ML5T Gearbox

[CitroJim]

Can someone please explain to me how an engine is on boost at cruise doing 70mph?

That's agreed Gareth. My thought is that at 70mph, the turbo will be spinning fairly well and making some pressure in the inlet tract that will be helping to increase the effective compression ratio a little. Also helping is that your throttle will be barely cracked open so the ECU will be delivering comparatively little fuel. You'll be running lean with a fair CR but still with a good volume of exhaust gas to spin the turbo.

The very opposite is the case when ploughing through town. Low gears, large dobs of throttle giving a rich mixture and a slowly spinning turbo causing the effective CR to be roughly the same as the natural CR.

[XantiaMan]

Digressing slightly, i was amused by a comment on the 406 forum, by someone who claimed that the injector duration doing 100mph at say, 5000rpm will be the same as revving the engine off load at a standstill, doing 5000rpm Also, that an engine will make boost without being subjected to load, it must do, because you can hear the whoosh of the dump valve when revved

[CitroJim]

Umm, indeed. Trouble is Gareth, that's been posed in a respected forum so now it'll be taken as gospel by some folks even though it's clearly incorrect. I hope it resulted in a lively discussion.

When making statements like that, one should always wrap a disclaimer around it that it is the belief of the poster only...

[XantiaMan]

Umm, indeed. Trouble is Gareth, that's been posed in a respected forum so now it'll be taken as gospel by some folks even though it's clearly incorrect. I hope it resulted in a lively discussion.

When making statements like that, one should always wrap a disclaimer around it that it is the belief of the poster only...

I gave up in the end, chap was having a 'no boost' problem which he was trying to diagnose with the car on axle stands... He even blamed me for wasting money on a boost gauge to help diagnose his problem. More pissing in the dark going on last time i read, with 'blocked cat mate' being suggested.

http://www.406oc.co.uk/viewtopic.php?f=1&t=8767&start=0

Have a read, he talks the talk but sadly that talk is just rollocks.

[CitroJim]

Well, I have to say Gareth, your posts in that thread were the only ones that were at all sensible and helpful. The rest, well...

Your last post in that thread summed up your feelings very eloquently

I'm sorry, I must take back the word "respected" I used earlier. I thought you were speaking of the 406 Coupe forum....

Why don't people go out and get a good book on the subject, do some reading and try to understand the subject a bit before making posts

[Deanxm]

the xm weighs in at 1500kg which will be a bit heavier than a xantia

Xantias are very heavy, an Activa weighs in at 1430kg , an XM CT Exclusive from figures i have found show it to be 1375kg...

No that is slightly wrong, my n/a weighs 1460kg then you have to add the turbo, all the boost piping, intercooler etc etc to that engine, it surprises me they weigh near the same though, i would have thought the xantia being smaller would have weighed much less

D

[HDI]

Digressing slightly, i was amused by a comment on the 406 forum, by someone who claimed that the injector duration doing 100mph at say, 5000rpm will be the same as revving the engine off load at a standstill, doing 5000rpm Also, that an engine will make boost without being subjected to load, it must do, because you can hear the whoosh of the dump valve when revved

Where do they get these notions !! He should fit a boost gauge so he can see the truth !! And I would be so happy if ECU's could be mapped without a dyno to load the engine !!
Going back to fuel consumption , I doubt many will have anything to match the thirst of my Evo 6 , around 10-12 MPG on the road and about 4 MPG on track !! Always seems miraculous when I use the Xantia and get 50 !!

[Deanxm]

From pug forum in question...................

The ECU has no idea what gear it's in unless the wheels are turning, and then it takes a few seconds to work it out from the speed sensor in the diff. Given how you can be on boost from a standstill in a working car, I don't think being in gear has any effect.

I can be a bit thick sometimes but this................its comedy gold

D

[XantiaMan]

From pug forum in question...................

The ECU has no idea what gear it's in unless the wheels are turning, and then it takes a few seconds to work it out from the speed sensor in the diff. Given how you can be on boost from a standstill in a working car, I don't think being in gear has any effect.

I can be a bit thick sometimes but this................its comedy gold

D

Someone else said pretty much the same, and that i should shame him publicly all over the internet world. To make matters worse, he is a moderator

[DickieG]

Someone else said pretty much the same, and that I should shame him publicly all over the internet world.

A quick link here would get things rolling in that direction

[addo]

DG, are you sure that Gareth's misspelling of publicly, wasn't deliberate?

I'd just assumed so!

[XantiaMan]

There are two ways of spelling it, i have an auto spell checker on Google Chrome which may have done the american version, and then someone thought it would be funny to edit my post further.

[DickieG]

There are two ways of spelling it, i have an auto spell checker on Google Chrome which may have done the american version, and then someone thought it would be funny to edit my post further.

No offence intended Gareth, I just corrected what I presumed to be a spell check error, even in this sentence Opera tried to change "offence to offense", blasted Yanks shud lurn to spoll write.

I didn't think that changing the spelling made any difference to the sentence, apologies

[xmexclusive]

Hi All

Interested in the original topic as I want to try streatching the top gears in a MG5T box as fitted to 2.5TD XM's. Jim's mention that changing 5th gear would be relatively easy is something that I would like to research further as it seems a good first step. May even fully meet my needs. My aim is to end up with gearing that would give me something close to 70mph at 2000rpm. That compares with the present 60mph at 2000rpm.
Once we get into likely engine economy and turbo cut in then I need someone with more expertise and knowledge to tell me the likely effects.
As I understand it a diesel is very different than a petrol in this respect. Am I right that very high revs on a diesel give an exponential increase in fueling rate to maintain torque therefore economy worsens considerably.


John

[HDI]

Not sure where this thing about the turbo needing to be on boost to improve economy on a Diesel comes from ?! If the turbo is on boost , this will be because the engine is under enough load to need it. Consequently more fuel will be injected and fuel consumption will increase. Volumetric efficiency will improve but all this means is a turbo Diesel on boost to achieve the power required will use less fuel than an NA Diesel under the same circumstances , but , fuel consumption still increases as more power is developed.
Diesel fuel consumption does not increase exponentially with revs , but it will consume almost as much fuel as can be injected if air is also provided. This is why tractor pullers use double and even triple compound turbos , that is one turbo blowing into another into another , to develop as much as 250 PSI boost.
Fuel consumption is proportionate to power but will still always be less than a petrol engine of the same power.