Not sure that we can. I would agree if you said "feels more responsive" but my BX sounds more powerful at the moment as it has developed an exhaust blow.mat_fenwick wrote: Anyway, a more powerful engine sounds better, we can agree on that can't we?
BX Power to Weight Ratios
Sadly Mat, it's Newton's 2nd Law of Motionmat_fenwick wrote:Oooh, now you've got me thinking - will it? I was told that it was related to the square of the mass, but not so sure now!I don't see why the lighter car would be quicker accelerating, they would both accelerate side-by-side until drag starts to dominate, when the heavier car starts to leave the lighter car behind
"The acceleration a of a body is parallel and directly proportional to the net force F and inversely proportional to the mass m"
i.e. a =F / m (and we are using BHP in BHP / tonne as an indicator of F)
[equation better known as F = m x a ]
I remember seeing an acceleration curve of a Porsche in a magazine a loooong time ago (they'd put an accelerometer in the passenger seat) and from memory it looked something like this (speed is vertical and time is horizontal)
Here is what we both agree on though, which is that the heavier car is quicker once drag comes into effect (on a raceway on the moon, both cars keep accelerting side-by-side for ever until the raceway ends. or the fuel/air supply runs out)
On the earth, the power to overcome drag, is
Drag power P = ½ ? C A v³
where
? is the air density in kg/m³ = 1.293 at STP
C is the drag co-efficient (dimensionless)
A is the cross-sectional area in m²
v is the velocity in m/s
P is in Watts (kg m²/s³)
I just hacked this into a spreadsheet to demonstrate the effect that drag starts to have
EDIT: I just realised, on the moon, the big car still wins, because the wheels on the smaller (lighter) car will start to lose traction with the surface as it starts to lift off due to "centrifugal" force. Eventually the bigger (heavier) car will too, but it will be travelling faster and will be further ahead. F**k knows how they're going to stop
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I'm not (now) disagreeing with you on the other point, if I'd actually thought about it a bit more I'd have come to the same conclusion. Wish I'd remembered Newton's laws when I was told (wrongly) about the square relationship.BX Meteor wrote:Here is what we both agree on though
(I think) I'm right in saying though, that bhp has no direct influence on acceleration. The force available is simply a product of the torque at the wheels and the wheel radius.
Where bhp comes into play is at higher engine speeds, as it's a measure of how fast the engine can produce that torque to provide a force acting against wind resistance.
So a (wheel) torque to weight ratio is probably more representative of a car's acceleration. But then if a car had the same torque as another but lower bhp, it must have a smaller rev range and therefore the 0-60 time would be slower due to more gear changes.
What was the question again?
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I agree, IMO a 2cv sounds really good and my BX14 I had sounded similar to this (a bit), all throaty and growly like. Comparing the 2CV & BX14 to a ZX 1.6 (same engine as BX16 but injection, sounds like sewing machine) very clear winner of noise to the 14/2CV.Way2go wrote:Not sure that we can. I would agree if you said "feels more responsive" but my BX sounds more powerful at the moment as it has developed an exhaust blow.mat_fenwick wrote: Anyway, a more powerful engine sounds better, we can agree on that can't we?
Similarly, my 1.4 astra which is carburetted, sounds nicer than the whole mk2 engine range - with 2 exceptions, the 2.0 16V 'redtop' and the big block 1.6 with the varajet carb (16SH) as found in mk2 cav's.
Doesn't the crank weight/shape etc affect the sound? (and obvious things like induction and exhaust etc). I remember a top gear episode with a tuned P38 V8 (by brooklands or someone like that) with a different crankshaft and bigger bore to usual was raced against a standard P38 of the same engine, the sound difference was incredible! Such a difference (in a good way).
I suppose you could say a more powerful engine sounds better if you were comparing V8's to straight 4's, but that's not very fair, a V8 is ALWAYS going to sound better! Quite like the straight 6 sound too mind.
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Mes voitures:
1985 Citroen BX19 GT
1988 Volvo 740 2.0 GL Hearse
2006 Lexus RX 400h SE hybrid
errrr .... hang on a minute , you've read what I said earlier and then made it out as if you're saying itmat_fenwick wrote:I'm not (now) disagreeing with you on the other point, if I'd actually thought about it a bit more I'd have come to the same conclusion. Wish I'd remembered Newton's laws when I was told (wrongly) about the square relationship.BX Meteor wrote:Here is what we both agree on though
(I think) I'm right in saying though, that bhp has no direct influence on acceleration. The force available is simply a product of the torque at the wheels and the wheel radius.
