Thanks : that's made it clear. I've made the point in a few posts that I wish between gear acceleration was given in car reviews or provided somewhere where it was simple to find. I need simplicity !

<<... the rate of acceleration is directly proportional to the power output - so a 1,000 kg car with 200 bhp will accelerate twice as fast as a 1,000 kg car with 100 bhp - conveniently expressed as bhp/tonne.

However, maximum power tends to show a peak on a graph so either side of that maximum it's a little lower - and frictional / aerodynamic drag increase as the speed rises. >>

The main factors preventing acceleration are (a) inertia = the deadweight of the car (plus occupants) and (b) rolling resistance = compression of tyre rubber, air resistance etc. Of course heavier cars require more bhp, which usually means a heavier lump under the bonnet, so there is always a trade-off somewhere. So acceleration should be very marginally better with the tank almost empty than full ?

Edited by Andrew-T on 23/04/2017 at 16:23

Power is a force over a distance in unit time. In car terms this is torque (the force) and distance (rpm). So if you half the torque and double the rpm you have the same power to the flywheel. The gearbox is used to multiply torque at the expence of higher rpm.

An engines torque is determined by its MEP ( mean effective pressure ) that is the average force the gas pushes the piston down by and the power is detemined by the number of times it does that in a given time.

Torque is seen as the main factor in acceloration and power the main factor in determining top speed.

Clear as mud or what ?

Power is still the main factor in acceleration - unlike torque it's the same at given rpm regardless of what gear you're in.

Of course automatics of any kind make "in gear" acceleration times redundant.

You say torque is the main factor in acceleration, but that is only true under limited criteria, eg, 50-70mph in top gear. Acceleration from 0-60 doesnt have much to do with torque at all as you are using the whole rev range, so a high reving naturaly aspirated car like an old honda civic type r can keep pace with a turbocharged car with the same power but much more torque. Same goes for acceleration through the gears iF the civic type r driver is using the neccessary gears to keep the engine in its sweet spot. The turbocharged equivalent may still do it slighly quicker due to having to make fewer gearchanges but the time difference will be much smaller than if both cars are just using one high gear.

There is no doubt however, that in the real world, torque is a much more useful commodity than horse power. Back in the early 90's i worked in a saab dealership and i distinctly remember a trip from Inverness to Aberdeen late one afternoon in a new 9000 2.3 turbo. On mainly (at the time) single carriageway roads with heavy traffic, i could immediately exploit the smallest clear straights to effortlessly blast past slower traffic using the huge torque from the saab's engine. Yes there is no doubt that a bmw 525 24v (which came out around the same time) would have been quicker round a race track due to its superior handling, but would have been hopeless on that road trip. They both had similar power, 200bhp for the saab, 192bhp for the bmw. But when you looked at the torque, the saab was in a different league altogether, with 243lb/ft of torque at 2000rpm whereas the bmw had (i think) about 180lb/ft of torque at about 4000rpm.

Another car which hugely impressed me with its torque was the then new VW polo 1.4tdi. This was back in around 2000, and i remeber being amazed that a car with only 75bhp could squirt past slower moving traffic so decisively!. The answer to that was of course the 144lb/ft of torque at 2100 rpm. I worked at a VW dealership at the time and we had one as a demo so i got to drive it quite a lot. Many times i got the better of much more powerful machinery through a combination of being in the right gear to exploit that torque, and knowing the roads like the back of my hand!.

Agreed, Cyd - and the other point is that the peak torque given in literature is just a point on a curve - to work out how fast a car will accelerate you need to see how steep the curve is. This is why big horsepower cars acclerate quickly as well as go fast in straight line - the rate of change of available horsepower is much higher, given that you might have several times the HP available at peak, but generally (for road going engines) at the same 6-8k rpm (I suspect if you differentiate the power curve you get to the torque curve, i.e. torque is rate of change of HP. Never tried or managed to prove that though!). F1 engines make a lot of power partly by turning fast - they would be horrible to drive in a normal car - 3k rpm 'tick over' and screaming everywhere at 10-12k rpm!

I'd be interested to know what some of you lot did for Physics at school.

TORQUE is what accelerates a car. The torque produced by th eengine is multiplied by the gearbox and transmission and converted into a FORCE at the contact patch of the tyre with the road.

The car then obeys Newtons Laws of Dynamics where F=ma.

Stanb had it correct way back up this thread.

Are educational standards dropping?

Torque at the wheels is dependent on the overall gearing as well as the individual gear ratio - and in the case of a torque converter, the torque multiplication factor.

Why is there Power-to-weight as a recognised measure of performance but no Torque-to-weight.

1 bhp = 550 lb ft per second

Edited by RT on 24/04/2017 at 14:27