The physics of cars - a question sequence.

[Vir Campestris]

At last he gets it right.

Any more argument must be just trolling.

Andy

[Vir Campestris]

Substitute torque for power and that is exactly right.

Andy

[Dave Plowman (News)]

In your second point you specified a road speed that coincided with maximum BHP. Therefore it would be impossible to be at the same speed in the same gear and at maximum torque. But perhaps you meant something else?

[Dave Plowman (News)]

And you don't seem to realise that using a different gear for each example simply confuses the issue.

Which is were we came in. I was talking about accleration *in any one gear*

How often do I have to say it?

[Dave Plowman (News)]

Right. So according to you you get peak accleration at peak BHP, and the same acceleration as the BHP drops off above that?

Forget reading what others say. Try reading what you do.

[Dave Plowman (News)]

FFS. Please read the posts you've quoted.

I'm only talking about the engine. Introducing different gearing simply confuses the issue.

[Roger Mills]

No-one disagrees with you that max acceleration in any one gear occurs at the engine's max torque point.

But you seem to be fixated on that point and unable to accept that, at the corresponding road speed, you can get *greater* acceleration by using a lower gear and running the engine near to its max power point.

BHP at the wheels = thrust (lbs) x speed (mph) / 375

OR Thrust = BHP x 375 / speed

So it stands to reason that the more power you can get to the wheels at a given speed, the greater the thrust and hence acceleration. QED!

I note that you haven't commented on my post about using a continuously variable transmission to maintain a constant engine speed!

[Dave Plowman (News)]

You jest, I assume? Read the vast number of posts which state this happens at maximum BHP.

Never ever denied that. But in that lower gear, you will still get better accelation at the peak torque point, not peak revs.

I really don't see how you can constantly confuse torque with power in the same post. It is the torque at the driven wheels which is important - not the 'power' ie BHP.

Because the type of transmission is totally irrelevant to my original point which dealt with the output characteristics of the engine.

[Roger Mills]

I have never confused torque with power!

If you want to maximise the acceleration of the vehicle, you have

*three* independent[1] parameters to play with - engine speed, engine torque and gearing. Hence my earlier reference to trying to treat a three dimensional problem as having only two dimensions. If you fix the gearing - which you seem to want to do - you restrict your options!

I fail to see why you struggle with the concept that, at a given road speed, you get *more* torque at the driven wheels by running the engine at max power and using a lower gear than you would by using a higher gear at the max torque point - assuming constant transmission efficiency. Seems blindingly obvious to me!

What part of that don't you understand?

[1] Power is not independent because it is the product of engine speed and torque.

[Clive George]

That's because when people are saying "max acceleration" they're not saying "max acceleration in any one gear", they're saying "how to get from one speed to a higher one the quickest".

Do you agree that when the task is to get from one speed to a higher one the quickest, max power is what you need to aim for, not max torque?

[bert]

Bearing in mind this is purely seat of the pants stuff, I would say acceleration initially dropped but then picked up as engine speed moved back up to and beyond max torque but again tailed off well before max power.

At 70 in top gear at about 3000 rpm acceleration was quite slow. (max power is 3500) (This IS a Land Rover we're talking about)

No The reason for not staying in the lower gear was because acceleration was decreasing and well before max power had been reached. I assume this is because engine speed had moved above max torque and so torque had decreased.

[bert]

On changing to the next gear the engine speed was then initially below max torque.

That would be more scientific but I don't feel sufficient desire to prove my point to justify the investment.

[bert]

Where does one measure thrust at a wheel? Presumably at the hub, or do you mean torque?

[Dave Plowman (News)]

You are assuming that given speed is such that in your chosen gear the engine can run at the maximum BHP point.

A very narrow set of conditions.

And again you choose to ignore that this discussion was about an engine's output - not the effect of different gearing.

[Dave Plowman (News)]

Then they need to read the original thread that started this.

If you now want to talk about getting the best acceleration through the gears, and where the best change up point is, it would depend on the individual car and gear ratios. Some with a wide ratio box and not many gears might get the best results by revving beyond peak power to get the engine on a decent part of the torque curve on the higher gear. Some may be better changing up at near enough max BHP.

But it's all about torque anyway. The amount of torque the engine is producing at near peak revs to the amount it will be producing when you change up. BHP is once again irrelevant.

[Clive George]

I did, and what I said is correct. You may think the discussion is still about one thing, but for most people it's moved on - thread drift is a perfectly normal thing. You're talking about one thing, everybody else is talking about something else. Both sides are correct about what they're talking about, which is why this argument has gone on quite so long - it's not a physics problem, it's a communication problem.

You could have just said "Yes" - it would have been quicker...

[Dave Plowman (News)]

Not so. Plenty are still saying the best acceleration occurs at peak BHP. It doesn't and never has.

That is what started the original discussion. With idiots throwing in gearboxes, etc.

[Clive George]

If you define best acceleration as "how to get from speed A to a higher speed B as fast as possible", which is an entirely reasonable definition IMO, it does.

I've pointed out that nobody in this discussion is being an idiot WRT the technical bits. A big part of the problem of this discussion is people dismissing what others have said out of hand, when actually what has been said is correct, only applying to a different thing the first person thinks it does.

When you're talking about the best acceleration in the way I and various others are talking about it, gear boxes are very relevant indeed.

[Roger Mills]

Indeed they are. The rate of acceleration is dependent on gearbox

*output* torque - not on engine torque. At a given road speed, output torque will *always* be higher when the engine is at max power in a lower gear than when it's at max torque in a higher gear.

Forget acceleration for a moment, and think about steady state thrust when (say) climbing a steep hill. If you want to make the most progress do you slog it out in a high gear with the engine at max *torque*, or do you change down to let the engine develop more *power*?

I know what I do!

[Roger Mills]

If you've got a smart-phone you'll already have one. There are lots of free apps for displaying the output in graphical form. [I don't know how useful they are for this sort of thing - but maybe worth a try.