Choking means the turbine can't pass more gas without either a decrease in temperature (EGT)or an increase in pressure.
Wastegated turbos run in choke and use the wastegate to increase/decrease drive pressure to regulate the power the turbine produces and thus regulate boost.
But on a VNT you've got no wastegate, so if your airflow requirements surpass the flow capacity for the turbine you're in for a lot of pain. The turbine starts to need a lot more pressure to pass more exhaust and this makes drive pressure spike and kills engine power but still delivers boost. Giving you the false impression that it because it's still making enough boost it should be working well.
It's a similar situation to clamping the wastegate on a turbo that needs the wastegate. You hit the wall power wise but still have plenty of boost. EGT can be okay but if you give it more fuel, it chokes even worse, hitting the wall at lower rpm.
On a 3B you'll probably be okay with a GT2256V. But for the 4BD1T I've run a couple of scenarios between a GT2256V and a GT2259 turbo with the same turbine but wastegated. The GT2259 was only a few psi behind at very low rpm and killed the V everywhere else. I've cut and pasted it here:
1. GT2256V VNT used on the merc ML270 (I have one of these turbos).
This turbo has a 0.64 A/R turbine housing and I've assumed the VNT can pull that down to about half the A/R.
2. GT2259 wastegated (used on Hino and Iveco diesels).
This turbo has a 0.56 A/R turbine housing and the compressor can flow about 10% more.
The results are interesting.
Low End.
Essentially the GT2259 turbo still spools just as quick. Both these turbos can provide enough boost to burn 140cc of fuel and produce over 600Nm of torque by 1400rpm.
And the wastegated turbo does with with less drive pressure.
Call it even.
Mid-range.
They are pretty much equal at 2000rpm, the wastegated turbo is running less drive pressure (because the turbine is more efficient without the extra vanes).
They can both provide more boost than needed through the mid-range, for the wastegated turbo that is no problem. You simply use the wastegate to cap boost and this lets the turbine breathe a bit easier.
But the VNT turbo still has to push that exhaust through the same turbine. The only way to get less power from the turbine is lower exhaust temp. The only way to do this is to run extra boost which costs power and spikes the exhaust drive pressure.
By 2,500rpm the VNT is running 5psi more drive pressure than the wastegated turbo.
Winner, the wastegated turbo.
Top End.
The VNT has a smaller compressor, it can only do about 27lb/min which starts to cap power at the 2,700rpm point. Now we've already had to drop off max fuel with this turbo as the turbine starts to spike and not be able to flow all the exhaust at max EGT.
So from here we're continually droppping fuel and boost to stay within the limits of the turbine and compressor.
In fact, we passed peak power (~150kw) at about 2,500rpm and have been losing ever since.
Drive pressures also keep spiking and are over 30psi by 3000rpm. Basically the party is over and it's time to change gear.
The wastegated turbo keeps producing more boost and power to beat the engines dropping VE. The larger compressor doesn't start to cap power until around 3,200rpm. At which point 140cc of diesel will be delivering around 180kw.
Drive pressures are comparable to the VNT, but the boost and power are well head.
Winner. Wastegated GT2259.
Summary GT2256V
Torque over 600Nm from 1400-2400rpm
Max power approx 150kw at 2,500rpm. 27psi here.
Limited by compressor and turbine flow limits.
Summary GT2259
Torque over 600Nm from 1400-2600rpm.
Max power approx 180kw at 3,200rpm. 26psi here.
Limited by compressor flow.
So this size VNT turbo has no real advantage on our engines unless you are already limited to lower power through other driveline concerns. We need bigger VNT's for these engines, a GT2359V might do it, but I have no info on those.
Clearly Hino, Iveco and the others running GT2259's on 4-5 litre diesels have already done this work. It's always good to agree with the big boys.
VNTs are my absolute favorite thing about TDIs
