It's because it isn't a logical thing to do. A fixed position that will give good low end boost will be restrictive. A fixed position that isn't restrictive won't give low end boost. Since there's no wastegate all the gasses have to go through the vanes regardless of what RPM and how much exhaust is coming out of your engine. High EMP at higher RPM becomes a concern as does overspeeding the turbo.
If you don't intend to use the vanes with some sort of control, be it mechanical or electronic, you would be MUCH better off selling the VNT and buying a wastegate turbo sized for your power goals and driving needs.
It's because it isn't a logical thing to do. A fixed position that will give good low end boost will be restrictive. A fixed position that isn't restrictive won't give low end boost. Since there's no wastegate all the gasses have to go through the vanes regardless of what RPM and how much exhaust is coming out of your engine. High EMP at higher RPM becomes a concern as does overspeeding the turbo.
If you don't intend to use the vanes with some sort of control, be it mechanical or electronic, you would be MUCH better off selling the VNT and buying a wastegate turbo sized for your power goals and driving needs.
I'm well aware of the cons. But I'm on a super tight budget, and I've heard more than one person claim that there's a "sweet spot" for a fixed position. They may be right, they may be wrong. I guess I'll have to go on a trial and error basis.
Keep in mind that I have almost zero dollars in this setup, which is about what I have to spend. I have to think that a fixed vane vnt has to be better than the 50 hp or whatever I was living with before...
I get the no-budget thing.
There will be a spot you can find with trial and error that will generally work for you. Your low end performance will suck, you'll have the equivalent of a cork in your exhaust at higher RPM but it will "work". The turbo may not last as long, but it will "work".
The yellow line in this graph is the vane position during a drive on my VNT (0=fully open, 100=fully closed) - there isn't really a 'sweet spot'. "Cruising" boost is around 4-5 PSI on my setup (graph is in kPa - 100 kPa = 14.7 PSI), depending on RPM that can be the vanes fully closed or it can be the vanes resting around 60%. With the pedal to the floor I'll hit about 30 PSI with the vanes anywhere from 40% to 20% depending on RPM. If I had them set at say "60%" all the time my turbo would explode absolutely 100% for sure. It would be 'safe' to have them at 20% but I'd have very little boost EXCEPT with the pedal to the floor at 4K RPM.
If you have no budget, then you probably can't afford to replace the turbo if you blow it up during your trial and error period. So
SELL it and use the proceeds to buy a wastegate turbo. You should come out about even, but have something that will last a long long long time.
You can also try building a mechanical VNT control system, there's a few versions lurking in the forum. That isn't particularly expensive, just time consuming.
Good info, thanks. I'm saving my pennies for a purchase from alcaid... Hopefully it won't take too long!