Right how is it that a diesel has more torque (in general) but less horsepower?
I know that is a massive generalisation but I'm trying to get my head round how a car can have more 'pulling power' but actually travel slower than a petrol car of the same type?

Is it the fact that the diesel has to change gear earlier and so the petrol is quicker to 60 because it can travel for longer throughout each gear?

Interested to see peoples views on this topic, never seen it come up on here before (please correct me if I'm wrong).

Can a mod move this to a technical area, didn't mean to post it in here

I cannot get my small brain round this but have a read of this, oh and good luck >> http://www.allpar.com/eek/hp-vs-torque.html

torque any day, i drove a 1.2 altea turbo and flipping me that flew for a 1.2!

Sometimes quite a difficult thing to explain as they do the same thing but are measured in different ways. I wish i had college notes to refer to some of this but i'll do my best!

With torque, imagine you have for say a bar of metal. When you apply a rotating force what you get is an angle of twist(degrees) For an example of how much force you apply Say you want to get a nut off a thread that is solid on. Say over 1ft you apply 100 pounds of force that would equal 100lbft. The more distance you apply a load over ( say 2ft ) you would then have 360lbft of torque being applied.

With power, refering back to the original torque example, is how far in distance you go with that applied torque.

Pulling this off a random internet page ( and i hope its not a lie as it looks familiar to the equation i was thinking of.

Say you were applying 87.5 lb-ft of torque over a distance of 6.28ft ( 2 x 3.142 x 1 ) [ 3.142 is pi ]

torque * RPM * constant = hp
87.5 lb-ft * 60 rev/min * X = 1 hp
X = 1 / (60 * 87.5) = 1/5250
torque * RPM * 1/5250 = hp
hp = (torque * RPM) / 5250

This is why when motoring magazines they will put the power figure @an rpm!

Basically in referance to your original question, a diesel has a larger compressed explosion of the chamber in the cylinder due to it being turbocharged and diesel being a more explosive material under compression compared to petrol, so with the explosion in the chamber being larger, it will rotate the piston more forcefully, hence the larger torque figures!









Any questions

I'll refer back to some notes tomorrow but im pretty sure thats right!

So why does a turbo produce more torque than a supercharger

you loose torque with the drive belt Adam, turbo uses exhaust gasses and generates huuuge amounts of pressure in the cylinders and on detonation pressure is increased twice fold and there's your torque.

As above. Also with a supercharger your turbine speed is limited and controlled by the speed of the engine, turbochargers can run a higher rpm as they are controlled by exhaust back pressure.

I think Alex pretty much summed it up

Just another view, i was able to understand this a little better......

Torque is a Force. In "horse" terms... it is how hard the horse can push with its legs.

Power is the rate Energy is used. In "horse" terms... it is how quickly the horse can move its legs while pushing at a given force.

So a big cart horse can generate a lot of push (Force) and shift a very heavy object, but it can't move its legs very fast so it doesn't make much Power (push x leg speed)

A race horse has lighter muscles and can't generate nearly as much push (Force) with each leg, but it can move them a lot, lot faster. So fast that it generates more Power (push x leg speed) but hook a heavy cart to it and it would grind to a halt.

Diesel engines tend to use higher combustion pressures on its piston than petrol engines (hence they are usually built stronger and heavier). They also tend to use longer stroke crankshafts allowing the pistons to apply more "leverage" to the crank. This means they can generate more torque (force) and do a good impression of a cart horse.

Petrol engines tend to use lower pressures, lighter materials, shorter strokes, and lots more revs.... not as much torque, but more power due to the speed they move. So they can move a light object faster..."

Right so diesels are technically more powerful but because of the extra weight and lower gear ratios they will be slower than their petrol equivalent?
But in gear the acceleration will feel quicker because of the additional pulling power?
So does that mean the optimum torque of a diesel is spread across the revs more evenly than a petrol?

it's easier looking at it this way

cannon A has a larger bore and filled with 3kg of explosive, it can fire 1 round 4 miles and takes 5 minutes to reload. Force is equal to 4532 psi

Cannon B has a smaller bore and filled with 1.5kg of explosive, it can fire 1 round 2.8 miles and takes 2 minutes to reload Force equal to 2353 psi

to put that in relation to a cars combustion cycle.

a diesel car uses a more combustible fuel in the cylinder creating an bigger kinetic reaction but takes a longer revolution (rpm) to recover it's orignal position as the force is greater.

A petrol car uses a less combustible fuel in the cylinder creating a smaller kinetic reaction but takes a shorter revolution (rpm) to recover it's original position as the force is less.

Which basically means that a petrol can spin the crank shaft at a much faster revolution compared to the diesel as kind of explained in the cannon theory above.

diesel has low down torque because like explained above there the rpm is slower then the petrol, therefore you gain more torque low down in the rev band, where a petrol needs to generate twice as much momentum or kinetic energy at high rpm to acquire its torque.

this is why diesels are more economic because the petrol combustion cycle requires more fuel per revolution of the crankshaft compared to the slower but more powerful revolution of the diesel.

i think I've explained that right, I did study the combustion cycle of an engine in science when I was at school.

I think we've covered it well here lads

Basically, torque wins races. Bhp is pub talk

But you need bhp to win a longer race...