Car Performance Measurements

Redisle

Think this deserves a thread as it's an interesting topic.

I'm spending a bit of time at the moment looking at various cars on Parkers as I want to get something a bit more interesting.

I'm looking at figures like bhp, weight, mpg and 0-60 times and using these to get a better informed idea of what car I would like to change to. I have to wonder though which measurement or figure of merit has the most applicability to how a car will drive in real life?

Just taking an example..

I had a spin in a 106 Gti before and thought it had a lovely feeling of acceleration when floored in 3rd gear. Proper head into headrest kind of thing. Now according to Parkers this car has a 0-60 time of 8.4 seconds. It has a weight of 950kg and a power output of 120bhp. This gives it a power to weight ratio of 0.126, call that 12.6%.

Now take another car, one I'm watching at the moment on donedeal, an MG ZR 120. Again according to Parkers this car has a 0-60 time of 8.6 seconds. It has a weight of 1145kg and a power output of 115bhp. This gives it a p/w of 10.04%.

I have not driven one but I just wonder, given the almost identical 0-60 time would this car really feel as quick as a 106 gti? Or would it feel slower due to that p/w ratio?

Or is a 0-60 time not really an accurate measure? I am guessing these runs are done by professional drivers in ideal conditions and would be very difficult to match?

Perhaps gear ratio has more to do with it, or Torque, or even aerodynamics?

Be interested to hear what others think, have you maybe driven cars that feel faster/slower than what the specifications would suggest? I know this probably has a lot to do with Torque, as I see this mentioned often in Diesel related threads but it seems to be a difficult thing to get an intuitive grasp on?

keithsfleet

Car layout is different in all cars and a big difference when it comes to handling.
The 106 you mentioned is a cracking car that screams to have it's engine pushed as hard as possible and, as you mentioned, is incredibly light so it feels a whole lot faster than a slightly heavier car.
Centre of gravity is another thing that makes a difference to handling.
In my experience test driving a car is you're best bet to finding out how it feels to cane it. Figures don't really mean much as the layout to pretty much every car is different. And at the end of the day figures are figures and don't really apply (imo) to the real world with day to day.

keithsfleet

Also, aerodynamics wouldn't be something I'd be bothered with unless you talking of driving it at 100mph on a regular basis.

CiniO

Of all the parametres 0-100km/h (0-62MPH) seems to be the most relevant measure for me when comparing how fast the car is.

Obviously smaller car with lower BHP and the same 0-100 measure will be different than bigger heavier car with higher BHP. The first one will usually be accelerating faster at lower speeds (f.e. 0 to 50km/h) while second car will accelerate better at higher speeds.

Chimaera

The problem with power figures, is that the published figures are only the peak power output of the engine i.e. it only tells you about the maximum the engine can achieve. To get a better comparison between two engines, you need the power and torque curves which show how power and torque vary across the rev range.

If you get 2 engines, one producing a maximum of 100 bhp at 2000 rpm, and the other producing a maximum of 100 bhp at 5000 rpm, the one with the power peak at high revs is going to feel much faster all other things being equal.

Another factor is gearing. Shorter gearing will make a car accelerate harder for a given level of engine performance, but at the expense of outright speed.

Aerodynamics play a small part at the speeds most people do on the road, but they play a part all the same. There's a reason manufacturers spend a lot of time in the wind tunnel with new models (mostly for economy reasons but it benefits performance too). Aerodynamic drag increases with the square of your speed. Going from 50 km/h to 100 km/h increases the drag by a factor of 4.