Turbo Diesel mod, open turbo fully for better coasting.

[Tesla]

My vehicle has a VNT (Variable Vane Turbo), it adjusts opening to match engine air requirements. A while ago I was doing some investigations on engine braking on steep downhill runs, my ECU opens the turbo fully at 3,000rpm, I noticed I had good braking till 3,000 rpm, then turbo would open up and it starts to run away. As the Turbo is the biggest restriction in the exhaust, I thought this may be useful for getting better economy, so I put together a temp set up where I could to either open or close Turbo fully depending on need, worked well, but as I was doing lots of other trials didn't do this consistantly. I am in the process of setting up a more permanent design with a microswitch on the throttle and some air solenoids to control the turbo actuator, this way I will be able to have either engine braking or free coast when throttle is off, most of the time the switch will be in the free coast position.
Those with a waste gate turbo may be able to open wastegate fully to coast or close it for engine braking.

[Ladogaboy]

Wow, I would think it should do that already. The wastegate on my turbo stays open under low engine loads naturally. In times of engine braking, it stays open too, but I've never minded that because I already slow down fast enough, even without the back pressure.

[Duffman]

I had a guy on a forum (cant remember which one) tell me that he got the best mileage with his 6.6L Duramax with the least boost (& backpressure) on his turbo at cruise. He had one of those customizable programmers and could adjust his VGT with it.

[oil pan 4]

I am putting together a HE351VE (the holset they use on 2007.5 to 2010 cummins) for use on a 6.5L and for cruise I know it will be running the VGT wide open.
Most of the time the VGT will be wide open.

[slowmover]

I've been quite interested in the VVT from the later CTD's as used with a controller by FLEECE for purposes of mpg. I've not contacted them as there are higher priorities this year on the truck, and the conversion expense is high. But am all ears in listening to what others have found.

As to mpg: lowest manifold pressure and exhaust gas temp pretty well equals best mpg (in a general sense). The test is when our vehicles are loaded (maximum vehicle efficiency) and the motor is asked to do what it was designed for: Full load under difficult conditions. If this issue is not addressed properly, then mods to the turbo control[s] are worthless.

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[Tesla]

Not sure how other vehicles are set up, but as I understand it the VNT/VGT generally speaking acts like a small turbo at low rpm & load by closing up and opens up fully at high speed/load, this gives better torque across entire RPM range. If the relationship was just engine & turbo, it would be a simple affair, but when you add in the emission controls like EGR particularly, then the ECU starts playing with the VNT to achieve a set of pre programmed parameters.
I did monitor the VNT solenoid for a while to get a handle on how and when it opens/closes & even went with a PWM unit for a while & bypassed ECU control of the turbo.
I removed the external intake snorkle recently and set up a cold/pressure air intake from front of vehicle, this helped significantly with both engine response & EGT's are down 20-30C as well as dropping boost about 2psi, there have also been other mods going on as well, but the cold/pressure air intake was a major improvement.
I know this proposed mod will also help, as I have already tested it, but to quantify improvement is not so easy.

One of the other things I have been pondering is would it also help coasting down if the intake was restricted a bit at the same time as opening the exhaust, so reduce air going in as well as free up air going out, what would be the ideal free coasting diesel engine set up in gear?