VNT is killing your fuel economy

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
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.
 

Mozambiquer

Vendor , w/Business number
Joined
Mar 21, 2015
Location
Versailles Missouri
TDI
1998 VW Jetta TDI. 1982 VW Rabbit pickup, 2001 VW Jetta TDI
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.

Sent from my S60 using Tapatalk
 

IndigoBlueWagon

TDIClub Enthusiast, Principal IDParts, Vendor , w/
Joined
Aug 16, 2004
Location
South of Boston
TDI
'97 Passat, '99.5 Golf, '02 Jetta Wagon, '15 GSW
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.
 

jayb79

Top Post Dawg
Joined
May 20, 2000
Location
Exeter,NH
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.
 

Mozambiquer

Vendor , w/Business number
Joined
Mar 21, 2015
Location
Versailles Missouri
TDI
1998 VW Jetta TDI. 1982 VW Rabbit pickup, 2001 VW Jetta TDI
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.

Sent from my S60 using Tapatalk
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
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.
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
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.

Sent from my S60 using Tapatalk

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.
 

Mozambiquer

Vendor , w/Business number
Joined
Mar 21, 2015
Location
Versailles Missouri
TDI
1998 VW Jetta TDI. 1982 VW Rabbit pickup, 2001 VW Jetta TDI
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.

Sent from my S60 using Tapatalk
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
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.
 

scooperhsd

Top Post Dawg
Joined
Aug 19, 2003
Location
Kansas City KS
TDI
NB, 2000, RED(5 Speed conversion) 2015 Golf SE
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.
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
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.
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis (where it's safe)
TDI
There are just too many to list....
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.
 
Last edited:

Lightflyer1

Top Post Dawg
Joined
Sep 13, 2005
Location
Round Rock, Texas
TDI
2015 Beetle tdi dsg
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.
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
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.
 

Lightflyer1

Top Post Dawg
Joined
Sep 13, 2005
Location
Round Rock, Texas
TDI
2015 Beetle tdi dsg
You could probably drop your speed to 65 or even 70 mph the whole way, from how you described your "commute", and have much better fuel economy and it would add mere minutes to your 50 mile one way commute. Not even close to half an hour.
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis (where it's safe)
TDI
There are just too many to list....
There is a sweet spot for highway fuel economy. If you go so slow the engine is below that RPM range when in high gear, there is diminishing returns. And time does add up.

We had someone who would drive SO SLOW to TDI fests that he had to take an extra day off work, and depending on where the fest was, it could be two or more extra days off work. Just to prove a point I guess, not sure. But I can tell you I'd be nervous as all heck if I was doing 50 on a modern interstate. I just got back from a long trip, some of it hanging near the 85 MPH speed limiter on the Sprinter, and people were still passing me. I was doing 75 or so most of the trip, and that was pretty middle of the road speed average, including most semi trucks.
 

Mozambiquer

Vendor , w/Business number
Joined
Mar 21, 2015
Location
Versailles Missouri
TDI
1998 VW Jetta TDI. 1982 VW Rabbit pickup, 2001 VW Jetta TDI
We had someone who would drive SO SLOW to TDI fests that he had to take an extra day off work, and depending on where the fest was, it could be two or more extra days off work. Just to prove a point I guess, not sure. But I can tell you I'd be nervous as all heck if I was doing 50 on a modern interstate
I have done that before... Once, I had plenty of time and was driving my old 2 door rabbit and I drove a few hundred miles at 45 mph. I got 47 mpg that tank, even though I had the four speed transmission that was not geared for economy... My rabbit pickup was a lot better, and I could get 55 mpg if I kept it to 55 MPH. A turbo changed that though...

Sent from my S60 using Tapatalk
 

Lightflyer1

Top Post Dawg
Joined
Sep 13, 2005
Location
Round Rock, Texas
TDI
2015 Beetle tdi dsg
No one is saying drive 40 on the interstate. But slowing down to 65/70 mph isn't all that hard to do and produced stellar results in my fuel economy in my old 2013 Passat, when doing lots of highway driving.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
iirc even with the vanes wide open at 70mph cruise mine was showing 8 psi exhaust manifold pressure with the stock turbo, that was with like 1-2 psi boost
the stock turbo in general is just terrible for fuel economy, it's like they sized it to be efficient if you're driving 50mph with a 1.0l motor

go to something like a td04hl15t-5cm from a junkyard saab 2.3hpt and you'll notice a significant increase in fuel economy and power

ETA: I've done the 55 in a 70 thing, mostly while towing really stupid overloaded trailers with decades old tires and no spares to speak of.
It's downright dangerous holding up traffic like that. Sure, you save some fuel but at the cost of getting rearended from time to time? No thanks. I'll stick with a couple mph less than traffic (so like 74 indicated), then I've only gotta pass maybe 2 people on my 70mi commute.
 
