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Discussion Starter · #1 ·
Can the EGR gauge indicate fuel consumption?

Might be a dumb question but I'm going on a logic collision of some sort. I'm no economy-nazi but I'm interested in some responses.

Scenario:

Speed: Constant 100kmh (90kmh on speedo thanks to 33's) - must fix that!

Gear: 4th
Revs: approx 2800-2900rpm from memory
Exhaust temp: 250°C

Gear: 5th
Revs: approx 2400-2500 from memory
Exhaust temp: 300°C

So in 5th the revs naturally drop which in ideal situations would lower the fuel intake required and potentially use less fuel. This of course depends a little on the motors natural torque and power band. But shifting to 5th as you can see from my numbers raises the EGT. If the EGT is rising then in theory its burning more fuel to rise. Now my logic is favouring the latter. Driving by the EGT gauge and keeping it at say 250°C would be the most economical gear at that speed. If the EGT is rising then so is the fuel use.

Am I correct?
 

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Diesels are the opposite to petrol motors...in a basic sense.
Petrol motor's run cooler when running rich mixtures and diesel's run hotter on a rich mixture.

What boost are you running in both scenarios? If you drop a few psi when you are in 5th that will contribute to the slightly higher egt's.
 

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Discussion Starter · #3 ·
Good point on the boost.

I'll check tomorrow but usually at road speed its pretty steady at 10-12psi regardless of gear from memory. I'll check. The dawes valve limits to 16psi.
 

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Y2KGUII ZD Wgn
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Can the EGR gauge indicate fuel consumption?

Might be a dumb question but I'm going on a logic collision of some sort. I'm no economy-nazi but I'm interested in some responses.

Scenario:

Speed: Constant 100kmh (90kmh on speedo thanks to 33's) - must fix that!

Gear: 4th
Revs: approx 2800-2900rpm from memory
Exhaust temp: 250°C

Gear: 5th
Revs: approx 2400-2500 from memory
Exhaust temp: 300°C

So in 5th the revs naturally drop which in ideal situations would lower the fuel intake required and potentially use less fuel. This of course depends a little on the motors natural torque and power band. But shifting to 5th as you can see from my numbers raises the EGT. If the EGT is rising then in theory its burning more fuel to rise. Now my logic is favouring the latter. Driving by the EGT gauge and keeping it at say 250°C would be the most economical gear at that speed. If the EGT is rising then so is the fuel use.

Am I correct?
No it doesn't work that way, as AV8TOR says they work totally differently to petrol's the AFR can move a lot with a diesel and it will still run, but not with a petrol, but when the AFR gets high, meaning not enough air for the amount of fuel going in then that is when EGT's climb.

Working out fuel consumption from it.... frayed knot :).

Frayed knot.jpg
 

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Discussion Starter · #5 ·
Afr?
 

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Discussion Starter · #6 ·
Diesels are the opposite to petrol motors...in a basic sense.
Petrol motor's run cooler when running rich mixtures and diesel's run hotter on a rich mixture.

What boost are you running in both scenarios? If you drop a few psi when you are in 5th that will contribute to the slightly higher egt's.
4th gear runs 12psi in the scenario mentioned. 5th runs 10-11psi
 

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1-2psi would barely make a difference.id say the biggest factor at play here is where the motor makes its torque,you have 33's with stock diff ratios i assume? Pushing bigger tyres at lower revs ie 5th gear,when the motor is working at the low end of the torque curve requires a bit more effort...more fuel,more heat etc. Keeping it in 4th makes it a little easier and thus the lower egts and possibly lower fuel consumption even though its at slightly higher rpm's
 

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Discussion Starter · #8 ·
1-2psi would barely make a difference.id say the biggest factor at play here is where the motor makes its torque,you have 33's with stock diff ratios i assume? Pushing bigger tyres at lower revs ie 5th gear,when the motor is working at the low end of the torque curve requires a bit more effort...more fuel,more heat etc. Keeping it in 4th makes it a little easier and thus the lower egts and possibly lower fuel consumption even though its at slightly higher rpm's
Your answer is spot on with my scenario and my logic. Thanks ;-)
 
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Glasshouse Bogan #1
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If I am sitting on 100kph in 5th on flat road say egt is 300 degrees and boost gauge is on 4 psi. If there is a headwind the boost and egt will rise as I put my foot down to keep at 100 so I use more fuel. If I just back off to say 90-95 kph the boost and temps go back down along with fuel usage.


Sent from AutoGuide.com Free App
 

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Discussion Starter · #11 ·
If I am sitting on 100kph in 5th on flat road say egt is 300 degrees and boost gauge is on 4 psi. If there is a headwind the boost and egt will rise as I put my foot down to keep at 100 so I use more fuel. If I just back off to say 90-95 kph the boost and temps go back down along with fuel usage.


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Yes i agree with all of that. But now if you did all that in 4th as well what will the difference be in EGT and fuel consumption.
 

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Y2KGUII ZD Wgn
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This whole argument is academic, sure there may be link between the two (taking into account that your AFR is good under normal conditions) but no way is there a relationship that can be calculated, like looking at your ECUTalk and noticing that your TPS is up by 2% on your normal 100k cruising speed on a flat section of road, from this you know your going into a headwind and going to use more fuel but that's about it.
 

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Discussion Starter · #13 ·
Yes head winds, tail winds, uphill etc etc naturally will effect fuel consumption. All I was asking was if EGT gauge can indicate fuel consumption at highway speed for me in my circumstances. I seem to have mixed responses. So in layman's terms it seems the answer is no because too many other factors. It could be yes, if my situation is right.
 
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