Optimum Gear Change

Oh good grief. Talk about opening a can of worms. If you google search, you'll find the complicated answer. The simple answer is - c) None of the above.

Chris.

Heh - the question that generates never ending answers :) For peak performance, generally somewhere around the redline, or a smidge below :)

Hi,

Depends on the engine / gear ratios. The ideal gear change for sporty driving is to rev up to max power, and on next gear the engine must "fall" downto the max torque revs.

Explanation : consider a Mk1 405 SRi (I do know this one well :-) ), max power @5500RPM, max torque @4500 RPM. You should rev to 5500, change gear, and on the next gear the engine must run at 4500RPM. This is the behavior given between 2nd and 3rd gear, then 3rd-4th then

4th-5th (not sure for the last one). Thanks Pug !

Regards, G.T snipped-for-privacy@worldonline.fr

205 Diesel & turbo-Diesel :

Uh oh. Here we go again. I hate to disagree with G.T., but that's not where you get maximum acceleration. I'm rubbish at explaining this stuff, but I'll give it a go. To acheive maximum acceleration, you need to keep as much power as possible across the rev range in each gear. So, let's look at changing from 2nd to 3rd in the example above. Peak power is at 5500 rpm. Let's say we have 125bhp at 5500 rpm - I dunno, I'm gonna have to make some numbers up here - but let's say we have an engine with a curve like this:

rpm 3000 3500 4000 4500 5000 5500 6000 6500 bhp 65 78 90 105 115 125 115 90

I think that's a reasonable power curve. I hope it's readable on everyone's newsreader.

Let's also say that changing from 2nd to 3rd will drop you 1000rpm.

If we change gear at 5500rpm, we'll drop down to 4500.

So, before the gear change, we had 125bhp. After the gear change, we have

105bhp. Now, let's say instead of changing at peak power, we kept on accelerating, up to 6000rpm. Before the gear change, we had 115bhp. After the gear change, we still have 115bhp and we'd still hit maximum power in second gear before changing. Now say we do the same thing from 3rd to 4th and again, we drop 1000rpm between gear changes. Since we waited until 6000rpm before changing into 3rd, we're at 5000rpm with 115bhp when we start accelerating in 3rd. So, as we accelerate, we get peak power and again, carry on past to 6000 rpm. When we change, we once again drop to 5000rpm with 115bhp. All the time, we're holding more power across the rev range in that gear because we waited until the point where bhp after peak power = bhp before peak power when changing gear. We have more power. So we're accelerating quicker than changing anywhere else in the rev range. If we'd have changed at 5500 from second to third, then from third to fourth, each time we would have dropped down to 105bhp and we'd have less power during acceleration.

So, you need to know what power curve you have and how many revs you drop in each gear to figure out where you should change. The redline can sometimes be a good indicator, but it rather depends on what the manufacturer was thinking when they put the little red line on the tacho and it's not normally there to help you know when to change gear for maximum acceleration.

Chris.

I answered this question in detail in another forum many years ago.

I've read you thorough article... and now I am wondering where it will be possible to find these cascade curves for my 406 HDi 110. I've tried searching Google and Yahoo but not found anything - do you know of any resources online that can help?

Thanks in advance.

The nearest I've got for you is this one (307 engines) - i tried hunting around for a few seconds for the obvious combinations, but got no further:

You can get several power-torque curves from

and

Hi Chris,

Why ? I'm not always right, and I'm not any better than anyone else here.

All right Chris, 125HP (DIN) @5500 RPM. You can see the (official) power curve at

Not that bad, the power curve shows : HP 68 80 94 108 118 125 117 around 108 I guess

Should be right, but you neglect the torque. You know there are lots of statistics you can do around an engine, like HP/litre, tore-bore ratio, and so on. One of them is called "smoothness" (or something similar, I don't know). It is given by the diff. between the max torque revs and the max power revs, and the bigger the better (you can state that Diesels are outstanding for this criteria).

