Best Turbo for Fuel Economy-GTC1446VZ, GTC1549VZ, VNT-15, or VNT-15/17 Hybrid?

benhart16

Well-known member
Joined
Jun 9, 2011
Location
Seattle, WA
TDI
2000 Jetta tdi
Hi,

I'm wondering which of the following turbos would give the best fuel economy for a BEW Golf, automatic transmission, stock or only a mild increase in power, driven in a fairly normal commuter fashion (no hypermiling):

GTC1446VZ
CW : 34,1mm inducer / 46mm exducer
TW : 34,5mm exducer / 39mm inducer

GTC1549VZ
cw - 36/49mm
tw - 37.57/41mm

VNT15
cw-33.7/49mm
tw-36.6/43mm

VNT17
cw-37/49mm
tw-38.2/43mm

GTD1449vz
cw-37/49mm
tw-exducer: 36mm



I think the answer to which turbo is best for efficiency, is going to be the turbo that generates the most boost for the least amount of increase in exhaust backpressure in the operating conditions where the most fuel is consumed throughout the commute. If it's all highway on level ground, then the engine should be producing about a constant 20hp, so the turbo should be sized to be efficient at this speed/power level without choking the engine during acceleration/hills. If it's mixed driving, then the turbo probably should be sized slightly higher as the weighted fuel consumption will be at a higher hp.

I think this above fact points towards going to a smaller turbo, but I've also read that a VNT17 can improve fuel economy over a vnt15 as it reduced exhaust back pressure. Any thoughts on this? Is this true? I'm skeptical, but if someone has real world experience, I'll listen to it.

Back in the 70s and 80s when adding turbos to diesels was becoming mainstream, the literature at the time stated that adding a turbo to a naturally aspirated diesel could improve fuel efficiency. This seems to be the case with the early 1.6 VW diesels. I believe the turbo 1.6D rabbit was EPA rated at 63mpg or something close, while the NA version was around 50mpg. The question is what mechanisms would allow this to occur? Probably the first guess would be the added torque would allow the engine to be operated at higher gears when climbing hills, and thus more efficiently overall (I think the transmissions were identical). Also, adding a turbo effectively increases the (dynamic) compression ratio a little bit. But I think the less obvious contribution is that adding a turbo basically converts the engine to a combined cycle system. The energy scavenged from the exhaust can be used to reduce pumping loses of the engine, and even provide a slight amount of power on the intake stroke as there will be positive pressure on the piston. At the minimum, it will reduce pumping loses, thus improving efficiency. Even if this is small, these small amounts do add up. New generation turbos can generate higher boost pressures than the increase in exhaust backpressure.

I read a thread where a guy had a BRM that he started adjusting the tunes on. He found at normal highway conditions, the variable vanes were kept excessively closed (probably for EGR). When he edited the tune to open them up a bit, his fuel economy went from ~36 to ~42mpg. I can see where a large turbo would have to close the vanes an excessive amount to spool the turbo at the low end of the range would result in poor turbine efficiency, thus requiring excessive backpressure. A smaller turbine operating in the middle of it's range would likely yield much better results. Also, newer generation turbos provide the same boost for less exhaust backpressure than the older designs.

When VW released their TDIe (efficiency or economy) in Europe, they spec'd the GTC1446vz making 130hp. They then installed the GTC1549VZ on the
CR170 and later the GTD1449VZ on the CR184 and 190. As near as I can tell, the Boras with the various engines all are rated at about 53mpg. The GTC1446vz has the smallest turbine housing of any of the turbos. The GTD1449vz has a similar turbine housing as the VNT15, but can flow much much more. That's what 15 years of engineering improvements will do.

I have a manual ALH jetta with a GTD1449vz on it (11mm pump, 0.216 nozzles, FMIC), and my plan is to place a pressure port over the egr port and compare backpressure to boost. I also have an automatic BEW (mostly stock) that is getting a VNT15 at the moment, but will probably be replaced by a GTC1446vz and then a GTC1549vz if I really want to go through the effort to have comparable data. (I'm not putting a GTD1449VZ on the golf, that's too much of a PITA)

Any thoughts? What have you guys noticed on your projects?
 

Andyinchville1

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

I have read that VNT 17 turbos can get better highway MPG .....

I have seen compressor maps before but for the life of me could not read them.

I'm wondering if a VNT 17 gets better MPG can you go bigger and get more ? I know at some point you won't.

Boost is very important for MPG's .... when I had a boost leak I could only get about 46 / 47 MPG if driving easy .... otherwise I had a bit of smoke and bad MPG.

Andrew
 

2002_auto_tdi

Veteran Member
Joined
Aug 3, 2020
Location
Virginia
TDI
03 5spd wagon and 02 01m sedan
HI,

I have read that VNT 17 turbos can get better highway MPG .....

I have seen compressor maps before but for the life of me could not read them.

I'm wondering if a VNT 17 gets better MPG can you go bigger and get more ? I know at some point you won't.

Boost is very important for MPG's .... when I had a boost leak I could only get about 46 / 47 MPG if driving easy .... otherwise I had a bit of smoke and bad MPG.

Andrew
Hey Andy,

I read that the VNT17s are collectively responsible or producing less mpg all things considered when poking around. Of course, you are more dedicated in that area than me. I have a VNT17 that is on the wagon and have almost been thinking of downgrading to the VNT15 from my sedan eventually because of this. I would be interested in your expanded thoughts as always.

Thanks.
 
