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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old June 28th, 2020, 03:01   #1
devon1996
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Default VNT is killing your fuel economy

Thanks to my consistent and traffic free 100 mile round trip commute I have been able to do quite a bit of experimenting on an 06 Jetta I've got. I write my own tunes and thus have been able to incrementally change the stock program and record the results.

My test track is about 20 miles of 72 mph and then another 30 of 82 mph of mostly flat highways. The car is an 06 brm with a new timing belt and cam kit with currently 200k miles running rotella T 15w-40. 5 speed manual trans. All accelerating is always done at full throttle and cruise control is set once I'm up to speed. So theres the background on what I was doing.

So the first thing I did was EGR delete. Just simply deleted the EGR hyst maps in the ecu and unplugged the valve. My baseline result was 36-38 mpg.

Next up was changes to the injection timing maps. Stock for emissions they're completely stupid where there is several areas of injection after tdc and the whole map is generally not very agressive. So out of lazynesss I copied a cold engine temp map and replaced the hot temp map with it. This change brought me up to a consistent 39-41 mpg.

My most recent change was the n75 duty cycle. So in Europe they have the SDI NA 1.9 and it makes 103 ftlbs of torque at 1800-2400 rpm. And what this meant to me is that you can get atleast 103 ftlbs of smoke free torque from one of these motors without any boost at all. Stock the turbo is wound up all the time with cruise n75 duty cycles at 70-90%. All that having the vanes closed like that is doing is increasing the effectiveness of the EGR. So I made a handy excel spreadsheet that calculated the air fuel ratio of different boost values and found that the stock boost command gave an air fuel of up to 100:1 under light load. For reference the smoke limit is acceptable down to 18:1. Naturally aspirated the math said about 20mg/stroke of fuel is still above the smoke limit at 22:1 air fuel ratio. So I went back into the ecu and changed the target boost from 1000-3000 rpm down to a little above atmospheric for any injected quantity under 20 mg/stroke. Then I went into the n75 map and did a couple things. First I set idle vane position to wide open. I figured it's good to exercise the actuator and no reason to be running basically an exhaust brake at idle. Then I went into the below 20 mg area again and pulled 50% of the duty cycle out across the map. So now the duty is about 30-45% while cruising so the vanes are open about 70% assuming it's linear. Cruising down the road I'm making about 1-2 psi of boost where before it was 6-10 psi. And based in the last trips the boost map paid off in dividends.

Changing the vnt to be less restrictive brought me up to 44-46 mpg. By changing just the boost basically I was able to gain atleast 10% more fuel economy. No driving changes at all.
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Old June 28th, 2020, 08:31   #2
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Interesting!
Just on a side note, I noticed you are using 15w40... That's a sure way to need another camshaft soon... Your car is designed for a certain oil for the specific lubricant properties it has. That spec is 505.01 Volkswagen spec, which is a 5w40 full synthetic. I used 5w40 synthetic which wasn't 505.01 in my brm and had the camshaft go out again after only 30k miles from the first replacement.

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Old June 28th, 2020, 09:43   #3
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Don't mean to rain on your discovery, but tuners have known this for years. Basically it's a trade-off between fuel economy and "right now" response to power requests.
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Old June 28th, 2020, 09:56   #4
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15w40 is more likely to be better for the cam, 5w40 was introduced around the time VNT became available for faster oil to the turbo at cold start.
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Old June 28th, 2020, 10:30   #5
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Quote:
Originally Posted by jayb79 View Post
15w40 is more likely to be better for the cam, 5w40 was introduced around the time VNT became available for faster oil to the turbo at cold start.
It's not necessarily that it's 15w40, but it's the lubricant additive package that is required for better camshaft lubrication.
I learned my lesson and have begun using liqui-moly with their cera-kote additive too to help decrease friction.

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Old June 28th, 2020, 10:36   #6
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Quote:
Originally Posted by IndigoBlueWagon View Post
Don't mean to rain on your discovery, but tuners have known this for years. Basically it's a trade-off between fuel economy and "right now" response to power requests.

Yeah I am aware of this and I knew it before. I was more putting it out there for people who might want to tune their own stuff and want a starting point.
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Old June 28th, 2020, 10:38   #7
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Quote:
Originally Posted by Mozambiquer View Post
It's not necessarily that it's 15w40, but it's the lubricant additive package that is required for better camshaft lubrication.
I learned my lesson and have begun using liqui-moly with their cera-kote additive too to help decrease friction.

