bhp/torque etc - what is reliable indicator of power?

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Caveat

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Firstly, apologies if this is a stupid question - until about 6 months ago I had zero interest in cars and am still coming to grips with some technical terms.

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?

If so, could anyone give me an idea of what a good torque figure would be for a small hatch - when I say 'good' I mean confident enough to easily overtake, reasonable acceleration from starting and feeling generally lively but not a racy performance type power.

can anyone advise?
 
Generally speaking you feel the power of torque more than you would feel bhp..

Turbo Diesel cars always have massive torque figures and are nearly identical to bhp figures. I have feck all torque in my car but its still an extremely fast car for the size of the engine as it produces massive amounts of bhp per cubic litre (100bhp per litre)

Where i miss torque is when going up hills. The car feels sluggish. But overtaking is no problem at all (as long as its flatish or dowhill :) )
 
Bhp sells cars, Torque wins races (and makes for a far more relaxing drive).

My car has pretty good bhp but lacks low-down torque, with the result that I need to change gear a fair bit. That's fine if I'm in the mood for a 'burn' but on a long journey it can get tiring.

Torque is what makes a car pull from low revs in a high gear. If you're in 5th gear doing 30kph, and you floor it, you'll be waiting till tuesday week to reach 100kph unless you have lots of torque. It saves you having to change down to get power to overtake.

If you're looking for a small hatch with good torque, look at the Skoda Fabia vRS...
http://www.pistonheads.com/doc.asp?c=145&i=8014
 
Caveat said:
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?
Click to expand...

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.
 
Thanks all

Am a bit clearer now on this - I suppose what this all illustrates is the importance of the test drive! Which would be a bit subjective anyway: something that feels 'just right' to me may have all sorts of niggles for more informed drivers and true enthusiasts.

Could I ask an opinion on the following:

(Civic '96 - '00 model)

1.4i : 90 bhp @ 6300 120 nm @ 4600
1.5lsi : 114 bhp @ 6500 134 nm @ 5500

...am interested in the latter. Have driven a 1.4i & found it fine for my needs in terms of power.
My question is, would I notice much of a difference with the 1.5?

Should maybe mention that the 1.5 is 80Kg heavier.
 
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I'd agree there's unlikely to be much difference - but only one way to find out: go for a (lengthy) test drive.
 
Trying driving a petrol and diesel of the same model of car back to back.

That will explain torque.
 
Power to weight ratio is significant. Explains why a (relatively) light car like a Caterham can fly powered by a 1960's engine (Ford Crossflow 1600).

Torque is also important. I've driven a Honda Civic Type R and it was tremendous when pushed very hard, 5,000+rpm - a real racing car - but something like a Focus ST (turbocharged and with VVT) pulls miles harder in any gear at more ordinary engine speeds. Hit 2500rpm and it's away.

Type R would be my choice on a racetrack, but for everyday the Ford please.
 
Thanks again to all

Slightly off topic, but I'm still thinking of going for the 1.5 civic, regardless of negligible power difference, simply because it's a Vtec engine. Hope this is wise. There doesn't tend to be much of a price difference.

Vtecs, I think, were developed by Honda to be extra efficient both in delivery of power & economy. In addition, I believe they are just all round better engines in comparison with the Civic non-Vtec?

Am I correct?
 
Just remember, dont put the foot down in 3rd or 4th while moving slowly and expect much to happen....vtec's are great, but are at their best from 3000rpm upwards to the red, take care of it and give them some revs to keep it happy :)
 
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