Horsepower vs. Torque vs. Weight

I think I understand the theory behind horsepower and torque, but have some difficulty seeing how it plays out in "real world" situations. Looking at diesel engines at the moment has just made things more confusing. My current car (Firebird) and an A6 weigh within 100lbs of each other. The Firebird has

200hp and 225lb/ft, whilst the Audi has 140hp and 280lb/ft. Also the peak torque of the A6 comes far lower down the rev range. Can anyone explain how this will play out in everyday driving, what differences I'll notice etc. 'And no "the Audi will go round a corner unlike the Firebird" comments please!

Cheers Dan

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difficulty seeing how

moment has just made

of each other. The

the peak torque of

play out in everyday

corner unlike the

the audi will corner unlike the firebird :) now that's done

well the higher torque will see it pick up quickly but die off as the revs get high so you'll probably not want to rev the bollocks out of it, firebird on the other hand has more bhp but slightly less torque meaning you'll have to rev the nuts outta it to get it really shifting.

the difference i've seen between torque comparing petrol and deisel is that the petrol will get up to speed quicker from standading but the diesel once at say above 40-50 will climb up another 30mph without much effort regardless of gear your in at the time be it 3rd, 4th or

5th. well going by my dad's volvo S60 D5 :)

you gave the torque and bhp of both cars but not the weight which could make it a whole different storey

Reply to
Vamp

Firebird will be faster, assuming transmission losses aren't horrific, and it's being revved. The A6 will be easier to drive having a better torque spread, but not as quick. I'm talking in terms of accelleration here, top speed is determined by power and drag.

Reply to
Doki

Ok, so pulling away from the lights the Firebird will be faster? Where does the extra torque of the A6 help me out?

Dan

PS The transmission loss on the FB may be horrific, it's an auto after all.

Reply to
<a>

Both weigh ~3400 lbs (1600kg I think). The A6 kindof has an excuse, being a big family car...

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<a>

Auto Firebird it might not pull away from the lights so good either. Depends on the shape of the torque curve and the torque convertor stall speed I think.

My Vauxhall Carlton auto sucks at starting from a standstill due to a high

1st gear ratio and peak torque being at 3600rpm which the engine cant reach till well after pulling away. Goes OK after that though.

Extra torque/turbochargey torque characteristics in the audi will mean there's less cog swapping to make reasonable progress. The pull (best balance of torque and power i'd guess) in turbot diesel happens at mid-low revs so when cruising, for a burst of speed you simply need to press the accelerator and its almost as good as changing down a gear so you dont bother. In a NA petrol to get it to the peak torque/power balance you have to drop a ratio or 2 to make the same progress.

My dad's turbodiesel audi a2 (75bhp/150lb/ft) could give my carlton (150bhp/160lb/ft) a run for its money up to about 30 or 40mph I reckon. Thats to do with weight and gearing though, I daresay. By the time we get to 50 the A2 will be well into 3rd and the carlton will still be in 1st.

Your wallet won't cringe when you floor the accelerator and you'll use much less juice overall in a diesel. However in the 140bhp diesel you'll make big sooty clouds from time to time and you won't be as fast as the 200bhp petrol.

I haven't proof read this as by rights I should have been in bed an hour ago (pesky clock changes) so I apologises if it doesnt make much sense.

Warmest regards Douglas

Reply to
Douglas Payne

In news:418573bd$0$1558$ snipped-for-privacy@news-text.dial.pipex.com, a decided to enlighten our sheltered souls with a rant as follows

Grunt, basically. The Audi will pull longer gearing with less hassle, but because the Firebird has an extra 60 bhp, you won't notice the difference.

From my experience, it works like this.

If you're in 5th gear in the Audi at say 80 mph on a big hill, the Audi will just plod up the hill at 80 without slowing down much, if at all. The Firebird will notice the difference more, but due to it's extra power the extra revs that are available are easier to get. So the Firebird will also go up the hill at 80, it'll just need a bit more gas than the Audi would.

If you give it full beans in both up the same hill from say 80 to 100, the Firebird should get there quicker.

That's one of the reasons I love the Sierra. It's got the power /and/ the torque to mean hills are pretty much irrelevant.

Reply to
Pete M

All heavily simplified, but this is how it works :

Power is the *rate* of torque - ie, it's a measure of the amount of torque, multiplied by it's frequency.

Take a car that makes peak torque of 100Nm at 2000rpm. Heavily simplified, it's peak power is 100 x 2000, which equals 200,000 "units".

Now, lets get a car that only makes half as much torque - 50Nm. But, it makes it at 8000rpm. Heavily simplified, it's peak power is 50 x 8000, which equals 400,000 "units".

So even though it only makes half as much torque, it makes double the power - because it's making it's half-as-much torque, four times as often.

