BX Power to Weight Ratios

Philip Chidlow · Post by **Philip Chidlow** » Wed Feb 22, 2012 11:09 pm

I was sorting (throwing out) some notes...

And found this I'd worked out during some idle moment:

(Figures are BHP/Tonne)

19TGD: 65.05
14TGE: 72.58
17TZD turbo: 79.65
16TGS: 89.78
19GTi: 110.61
16V: 135.65

Don't know how those figures compare to a modern equivalent?

BX Meteor · Post by **BX Meteor** » Wed Feb 22, 2012 11:11 pm

bears out what I always thought, which is that the BX 1.6 petrol is quicker than any of the BX weasels

Mickey taker · Post by **Mickey taker** » Thu Feb 23, 2012 12:02 am

$\:D/$

we need a like button

Mothman · Post by **Mothman** » Thu Feb 23, 2012 6:23 am

My Mazda comes in at 140, mind you she is 11 years old now.

Andy

Post by **Tim Leech** » Thu Feb 23, 2012 8:03 am

Philip Chidlow wrote:I was sorting (throwing out) some notes...

And found this I'd worked out during some idle moment:

(Figures are BHP/Tonne)

19TGD: 65.05
14TGE: 72.58
17TZD turbo: 79.65
16TGS: 89.78
19GTi: 110.61
16V: 135.65

Don't know how those figures compare to a modern equivalent?

That 16v is probably a cat equipped one?, the non catalyst model had 160bhp and weighted just over a tonne.

Philip Chidlow · Post by **Philip Chidlow** » Thu Feb 23, 2012 9:28 am

I havent checked the figures as they weren't supported by the calculations. I have looked at them again...

I got it wrong - a bit. The 16v must've been a cat-quipped one, yes.

Recalculated using bhp/1000kg (tonne) (I think I was using imperial before?):

17TGD: 60.6 bhp/tonne,
19TGD: 71.7 bhp/tonne,
14TGE: 79.99 bhp/tonne,
16TGS: 98.94 bhp/tonne,
19GTi 8v: 121.95 bhp/tonne,
16v non cat: 149.53 bhp/tonne,
16v cat: 138.3 bhp/tonne.

So I was a bit off. However, the above still show the range - and how well spread out it was.

Regarding the 16v it had better power to weight than a Renault 5 GT Turbo.

A 2009 Ford Mondeo 2.0 TDCi is about 89 bhp/tonne.

Post by **Tim Leech** » Thu Feb 23, 2012 9:35 am

Makes for some interesting reading! The GT which is 105bhp and weights about 950kgs is still a quick car even at 27, on the long bypass near my house which isn incline I opened the taps on the way home from the MOT and it flew up the hill passing about 5 cars on the way and was still pulling hard at 80!

.

Philip Chidlow · Post by **Philip Chidlow** » Thu Feb 23, 2012 10:56 am

That would make the GT's ratio about 110.5 bhp/tonne then. Which is certainly up there with the likes of a Mini Cooper, Smart Roadster Brabus and Audi A4 1.8T Sport... to name but a few (100-120 bhp/tonne seems to be a 'popular' power class).

To put it all into perspective though, a Citroen Saxo VTS has an impressive 130 bhp/tonne and a Porsche Carrera GT 441 bhp/tonne...

Post by **mat_fenwick** » Thu Feb 23, 2012 10:59 am

But if bhp was the only consideration, a 1.6 BX would be quicker than a Mondeo TDCi! It would be interesting to compare torque to weight ratios, which I suspect would be more meaningful in the real world.

EDIT - another point to think about is that it's not a linear relationship i.e. a 150bhp 1000kg car on paper is the same as a 300bhp 2000kg car. However in real life the lighter car will be quicker accelerating but (all else being equal) a lower top speed as aerodynamics are more of a factor than weight at high speeds. Not trying to criticise, it is interesting!

Philip Chidlow · Post by **Philip Chidlow** » Thu Feb 23, 2012 11:14 am

I'm not suggesting the power to weight ratio is anything other than another indication of the advantages of the BX's low weight. Interesting though, about torque to weight ratios:

19TGD 90.9lb/ft (124nm) per tonne

16v non cat 124 ft/lb (170nm) per tonne

17TZD Turbo 130 ft/lb (178nm) per tonne

... and the humble Mondeo 2.0 TDCi?

236 ft/lb (321nm) per tonne

The BX is thoroughly beaten on that score, then.

BX Meteor · Post by **BX Meteor** » Thu Feb 23, 2012 11:58 am

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).

Post by **mat_fenwick** » Thu Feb 23, 2012 1:18 pm

BX Meteor wrote:I don't agree with Mat's analysis saying that where BHP/tonne is equal, the lighter car will be quicker

I did point out that's with all else being equal (i.e. gearing/drag), so I think we are in agreement! Otherwise, the BX TD would be very similar to the 16v in performance. It is in terms of engine torque/weight, but the gearing on the TD is much higher, something like 28mph/1000rpm in top versus ~20mph for the 16v.

BX Meteor · Post by **BX Meteor** » Thu Feb 23, 2012 1:24 pm

/\ Well you wrote "a 150bhp 1000kg car on paper is the same as a 300bhp 2000kg car. However in real life the lighter car will be quicker accelerating but (all else being equal) a lower top speed as aerodynamics are more of a factor than weight at high speeds", and 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

Philip Chidlow · Post by **Philip Chidlow** » Thu Feb 23, 2012 1:45 pm

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.

Post by **mat_fenwick** » Thu Feb 23, 2012 1:48 pm

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!

The lighter car will have less inertia, but correspondingly less force (assuming same gearing, and that the torque discrepancy is the same as the power discrepancy) to overcome that inertia.

Intuitively the lighter car feels like it should be faster, and it's certainly felt like that with cars we've had. But that's hardly a controlled scientific environment. Hmmm...

Anyway, a more powerful engine sounds better, we can agree on that can't we?

BX Club Forum

BX Power to Weight Ratios

BX Power to Weight Ratios

Re: BX Power to Weight Ratios