Horse Power and Acceleration - scot22

Looking to increase my knowledge in what is my equivalent of a motoring university.

Is there any direct relationship between BHP and acceleration between gears ?

Horse Power and Acceleration - RT

Looking to increase my knowledge in what is my equivalent of a motoring university.

Is there any direct relationship between BHP and acceleration between gears ?

Yes - 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.

Traditional times for 0-60 mph and 0-100 kph (0-62 mph) are of course affected by the gear change points.

Edited by RT on 23/04/2017 at 14:22

Horse Power and Acceleration - scot22

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 !

Horse Power and Acceleration - Graham567

What Car magazine quote in gear acceleration times in their triple car tests every month if that's what your looking for.

Horse Power and Acceleration - Andrew-T

<<... 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

Horse Power and Acceleration - Stanb Sevento

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 ?

Edited by Stanb Sevento on 23/04/2017 at 15:33

Horse Power and Acceleration - RT

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.

Horse Power and Acceleration - gordonbennet

Some of this bhp lark can be rather misleading, quoted bhp and torque figures can be impressive but unless the engine and gearbox is responsive, and the torque and power developed quickly and with useable torque rise from low enough revs, all the numbers in the world end up meaningless.

ie some turbo Diesels might have a decent BHP figure and the accompanying torque figure might be equally good, but if it takes a second of two for the engine to spool up and respond and then some quirky overrated modern non torque converter gearbox takes its own sweet time to do it's thing as well, together they can ruin what should on paper be a decent engine, especially if its all over 1500rpm later and change up time, i like lowe revving luggers with effortless low speed torque.

Horse Power and Acceleration - Stanb Sevento

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

Dont know if I agree with that RT. Two indentical car one with 100 units of totque and 100 units of power, the other with 50 units of torque and 200 units of power. Stick them both in the same gear at the same speed and its the high torque one that will accelorate fastest,with the second going on to a higher speed. In practical terms you would need to change down but the original question was about in gear acceloration.

If I stuck my 100ps motorbike engine in my wifes 65ps Panda would it accelorate quicker? Not a hope, its just not got the torque. It may do 120 on the motorway though.

Edited by Stanb Sevento on 23/04/2017 at 17:26

Horse Power and Acceleration - badbusdriver

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!.

Horse Power and Acceleration - galileo

Lots of detailed relevant data in this website www.automobile-catalog.com/

For example, my 2009 1.4 petrol Hyundai i30 has very similar power and torque values to a 1963 1.6 Lotus Cortina. (I had a ride in a new one of these in 1963,well iimpressed by how it went)

However, the Lotus Cortina does 0-60 8.6 seconds vs the i30 11.9. The relative GVWs explain why, 3792 lb i30 and 2612 Lotus.

I suspect aerodynamic improvements explain why the i30 is quoted as 12mph faster.

Horse Power and Acceleration - Stanb Sevento

We are talking about different things badbusdriver Im talking about the basic torque of the engine, at the flywheel before the gearbox and final drive gearing get in the way and you are talking about the torque at the road wheels after the gearing.

Got to agree there is something special about a big torquy engine, effortless grunt anyt time you want it. puts a big smile on your face

Horse Power and Acceleration - scot22

Thanks for all of the posts :even those that left me a little confused - said I need simplicity ! Two suggested sources of information are very helpful ; I'll look in more detail during the week.

Horse Power and Acceleration - focussed

As a footnote -

Torque wins races on Sunday - Horse power sells cars on Monday.

Horse Power and Acceleration - RT

That's why F1 teams focus on horsepower so much.

A high torque engine is more relaxing to drive, particularly a turbo-diesel.

Horse Power and Acceleration - Pauly

Remember that power = torque x rpm, that explains everything ;-)

Horse Power and Acceleration - SLO76
To further muddy the waters, gearing is hugely important here too. Powerful cars can be slow off the mark with overly high gearing which is intended for refinement and economy while a lower geared car will accelerate more quickly.

Thinking of examples I remember when 6 speed gearboxes were in their infancy and you could buy a Mk I Fiat Punto 1.1 with either a 5 or a special edition with a 6sp box. They were both identical in weight, power (55bhp) and torque yet the lower ratio 6sp car had a shorter 0-60 time. Believe the Toyota Corolla 1.3 G6 of similar vintage was the same.
Horse Power and Acceleration - Cyd

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.

Horse Power and Acceleration - RichardW

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!

Horse Power and Acceleration - RT

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!

