As I understand it, a bhp figure is only meaningful if you have some idea of the weight of the car. e.g a small hatch with 80 bhp could feel more perky than a much heavier car, of equivalent size with say 100bhp. That is, the figure is purely a measurement of power output from the engine alone.
If I am right, is there a more meaningful figure that would give an overall idea of a car's power, i.e., adjusted for things like weight and maybe other factors? E.g. is torque a more reliable indicator?
can anyone advise?
You're completely correct in stating that the power is only meaningful in the context of other factors, most notably weight.
bhp is a measure of power (rate of doing work). Torque is a measure of rotational force. Without getting too technical, both give some indication of the performance of an engine. Neither takes into account the other factors that would determine things like maximum speed, acceleration.
bhp typically increases with engine speed: you can therefore have a very powerful engine that unless you rev it will do very little for you. Racing cars will typically be tuned to have very high bhp, as they spend their time at maximum revs (a bit noisy and not so fuel efficient for road cars).
Torque also varies with engine speed, but to a much lesser extent. An engine with high torque will pull well from low revs (e.g. putting your foot down without changing gear, pulling well up a hill etc.), and tends to give a smoother drive.
All things being equal, engines with larger capacities tend to have higher torque: almost all sub-2 litre (petrol) engines have low torque. Diesel engines also have higher torque, although the effect in most is due to turbocharging, which gives the highest boost to torque possible. The problem is that diesels don't like to rev., so will tend to run out of "puff" once the revs go beyond a certain point. This is why many diesels have 6 speed gearboxes, to try and keep them in the right rev range.
This is starting to get a bit obscure, so to try and summarise:
- both torque and power measure different aspects of an engine's performance
- they each have different characteristics: "boy racers" like high bhp, everyone else likes high torque (even if they don't realise it)
- neither gives a complete picture to expected performance
Personally, the figure I'd tend to use for overall real-world performance (the usual top speed and 0-60 acceleration figuers quoted aren't that meaningful) would be the time taken to accelerate from say 30 to 50mph, both in-gear and with gear changing. You can usually get this, or something like it, from the various car mags.