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benhart16

Well-known member
Joined
Jun 9, 2011
Location
Seattle, WA
TDI
2000 Jetta tdi
Hey Andy,

I read that the VNT17s are collectively responsible or producing less mpg all things considered when poking around. Of course, you are more dedicated in that area than me. I have a VNT17 that is on the wagon and have almost been thinking of downgrading to the VNT15 from my sedan eventually because of this. I would be interested in your expanded thoughts as always.

Thanks.
I think IndigoBlueWagon and others followed this exact path. They had larger turbos, then went back to the VNT 15 for better low end response and torque. Any of the other turbos I listed will run circles around the VNT15 while giving as good or better low end response. The only reason not to go to one of the other turbos is fabrication work is required as they are not bolt on. And this is absolutely a valid reason to stay with a VTN15 (the GTC turbos require less work than the GTD). The GTC1446vz has a smaller turbine than the VNT15, so it should give even better low end response, yet came on an engine rated at 150hp from the factory. Good luck getting a VNT15 to push 150hp.
 

2002_auto_tdi

Veteran Member
Joined
Aug 3, 2020
Location
Virginia
TDI
03 5spd wagon and 02 01m sedan
I think IndigoBlueWagon and others followed this exact path. They had larger turbos, then went back to the VNT 15 for better low end response and torque. Any of the other turbos I listed will run circles around the VNT15 while giving as good or better low end response. The only reason not to go to one of the other turbos is fabrication work is required as they are not bolt on. And this is absolutely a valid reason to stay with a VTN15 (the GTC turbos require less work than the GTD). The GTC1446vz has a smaller turbine than the VNT15, so it should give even better low end response, yet came on an engine rated at 150hp from the factory. Good luck getting a VNT15 to push 150hp.
Thanks for that. What are your thoughts on fuel economy with the turbo beyond what you posted? I would imagine that the VNT 17 (for example) would stay out of boost longer and therefore require less fuel compensation at the same time. This idealism may not apply with VW(s) and TurboDiesels but nevertheless a fun discussion.
 

benhart16

Well-known member
Joined
Jun 9, 2011
Location
Seattle, WA
TDI
2000 Jetta tdi
I honestly don't know which turbo would give the best fuel economy, which is why I started this thread to get input to see if anyone had knowledge on this topic. If I had to guess, the newer designs should perform better. I'd put my money on the GTC1446vz or the GTD1449vz. Like I mentioned earlier, the GTC1446vz was marketed as the TDIe for efficiency in Europe. Obviously, I don't know all the engineering details behind this, but I'm assuming there's a reason. When I get time, I'll take exhaust pressure vs boost pressure measurements on my ALH Jetta with the GTD1449vz as well as my other ALH Jetta with a stock VNT15 and see how they compare. Less back pressure for a given boost pressure should equal better fuel economy.
 

2002_auto_tdi

Veteran Member
Joined
Aug 3, 2020
Location
Virginia
TDI
03 5spd wagon and 02 01m sedan
I honestly don't know which turbo would give the best fuel economy, which is why I started this thread to get input to see if anyone had knowledge on this topic. If I had to guess, the newer designs should perform better. I'd put my money on the GTC1446vz or the GTD1449vz. Like I mentioned earlier, the GTC1446vz was marketed as the TDIe for efficiency in Europe. Obviously, I don't know all the engineering details behind this, but I'm assuming there's a reason. When I get time, I'll take exhaust pressure vs boost pressure measurements on my ALH Jetta with the GTD1449vz as well as my other ALH Jetta with a stock VNT15 and see how they compare. Less back pressure for a given boost pressure should equal better fuel economy.
Yes, sorry to answer your question with a question. I look forward to your measurements. Pulling out the popcorn. I hope that others join this thread as well soon.
 

benhart16

Well-known member
Joined
Jun 9, 2011
Location
Seattle, WA
TDI
2000 Jetta tdi
Check out the link below, it has TURBINE (not compressor) maps at the various vane positions:


I think this will shed a little light on the subject. The bottom graphic appears to confirm there is poor turbine efficiency at the low end, when the vanes are required to be nearly closed. Best turbine efficiency is achieved when the vanes are 60% open. Given the drastic difference in turbine efficiency (about 3x) between these two conditions, it would be reasonable to expect this to effect fuel efficiency. This also suggests the smaller turbos would provided better fuel efficiency, provided they aren't so small they choke the engine when higher power is required during acceleration and climbing hills.
 
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Andyinchville1

Veteran Member
Joined
Apr 7, 2016
Location
Virginia
TDI
2003 Jetta TDI wagon, 5 sp, 226K miles
I think they are out there and I've seen one awhile back but boost maps are helpful tools to look at ( i could not figure out how to read them tho...).

For max mpg you want maximum turbo efficiency in rpm range you operate in.... the turbo boost maps if you can read them should help.

Anybody have access to them?

Andrew

Ps - good thread .... my turbo has 445k miles on it so i am thinking of replacing soon (no current issues as far as i know .... just tdi guru felt like it may be prudent to swap out at some point

Re edit : i found 2 boost maps .... ill try to post ..
 
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Andyinchville1

Veteran Member
Joined
Apr 7, 2016
Location
Virginia
TDI
2003 Jetta TDI wagon, 5 sp, 226K miles
I found 2 boost maps.

I'll try to post them here.

Actually , i tried to put pics here with no success .... I have done pictures before but usually it's hit or miss.

I did upload the maps to the media Gallery though

Maybe somebody could find those and post them here?

But getting back to the Boost Maps ... I think there are ways of reading boost Maps so you can figure which turbo would be best for you based on RPM, boost levels and efficiency excetera?

Does somebody know how to do that as well?

Andrew
 
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