VNT is killing your fuel economy

PakProtector

Veteran Member
Joined
Jan 5, 2014
Location
AnnArbor, MI
TDI
Mk.4 and the Cummins-es
Ultimately, thermal efficiency is bounded by compression ratio( effective compression ratio, boost, or vacuum, or what ever). As just written about, the cost of creating this additional compression ratio with the exhaust turbine creating back pressure is costing more than it is worth. Not much of a surprise; read some of the efficiency measurements on VNT's...they kinda suck even if they deliver the 'right NOW' boost that is often desired.
cheers,
Douglas
 

MAXRPM

Veteran Member
Joined
May 7, 2008
Location
US
TDI
00 Jetta and 99.5 Golf, 2015 Passat TDI,BMW 2
This is like if you drive like a granny you'd get up to 60mpg on the highway if you drive it like you stole your own car you would get 38 to 40, do the math, do you like to stump your car and go like a missle of course you'd take a hit, why do we bump up our cars to get better mpgs? No, we knew all along about this.

Fact is do you drive for mpgs or do you drive it and punch on the pedal all the time, for me I want both, I get it accomplished by 2 different ways,1) ECU with switchable programs where 1 will give me the most and the other restrict the limit on my car,,,, 2) drive it like a granny and I get 46 to 48 mpgs, so even leaving the ecu at ultimate setting i will accomplish the 46 to 48.
 
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devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
This is like if you drive like a granny you'd get up to 60mpg on the highway if you drive it like you stole your own car you would get 38 to 40, do the math, do you like to stump your car and go like a missle of course you'd take a hit, why do we bump up our cars to get better mpgs? No, we knew all along about this.

Fact is do you drive for mpgs or do you drive it and punch on the pedal all the time, for me I want both, I get it accomplished by 2 different ways,1) ECU with switchable programs where 1 will give me the most and the other restrict the limit on my car,,,, 2) drive it like a granny and I get 46 to 48 mpgs, so even leaving the ecu at ultimate setting i will accomplish the 46 to 48.

Yeah I am pedal to the floor any time I'm accelerating. City mileage isn't great but my mostly highway commute is still 44-46 mpg going reasonably fast
 

atikovi

Veteran Member
Joined
Jun 22, 2002
Location
Suburban Washington DC
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.
No idea if this setup is like what I have on my '06 but the little hose to the turbo is off and plugged for other reasons and I average 55 mpg overall

and got 65 mpg on a 1,000 trip to Florida last week at around 70 mph.

Engine has a RocketChip tune which may or may not have anything to do with it.
 

devon1996

Well-known member
Joined
May 7, 2020
Location
Denver
TDI
08 A4 tdi swap
It doesn't matter what the computer is telling the turbo to do if it's unplugged haha. So there it is if you have no boost ever you'll get 55 mpg with the vanes open all the way all the time.
 

Andyinchville1

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

Just curious but is that a hard mod to do? ... the turbo that is

Want results can be had by doing it?

How about a 17 series turbo or a 20 series turbo which seems more "normal' as far as mods?

Thanks

Andrew
 
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