Last edited:

d24tdi

Veteran Member
Joined
Jan 6, 2019
Location
MT
TDI
96 B4V
Nice to see some real-world numbers on these particular engines to back up the theoretical aspects, thanks for sharing this.

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.
Yes, this is where I think you really are right on, and surprised others replying haven't hit on this more. The real motivating application with VGT tech is not just response but the ability to raise exhaust manifold pressure even when the engine is running at relatively light load and thereby drive EGR mass flow to manage NOx. Without VGT (e.g. with a simple wastegated turbo or no turbo or wastegate at all) there is no simple way to do this other on a diesel that has no intake manifold vacuum, other than to install a butterfly valve in the exhaust and/or a throttle valve for the intake, which some other manufacturers did resort to -- including VW of course, in the case of the throttle valve that was introduced starting with the PD motors and whose job is not just to control shutdown shudder but also help draw in more EGR.

The fact that running the vanes in the closed position under light load also greatly improves transient response is a very nice side benefit for low speed driveability and performance. But I would wager that when the decision was made to implement costly VNT tech for economy-car TDIs and many other diesels, the real driving force was emissions compliance that demanded increasing use of EGR, both increasing in terms of max flow potential and also broader operating scenarios where it would be needed.

The same is true with VNT introduction on the Ford/IHC 6.0L versus the 7.3. 6.0 had EGR for the first time and in order to make it work, control of backpressure with VNT was necessary. Again, it did provide a dynamic response windfall in terms of performance too, but I suspect if it hadn't been for the need for exhaust backpressure for EGR, they would have stuck with a tried and true solution that cost less to implement and delivered the same ultimate output.

It's just another case IMO where increasing emissions stringency actually has *improved* performance potential over the long run. People whine about emissions regulations and yes there are sometimes headaches when new technology is first introduced and not yet mature, but tightening emissions laws helped bring us electronic fuel injection in gas cars, common rail in diesels, and most of the other things that enable the remarkable performance and reliability (mostly :p) of modern internal combustion engines. Not to mention cleaner air to breathe.
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
iirc even with the vanes wide open at 70mph cruise mine was showing 8 psi exhaust manifold pressure with the stock turbo, that was with like 1-2 psi boost
the stock turbo in general is just terrible for fuel economy, it's like they sized it to be efficient if you're driving 50mph with a 1.0l motor

go to something like a td04hl15t-5cm from a junkyard saab 2.3hpt and you'll notice a significant increase in fuel economy and power

ETA: I've done the 55 in a 70 thing, mostly while towing really stupid overloaded trailers with decades old tires and no spares to speak of.
It's downright dangerous holding up traffic like that. Sure, you save some fuel but at the cost of getting rearended from time to time? No thanks. I'll stick with a couple mph less than traffic (so like 74 indicated), then I've only gotta pass maybe 2 people on my 70mi commute.

On my A4 bhw swap I am very tempted to go to a wastegated turbo for that reason. But as others said the vnt has major benefits for transient response. If you want a larger gated turbo to spool quickly your gonna have to deal with a plume of smoke before it lights. On the 5.9 common rail Cummins with their "in cylinder EGR" they used an electronic wastegate and a super tight exhaust housing to force higher exhaust backpressures so they're retarded cam timing would allow for more exhaust to flow back into the engine. But that killed fuel economy too where a 2001 5.9 gets 19 mpg the 2003 only got 15. But man those turbos spooled quickly. It's all a trade-off between response and fuel economy and emissions with emissions obviously being the most important.
 

Lightflyer1

Top Post Dawg
Joined
Sep 13, 2005
Location
Round Rock, Texas
TDI
2015 Beetle tdi dsg
ETA: I've done the 55 in a 70 thing, mostly while towing really stupid overloaded trailers with decades old tires and no spares to speak of.
It's downright dangerous holding up traffic like that. Sure, you save some fuel but at the cost of getting rearended from time to time? No thanks. I'll stick with a couple mph less than traffic (so like 74 indicated), then I've only gotta pass maybe 2 people on my 70mi commute.
You are doing some really dangerous things if you are doing as you said. Speed should be the least of your worries. Maybe get your overloaded trailer with the old worn out tires off the road. As I said before you don't have to slow to insanely slower speeds. 65 to 70 is usually enough to see a decent return. If you are seriously holding up traffic then speed up or take another road or wait. Doesn't anyone have any common sense any more?
 

