Except JBs post, which I think sums it up nicely.

Where are all the mappers - The Gaffer or JGM, can they give a laymans explanation?

 

Such an important point that everyone addressing this subject always misses.
- The acceleration that you feel when driving is due to torque AT THE WHEELS
- Cars have gears so this is different to torque AT THE ENGINE

So for instance
- BMW 320D driver floors it at 30MPH, the engine revving at 1500RPM with 150ft/lb of torque at the engine.
-Honda S2000 driver is alongside floors it at 30mph with engine revving at 3000rpm and 75ft/lb of torque at the engine

They will both accelerate initially at the same rate as the gearing at the Honda is half that of the BMW so although the torque is double for the BMW, the torque AT THE WHEELS is the same.

And guess what... the power output of each engine will be the same as well at that point, you see it is the power that translates into acceleration as this is the rate of work. The torque just tells us HOW this is delivered at the engine, but with gearing we can make it delivered to the tarmac exactly the SAME.


So why do torquey engines feel faster?
It s because they are often delivering more power over a wide range of rev. If you are driving on the road you will be using 2000-5000 revs generally so the average power put down by a torquey engine across thos revs will be high, whereas an all or nothing race engine by be putting b***er all at the bottom end bringing the average down.

On the track you might be using 4500-7000 revs so things like the S2000 come into their own.

That is why we should be wary of the single Highest output bhp on the rollers, it doesn't tell us the average that the car is putting down when driven, that is why some slightly lower bhp cars are faster (even on the drag strip) as their average power they put down is higher.

 

So no one can really explain it then!

I'll stick to my high torque, reasonable bhp dervs.

 

I feel it has been explained well overall, however, the key problem is the sheer number of variables when applied to a vehicle for the purpose of assessing performance.

Variables being:

Peak Torque
Torque Curve (area under)
Peak BHP
Engine speed
Gearing (transmission)
Gearing Other (wheel size)
Weight
Transmission losses
Drag Co
...and so on.

 

Okay then so it can be explained but it's damn complicated.

 

I think of it like this:

Torque - the turning force or 'strength' of the engine i.e. how easy it is for the engine to 'turn over' or make a revolution.

BHP - the speed at which the engine can turn, or the number of revolutions (revs) that can be made in given time period which is usually a minute. This is how you get rpm - revolutions per minute.

So, if you have an engine with high torque and high horsepower ratings it means it will turn over with a lot of 'oomph' and whilst turning over with a lot of oompth it will also be able to do it very quickly.

If you think about it Torque is what will get you moving from stationary - A lot of oomph to turn the engine over (whilst in gear) to move you off quickly, and then the ability of that engine to increasingly turn very quickly will translate to the wheels turning faster, and you breaking the speed limit in 3rd.

Torque is why a diesel will nail you off the line (if 4wd) and BHP is why you will catch it up and pass it later on .

Hope this helps.

Used 'oomph' istead of 'power' because it can get confusing.

Year 9 Physics - so there may be some corrections added later!

 

In car terms, acceleration is all about power to weight. So if you have 2 cars of the same weight and the same gear ratios, the engine with greater spread of power will accelerate faster. So at any given speed and assuming you can select the correct gear to use the maximum area under the power curve for the acceleration, power is what you need.

Power is a function of torque but its ultimately the power number you are looking for.

An f1 engine has less torque than a TDi but put both engines in the two equal weight and geared cars which one would accelerate faster?

The one with 750HP or 170HP?

 

Ok, so why does a 535d (299 BHp) accelerate faster to 60 mph than a 535i (306 BHP) ie 5.7s vs 6.1s

Thanks to everybody for their explanations. I'll have a proper read tomorrow when I'm not so knackered




T

 

Could be many reasons - gear ratio, tyre choice, aero, traction, power to weight ratio, set up etc

 

Probably area under the power curve on the diesel better for the gear ratios used

 

Ok, so in otherwise physically identical cars (ie 535d, 535i) the lower powered car can be significantly faster than the more powerful because of a combination of greater torque and gearing?

Actually doesn't it go to show that if you have two identical cars of near identical BHP ie (306 vs 299) the car with better torque will accelerate faster? (to 60 at least)

 

correct, the car with the better or more efficient TORQUE CURVE will accelerate quicker assuming everything else (gearing, weight etc) is like for like.

