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Old 12th June 2011, 12:47   #46
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Default Re: When do you shift gears?

Until the point the turbo 'kicks in', it consumes power from the engine. The larger the turbo, the more power it consumes.

I always upshift in the turbo 'zone'. The car's efficiency is highest in that band. I obviously don't keep my eyes on the tacho, but my upshifts happen ~2.5-3k rpm and downshifts around 1500-1800. If I'm pottering about inside the colony, I normally keep the car at around 1200-1500 rpm. Doesn't seem to help from a fuel efficiency point of view.

For my TJet, I find the best fuel efficiency is best regulated by using gentle throttle input rather than the actual engine speed (which is difficult to focus on in real-world driving, given the amount of nonsense a typical drive in Delhi throws at you).

My understanding of the original question which you repeat here

Quote:
Is it compulsory to let the Turbo spool in every gear? No, it depends on your driving style.
Letting the Turbo spool will increase or decrease the life of engine and Turbo? Pretty sure that as long as temperature and lubrication is in order the life of the turbo is not reduced. anyway in an engine there are many bits that are under far more mechanical stress than a turbo and will give in sooner.
Also if the RPM is increased such as to spool the Turbo, the soot formed will be less, then will the emissions be less or not? Again, this will depend on engine geometry and configuration, also the quality of fuel etc. But very clearly - when more fuel is being consumed (higher engine speed) more emissions are produced. Switching off a turbo will impact emissions, though I'm not sure it'll be a positive impact. Thinking about it, it would seem the engine efficiency would go up with a turbo rather than down, given that the engine breathes better.
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Old 12th June 2011, 13:38   #47
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Default Re: When do you shift gears?

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Originally Posted by swiftnfurious View Post
The upshift at 1500 rpm was a general statement [and not in respect to the turbo lag]. I try and upshift 2 & 3 gears very early and the car doesn't feel strained / slow [the car doesn't feel running out of steam]. I am not a techie - so dont know how to explain it, but the explanation to reduce lag by upshifting early is provided in some thread - may be the same one. Will have to dig a bit deep.
it may not reduce lag, but the power curve is very steep and at 1500 you probably get enough power anyway to drive around in 2nd.

At the same time while running at 2.2k even though the engine can generate more power - probably it wouldn't if you are in the first gear - simply because you don't need it (gear ratios are reducing your acceleration) and also it will be hard for you to feel that power, because the "feeling" you get comes from acceleration, not actual power.


In addition, the turbo may be spooling slightly faster in higher gears anyway - in the first gear at a given rpm you don't need much power so engine will burn less fuel as a result exhaust gases will have less leftover heat to expand in the turbo. In second gear at the same rpm you need more power (higher gear ratio means more torque needed) and as a result the exhaust will have more leftover heat too - easier to spool turbo.



From what I have noticed on my Figo, there is no "trubolag" in the first gear whatsover - at low rpm it has enough power, high rpm it starts making noises and I don't feel the speed so my mind forces me to upshift.

In second gear at very low rpm it does stall.
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Old 12th June 2011, 19:34   #48
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Default Re: When do you shift gears?

Very informative thread this one is.

I have a question which I hope is not OT. What is the difference in torque output between a VGT version and FGT version of same engine at RPMs below the turbo kick RPM. Most threads I have gone through talk about VGTs producing torque and power to a higher RPM range compared to FGT. Is a VGT better at lower end? Else how one expects a 1248cc MJD engine to pull heavy cars like SX4 from standstill without racing the engine?
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Old 13th June 2011, 12:45   #49
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Default Re: When do you shift gears?

Quote:
Originally Posted by vina View Post
... engine will burn less fuel as a result exhaust gases will have less leftover heat to expand in the turbo....
.... more leftover heat too - easier to spool turbo....
Hi Vina,

Could you please elaborate on the effect of temperature of gases on the turbo?

Thanks,

Dhanush

Quote:
Originally Posted by mohan41 View Post
... Is a VGT better at lower end? ...
Yep!. The main purpose of VGT is to improve low end driveavlility, and to reduce turbo lag.
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Old 13th June 2011, 18:47   #50
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Originally Posted by dhanushs View Post
Hi Vina,

Could you please elaborate on the effect of temperature of gases on the turbo?

Thanks,

Dhanush

I may be wrong here - but here's what I have read on message boards (and interpreted from more authoritative sources):

turbo works as a turbine and uses two modes of energy - pressure from the flow of the exhaust gases AND leftover heat in the exhaust gases.

the first one is obvious - it is like a windmill, the momentum (kinetic energy) of the exhaust is used. The second one is similar to how turbine engines work (except in those this is the only way power is generated) - hot gases expand inside the turbine, and this expansion causes the turbine to rotate.

