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Discussion Starter #1
From Miles to Empty thread

Looking at the 0-60 times for petrol & diesel, they look similar. But break them down, petrol is quicker 0-30 but diesel is better above 30 - as expected.

It offers an easy technique. The diesel has an early change in first so the petrol driver sees an early advantage. And again, they see the diesel falling back going from second to third (no need to take it to high revs for this shift). Just hold station behind for a moment. At that point the petrol driver thinks "I've won" and changes up. That's the moment to step on the power.
Sorry but thats utter tosh!

0-62 drag race petrol wins according to Kias figures and the same for top speed (In like for like 89hp cars)
The 0-60 times might be the same but the smaller increments are not. The diesels are slower 0-30 but quicker 30-60.

Surely any race you keep it pinned until you cross the line? Why would you want to "Step on the power"? surely youre foot to the floor all the way or you wont be geting your best performance will you? Youre assuming a surfiet of power where you can chose the point at which you pass, not cars with identical power but different power characteristics.
TD and petrol of similar maximum power have differently shaped power curves and different shift points. The diesel's low down disadvantage is due to needing much earlier gearchanges in 1st & 2nd. It then gets a convincing "surfiet" through 3rd, 4th, 5th. I am not trying to describe getting "best performance" (I don't even raise revs before lifting the clutch) but playing a trick to get the other driver to change into 3rd too soon.

To put the characteristic into perspective and comparing both high power Ceeds, then at 40mph, the D has about 100bhp available - in either 2nd or 3rd. The best the P can offer is 90bhp in 2nd only; in 3rd, it drops to just 60bhp. Only by holding on to the red line in 2nd does the P get a second wind between about 55-60 where 120bhp momentarily trumps 113.
 

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Don't get hung up on bhp (horsepower), its a calculated figure for which the formula was derived when steam engines were invented so that people could visualise how powerful they were, everybody know what a horses looked like. You cannot measure bhp, what you measure on a dyno is what an engine produces i.e. torque, you divide the torque figure by 5252 and multiply it by the rpm the torque is achieved at, e.g. in a Ceed CRDi you have 186 lbs ft at 2000 rpm therefore at 2000 rpm you have 71 bhp. The formula works for all fuel types.

Because of the way the different (petrol v diesel) produce their power you will always find that in a 1-30 or 60 sprint in equally powered petrol and diesel cars the petrol will mostly be quicker mainly because of its far wider rev band and ability to hold gears a bit longer. But in the real world where most acceleration takes place above say 20 or 30 in one gear the diesel is way quicker mainly because of the turbo.

Now to throw a spanner in the works, before I bought the Ceed I drove a Skoda Octavia with the 1.4 122 bhp turbo petrol engine. 0 - 60 like a petrol, 30 - 60 like a diesel, the best of both worlds. Obviously it also has the fuel economy of a petrol which was one of the reasons I did not buy it but what a superb engine.

In summary, its not the fuel that makes the real difference, its the turbo.
 

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From Miles to Empty thread





The 0-60 times might be the same but the smaller increments are not. The diesels are slower 0-30 but quicker 30-60.



TD and petrol of similar maximum power have differently shaped power curves and different shift points. The diesel's low down disadvantage is due to needing much earlier gearchanges in 1st & 2nd. It then gets a convincing "surfiet" through 3rd, 4th, 5th. I am not trying to describe getting "best performance" (I don't even raise revs before lifting the clutch) but playing a trick to get the other driver to change into 3rd too soon.

To put the characteristic into perspective and comparing both high power Ceeds, then at 40mph, the D has about 100bhp available - in either 2nd or 3rd. The best the P can offer is 90bhp in 2nd only; in 3rd, it drops to just 60bhp. Only by holding on to the red line in 2nd does the P get a second wind between about 55-60 where 120bhp momentarily trumps 113.

Enjoyed reading you’re posting ace Demon, it explained to me what I had wondered about for quite a while. Remembering my first diesel, Peugeot 450 and its horrible refusal to accelerate in the low gears then my next, an Octavia 2ltr diesel complete with Turbo that drove much more like a petrol engine in the lower gears was a revelation to me. OK, granted they still are a wee bit slower low down the box, but the extra torque in the low gears towing a caravan or trailer more than makes up for it. Diesels are a bit noisy, but as I grow deafer I can live with it. Diesel oil is dreadful stuff when spilt and I get annoyed to find it beneath my feet at a pump. I get annoyed at the clown that spilt it though. Would I go back to petrol now, it’s a big No! But I have to admit the sound of a sporty petrol engine still thrills. To each his own, I suppose.
 

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Many modern turbo diesel cars have their torque limited in first and sometimes second gears thus all the power you have purchased is not available until you hit 3rd resulting in slightly slower 0 to whatever times, . Manufacturers do this for a good reason, the gearboxes will not withstand the full available torque from a standing start. Many of the re-chips and tuning boxes remove this "safety" feature but you use the extra get up and go at your own expense.
 

