2.1 td should i still be runnin on turbo after 3k revs

zxb29 · Post by **zxb29** » 16 Jun 2006, 12:26

Hi all i have a peug 406 2.1 turbo dies 12v 1996 which i believe is the same as the xantias.

turbo boost while driving is fine from 2000 to 3000 revs but after that seems to pull on engines own power rather then the turbo doing the main pulling

is this something normal to expect ?

also just changed rear brake pads now when i brake pedal seems to be a lot further down but if i brake let go and brake again seems to be at the top and more effective is this a sign of the brakes need bleeding from the back?

thanks in advance

Robin · Post by **Robin** » 16 Jun 2006, 13:46

I think you may have the action/purpose of the turbo a bit wrong here. The turbo compresses the intake air to provide a pressure increase in the intake manifold to improve efficiency. The turbo is driven by the exhaust gasses and not by the engine directly with a belt or similar. That system is supercharging generally. The maximum torque, the ability of the engine to accelerate in principle is at it's highest between 2 and 3 k rpm so beyond those figures the torque drops away and performance seems to ebb. In most driving conditions there is little to gain by revving much above 3k.

The brakes, sounds like bleeding the rears would be a good start but try the fronts too incase some air got in via the resevoir while the system was being worked on.
Cheers Robin.

zxb29 · Post by **zxb29** » 16 Jun 2006, 14:48

hmmmmm i miss my zx td much more torque

thanks for tha advice have borrowed work colleuges gunsons bleedin kit, so will give it a go tonight.

thanks

deian · Post by **deian** » 16 Jun 2006, 16:43

My 2.1td xantia is the same, it normal behaviour.

I've also noticed that if you rev beyond 3000rpm fuel consumption is horrendous. So I drive between 1500 and 2500, and just rev her up to 3000 if i'm in a rush or overtaking, suits me fine that way.

Don't worry about it.

HST · Post by **HST** » 16 Jun 2006, 18:04

Robin wrote:I think you may have the action/purpose of the turbo a bit wrong here. The turbo compresses the intake air to provide a pressure increase in the intake manifold to improve efficiency. The turbo is driven by the exhaust gasses and not by the engine directly with a belt or similar. That system is supercharging generally. The maximum torque, the ability of the engine to accelerate in principle is at it's highest between 2 and 3 k rpm so beyond those figures the torque drops away and performance seems to ebb. In most driving conditions there is little to gain by revving much above 3k.

The brakes, sounds like bleeding the rears would be a good start but try the fronts too incase some air got in via the resevoir while the system was being worked on.
Cheers Robin.

"Seems to ebb" is exactly right. The engine develops maximum power (basically torque x revs) above 3000rpm, so in a given gear must accelerate more strongly above 3000. It doesn't feel like it because the power builds up gently: you feel a surge at low revs as the turbo kicks in, but above that the power stays fairly constant, but IS increasing. It's because of the high mid range torque that the high rev power of diesels doesn't feel like much. Watch the speedo though, and if you're engine is working properly, you'll see the acceleration. Max power (ability to do work) = max acceleration - basic physics.

Here's an extreme example to illustrate my point. I have three vehicles with the following approximate torque figures for their power unit: 220, 180, 80lbft. Right, which one will accelerate 50 - 70 quickest?

Answer the vehicle with 80lbft is quickest, the one with 220 slowest. Surprised? The explanation? Well, the vehicle with 220lbft is a pushbike, loads of torque with me standing on the pedals, but very few revs, therefore little power: won't even do 50mph. The 80lbft one is a motorbike - lots of revs, therefore lots of power when delivering the torque (and the advantage of less weight). The other a diesel car.

The advantage of high torque figures at lowish revs is driveability, for outright performance power wins every time (unless delivered over an extremely narrow rev range).

I went from a petrol 2.5 V6 to 2.1TD. Above a certain speed driveability of the TD is quite similar, though the V6 was more flexible (pulling smoothly from less than 500rpm in any gear) and would trounce the diesel if revved. The V6 had a bit less torque, but much more power. And a much bigger thirst.

Kowalski · Post by **Kowalski** » 17 Jun 2006, 00:37

HST wrote: "Seems to ebb" is exactly right. The engine develops maximum power (basically torque x revs) above 3000rpm, so in a given gear must accelerate more strongly above 3000.

