The physics of cars - a question sequence.

That's a technique I used to use myself over two decades ago when driving the works van over short motorway sections on job to job journeys.

Assuming the clutch can tolerate such abuse[1], it's a valid way to wring every last drop of acceleration performance out an engine power train.

No - the correct term *is* power when talking about that commodity. BHP is just one of the units in which it is measured - along with kW and PS.

Just as "speed" is a valid term - or velocity if you also want to specify the direction of travel - regardless of whether you are measuring it in MPH, KPH, FPS, etc.

That's fine if you have sufficient skill - and are driving someone else's car! You don't have to hang around making the gearchanges, though.

I'm getting all confused now. Much more of this and I'll need counselling.

michael adams

...

I am reading this, and I'll try one last time.

Torque is irrelevant to performance. I know it's hard for you to understand, and it doesn't agree with your prejudices, but it's true.

For a car with a gearbox - and that is all of them - to get highest performance you ignore peak torque, and keep the engine at maximum power, or as close as you can get.

You can easily see this if you just work out the torque at the gearbox output for these two cases

It's true that if you can't change gear you'll get the highest acceleration at peak torque. But that is not relevant to getting the highest performance.

If you compare cars with similar peak torque outputs, but different power outputs the performance won't match. But if you look at cars with similar power outputs, but different torque outputs the figures will match.

Look here - BMW's 5 series

Model 0-62 peak power peak torque

Now explain how the 528i with _less_ torque than the 520D is so much faster. And the two 520s match.

Andy

Yes, and in every gear the engine RPM is directly and irrevocably proportio nal to road speed.

The speed/time graphs are exactly the same shape in every gear. Acceleratio n is greatest at the highest TORQUE regardless of which gear is selected.

The most ridiculous statement of the thread (so far).

Well, the two 520s are Diesels, and don't have the RPM range that the Petrol 528 has; they are governed to 4000 RPM.

Nice one!

While I agree it is not entirely irrelevant to performance it is a consequence of engine power and it's ability to supply the energy to the car ending up in kinetic energy.

It would be similar to claiming that the speed of sound is dependent on temperature.

While peak power is an indicator of engine power, the acceleration calculation would be of the form:

For the 528i, energy of car at 62mph (0.5 x m v^2) = integral(0 to

where engine power is that supplied at any instant of the acceleration test.

I might agree that maximum acceleration in any gear would occur at the maximum engine torque in that gear, the fact in itself is most unhelpful in determining 0 to 60mph times.

I don't somehow think that the 520i is a Diesel *or* that produces 135 HP without going above 4000 RPM. Try again!

well no, *only in that gear*. In a lower gear at peak power, acceleration will be greater.

You are Dave Plowman and I claim my £5

ITYM 4500. I have that diesel in my freelander

That's only true for a manual transmission. At the risk of being accused by you know who of introducing another red herring, as soon as you insert a torque converter between the engine and gearbox, your cosy relationship goes out the window! So we ignore that, and assume that we have a conventional manual transmission.

They're not, actually. They get considerably less steep in each successive gear.

Gearbox output torque vs road speed graphs are essentially the same shape in each gear but are, of course, scaled differently.

This is where we go round in circles! It's true that peak acceleration in each *gear* coincides with the engine's peak torque. But, in many cases, you can obtain a *higher* acceleration at a given road speed by using a different gear and running the engine at max power.

That acceleration will be lower than you would get in that 'different' gear at peak engine torque but it would be at a higher road speed. For maximum accelerative performance you need the maximum available acceleration all the way up the speed range - so you need to run the engine as close to max power as possible.

Didn't some Morgans

Try reading the post I was replying to before continuing. Which you've conveniently snipped. Here it is:-

Ah! I see your problem now. If, as you've seemingly posed the question, we're discussing, "When does the maximum instantaneous acceleration over the operating rpm range of the prime mover occur in any *one* gear ratio at a time?", then the answer to *that* question *will indeed* be at maximum torque rpms.

Which makes it pointless you commenting on my post, since you simply can't accept the basic principle of doing the test in only one gear.

And Vir is the one who started this nonsense months ago. But won't be told.

What any car's 0-60 time may be is totally irrelevant to where the maximum acceleration occurs anyway. Unless comparing apples to oranges.

Except that if you checked the best 10 mph increment in speed between the three, the diesel probably wins...

And in the original discussion, 0-60 times weren't mentioned. Just the point in an engine's output where the best acceleration occurred. Which of course is at maximum torque, not maximum BHP.

Actual 0-60 times depends on so much more.

Ah. A Freelander. Explains why you don't understand performance.