I've been reading "Forced Induction Performance Tuning" by A. Graham Bell. In Chapter 8, which discusses 'Turbo anti-lag strategy', some of the technologies to reduce lag included: ball bearing turbos, ceramic turbines and most significantly the implementation of "Variable Nozzle Technology" (VNT).

Apparently Garrett make a multi-vane type VNT turbo. The vanes close at low RPM's (when exhaust flow is low) to provide a small nozzle size so as to maximize turbine wheel acceleration and speed. The vanes then progressively open as exhaust flow increases to minimize back pressure, control boost pressure, improve fuel economy and increase horse power. In effect, it works as a small A/R turbine at low speeds and like a big turbo with a larger A/R turbine housing at high speed.The book claims that boost response times of Multi-Vane VNT turbos are dramatically reduced when compared to ordinary turbos of similar size. Ordinary turbos take 70% to 90% longer to produce the same boost pressure.

This technology, although not new, seems to provide all the benefits of both large and small turbos. Has any one had experience with VNT turbos or know if Garrett is planning to make an after-market application for the 996TT?

Mike

 

Turbine Picture

 

Boost Chart Compare

 

Does the term 997TT mean anything to you?

 

Uggghhhhh..... no it doesn't. Are you saying the 997 will be using VNT?

 

Audi has already got this in production on a vehicle
The new diesel engine for the current (new) body style A8 uses this technology, with the exception that the variable nozzle is actuated by a electric motor rather than other means. If you would like to get more info on these checkout "Audi Driver" magazine with the Champion motorsports R8 on th cover. I believe this engine is a 3.7 liter tdi that gets the rather large A8 to 60 mph in less than 7 seconds not bad for a diesel.... Unfortunately this is an engine only available in Europe. Some Miata people have tried VTN turbos in the USA via a company called Aerodyne which produces this style turbo for smaller displacement applications (miata's, motorcycles etc.) As per the Miata people it seems as if the turbo eithers works well for a long time or self destructs shortly after installation. Take all this with a grain of salt as it has been filtered by many different people.

Colin Miller

 

I dont really know anything about engines etc, but I think the X5 3.0d has the turbo mentioned above as well ?


"BMW's straight-six 3.0 liter engine has a turbocharger with variable vane geometry, four valves per cylinder and connecting rods manufactured by BMW's innovative "cracking" process. The same engine is already featured in the 330d and 530d models. The engine produces 135kW of power at 4000 rpm and developing a maximum torque of 410Nm between 2000 rpm-3000 rpm. "

 

A while back, Chrysler had done a series of cars vith Garret T25's with VNT's. They were pretty cute. They had all kinds of longevity problems with them though. The nozzles would stick all the time. They just couldn't keep the pivot mechanism's alive in that environment.
That's pretty cool that Audi's trying it again though. I'm sure Weissach has something to do with the development and can get around some of the issues with them.
One biggy to consider though. If you think you're going to throw on a set of T3/T04E hybrids with a VNT turbine and get better boost response than the stock turbo with the 700+hp capability, you're dead wrong.
You have to consider the compressor side. And the compressors typically follow right along their surge line to produce boost as it is, even with a standard turbine housing. This is referring to the big compressors for high end stuff.
If you run a tighter turbine housing or a VNT and try to spool up the compressor too fast, it will just run into surge, and reverse flow for all kinds of drivability issues. We've seen this with some of the older generation T04B compressors mounted to standard -72 T3 turbine wheels. Just climbing a light grade would send both turbo's into a continual surge pattern back and forth. Kind of fun to hear, but if you stood on it, it couldn't get out of it and you had to let off, then get on it again to get the boost up.
The VNT's would easily have the capability of getting the most out of any compressor though. Especially with todays electronics. But you would need to completely map out the range and have the electronics control the turbo to keep it out of surge and keep it from overspeeding on the other end.
I guess, basically, they worked great on the little turbo's, but when you get into the big power numbers, there's just going to be some kind of compromise.
Hmmm. Maybe dual nozzles, one set on the turbine, one set on the compressor Come on Weissach, get to work

 

Seems as thought the down fall of a variable turbo is that there is a very small window of efficiency for this type of a turbo. Which is why they tend to be found on diesel, and work type of engines where the functional rpm range is rather narrow (1000-3000 rpm)
this is where the viability of a variable turbo come in to question on a car like the 996 tt where this engine may need the fast spool up down low, the variable turbo would not be able to flow enough air to feed the hungry monster behind the rear seat if you know what I mean. The theory is very good but the realistic application is limited to, as I said before diesels or work engines that have a small operating range and the turbo can be constructed to operate within this range.

