 

 

 

Edited by - Nifty on 3 Oct 2005 00:24:50

[andy couchman]

The squat theory is appropriate.

 

To get a good feeling of acceleration swivel your office chair backwards quickly...

 

Now pick yourself up off the floor and try it again but don't go over backwards this time. Fast innit?

 

You'll need a race car on slicks to better a 0-64 feet time of 2.0 seconds and that is 'only' 1.0G acceleration so if the Scooby squats on the back and the front rises it is already well on the way to a 1G passenger/driver experience.

 

The 7 on the other hand remains pretty level so you only feel the pure acceleration.

 

More or less.

 

Andy

[Richard J Darnell]

I read elsewhere that us humans are only good at detecting a change in the rate of acceleration.

Think of an aeroplane on take off - you get an initial surge and then constant acceleration that doesn't feel that great but I bet the acceleration above 100 mph (say) is faster than most cars.

 

[ashaughnessy]

Forgive me for hijacking this thread but I'm fascinated by the tractor-related portions of it. What are the characteristics of a tractor (it's engine, transmission, etc) that make it able to pull in the way it does and why can't a high powered/high revving petrol engined car do the same? When you look at tractors the engines don't seem so big so where do they get the pull?

Of particular interest: What do the power/torque curves look like compared to a typical car? What size engines do tractors and similar vehicles (e.g. lorries) tend to have? What gearing is used to convert the engine torque to a force between tyre and ground?

I'm presuming one of the major reasons why a tractor can pull is clutch and transmission strength. A high revving petrol engined car would burn out it's clutch trying to move a heavy load from a standstill.

Yours in schoolboy-style anticipation - Anthony

[Farmer_Terry]

The tractor thing is simply torque. Any vehicle could actually do it if it had a similar gearbox - i.e. capable of a very low gear. My tractor in low/first goes at very slow walk while the engine is nearly flat out at 2000rpm

 

As I said earlier, a gearbox will increase the torque while decreasing the revs (or vice versa) which keeps the power in = power out.

 

Any 90hp vehicle with a) the right gearing, b) the large diameter tyres, c) the chunky tread and d) the weight to hold the whole lot down would do the same thing.

 

A 7 fails on all of those, a serious off-roader would get one-and-two-halves of them and so is halfway between the two.

 

Does that help?

 

Farmer Terry

[Nifty]

You've missed the importance of the engine design Terry.

 

Your 90 hp tractor engine is going to be in the order of 4 litres, whereas my 128hp Seven is 1.4 litres.

Your tractor engine will have much more torque and a very flat torque curve.

 

For info on the statistics of the Steiger's engine see here

 

The STX 500 in my photo has a rated horsepower of 500, with a peak figure of 550 and a maximum torqur figure of 1743 lbft (from a 15 litre Cummins straight 6 turbo diesel)

 

Keep off the straight and narrow 😬

 

Edited by - Nifty on 5 Oct 2005 08:59:02

[Violet Elizabeth]

More to do with longevity I would have though, and the price of farm diesel. But Terry has it right - gearing can do wonders if you have the revs to play with.

 

A similar topic was covered in Peter Cs's brilliant post comparing the caravan pulling abilities of a BMW turbo diesel vs his (old) K series.

 

Think it was called A Fair Fight, or something. Will dig it out. Well worth a read.

 

Basically a high reving K can produce as much 'van pulling torque as a Beemer turbodiesel, given the right gearing. Although Nifty's tractaaaah might be a more difficult prospect !

 

There was another theoretical comparison I read, of a waterwheel vs a petrol engine. The waterwheel producing 10,000 ft.lbs or something crazy - seeming unstopable. But geared down to half a revolution a minute, a 1.6 petrol engine could easily send the wheel backwards.

 

 

[ashaughnessy]

But when you connected the 1.6 litre petrol engine up to the water wheel and tried to oppose the water wheel, it will burn out it's clutch trying to apply enough engine speed to stop the water wheel. Doesn't a tractor or similar device need to be able to produce prodigious torque at very low engine speeds in order to get the load started?

[Farmer_Terry]

Not really because the very high torque is at the BACK of the gearbox. The front of the gearbox is perfectrly normal. Regular engine speeds, regular clutch. The gear-down takes care of the rest.

 

The agument that the clutch would burn out trying to stop the water wheel is not right (IMHO). Get the gearing right and it would stop the water wheel dead.

 

Farmer Terry

[Mr Locust]

Nifty,

 

Your tractor engine will have much more torque and a very flat torque curve.

 

Sorry mate that's not strictly true........(I always seem to get to pick you up on these points don't I 😬)

 

Good off highway diesel engines tend to aim for torque backup or torque rise characteristic. These engines quote maximum power at maximum rated rpm but as you work back down the rpm range from max rpm, the torque will rise. The likes of John Deere will typically quote 25% torque rise when the engine is loaded from maximum rated power rpm to max torque rpm. For example a typical calibration curve for a Perkins 1100 series 4.4l engine will be rated/max power at 2300rpm but with max torque at 1200rpm and torque will rise 20-25% over that range.

 

That is what makes the engine 'feel' gutsy, powerful etc. because the machine 'hangs on in there' under high load. I have found from bitter experience that flat torque curves are particularly badly suited to hydrostatic transmission machines due to the rapid onset of engine stalling as transmission load rises!

 

 

Ian

 

Green and Silver Roadsport 😬

 

Edited by - Mr Locust on 5 Oct 2005 09:50:52

[ashaughnessy]

I'm thinking of the case where the water wheel is already moving (and has a lot of momentum already) and you try and connect up your engine in opposition to stop it. This is analagous to the case of trying to pull a load from standstill.

 

It would be different if you got your engine all connected up to the water wheel and you were driving the water wheel the wrong way and then you started the water flow. In that case, no problem.

 

Anthony

[Nifty]

Ian,

 

I actually meant the same as you.

My reference to the "flat" torque curve was relative to the steeply climbing graph of a normal car like the sheet Arnie has on his website. Compared to this the torque curve of a tractor (and possibly lorries too) are reasonably flat but with max torque usually at much lower revs than max power and a gentle slope from max torque down to rev limit - thus going backwards you get the torque rise.

 

A car's torque curve looks like a mountain whereas a tractor's looks like an escarpment with the gentle slope down as revs rise.

 

(Why can't we draw pictures on here *tongue*)

 

Keep off the straight and narrow 😬

 

Edited by - Nifty on 5 Oct 2005 10:19:49