Wastegated vs. VNT for forced induction power

[TimpanogosSlim]

I've seen conflicting opinions about turbo suitability for performance so i thought i'd start a thread.

I want to be clear that the question is: over-all power and performance regardless of complexity. We know that the VNT requires more involved control methods.

I've seen it alleged that one can "make more power with a wastegated turbo" and that the VNT was introduced as an emissions device.

As for myself i think it is somewhat clever that they can make it behave like a smaller turbo at lower RPMs when a larger turbo might have trouble spooling up.

I can see where having enough boost to burn all of the fuel at low RPM could be considered an emissions concern, but having low-rpm boost is also a drivability and performance concern.

At the drag strip a turbo that kicks in at 3000rpm might be fun but not so much on the street.

What do you all think?

[bbob203]

I think the holset turbos Alcaid is pushing on paper seem to be the best answer to low end power and top end flow without needing a vnt. Vnts are cool but to convoluted to me as much as I wanted to set one up I'll just go for the holset.

[TimpanogosSlim]

alright, but what is it about the holsets that makes them that way?

[RabbitJockey]

alright, but what is it about the holsets that makes them that way?

it's just that they're newer technology, even newer garrett or borgwarner stuff would spool way faster than the antique stuff most people are running.

but when it comes to wastegate vs vnt, properly sized vnt is going to win, as it will allow for a broader power band, i've seen people say that a vnt turbo is more of an emissions device(quite possibly true but i have no idea), but i haven't seen anyone say you can make more power with a wastegated turbo.  vnt is better... but if you're going for just performance, i think most people will be plenty satisfied with a properly sized modern turbo, like the case is with schnell snail performance up in new england, he has a tdi in a mk2 gti with a gt2052, everyone that drives or rides in it says it is no slouch.  there are a lot of things to consider when you're talking a vnt and performance though.

[745 turbogreasel]

BMW's 380 HP 3.0 diesel comes with a  VNT equipped from the factory.  Maybe they are there for emissions only?
So is the R18 3.7 530HP, and I think it's ony emission requirement is don't fog the trackand make other drivers crash.
Also the Pugeot 908 is VNT

I don't think  Banks 7 sec drag truck does, but they use a lot of NOS instead.

[vanbcguy]

It's really difficult to compare apples to apples as there's so many differences from one technology generation to the next even within the same family - a first generation VNT is not the same as a 3rd generation VNT.  A 1970s era K24 is not the same as a brand new Garret GT series.

I wouldn't call a VNT turbo an emissions device, though it is certainly easier to meet emissions standards with one than with a wastegated turbo.

A properly sized wastegate turbo will have a "sweet spot" where the exhaust flow from the engine is perfectly matched to the size of the turbine and the desired intake airflow.  A VNT turbo will have a much larger "sweet spot" since it is able to adjust the shape of its turbine housing to match the needs of the engine.  But the maximum horsepower either design can make is still based on the amount of air it can push through the compressor, and the amount of exhaust that can exit through the turbine / wastegate.  The compressor side of VNT vs non-VNT turbos is identical, so there's nothing really to be said about that.  On the exhaust side you can probably get more flow through the combination of the wastegate and the turbine itself from a wastegate turbo than a VNT of exactly the same size, but the VNT allows you to go for a larger turbine with better flow characteristics without losing your spooling capabilities since it is capable of shrinking itself down when needed.

Really it comes down to what you are trying to do - if your goal is to make an engine that produces insane power at a fairly narrow power band then a really big wastegate turbo would probably do you great.  If you want the best overall driveability rather than peak horsepower numbers at a particular engine RPM then a VNT is the best choice. 

[TimpanogosSlim]

That's all really what i figured.

and, heck, "K03" can mean a lot of different things too.

my 1.6td is starting out with a k03 off of a mk3 jetta tdi, and should probably be very drivable from what i hear.

some day maybe a vnt15/17 hybrid? Or maybe a holset 211w is about the same money and better. I dunno.