Where bhp comes into play is at higher engine speeds, as it's a measure of how fast the engine can produce that torque to provide a force acting against wind resistance.
So a (wheel) torque to weight ratio is probably more representative of a car's acceleration. But then if a car had the same torque as another but lower bhp, it must have a smaller rev range and therefore the 0-60 time would be slower due to more gear changes.
What was the question again?
And next post by magic (I've been slaving away in a spreadsheet for a couple of hours)I wrote:Hold on, that's torque from the engine, it depends on gearing and wheel size to give torque at the tyre surface on the tarmac (wheel radius).
Force = Mass x Acceleration, and the Force is that torque at the wheel radius.
A superior BHP/tonne figure is an indication that the car's gearing will give more "longevity" in lower gears, over an inferior BHP/tonne i.e. in the car with the inferior BHP/tonne, one will be changing gear sooner, and that is the key.
I don't agree with Mat's analysis saying that where BHP/tonne is equal, the lighter car will be quicker, it depends purely on the torque curves and gearing. If the engine torque curves are the same, I would expect the gearing to give equal torque at the wheel radius, so neither will be quicker.
However, as the cars go past 60mph, then wind resistance starts to dominate, and the car with the higher BHP, assuming its drag (drag-ratio x cross-section) is not much bigger than the car with lower BHP, will start to dominate, because it is BHP to drag which determines top speed, and if you have more BHP than a car with identical BHP/tonnne but lighter, there is more power still available to overcome the drag, and if the drag is not significantly more on the heavier car, it wins.
That is what I find in my Honda Accord, 190 BHP, there is a dual carriageway near me, and smaller cars that are obviously trying to outrun me, lose out once we get past zzz mph (Plod might read this, at least one of the z's is a zero eg Plod 055).
Here is how the gearing affects the acceleration.Philip Chidlow wrote:I guess the question is, how do you compare a BX with a 'modern' car like the Mondeo in terms of potential. Factors like economy/efficiency and aerodynamics/gearing are critical, so maybe there could be a formula taking a number of factors into account to give a 'score'? I bet something like this exists already.
All the figures in yellow are from the Book of Lies
From the max torques I've worked out the BHP at that rpm.
From the max BHP I've worked out the Torque at that rpm.
I used the Max Engine Torques in Nm along with the gear ratios to work out the Max Wheel Torqques. I've then used the wheel radius to work out the Force in N. From that I've given the peak acceleration.
Note however that the Book of Lies seems to have the wrong max Torques for the 1.9 engine.
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They have,BX Meteor wrote:
Note however that the Book of Lies seems to have the wrong max Torques for the 1.9 engine.
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Oh yeah, so I have I didn't read much past the point at which you pointed out I was wrong. I find that putting my own understanding into words makes it clearer for me, so no, I wasn't trying to steal your glory!BX Meteor wrote:errrr .... hang on a minute , you've read what I said earlier and then made it out as if you're saying it
As for more powerful engines sounding better, I'll retract that (tongue in cheek) comment too - compare an 85bhp motorbike engine with a BX diesel!
Last edited by mat_fenwick on Thu Feb 23, 2012 6:01 pm, edited 1 time in total.
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And there was me about to say I only understand about 50% of this thread (mainly the conjunctions and prepositions). While nosing around for translation, I found this, which kinda seems logical, can't vouch for it's correctitude though!Grenman wrote:Can you explain all this in simple English so that dummies like me can understand please?
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1991 TZD hatch (Triton Green)
1992 Hurricane
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The point - if there ever was was one - that extrapolated from my original post was to see if there was a benchmark by which all cars could be judged with regards to their power to weight. The BX is a light car so initially I thought that a 2010 BMW - even though it had a more powerful engine might prove that newer cars are sacrificing efficiency (being forced to use bigger, more powerful engines) because of their ever-increasing bloatiness.
But the above will only mean anything if we can, a) compare the results to the BX's contemporaries and b) to modern 'equivalents.
Maybe
Either way it's a mental exercise - and who ever said there had to be a point?
But the above will only mean anything if we can, a) compare the results to the BX's contemporaries and b) to modern 'equivalents.
Maybe
Either way it's a mental exercise - and who ever said there had to be a point?
• 1992 Citroen BX TZD Turbo Hurricane
• 2006 Xsara Picasso 1.6 16v
• 2006 Xsara Picasso 1.6 16v