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Many modern turbo diesel cars have their torque limited in first and sometimes second gears thus all the power you have purchased is not available until you hit 3rd resulting in slightly slower 0 to whatever times, . Manufacturers do this for a good reason, the gearboxes will not withstand the full available torque from a standing start. Many of the re-chips and tuning boxes remove this "safety" feature but you use the extra get up and go at your own expense.
That suits me fine, I spend much more time driving at cruising speed than doing full power standing starts. In fact I don't think I have done a full power standing start since I was a teenager. You are certainly not going to win many traffic light GPs in a Ceed anyway.
 

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You are certainly not going to win many traffic light GPs in a Ceed anyway.
That all depends on how you choose your opponent! :p Pick a comprable car and it can be fun. Pick a Imprezza Turbo and you know it will end badly!

Ive had some great smiles with 17year olds in their Saxo etc with a noisy exhaust and an even noisier stereo. They dont like seeing a Kia win! :D

I have similar fun on my 26year old bike that may or may not have approx £3000 of engine work! Newer bikes and "Hot" car drivers seem quite perplexed when a bike may be older than them doesnt get left behind and can usually leave then for dust! You gotta love a sleeper!
0-60 in less than 3 seconds and 0-100 in about 6 seconds isnt to be sniffed at even with todays technology and i dont have any electronics telling what horsepower im allowed to have, just a throttle cable and 4x 40mm carburettors. And if i choose too much power its only me to blame when it bites me!
 

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Discussion Starter #8
Many modern turbo diesel cars have their torque limited in first and sometimes second gears thus all the power you have purchased is not available until
The thing that ultimately prevents the engine from producing full torque will be wheel spin. It is inherently self limiting.

A front wheel drive car can only accelerate at about 0.6g due to wheel spin. If it weighs 1500kg, that's just short of 9000N at the road. Tyres, about 0.3m circumference, gives 2700Nm at the driveshafts so with a 13:1 gear ratio, we end up with 200Nm at the flywheel. And that's in the dry.

There's absolutely nothing the tuners can do about it.
 

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Discussion Starter #9 (Edited)
Don't get hung up on bhp (horsepower),
It would be better if people didn't get hung up on torque because it is meaningless without context.

When I gave power outputs at 40mph in various gears between the two engines, whatever you think a horsepower is, it is a consistent measure. It's the only meaningful way to compare.

In summary, its not the fuel that makes the real difference, its the turbo.
The fuel does make a difference; it determines the options available to the engine designers. Anyway, this is not relevant. An engine is a thing that converts fuel to power, heat and noise; it's the numbers that matter.
 

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Discussion Starter #10
Enjoyed reading you’re posting ace Demon,
Much appreciated

Would I go back to petrol now, it’s a big No! But I have to admit the sound of a sporty petrol engine still thrills. To each his own, I suppose.
When I last changed I would have preferred to have stayed with petrol. The trouble is, the market the Ceed is in is short of decent engines. When I last changed car I was looking for something like the Citroen BX I had many years ago. This was a 1000kg car with a 90bhp 1.6 engine.

Now there is nothing under 1300kg. OK, so manufacturers give them 120bhp engines; 0-60 is the same, top speed has gone up. Job done, supposedly. Except they're still 1.6 which means that below 4000rpm it is no better than the old BX and this is where the weight really bites. The sweetspot of a small engine in a heavy body is frustratingly narrow. (And I am not that keen on the noise.)

To make up the difference I want a lazy 2 litre. A 1.4 turbo should work decently too. But these are a bit thin on the ground so getting a TD was just the best of a bad job, really. I didn't go diesel for the possibility of long term money saving.
 

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It would be better if people didn't get hung up on torque because it is meaningless without context.
The context is simple, the rev band where we normally drive should contain the area where maximum torque is produced thus making the car an easier and nicer drive. Max power can be totally meaningless, ever driven a Honda Civic Type R, max torgue at 6000rpm max bhp at 8000 rpm, it required some concentration to say the least to keep it between those figures for rapid overtaking, easy peasy in a turbo diesel (or turbo petrol).

The fuel does make a difference; it determines the options available to the engine designers. Anyway, this is not relevant. An engine is a thing that converts fuel to power, heat and noise; it's the numbers that matter.
The fuel does make a huge difference, a good turbo diesel has a spead of power from say 1500rpm to 4500 rpm before it tails off, a good turbo petrol has a spread of power form 1500 rpm to at least 6000 rpm.

For the time being I will stick with a turbo diesel but every turbo petrol I drive produces a compelling reason to change except for the economy.
 

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It would be better if people didn't get hung up on torque
Thats a crazy thing to write, torque is all important, John Franklyn said that earlier.