In high gears (i.e. high speed, and higher power needed by the vehicle) at reasonably high rpm numbers, engine does not extract all the energy from the working fluid (the exhaust gas) - the gas can still expand significantly, but doing so will reduce the engine's specific-power so the designers don't do it. In a turbocharged vehicle some of this wasted power can be reused by running a secondary turbine on this power and using that turbine to then improve the breathing of the engine - aka turbocharger.



Coming to what I was saying in the previous post:

Let us take two situations (and again I may be wrong):

(1) First gear @1600rpm (for Ford, or 2000rpm for MJD)
(2) 5th gear @1600rpm (for Ford, or 2000rpm for MJD)

both on level ground, no wind, etc. reasonably loaded vehicle, constant speed.


In the first scenario the car will barely do 10kmph or thereabouts, there is hardly any resistance to motion - even though rpm is 1600 which means engine can generate a lot of power, ECU will almost certainly starve it of diesel. This in turn will also mean that the exhaust gases would have utilized most of the heat in expanding within the engine's cylinders (compared to scenario below) and not much will be available for turbo.

In the second scenario on the other hand, the car is cruising, there is air resistance and the engine does need to produce significant power (though probably still not the peak power it can generate at this rpm) - this also means it will be burning far more diesel per cycle, which means the exhaust gases wouldn't have expanded as much as they could - they'll have far more leftover energy than in case (1)

The above scenarios hold even when the engine gets same amount of air (initial pressure) at the intake. Of course if the analysis is correct there will be a "positive feedback effect" (perhaps one reason why turbo spooling is so sudden) in the second case - the intake pressure itself will be higher thanks to the fact that turbo has more energy to work with and as a result turbo will have even more energy to work with.


that is why I said that may be in higher gears turbo can get spooling at lower rpm numbers than in lower gears.

Quote:
Originally Posted by mohan41 View Post
Very informative thread this one is.

I have a question which I hope is not OT. What is the difference in torque output between a VGT version and FGT version of same engine at RPMs below the turbo kick RPM. Most threads I have gone through talk about VGTs producing torque and power to a higher RPM range compared to FGT. Is a VGT better at lower end? Else how one expects a 1248cc MJD engine to pull heavy cars like SX4 from standstill without racing the engine?

I think this question belongs on the Turbo Lag 101 thread (Turbo Lag 101) (and the moderators may please shift the posts to that thread if they feel the same) - but anyway:

A turbo can be tuned for a narrow range of rpm only. Small turbo will spool at low rpm, but will not provide enough boost at high rpm, while large turbo will provide enough boost at high rpm but will not be able to spool from the low exhaust energy available at low rpm.

VGT is a mix of both - it acts as a small turbo at low rpm requiring less energy to spool up (but providing less air flow too - more air flow not needed anyway at low rpm) BUT as a large turbo at high rpm (for the need for more air flow as well as available energy in exhaust are both higher)

Last edited by Technocrat : 13th June 2011 at 23:21.
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Old 6th July 2011, 19:25   #51
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Default Re: When do you shift gears?

The following is printed on Page 9 in Environmental Protection in the Indigo LS Owner's Manual--
"Shift to higher gears as soon as it is possible. Use each gear upto 2/3rd of it’s maximum engine speed."

Why TML is advising to shift the gear after raising the Engine upto 2/3rd of its maximum engine speed?
The Indigo's Tachometer Redline starts at 5000 rpm. So according to TML, the Engine should be raised upto 3500 rpm (2/3rd of 5000) in each gear.

Would not the Engine wear out faster or does it have some thing to do with the Turbocharger?

Please enlighten on this.

Last edited by IndigoXLGrandDi : 6th July 2011 at 19:33. Reason: Typo error corrected
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Old 6th July 2011, 19:29   #52
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Default Re: When do you shift gears?

Quote:
Originally Posted by IndigoXLGrandDi View Post
The following is printed on Page 9 in Environmental Protection in the Indigo LS Owner's Manual--
"Shift to higher gears as soon as it is possible. Use each gear upto 2/3rd of it’s maximum engine speed."

Why TML is advising to shift the gear after raising the Engine upto 2/3rd of its maximum engine speed?
The Indigo's Tachometer Redline starts at 5000 rpm. So according to TML, the Engine should be raised upto 3500 rpm (2/3rd of 5000) in each gear.

Please enlighten on this.
Actually, both the statements are contradictory.

"Shift to higher gear as soon as possible" would mean you shift at 1.5-2k rpm itself; the rpm at which the engine can somehow sustain itself albeit with adverse consequences if done consistently over a long period.

