Thread: VVT Control

That page and it's graphs are just a touch mis-leading. Modern cars use PID control of the VNT solenoid and vacuum can to give a boost curve that's closer to the square edged ideal graph than any of the others he's shown.

Yes they have a small overshoot, but that's not a bad thing, with any PID tuning (especially with pneumatics) there is a tradeoff between the fastest rise time and overshoot. Car manufacturers leave the overshoot and even market it as "transient overboost", giving a little extra torque available during acceleration.

Personally I think the dawes valve and pressure actuators are over-rated. The GT2256V I have in the garage (still not fitted) has a pressure actuator I fitted years ago when I thought that was the only way. But it will be going back to a vacuum actuator before it gets installed.

Double post. The servers are quite busy lately.

Dougal is never going to get around to fitting that vnt.....

To answer your question, no, it's so quick you can't notice, pretty graphs etc are great to aim for perfection, just that in the real world it don't matter a lick, the Elec over vac systems will just be on/off, I'd suggest the term variable is a little misleading, yes it overshoots a few psi on changeover when going from coast to full load rapidly, but again, in reality it has no I'll effects, except occasional alarms from 30psi overboost on the madman gaugey do dad

Originally Posted by Dougal

That page and it's graphs are just a touch mis-leading. Modern cars use PID control of the VNT solenoid and vacuum can to give a boost curve that's closer to the square edged ideal graph than any of the others he's shown.

Yes they have a small overshoot, but that's not a bad thing, with any PID tuning (especially with pneumatics) there is a tradeoff between the fastest rise time and overshoot. Car manufacturers leave the overshoot and even market it as "transient overboost", giving a little extra torque available during acceleration.

Personally I think the dawes valve and pressure actuators are over-rated. The GT2256V I have in the garage (still not fitted) has a pressure actuator I fitted years ago when I thought that was the only way. But it will be going back to a vacuum actuator before it gets installed.

Yep thanks Dougal.

My little project is going to have a fandangaled algorithm implanted on a microchip that even Alby Einsten will struggle to work out. Might even draw some pretty little graphs to go with it when I am done

Originally Posted by the_grubb

But you don't need the vanes to change/any control to bring it to full boost.... the vanes could be positioned for minimum nozzle area, maximum nozzle area or anywhere inbetwen and it will still reach full boost.

The variable vanes allow you to vary the rate at which it reaches full boost whilst allowing it to not over boost.

Originally Posted by rovercare

Ok so that explains why with the exact same fueling the vnt on mine with vanes locked shut will obtain 18psi, locked open makes 0 psi, but with a pressure can changing the vanes, makes 30 psi, ta for that

So for those others that are interested;

If Rovercares turbo was put on say a larger capacity engine that was required to make say 15psi. Then with vanes closed making 18psi it would be over boosting and with vanes open and a big enough engine, there would be enough flow for the boost to rise.

What I said was a very simplyfied generic statement so people could understand a Variable Vanes operation.

But Rovercare is right - real world matters.

Originally Posted by the_grubb

So for those others that are interested;

If Rovercares turbo was put on say a larger capacity engine that was required to make say 15psi. Then with vanes closed making 18psi it would be over boosting and with vanes open and a big enough engine, there would be enough flow for the boost to rise.

What I said was a very simplyfied generic statement so people could understand a Variable Vanes operation.

But Rovercare is right - real world matters.

Correct, problem with the VNT is I'm limited in regards to fuelling to 650C pre turbo as with a non wastegated turbo, I can only control boost with fuel, 30psi, 650C is Ideal for a reliable setup up, but I'd like to abuse it, but that means machining the manifold to accept an external wastegate, which I don;t have the time for at the moment unfortunately, so its not quite perfectly matched for the engine size, but performs better than the previous, more go, less smoke

Either way, my advice might be wrong in theory, but its working in the real world

Originally Posted by the_grubb

Yep thanks Dougal.

My little project is going to have a fandangaled algorithm implanted on a microchip that even Alby Einsten will struggle to work out. Might even draw some pretty little graphs to go with it when I am done

Pretty sure he'd struggle, being dead and all

Originally Posted by the_grubb

Yep thanks Dougal.

My little project is going to have a fandangaled algorithm implanted on a microchip that even Alby Einsten will struggle to work out. Might even draw some pretty little graphs to go with it when I am done

Unlikely to be an algorithm unless you refer to look up tables as an algorithm. Most modern control systems such as fuel injection etc have maps/look up tables where the response has been predetermined as a result of some research/tuning by the engine/control system manufacturer.