 

In the 0-60 example the 535i is probably harmed by the fact that it's optimal rev range is higher and it'll take a little bit of time to get the engine into that range from a dig. OTOH, the diesel is pulling like a train from the moment drive is engaged because it's peak power and torque is much lower down. A more telling stat would be something like 20-80, where the petrol is probably faster. It might even be faster from 10-70 depending on how high it would be revving at 10mph in 1st gear.

 

ok

 

http://www.astraownersnetwork.co.uk/...ad.php?t=15459 <- more indepth.

 

Here's an odd one - the 335i is a fair bit faster to 60 and 100 than a 335d (5.4 for 335i, 5.7s for the 335d) yet the 335d is quicker around Bruntingthorpe (against a manual 335i) because it it's torque.

So it's not always clear cut!

335i vs 335d is a very good comparison of diesel vs petrol - same size engine, both twin turbo, similar bhp.

 

there seems to be two things being discussed here

1. the theory - Torque = unit of force and BHP/power = rate of application of that force


and 2. the application of the theory in car engine/transmission design - which is affected by such things as gearing (giving mechanical advantage) etc

High Torque engines usually have a long stroke (to get the mechanical advantage) but long stroke engines do not rev very well due to large distance the pistons have to travel - compared to high power short stroke engines

Simply think of it as this

If you are undoing a very very tight wheelnut with a small wrench (low torque) you need a lot of power to undo it

If you put a wrench on with a 6ft bar – then my 4 year old son would be able to undo it (high torque – low power)

 

"Horsepower is measured using the units, foot pounds with one horsepower equal to 33,000 foot pounds"

so 2 * pi * t * RPM / 33000 = t * RPM / 5252

That explains it.

Now to find out why 1bhp = 33ftlbs

 

comparing lap times is super dangerous - confusing when discussing the differences between torque and power. As there are many many other factors.

Just one for example, due to the different gearing between derv n petrol, perhaps in the derv you can save one gear change, which will cost some time to the petrol. However is a direct relation to the gearing rather than the power delivery of the engine.

Its all interesting stuff eh.

read a few good exmaples now. plenty of info on the interweb.

 

It must be the 335 diesel's superior gearbox Matt

 

Here's a poor copy of the lap write up:

 

I was always told, might be wrong though.

To decrease acceleration times increase torque, to increase top end, increase BHP.

 

Ignore me. I'm being a retard.

 

Thought most people did

 

After driving them both I'd have thought the 0-100 times would be closer than over 1 second. Do you know what the 335i's 0-100 is with the dct box?

 

Jim - afraid I don't! The times I have seen never specify DCT, auto or normal manual annoyingly.

Also shows how different times can be - the "press" 335i is quicker than the "press" 335d but the readers 335d is quicker than the readers 335i!

 

I wondered what the none * car was. I wonder if it was the readers driving it. I didn't think there was much in it on my test drives but both cars I drove were 6 speed auto's. I'd never drove a 7 speed dct before buying mine . Do you think the manual could be slower than the automatic driven in anger?

I think mine is quicker in auto sport, but I put it in manual as I enjoy the experience more.

 

Jim - I suspect yours is the quickest 'box of all. Manuals are almost always quicker than full autos, but semi autos usually seem even quicker.

The 335d beat the 335i (manual) simply by having more torque to fling it out of bends. I should think a straighter track would give the 335i an advantage.

In that test ** is Evolve tuned, * is for press cars, no star is readers cars. AFAIK the same driver did all the laps - a Performance Car editor or something. Think he's a "proper" driver from what I've read.

Enjoying your car? Looked very nice in the pics you posted! Still loving mine - just cleaned and polished it. In this weather it should stay clean for about 23 seconds...!

 

I am loving it mate and the gearbox and engine are awesome, it's the best car I've ever owned and feel it's not too flashy but still looks sporty. I haven't posted any pics up as I don't know how to blur the number plate.

The only problem I have is driving like a fanny near pot holes in the fear of cracking an alloy. I can't seem to get an answer to if it's the poor alloys, tyres, suspension or something else. I only want to know so I can either change the tyres or buy a different set of alloys.

 

No, its power, it happens to be a function of torque. Capital letters doesn't affect the fundementals of physics