Thread: Ideal boost at cruising speed

I have a 2256V, currently controlled by a Dawes valve. As I mentioned I think the vacuum on the actuator is too much so the boost isn't allowing the arm to drop away. I could fix this by reducing the vacuum so the arm drops away more readily under boost, this should reduce my boost at cruising as the vanes are more or less correct.

Now when I look into this more I can see that VNT control is actually fairly complex if you want to do it right. The actuator needs to rely on boost, rpm and throttle position. From this you can create a VNT map that is specific to the engine. So do I reduce vacuum for what is clunky VNT control or should I turn my 4BD1 into an engine with sensors hanging off it so I can control the VNT properly?

Surely you do the easiest mods first.

Originally Posted by inside

I have a 2256V, currently controlled by a Dawes valve. As I mentioned I think the vacuum on the actuator is too much so the boost isn't allowing the arm to drop away. I could fix this by reducing the vacuum so the arm drops away more readily under boost, this should reduce my boost at cruising as the vanes are more or less correct.

Now when I look into this more I can see that VNT control is actually fairly complex if you want to do it right. The actuator needs to rely on boost, rpm and throttle position. From this you can create a VNT map that is specific to the engine. So do I reduce vacuum for what is clunky VNT control or should I turn my 4BD1 into an engine with sensors hanging off it so I can control the VNT properly?

I wonder if there's a reasonable middle road between the Isuzu-ish (agricultural) Dawes setup and the full on electronic version with multiple inputs.
Maybe something like a simple switch controlled solenoid that bleeds vacuum/pressure giving you something along the lines of a cruise/power selection?

Steve

Originally Posted by steveG

I wonder if there's a reasonable middle road between the Isuzu-ish (agricultural) Dawes setup and the full on electronic version with multiple inputs.
Maybe something like a simple switch controlled solenoid that bleeds vacuum/pressure giving you something along the lines of a cruise/power selection?

Steve

Yes there are plenty of purely mechanical/pneumatic options. Feel free to try some and let us know.

Originally Posted by Dougal

Yes there are plenty of purely mechanical/pneumatic options. Feel free to try some and let us know.

Definitely on the todo list, but a bit difficult right now while my VNT is still on the bench

Steve

Originally Posted by Dougal

Yes I suspect both these guys are running simple boost actuator cans. Can you not adjust the closed vane position with a set-screw on those? I understand you ran one on a 300tdi for a while.

Many years ago when I replaced the carbie V8 with a 300Tdi in my rangie, i used a GT2256V and manifold from the Brazilian 2.8 litre update of the 300Tdi. This turbo was fitted stock with a boost pressure actuator, taking the boost signal from the compressor outlet. Yes they have a stop screw for the vane position, but I never touched it and because I didn't know then what I know now I never attempted to make it more sophisticated than what the Brazilians had done.

Originally Posted by inside

I have a 2256V, currently controlled by a Dawes valve. As I mentioned I think the vacuum on the actuator is too much so the boost isn't allowing the arm to drop away. I could fix this by reducing the vacuum so the arm drops away more readily under boost, this should reduce my boost at cruising as the vanes are more or less correct.

Now when I look into this more I can see that VNT control is actually fairly complex if you want to do it right. The actuator needs to rely on boost, rpm and throttle position. From this you can create a VNT map that is specific to the engine. So do I reduce vacuum for what is clunky VNT control or should I turn my 4BD1 into an engine with sensors hanging off it so I can control the VNT properly?

AFAIK the GT2256V I have is the only production VNT that has a boost pressure actuator - as per one of my posts a little earlier, OEM's are controlling them in ways that allow them to comply with emission requirements of the day (unnecessary for a 4BD1 powered 110 or even a tD5 version).

It has been a few years now (after removing the 300Tdi) that I came to the belief that the issue of high drive pressure at cruise (due to vane position as a result of lower boost pressure) is something that is worthwhile addressing.

Many people (with waste gate or VNT turbo's) only look at maximum performance results (peak boost and spool time), and are unaware of the implications of what is happening with the turbo system for the majority of it's operation.

With a waste gate turbo, it is the compromise that you make with the size of the turbine housing, but the VNT changes the game plan.

The simple Dawes valve system with a vacuum actuator, IMHO is not as good as the boost actuator on my GT2256V. The reason is that it doesn't reduce the vacuum until the boost pressure reaches the pressure required to open the Dawes valve.


A dual pressure system using 2 Dawes valves would be better - if it was arranged so the high pressure setting comes into play only when the accelerator movement is near full travel, and the lower setting for the rest of the time.


The programmable controller is going to be the most versatile, and the best solution for someone who is comfortable with working with the electronics and programming.

OK
So we are all talking boost pressure etc. what about discussing tube sizes????? Recently I changed all my tube from my turbo to intercooler and intercooler to manifold from 2" to 2.5". The intercooler is 2.5 in & out.

So now I have dropped pressure and reduced the heat generated into the pipe work. Now without testing the system I would assume the volume of air would roughly be the same??? Its just not being forced through the smaller tube.

The Rangy still performs just as well. EGT could be a whisker less possibly.

So what would be the prefered tube size then???? (open can of worms here)


Justin

Garrett also sell intercoolers and in their catalogue the say the piping to/from should be sized for a velocity between 200 and 300 ft/sec

So you need to calculate you volumetric flow then determine a suitable diameter to give the required area for the velocity. I have done this in a spreadsheet if you want to try it (zip file attached here).