Thats just plain wrong, power is a function of torque x revs but accelaration depends on torque, torque is translated into an accelarating force which is proportional to power divided by speed. Once the torque starts fading above 3000rpm the accelaration starts to fade too.

It doesn't feel like it because the power builds up gently: you feel a surge at low revs as the turbo kicks in, but above that the power stays fairly constant, but IS increasing. It's because of the high mid range torque that the high rev power of diesels doesn't feel like much. Watch the speedo though, and if you're engine is working properly, you'll see the acceleration. Max power (ability to do work) = max acceleration - basic physics.

The surge you feel at low revs is a big increase in torque as the turbo comes in, its also a large increase in power. What you feel above 3000 rpm is the torque fading. If we take the simple physics view of things, in any gear maximum accelaration occurs at maximum torque, not maximum power, thats not to say that better accelaration can't be had in a lower gear where there is more power.

Here's an extreme example to illustrate my point. I have three vehicles with the following approximate torque figures for their power unit: 220, 180, 80lbft. Right, which one will accelerate 50 - 70 quickest?

Answer the vehicle with 80lbft is quickest, the one with 220 slowest. Surprised? The explanation? Well, the vehicle with 220lbft is a pushbike, loads of torque with me standing on the pedals, but very few revs, therefore little power: won't even do 50mph. The 80lbft one is a motorbike - lots of revs, therefore lots of power when delivering the torque (and the advantage of less weight). The other a diesel car.

You don't quote weight figures for those torque figures, so they are utterly meaningless on their own.

Homer · Post by **Homer** » 17 Jun 2006, 07:52

zxb29 wrote: thanks for tha advice have borrowed work colleuges gunsons bleedin kit, so will give it a go tonight.

There is no need for a bleeding kit on a Xantia, just run a length of hose from the bleed nipple to the reservoir.

Most of the bleed kit will consist of something to provide pressure in a conventional brake system via the brake fluid reservoir, attaching it to the LHM tank simply won't work.

jeremy · Post by **jeremy** » 17 Jun 2006, 08:46

Homer - he's got a 406

HST · Post by **HST** » 17 Jun 2006, 18:17

You don't quote weight figures for those torque figures, so they are utterly meaningless on their own.

Meaningless? Well, sort the vehicles quoted into order of weight. It's not too hard is it?? There's an order of magnitude difference, unless the cyclist is a real lardass.

deian · Post by **deian** » 17 Jun 2006, 19:54

Regarding the turbo not pulling over 3000rpm, try doing 80mph on a motorway and floor the accelerator, u will hear the turbo spin up and you will still feel the car pulling like a train, this even happens at speeds over about 100mph (of course this is truly illegal, and not recommended).

HST · Post by **HST** » 17 Jun 2006, 23:50

HST wrote:
You don't quote weight figures for those torque figures, so they are utterly meaningless on their own.

Meaningless? Well, sort the vehicles quoted into order of weight. It's not too hard is it?? There's an order of magnitude difference, unless the cyclist is a real lardass.

I should add, if the cyclist is really fat it will greatly increase the torque (500lbft isn't too hard to imagine for a particularly corpulent Texan) but probably wouldn't help the 50-70 time much, even with a low overall weight cf a car. Point made.

Post by **Peter.N.** » 20 Jun 2006, 20:26

If you had better performance from a 1.9 td than a 2.1, you have something very wrong. The power on the 2.1 is up about 30 bhp and 50 lbs/ft and the 2.1 should pull much better at low revs.

deian · Post by **deian** » 20 Jun 2006, 21:08

If what i've read is correct... 1.9td is 90bhp and 2.1td is 110bhp, thats 20bhp difference right?

Still it's a big difference! So I agree PeterN

Post by **Peter.N.** » 20 Jun 2006, 22:57

Sorry

a senior moment, I had the right figures but failed in my mental arithmatic!

zxb29 · Post by **zxb29** » 22 Jun 2006, 15:37

Hi chaps didnt expect so many replies thanks all so much for their input and maths etc

my main point was that i was expecting more torque on pull then a citroen zx td but pick up feels sluggish but this could just be the that the citroen was too quick as the turbo kicking in would push u back in your seat, not getting the same thrills on the 406

il give the 80mph test on the motorway on the way home today

dein it pulls fine up to 3k but after that its really sluggish only option i have is to change gears at 3k revs. i can hear the turbo fine sounds like a hdi for some reason ie whistles at 2-3k