Ideal boost at cruising speed

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Originally Posted by taylorslandy

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

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I'd be pretty happy with that.
Any chance of a bit more info on your setup?

• Which model turbo are you using?
• EGT measurement pre or post turbo?
• What size/type of intercooler?
• What sort of control system for the VNT?
• Is 18psi your max boost, or just what you're getting at cruise?
• What are you seeing for max EGT?

Steve

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Originally Posted by steveG

I'd be pretty happy with that.

Any chance of a bit more info on your setup?

• Which model turbo are you using?
• EGT measurement pre or post turbo?
• What size/type of intercooler?
• What sort of control system for the VNT?
• Is 18psi your max boost, or just what you're getting at cruise?
• What are you seeing for max EGT?

Steve

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Hi Steve, I can't take the credit for setting the turbo up as I bought the vehicle with it done, and therefore don't know all the details but I will find out. I'm sure you will know of the vehicle, it was "rovercares" old extra cab ute which I've made into a dual cab 130.

• turbo is a Garrett VNT off a hino truck (i think?) cant see numbers
• EGT is pre turbo
• intercooler is a big bastard "plazmaman pro series" water to air
• VNT control is simple boost pressure actuator (pretty sure, will check)
• cruise at 18psi but max boost is 30psi, often mid-high 20's up hills
• max EGT 750 but don't let it get over 700 much.

hope this helps, Andrew.

Ahhh - that makes sense now.
VNT is GT2260 from Iveco turbo daily and control is boost actuator with a bleed off valve from memory.
I'm impressed at the economy you're getting out of it, considering its fueled up as high as possible. I got mid 13's average over the 6000k's of our Cape York trip last year - not towing and weighing just under 3T.
The 18psi cruise you're getting compared to my 7psi doesn't seem to be killing your economy. In your position I'd love it and leave it alone.

Steve

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Originally Posted by steveG

Ahhh - that makes sense now.
VNT is GT2260 from Iveco turbo daily and control is boost actuator with a bleed off valve from memory.
I'm impressed at the economy you're getting out of it, considering its fueled up as high as possible. I got mid 13's average over the 6000k's of our Cape York trip last year - not towing and weighing just under 3T.
The 18psi cruise you're getting compared to my 7psi doesn't seem to be killing your economy. In your position I'd love it and leave it alone.

Steve

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Thanks Steve that jogged my memory, I was nearly right, hino/ iveco, still a truck! Good to have model of turbo. It does seem funny how such high boost, max fuel seems to give great economy, but I'll take it! Like i said you really only need 1/4 throttle at cruise. If motor blows up I'll know it wasn't a good idea I suppose. Andrew.

As I said before, we need to know what the drive pressure is compared to boost pressure.

Without that we are fumbling in the dark.

High drive pressure at cruise is a problem that VNT turbos are susceptible to. And many of the VNT turbos that people are finding and slapping on alternative engines have small turbines because they are more common.

GT22xxV is as tight a turbine as I would be pleased to put on a 3.9 litre diesel that is going to do a lot of high speed km's. There seem to be more VNT smaller than this, and those with the GT25xxV turbine are like the proverbial hen's teeth - here I have used xx for the compressor size, where 56 what I consider minimum, 59 or 60 preferred for a high performing 4BD1 (fuel rate at or near maximum and pulling a load).

These larger VNT turbos are what BAS and others are supplying for 2.5 litre diesels like the 200/300 Tdi and TD5, also IIRC are stock on the 2.7 litre V6 in the likes of Disco 3.

Edit: Compressing air consumes considerable power. Here are some figures of the HP required to drive the turbo compressor (not including mechanical/friction losses in the turbo, or turbine efficiency). For a 3.9 litre diesel, assuming VE 0.85, and intercooler effectiveness 0.65

At 2200 rpm cruise, 18 psi boost, and compressor efficiency 0.7, requires 21.4 HP
At 2200 rpm cruise, 18 psi boost, and compressor efficiency 0.65, requires 23.2 HP
At 2200 rpm cruise, 12 psi boost, and compressor efficiency 0.7, requires 12.9 HP
At 2200 rpm cruise, 18 psi boost, and compressor efficiency 0.65, requires 13.9 HP

At 2400 rpm cruise, 18 psi boost, and compressor efficiency 0.7, requires 23.4 HP
At 2400 rpm cruise, 18 psi boost, and compressor efficiency 0.65, requires 25.3 HP
At 2400 rpm cruise, 12 psi boost, and compressor efficiency 0.7, requires 14.0 HP
At 2400 rpm cruise, 18 psi boost, and compressor efficiency 0.65, requires 15.2 HP

Now we are given to believe that this power is free because it is recovered from the heat in the exhaust gasses, but this is only true for that part of the drive pressure below boost pressure. If drive pressure is greater than boost then some of the engine power pumping the exhaust out is not recovered.

With my set up of GT2559V with air to air intercooler the size nearly of a radiator.

Using my personal dyno (the feel in your back from the seat - highly technical I know)

I find that 7-8psi is perfect when cruising on flat road at 100Km/hr.

Any more and it doesn't improve anything, but any less and it becomes sluggish.

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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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As a simple solution I would combine a by-pass with the dawes valve. A switch which operates a solenoid (ie. ARB airlocker solenoid) that gives the actuator full vacuum to open it up. Then you could plant a throttle valve to bleed a little pressure to tune to desired boost (crude still).

'Normal' driving uses dawes, then cruising flick switch. Plant foot for hill/overtake/what ever flick switch back.


Personally don't see the issue with a complete electronic control VNT. It's one thing say to drown the ECU of your electronically injected diesel engine and be stranded. It's another thing to drown the stand alone VNT electronics and still be able to drive home with reduced VNT control on your mechanical injected diesel.