[theman53]

I have driven/ridden with a few different turbos in different cars. Just about every time, in every different scenario, and all the vehicles, I like the bigger turbo better than a smaller one all things equal. I have a 2256v that I want to put in the jeep TDI I am building, but at the same time I want another of these Holset he221w that I have in the 1.6 jetta. I am just worried that the 2256v won't be big enough. As far as power is concerned, I daily a MKIV jetta with the factory vnt 15 and I don't think it is big enough. Tons of bottom, but it runs out of guts for me up top. It is hard to explain until you have sat in the seat with all sorts of different turbos and your personal taste before you find the best for you. I think if a big enough VNT were used it could be fun, but the small frame holsets like the he221w and the he211w are the way to go for me. I wouldn't even consider a vnt 15 or 17/22 unless I had a need and it was cheap enough I could save for a turbo I really wanted.

[745 turbogreasel]

Twin 15s probably be plenty, and cheap someday too.

[TimpanogosSlim]

I have driven/ridden with a few different turbos in different cars. Just about every time, in every different scenario, and all the vehicles, I like the bigger turbo better than a smaller one all things equal.

So, what I'm hearing is that your driving style has an emphasis on top-end power. Correct me if I'm wrong?

I, for one, hate turbo lag. I also live at an altitude where air is fairly thin, so a diesel without boost is pretty pointless for me.

My daily is currently a Mk5 GTI 2.0T currently at about 250hp. The factory k03 starts to spool at about 1850rpm and i have meaningful boost before 2000 rpm.

Previous daily was a 2003 GTI 1.8T and bone stock. I liked it, but even with the late-mk4 1.8T and k03, i was acutely aware that my right foot was just one variable in the drive-by-wire programming, and delivery of torque to the front wheels from a dead stop was something i ultimately only had an advisory role in.

I also test drove a 2001 GTI 1.8T - early mk4 with, what, 30 or 40 less horse than the late mk4, and i found the lag unacceptable.

I've also test driven a 2004 1.8T with a K04, and found it equally unacceptable.

So, the comments about having a 'wider sweet spot' kind of speak directly to me.

my current 1.6td project w/ 1z-spec K03, I realize i may still have some lag issues, but it was a financial decision.

I just know that at 5000 feet above sea level the 1.6 NA is too anemic.

[theman53]

If you feel the k03 doesn't spool fast enough then you will never have a turbo that does spool fast enough for you. The older designs don't spool as well as the newer stuff. My he221w spools better than either one of your 1.8T you have listed above, granted I am only around 1,300 above sea level.

The thing I don't like about the vnt is it is on or off and once on it is pretty much done. You get full pulling power almost instantly, but that is it. A bit over 3,000 rpm and mine is noticeably falling off. Where the holset in a much smaller engine setup starts to pull and keeps pulling as long as you ask it for more.

The question is when you ask about power, what power are you looking for?

[TimpanogosSlim]

When you say "The K03", understand that "K03" appears to refer to several different turbos made over the last 20 years or more.

[theman53]

I wasn't aware that the 1z TDI came with more than one option for that engine?

[vanbcguy]

There are VNT turbos that are much much larger than a VNT-15. The GTB1756VK I'm using is quite a bit larger plus it is good to a bit over 30PSI. Definitely can't say that of a VNT-15 or even a regular VNT-17. Plenty of folks over on TDIclub running GTB2260vks which are significantly bigger yet again.

There is nothing inherent in VNT technology that makes them run out of steam at 3K RPM. If your turbo runs out down low and you want more then you need a larger turbo, regardless of technology.

Sent from my HTC One XL using Tapatalk

[TimpanogosSlim]

I wasn't aware that the 1z TDI came with more than one option for that engine?

You know, I am myself confused.

It was sold to me as being from an AHU, but i thought i read that the AHU came with the GT15.

It's coming with the manifold that has a triangular flange for the turbo and a weird cast iron leg for EGR.

It has a big "KKK" logo on it so i'm pretty sure it's not a GT15.

Said to have come out of a canada-spec late 90's Jetta TDI.