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I won't get into it too much but at the end of the day rotella T4 has substantially more antiwear additives than the VW spec.
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Old June 28th, 2020, 17:32   #8
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I would run a zinc rich 15W40 way before I would run any silly 5W30 VW oil that ruins cams.
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Old June 28th, 2020, 18:30   #9
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Ok... I didn't mean to get in a which oil to use war... I just had an experience where I got bit using the wrong oil and now I want to be sure that people know what can happen. Obviously it's working for you, and I won't tell you how to live your life, I'm going to keep using the OEM specified stuff in my car, plus cera-kote.

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Old June 29th, 2020, 13:34   #10
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Anyways I was pretty surprised how little of an effect running higher injection timing has vs removing the boost. I figured timing would have been worth atleast 3-4 mpg. I think I'm going to adjust my torsen setting since it's at -2.7 kw. So I'll report on if thats worth anything. This car is just a big science experiment anyways. I already figured out how to make it faster now I just gotta get the maximum fuel economy out of it. I'm really going for that elusive 50 mpg. Might be tough at 80 mph though.
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Old June 30th, 2020, 10:31   #11
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The turbocharger just magnifys what happens in non-turbocharged engines - the harder your drive it, the worse your MPG. OTOH - driven more gently - the turbocharger makes our TDIs actually work pretty well - enough zip to keep up with traffic (even if the HP isn't what the gassers have), and yet they provide enough air to help keep the exhaust "clean" (not sooty - unless you have a tune). Steady state driving with a turbo is just like steady state normally aspirated - only magnified.


Devon - the general rule on TDIs is speed + MPG = 120 (or so). So, if you're pulling 40 MPG at 80 MPH - you're not doing too bad - if you want 50 MPG - slow down - drive like you have an egg under your throttle foot. One thing I like about my TDIs is that I can drive like I like to and yet still get decent MPG.
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Old July 2nd, 2020, 04:33   #12
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Quote:
Originally Posted by scooperhsd View Post
The turbocharger just magnifys what happens in non-turbocharged engines - the harder your drive it, the worse your MPG. OTOH - driven more gently - the turbocharger makes our TDIs actually work pretty well - enough zip to keep up with traffic (even if the HP isn't what the gassers have), and yet they provide enough air to help keep the exhaust "clean" (not sooty - unless you have a tune). Steady state driving with a turbo is just like steady state normally aspirated - only magnified.


Devon - the general rule on TDIs is speed + MPG = 120 (or so). So, if you're pulling 40 MPG at 80 MPH - you're not doing too bad - if you want 50 MPG - slow down - drive like you have an egg under your throttle foot. One thing I like about my TDIs is that I can drive like I like to and yet still get decent MPG.

Yeah I don't drive gentle. Full throttle for every acceleration. The 45-46 I'm getting with the change in tune is excellent for 80 mph. Just want to squeeze every ounce of energy out of that gallon of diesel. Goes to show though how much VW had to leave on the table to satisfy the government.
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Old July 2nd, 2020, 05:25   #13
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Quote:
Originally Posted by IndigoBlueWagon View Post
Don't mean to rain on your discovery, but tuners have known this for years. Basically it's a trade-off between fuel economy and "right now" response to power requests.
I was just thinking that same thing, LOL.... "Welcome to 20+ years ago".

But yes, it is no secret that fuel economy does NOT necessarily go lock step with emissions compliance. And I can guarantee your ~5 MPG improvement made the emissions much, much worse with regards to NOx.

But I am in agreement with the silliness of the standards in general, and I also want the highest fuel economy as possible.

And this is something that really tears at me in some respects as I also like smokeless, odorless, clean tail pipes, too. But the baggage that carries with it has proven to be less-than-ideal for cost and durability. Not that the BRM could ever claim any of that to begin with, though.
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Old July 2nd, 2020, 06:20   #14
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Quote:
Originally Posted by devon1996 View Post
Yeah I don't drive gentle. Full throttle for every acceleration. The 45-46 I'm getting with the change in tune is excellent for 80 mph. Just want to squeeze every ounce of energy out of that gallon of diesel. Goes to show though how much VW had to leave on the table to satisfy the government.

Everyone says they want the best fuel economy possible, yet they are driving 80 mph+. If you really wanted to squeeze every ounce of energy out of a gallon of fuel you would slow down.
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Old July 7th, 2020, 10:39   #15
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Everyone says they want the best fuel economy possible, yet they are driving 80 mph+. If you really wanted to squeeze every ounce of energy out of a gallon of fuel you would slow down.

I guess a better way to say it is I want the engine to run with the maximum thermal efficiency possible. Going 55mph might get me 52 mpg but that isn't worth adding half an hour to my commute.
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