"Use each gear upto 2/3rd the max. speed" would mean you upshift around 3.5k rpm, which is very high for normal driving.

IMO, upshift should happen ~2.5k rpm (in the range 2.5-3k rpm).
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Old 6th July 2011, 19:32   #53
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Default Re: When do you shift gears?

Quote:
Originally Posted by IndigoXLGrandDi View Post
The following is printed on Page 9 in Environmental Protection in the Indigo LS Owner's Manual--
"Shift to higher gears as soon as it is possible. Use each gear upto 2/3rd of itís maximum engine speed."

Why TML is advising to shift the gear after raising the Engine upto 2/3rd of its maximum engine speed?
The Indigo's Tachometer Redline starts at 5000 rpm. So according to TML, the Engine should be raised upto 3500 rpm (2/3rd of 5000) in each gear.

Please enlighten on this.

What you have in bold is an advice on max rpm in a gear - though I don't think TML wll have any objection if you redline from time to time.

TML is NOT asking you to "So according to TML, the Engine should be raised upto 3500 rpm (2/3rd of 5000) in each gear." - in fact quite the contrary. When they say "Shift to higher gears as soon as it is possible" that means that you should try to keep engine rpm as low as possible (power requirements and idling) at all times.
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Old 13th March 2017, 09:09   #54
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Default Car speedometer red line

I have noticed this from quite a time now in almost every car but never thought much about it. Almost every car has red line at 30 km and 50 km mark. Is this due to marking some ideal speed to gain most mileage or is it just pure asthetics? Has to be some meaning to it. Can anyone shed some light over this?
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Old 13th March 2017, 11:19   #55
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Default Re: Car speedometer red line

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Originally Posted by navdeep.rana View Post
I have noticed this from quite a time now in almost every car but never thought much about it. Almost every car has red line at 30 km and 50 km mark. Is this due to marking some ideal speed to gain most mileage or is it just pure asthetics? Has to be some meaning to it. Can anyone shed some light over this?
My cars do not have these marks on their speedometers.

I am only guessing here (and could be way off the mark) but I think it may have something to do with mileage or gear changes @ a particular road-speed and RPM. You would find the best guidance in your owners manual.

PS - Since this thread is about shifting gears, I generally shift gears between 2000 and 3000 RPM depending on where I am driving and traffic conditions.

Last edited by R2D2 : 13th March 2017 at 11:32.
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Old 13th March 2017, 11:32   #56
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Originally Posted by R2D2 View Post
My cars do not have these marks on their speedometers.

I am only guessing here (and could be way off the mark) but I think it may have something to do with mileage or gear changes @ a particular road-speed and RPM. You would find the best guidance in your owners manual.
Thanks for your input on this. Did a little research on my end. Some says it's to remind you of city speed limits in some countries other says its ideal gear shift speed for 1st and 2nd gear. Not very sure about this. I think my car will touch 30 kmph in first gear in around 1800-2000 rpm. Will have to see for sure when I go for a drive next time.
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Old 13th March 2017, 11:36   #57
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Default Re: Car speedometer red line

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Originally Posted by navdeep.rana View Post
I have noticed this from quite a time now in almost every car but never thought much about it. Almost every car has red line at 30 km and 50 km mark.
Did some Google search and got to know that they are speed limit marking.

Quote:
These marks are speed limit reminders. Depending on the car it may have marks at one or all of 30, 50 and 130 km/h. 30 km/h is a common speed limit in residential areas in Europe, 50 km/h is a common 'default' speed limit in cities and towns around the world.
EDIT: Just saw you posted the same.

Speed limit theory makes sense IMO.
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Old 13th March 2017, 20:50   #58
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Unhappy Re: Car speedometer red line

Quote:
Originally Posted by R2D2 View Post
My cars do not have these marks on their speedometers.

I am only guessing here (and could be way off the mark) but I think it may have something to do with mileage or gear changes @ a particular road-speed and RPM. You would find the best guidance in your owners manual.

PS - Since this thread is about shifting gears, I generally shift gears between 2000 and 3000 RPM depending on where I am driving and traffic conditions.
Quote:
Originally Posted by a4anurag View Post
Did some Google search and got to know that they are speed limit marking.



EDIT: Just saw you posted the same.

Speed limit theory makes sense IMO.
Yes! Just drove the car and the RPM reached all the way to 3000+ in first gear in order to reach 30 kmph mark. So I don't think that its marked for ideal gear shifting speed. This might me more like upper speed limit which you shouldn't cross while driving and should change gear under this mark. But then again I would just be guessing.
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