 

Yup..
Either that, or typical, low power oem applications that don't have to cover such a monsterous range.
Consider this.
Even if you had an electric motor driving the compressor directly at full rpm's like Audi did for their hill climb car, you'd run into horrible surge problems at low speed. They got away with it due to the high rpm's utilized all the time, thus giving them boost on all the time. But forget about it if you have to drive the car around the grocery store parking lot with the wife

 

I agree doctor!!!!

 

Along the same lines, we had fooled around with sequential twin-turbo's to negate the poor effects of VNT stuff and utilize a single compressor on the low end and two for the high end stuff.
Interestingly enough, at low boost, the system was phenomenal utilizing only backpressure in the exhaust to switch it from low to high and maintaining about 1/1 boost/backpressure throughout the rpm band.
The thing drove like a truck though, with so much low speed torque and throttle response, it was actually difficult to pull away from a stop. As you lightly accelerated, it produced so much boost against the throttle plate that it would accelerate more and you'd back off, so it'd make more against the throttle plate and so on. Kind of a run away effect.
But for a 3.6, running on the single t3 at low speed, it would only produce about 450lbsxft of torque, even at 1.2 bar. But when it switched over to 2 turbo's the torque went up to 650lbsxft of torque at the same boost level.
The transition was the main problem at higher boost. Like I said, like the oems, at low boost, it was perfect. But anything over about 7-10psi, the transition was horrible. It would ramp to 1.2 bar, then drop to .5 bar, then ramp back to 1.2.
That's probably also the reason for all the rx7's and supra's going to a single turbo setup vs. trying to workout the sequential setup for high power.

Anyhow, enough rambling.
Just thought it would be interesting as it's along the same topic of trying to get better boost response.

 

I have a Variable Vane Turbo by Aerodyne (Instaboost ) on my M3. I must say it is quite a kick in the pants. I have full boost 11psi at 2100 rpms. Very different feeling than my PCar. This turbo has been on my car for 4 years (50K miles)now and has performed very well. An added benefit IMO is that there is no need for oil lines from the engine to feed the unit (it has a self contained oil wick that needs to be changed every 15K miles). Albit the unit was about 40% more expensive than a plain turbo it has its advantages.

Here is a DYNO sheet at 10psi: This is my 95 M3 (3.0L) stock dyno at the wheels was 194hp @ 6700 rpms it now makes 200hp @ 3500rpms. Check out the torque #'s! RWHP 327/ 360 ft/lbs. I have seen this type of turbo produce 430rwhp at 16psi.

 

Again, I'm hearing about this "zero clearance" stuff....
What gives there?
Obviously, "zero clearance" means nothing moves, so it's some term that's being used as advertising hype.
"Very close tolerance/clearance" might be more correct there. But again, what are they trying to get so close, the compressor to housing clearance? the turbine to housing clearance? the shaft to bearing clearance? the compressor to backplate clearance?
The efficiencies should improve a bit with less clearance between the wheels and the housings, but how about reliability in varying temperatures with aluminum expansion rates being so high. And the side loading of the bearings under stall/surge....

I assume you're "Kevin at Alamo turbo" that Duane has referred to building his turbo's for him. He's quite excited about them and read me off a brief description, but it also was a bit lacking.

Help clarify / correct this misnomer of "zero clearance" please...

 

What oils are they requiring to work with the clearances that are so much tighter??? It sounds almost like they'd be running dry if you're setting them so tight.
Also, is this just the shaft od to the bearing id, or the bearing od to the housing id also? I'm assuming again that you're still referring to the tin/floating bearings typically used in these.
And what's your theory on the 10-15% improvement in efficiency by tightening up the clearance?
Typically, when turbo's show up on our doorstep and they're too tight, they take a considerable time to break in and start performing properly, and this seems to go against your "improvement"???
I've just had two customers tell me about these, and I understand your excitement about new technology. It just doesn't make sense to me yet as I'm not quite grasping the underlying fundamental issue we're talking about yet.