Mozambiquer

Vendor , w/Business number
Joined
Mar 21, 2015
Location
Versailles Missouri
TDI
1998 VW Jetta TDI. 1982 VW Rabbit pickup, 2001 VW Jetta TDI
I wonder if changing the vnt settings on my b5.5 Passat would help with the fuel economy, and if any of the tuners do that in their tunes.

Sent from my S60 using Tapatalk
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
I wonder if changing the vnt settings on my b5.5 Passat would help with the fuel economy, and if any of the tuners do that in their tunes.

Sent from my S60 using Tapatalk

I can't post pictures but I pulled up a "pro tuned" file that I have. They cut back on n75 duty through pretty much the whole map. So atleast this tuner did do that.
 

Andyinchville1

Veteran Member
Joined
Apr 7, 2016
Location
Virginia
TDI
2003 Jetta TDI wagon, 5 sp, 226K miles
Hi

Just curious but alot of time when I'm driving 65 to 70 I notice I have about 2 to 3 lbs of boost even steady cruising.... if it could be molded to get near zero ok boost under cruise conditions , what could I expect in terms of additional fuel economy?

I have read online that the biggest restriction in the exhaust is the turbo...

I'm wondering if one could rig a manual bypass on the exhaust to route exhaust around the turbo when wanting max mpg (of cbb ourse that may also require a routing of intake around the turbo also?l)

I would like to seek out more mpg...

What do you think?
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
Hi

Just curious but alot of time when I'm driving 65 to 70 I notice I have about 2 to 3 lbs of boost even steady cruising.... if it could be molded to get near zero ok boost under cruise conditions , what could I expect in terms of additional fuel economy?

I have read online that the biggest restriction in the exhaust is the turbo...

I'm wondering if one could rig a manual bypass on the exhaust to route exhaust around the turbo when wanting max mpg (of cbb ourse that may also require a routing of intake around the turbo also?l)

I would like to seek out more mpg...

What do you think?

You could use the EGR port as a dump valve but you probably wouldn't see much increase in economy. You wouldn't need to bypass the intake side of the turbo though.

Your already tuned so they took pretty much all the boost out of cruise already. Stock I was at like 6-8 psi.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
You are doing some really dangerous things if you are doing as you said. Speed should be the least of your worries. Maybe get your overloaded trailer with the old worn out tires off the road.
hey hey hey hey
hey
they have plenty of tread, just cracks that you can see the cords through
they'll let go eventually, then I've got another pair that were on the garbage tire pile that I'll mount on the wheels that are quite a bit fresher
ain't got a tire machine at the moment, always easier to dismount a tire that's been blown up and driven on a little while :p
On my A4 bhw swap I am very tempted to go to a wastegated turbo for that reason. But as others said the vnt has major benefits for transient response. If you want a larger gated turbo to spool quickly your gonna have to deal with a plume of smoke before it lights.
ehhh, above 3k rpm it lights off instantly, in fact, high idle (mechanical governor terminology here) my td04hl19t-5cm would make 15-20 psi of boost, and when it is cold out, the combustion temperatures get so low that it actually starts misfiring and pouring white smoke
putting around town, it drives only a tiny bit more sluggish than a stock ALH, certainly faster than one with a plugged up intake will do flat out, and after a couple seconds it's gone and spooled up making twice what a stock ALH would make peak. I'd rather wait a second for more poop than just have pitiful poop instantly, nahmean?
You could use the EGR port as a dump valve but you probably wouldn't see much increase in economy. You wouldn't need to bypass the intake side of the turbo though.
pre-turbine exhaust leaks typically kill MPG
saab did it long ago, holding the wastegate open at light load, but that is a vacuum-throttled gasoline engine where you don't see the decrease in pumping losses from the (small) boost made at steady-state cruise
 

bigsexyTDI

Veteran Member
Joined
Sep 28, 2005
Location
Kentucky
TDI
'98 NB, '04 Jetta
I gotta admit, I like seeing the newbies actually doing research on this...

Everything you seek is all here young man. You just might have to really dig for it...
 
Top