Modern turbos aren't allowed to reach their peak torque - to protect the transmission it's electronically limit, hence the flat torque "curve" across a range of rpm.

Horse Power and Acceleration - RichardW

>>hence the flat torque "curve" across a range of rpm.

Which given that power is a direct function of torque and (engine) speed, gives a nice linear acceleration, gears not withstanding.

Horse Power and Acceleration - RT

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

Horse Power and Acceleration - Stanb Sevento

For heavens sake, we are getting a bit bogged down in this, talking about different things. If you dont say whether its torque at the flywheel or torque at the back wheels then the argument is meaningless.

Torque is no use without power and power is no use without torque. You need a ballance.

If you took a bog standard car engine and wanted to tune it for racing which would you go for?

If it was a street circuit with lots of slow bends and short straights then you would tune for torque to get more punch out of the corners. If it was a high speed oval track of something you would tune for power to give you a higher top speed. Nearly everything that you can do to an engine that gives a big increase in power will reduce torque or at least push it well up the rev range. The things that increase one tend to reduce the other.

That is the sum total of what I ment by " torque is the main factor that determins acceloration" and if anyone tries to complicate this further by mentioning turbos I will go away and kick the dog.

Horse Power and Acceleration - Cyd

Oh dear oh dear oh dear! Having sung your praises earlier, you've gone and blown it now!

If you dont say whether its torque at the flywheel or torque at the back wheels then the argument is meaningless.

What utter t***. The only difference between the two is the mechanical losses in the transmission system, largely generated by friction between moving parts. If both curves were drawn on a graph they'd be one on top of the other and the same basic shape. The area between them would be the losses. The difference is merely numerical, not fundamental.

Torque is no use without power and power is no use without torque

Hhhmm! OK, then. Electric motors.

Electric motors produce maximum torque at zero speed. As a consequence of the physics, this means they produce maximum torque simultaneously producing zero power.

Actually the last point is why Tesla and Future Faraday machines are so bloomin' fast down the drag strip. Maximum traction (and hence maximum acceleration) instantaneously at zero speed. It really does feel like being shot from a catapult.

Steam locos also produced maximum torque at zero speed - hence all that wheel spinning in old Westerns

Horse Power and Acceleration - 72 dudes

Poor Scott, look what you've started!

Basically, yes there is some correlation between engine power and in gear acceleration times, but all things taken into account, it's the torque figure which is more important and the RPM at which it's delivered.

So a diesel engine with say 75 BHP might be quicker than a petrol engine with 138 BHP in 4th gear in the 30-50 MPH range because it has greater torque lower down the rev range.

A good example of a weak 138 BHP engine is the 2 litre petrol in various cars including the Volvo S40, Ford Mondeo/Focus, and the Peugeot 407. Rice pudding blah, skin etc.

How did you get on with the test drives in the two C-Max?

Edited by 72 dudes on 25/04/2017 at 16:35

Horse Power and Acceleration - Stanb Sevento

LOL I thought you were on my side Cyd. Point by point.

Torque is a force, a rotational force, it can be static, a coil spring in a watch has torque even when its not doing anything. It has no frictional losses I have been talking about the basic torque generated by the engine at the flywheel, a fundamental characteristic of the engine.

Torque only makes power when it moves If say an engine makes 100 units of torque at the flywheel, in a car that may produce 100 units at the back wheel in first gear 70 units in second 50 units in third, So to say you increase an engines torque by changing gear is not true you are changing the torque at the wheels. the engine torque is the same and in fact power could be the same throughout Power is force X Distance X time, torque is only the force bit. You cant have power without torque.but it also needs to move.

On a rolling road, usually done at max throttle, torque climbs at first as the engines volumetric efficiency increases ( it fills the cylinders better ) then eventualy falls because at higher revs there is less time to fill the cylinders so they fire at less than max charge. Power can keep increasing due to the higher revs ( increasing the X distance part ) more than compensate for the reduction in torque. Power to eventuallly falls as the increasing lack of charge in the cylhindes takes it tolle. The printout shows two curves but remember each curve is measured in different units.and scale.

Electric motors have different caracteristics, torque is max from rest and decreases as it gets faster but power is roughly constant. No need to build up revs to get torque. And yes they can have max torque and no power, Its just using a lot of juce to produce a force thats going nowhere.

Its not t*** really. In fact Im not sure what you mean by t***

Edited by Stanb Sevento on 25/04/2017 at 18:08

 

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