Anyway, it seems you're right here, I've just ran a simulation using Powerdyn, and the 80-180KPH (starts @80KPH in 3rd), with 0.5s to change gears (c'mon, I need less with a BE1 indeed) takes : - 25.51s if changing at 5500RPM ; - 23.99s if changing at 6000RPM. I'm pretty surprised as maths seems to be against my feelings... You know how unpleasant it is to bring a XU9J2 above the max power revs :-)

Yeah, keep in mind that there are no redlines on a Clio for example (OK, not a car), and the revcounter max mark is at 8k, even for Diesels.

Damn, I stand corrected :-)

Regards, G.T (I hate maths) snipped-for-privacy@worldonline.fr

205 Diesel & turbo-Diesel :

thanks for the reply folks.

This is where it's easy to get confused when you start seeing torque and power curves on the same chart, like they're two different things. I haven't really neglected the torque, because power is a function of torque anyway. Check Dave Baker's post - he states that:

Power = torque x rpm / 5252

So, by only concentrating on the power of the engine, you're still working with the torque curve as well.

I haven't looked at that before. At a guess, I suppose that if you have a large gap between max torque and max power, then the torque curve would have to be quite flat and not fall away sharply - which of course would be good ;)

I know! I admit, even knowing this, it feels somewhat wrong to be revving past max power. The petrol XU9 engines lose power quite sharply after hitting max power (as with most petrol-engined cars), and you can really feel it.

Since most cars are rev-limited these days, it annoys me that manufacturers don't just mark the rev-limit point on the tacho.. still, the 'red line' always was a bit of a vague notion and can often give people the wrong impression of what's going on with their car.

I actually used to think exactly the same way as you about when to change gear - it wasn't until I started reading into it that I realised what was really happening. Still, it all makes sense when you think about it, which is good ;)

Chris.

Hi Chris,

True. It commonly admitted that power doesn't exist itself, as it's just a function of torque & revs.

To tell the truth (and I'm pretty sure Dave agrees with me), the "real" power formula is : P=T.w with P power in watts (W), T torque in Newtons per metre (N.m), and w the angular speed (rad/s). If working with metric system, the formula becomes P=T*RPM/9.549. I guess the 5252 factor is for imperial system, with power in BHP and torque in lb.ft. BTW, 1HP is almost 1BHP, as 1HP (DIN / ISO) is 736 W when a BHP equals 746W.

True, damn true. Indeed, the power curve's interrest is to show how long the torque drop is compensated by the revs increase.

That tends to dismontrate that, in fact. For example, Diesels "smoothness" is typically around 2kRPM. For example a 205D (1.7), max torque @2k, max power @4.6k -> smoothness=2600RPM. This criteria tends to show the useable range of the engine, too - for road drivers like us, rather for sports driving, eh :-) To go further in the example, the 205TD, max torque @2.1k, max power @4.3k, smoothness=2200RPM. You've just seen this also tends to show that the engine is more "peaky".

That's the reason why I tend to change @5.5k on mum's car :-)

I do totally agree with you on this. The most important thing is, we now know what is right. Oh, now the question is how could we use this brand new knowledge without being photographed by a hidden speedtrap :-)

Regards, G.T snipped-for-privacy@worldonline.fr

205 Diesel & turbo-Diesel :

So all I need to find now is a power curve for my 180 then........ Or get one done for mine...............

Thanks for your replies

Hello,

You could also use Powerdyn, a simulator / data acquisition software (I guess it's a shareware now), which I guess the guy developped an english version. From memory, the url is

or try a Google search.I used Powerdyn on v1 (I contributed to the development at versions .98 and.99, then 1.0), and that provided me the results of the accel times I gaveabove.

Regards, G.T snipped-for-privacy@worldonline.fr

205 Diesel & turbo-Diesel :

That looks a bit involved. Could only get about halfway down the page before the Google translator gave in. It's going on a Rolling Road soon, so I think I'll wait :-)

BTW Thanks for the reply

You were wrong to use its. Should be it's. Sorry, couldn't resist.

A.