BX Power to Weight Ratios

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Way2go
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Post by Way2go » Thu Feb 23, 2012 1:56 pm

mat_fenwick wrote: Anyway, a more powerful engine sounds better, we can agree on that can't we?
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. :wink:
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Post by BX Meteor » Thu Feb 23, 2012 2:40 pm

mat_fenwick wrote:
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
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!
Sadly Mat, it's Newton's 2nd Law of Motion
"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)
Image

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 ρ C A v³
where
ρ 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
Image

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 :lol:

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Post by mat_fenwick » Thu Feb 23, 2012 3:31 pm

BX Meteor wrote:Here is what we both agree on though
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. :oops:

(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? :roll: :lol:
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Post by saintjamesy89 » Thu Feb 23, 2012 4:09 pm

Way2go wrote:
mat_fenwick wrote: Anyway, a more powerful engine sounds better, we can agree on that can't we?
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. :wink:
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.

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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Post by BX Meteor » Thu Feb 23, 2012 5:32 pm

mat_fenwick wrote:
BX Meteor wrote:Here is what we both agree on though
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. :oops:

(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? :roll: :lol:
errrr .... hang on a minute !=!! , you've read what I said earlier and then made it out as if you're saying it :shock:
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).
And next post by magic (I've been slaving away in a spreadsheet for a couple of hours)

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Post by BX Meteor » Thu Feb 23, 2012 5:39 pm

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.
Here is how the gearing affects the acceleration.

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.

Image

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Post by Philip Chidlow » Thu Feb 23, 2012 5:46 pm

BX Meteor wrote:
Note however that the Book of Lies seems to have the wrong max Torques for the 1.9 engine.
They have,

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Post by BX Meteor » Thu Feb 23, 2012 5:52 pm

/\ If you've got the gear ratios I'll do a complete table of the whole BX fleet

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Post by mat_fenwick » Thu Feb 23, 2012 5:57 pm

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 :shock:
Oh yeah, so I have :oops: 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! :lol:

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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Post by BX Meteor » Thu Feb 23, 2012 6:01 pm

OK, at least we've both thrashed it to death here ...bonus :lol:

I'd like to know who told you acceleration was a function of the mass squared though, was it a woman :lol:

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Post by mat_fenwick » Thu Feb 23, 2012 6:03 pm

It was a guy at college, one of the ones who always got top marks. Maybe if I'd paid more attention in class I would have been able to point out his error!
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Post by BX Meteor » Thu Feb 23, 2012 6:23 pm

If I get all the gear ratios I can quickly complete the table, then I will be able to construct some approximate acceleration curves for each BX

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Post by Mothman » Thu Feb 23, 2012 7:19 pm

Just a quick question if i may? Can you explain all this in simple English so that dummies like me can understand please?

What point are you trying to prove? Which model BX goes the fastest?
Well thats obvious isnt it?

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Post by Caffiend » Thu Feb 23, 2012 8:21 pm

Grenman wrote:Can you explain all this in simple English so that dummies like me can understand please?
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!
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Post by Philip Chidlow » Thu Feb 23, 2012 8:22 pm

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

:?: :lol:

Either way it's a mental exercise - and who ever said there had to be a point?
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