Power is what gets you places, and is the all important figure. It doesn't matter how much torque you have - if it's not getting made very often (rpm) then the peak power figure isn't gonna be very high.

Because diesel engines only rev to 5000rpm, they need *huge* torque outputs to match the power of a petrol engine. If you're only making the torque half as often, then there needs to be twice as much of it, in order to make the same power !

Yes, as long as you have traction, and you hover the revs around the peak power figure (6000rpm or whatever).

It doesn't !

It's 60bhp down on the Firebird. As long as you keep both making peak power (6000rpm and 4000rpm or whatever) then the Firebird will be lots quicker !

Reply to
Nom

Agreed.

Torque makes an engine nice to drive (ie, a diesel) - but it's power that gets you places quickly.

Reply to
Nom

Yes, providing you're in the right gear (ie, keeping the revs somewhere around peak power in both cars).

Here's a different comparison :

Take a 2.0 Turbo car, making 200bhp and 280Nm as standard.

Now take a Civic Type-R of similar weight. It also makes 200bhp, but it only makes 200Nm (can't remember the exact figure).

Now even though the Civic is way down on torque, it's no slower at getting places (because it sits at 8000rpm, so the lower torque is made more often than in the Turbo car) - as long as you keep it around 8000rpm all the time, and continually change gear.

The Turbo has lots more torque - but it's no quicker (neither top speed nor acceleration) - because both cars make the same power.

Power-to-weight gives acceleration. Power gives top speed.

Torque is merely the means to produce the power - the more torque you make, and the more often you make it (either one, the other, or both) - then the more power you make.

Reply to
Nom

Power is a combination of torque and engine revs. If you put the two engines into the same car, the audi engine will likely beat the firebird off the line as it has more torque (turning potential). The firebird will eventually overtake the audi as it has higher power and hence higher top speed. This power is only achieved though at high engine revs. You must have seen boy racer cars where the guy has to rev the engine to 2500rpm just to get the car moving because he's put a wild cam in and the car has higher overall power but no torque at low revs. real fun in traffic jams.

Reply to
jeremy

get high so you'll

hand has more bhp but

it really shifting.

Yep, true, but also take into account that in the Audi it's quite gutless off-boost, so you don't get as progressive power delivery as a petrol below

2000rpm. But there's a solution to that - don't drive it off-boost. It's got quite a short first gear which gets you up to the boost quickly, then just keep it on boost! And the 5-pot 2.5TDI will happily rev up to its redline, but obviously it's a lot lower than in most petrols (around 4500rpm)

that the petrol will get up

will climb up another 30mph

or 5th. well going by my

make it a whole different

Oh, and you will get a reply to your email - often I leave replies to emails until I can actually give a vaguely meaningful reply, whereas I talk any old crap on here :-)

Peter

-- "The truth is working in television is not very glamorous at all. I just go home on my own at night and sit alone and eat crisps."

Reply to
AstraVanMan

Towing a horse float. On a more serious note why not drive it and see if you like it. There is so many factors like gearing and aerodynamics that it is really the only way to see if you like a car.

Fraser

Reply to
Fraser Johnston

Another thought: The firebird is going to be RWD, and the Audi FWD, unless it's a Quattro. RWD will always have the jump at a standing start.

Reply to
Doki

What the f*ck are you on about? If you want to get something off the line fast, you rev the nuts off it and slip the clutch, or in the case of the Firebird, hold it on the brakes, revving against the torque convertor. You can make up for the fact that you don't have oodles of low down torque because first gear is generally so short that you're into the powerband very quickly.

Reply to
Doki

If you're a chav.

Reply to
jeremy

I marked for this bit :)

Reply to
dojj

Yep.

The firebird will over take the Audi *very* quickly - if it even falls behind at all ! This is assuming you've made it's start to the best of it's ability - ie, kept the revs high, and kept good traction. The firbird will then proceed to have a higher top speed, and a higher acceleration at just about any speed than the Audi (providing you change gear appropriately).

Yes.

And that is why low-torque engines aren't nice to drive.

But high-torque is no substitute for high-power !

Take a high-power engine. The more torque it has, the nicer it is to drive.

Take a low-power engine. It's shit to drive - doesn't matter how much torque it has, cos it doesn't have enough power to get you places at the required speeds :)

Reply to
Nom

Guess it depends what he means by everyday driving. I drive a v8 van, so pedal to the metal isn't something I can afford to do all that much. I do though appreciate the fact that it pulls away like a train without any screaming. I'd have it anyday over a high power high revving little engine. On the other hand, a caterham with a bike engine on a quiet road is pretty much unbeatable for the price, but wouldn't make the nicest daily commuter.

Reply to
jeremy

LOL!

Reply to
DervMan

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