Ideal boost at cruising speed

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Originally Posted by Bush65

Dougal, you, I and others here know that the energy/enthalpy in the exhaust gas (that is converted to torque by the turbine) is a function of temperature and pressure.

What I was saying is that at cruise the boost pressure should not be high - not as high as in the case of the 2 members with VNT turbos who have asked about their high boost at cruise in this thread. I agree with what you say, but feel you are making arguments that might persuade them into not to look into improving the control of the vnt turbo with respect to reducing drive pressure at cruise - cruising here 700+ km days are regular for outback trips, where fuel prices per litre can be staggering.

I know the EGT will be down at cruise, this is natural because we are not burning as much fuel. Because we are not burning as much fuel we don't need as much air/oxygen to burn it, thus do not need as much boost.

If we suddenly plant the right foot we would need more boost to burn the extra fuel that we have just caused to be injected, but EGT will be low (until the extra fuel is burnt) then the VNT can restrict the exhaust gasses to increase the drive pressure.

I'm guessing the Pete Bell vnt kit doesn't control the vanes via the TD5 ecu (I assume the ecu doesn't have unused outputs for that).

One reason most newish diesels have gone to vnt turbos is so they can control the boost and drive pressures to allow more egr events to meet emission targets.

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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.

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.

Quote:

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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?

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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

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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

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

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Originally Posted by Dougal

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

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

Steve

Quote:

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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.

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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.

Quote:

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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?

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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.
https://www.aulro.com/afvb/images/im...013/02/703.jpg

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.
https://www.aulro.com/afvb/images/im...013/02/704.jpg

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):D


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).

Hi, this thread has been interesting reading as I have also been wondering about my set up. It seems that we should aim for a low boost pressure at cruise for efficiency etc but I have just found the following to give you all something else to think about.

Firstly my vehicle is a 130 dual cab, 4bd1 with VNT intercooled, 3" exhaust. I've only just got it on the road and have done just done a few road trips.

Cruising at about 95kph (105 on speedo) on flat road the boost is 18 psi and EGT about 380 degrees. Was pleasantly surprised to check fuel economy was;
* 9.3 L/100 (family of 5, light load, hwy driving)
* 10.5 L/100 (heavy load, hwy driving)
* 14.4 L/100 (heavy load + towing overloaded tandem stock trailer with tall load, hwy driving)
* 11.5 L/100 (towing empty tandem stock trailer, hwy driving)

Was good test as each leg was over 350km.

I know the boost seems high but the thing gets great fuel economy so if it ain't broke don't fix it?

Only reason I can think of for good economy is that it produces massive torque off idle so the throttle is barely a 1/4 depressed while cruising (need to fit the cruise control cos foot gets sore ricked up in the air!)
Cheers, Andrew