Some years ago we had a Golf TDi, 90 bhp and 148 lbs ft of torque. Also had a Polo diesel at the same time, no turbo, 65 bhp and 100 lbs ft of torque. Overtaking in the Golf was a simple, press pedal and go but in the Polo you needed a fortune tellers advice before attempting such a manouvre.

Unless you revved the nuts off them (difficult in both due to the noise created) you did not approach max bhp, it was the extra torque in the Golf that gave the the performance.

There was another side to the coin however, the Golf was bloody awful in town, the turbo came in less progressively than they do today and it had pretty high gearing, the Polo had linear power delivery and sensible gearing making urban driving almost a pleasure.
 

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I don't give a flying f**k about the performance of my Ceed CRDI 1.6.
Never took it above 100 and never floored it on acceleration either.
But it does give me good economy.
If i want performance i get out my Triumph 955i and it would show a clean pair of heels to any Kia. Period.
:eek::eek::D:D
 

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Discussion Starter #15
The context is simple, the rev band where we normally drive should contain the area where maximum torque is produced thus making the car an easier and nicer drive.
Those are just words. Torque is expressed as a number. So meaning can be expressed numerically.

Can you, for instance, derive the connection between torque & power from first principles, or is the formula just something you read?

Max power can be totally meaningless,
Agreed. I look for power that is immediately available in typical conditions.

Knowing the max power, revs at max power, max torque, the gearing plus a few assumptions, allows estimates to be made. All four pieces must be known. When all are known it is possible to work out (in numerical terms) how different configurations will behave. (Thus, I know the TD Ceed has more power at 40mph than the P Ceed.)

When only one piece is known, only max power is useful; that is why it is commonly quoted. If the only known piece is max torque, it means absolutely nothing.

In summary, its not the fuel that makes the real difference, its the turbo.
The fuel does make a huge difference,
It seems you are having difficulty deciding.
 

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Can you, for instance, derive the connection between torque & power from first principles, or is the formula just something you read?
Its a formula we were given in my early days at college and is available in any decent text book.


It seems you are having difficulty deciding.
Read both my posts and it will be obvious I have no difficulty deciding, the only problem I have is people picking words out of posts without quoting the whole line.
 

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Its a formula we were given in my early days at college and is available in any decent text book.
I'm not questioning the formula. I was wondering whether you know how to derive it from first principles.

The point is, I provided some facts about power in my original post which you took as a cue for a physics lesson. Either you understand the subject and are attempting to talk to a non-technical audience or you are just repeating stuff you have picked up. I can't decide which.

Read both my posts and it will be obvious I have no difficulty deciding, the only problem I have is people picking words out of posts without quoting the whole line.
I had already replied to the "fuel makes no difference" one (ie, I disagreed with you) before you said that "fuel makes a difference". The example behaviour you gave for the TD need not be exclusive to a TD, incidentally.
 

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I'm not questioning the formula. I was wondering whether you know how to derive it from first principles.

The point is, I provided some facts about power in my original post which you took as a cue for a physics lesson. Either you understand the subject and are attempting to talk to a non-technical audience or you are just repeating stuff you have picked up. I can't decide which.
Don't really feel the need to justify my credentials but if its helps whilst training as a mining engineer we obviously studied motors which for underground use were 99.9% electric with the occasional diesel loco. The formulas for calculating motor sizes required etc were obviously part of the course as was the conversion of torque to power. Back in my day we talked about the horse power of motors and lbs ft of toque, no doubt today its kilowatts of power and newton meters of torque.

All wasted, work in civils now and never need to use any of the stuff I learned.

Don't really understand your last comment.
 

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Discussion Starter #19
Don't really feel the need to justify my credentials but if its helps whilst training as a mining engineer we obviously studied motors which for underground use were 99.9% electric with the occasional diesel loco. The formulas for calculating motor sizes required etc were obviously part of the course as was the conversion of torque to power. Back in my day we talked about the horse power of motors and lbs ft of toque, no doubt today its kilowatts of power and newton meters of torque.
Well, perhaps you have revealed more about yourself than you might have done by saying "I can/can't derive it from first principles".

All wasted, work in civils now and never need to use any of the stuff I learned.
As do most people when they move from learning to work.

Either you understand the subject and are attempting to talk to a non-technical audience or you are just repeating stuff you have picked up. I can't decide which.
Don't really understand your last comment.
Technically competent people often make a mess explaining things when they try to dumb down for the masses.

....

For the record, if we stick with Imperial, the frig factor of 1/5252 is 1/60*2*pi/550 ie, when rpm is expressed as angular velocity and the 550 is the standard definition of the number of ft-lbf/sec in a horsepower (whether or not it is the power of an average horse).

I don't understand why you consider horsepower is something "not to get hung up on". It is tangible and the "per second" bit is absolutely essential (other than at standstill).
 
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