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I know this thread is a bit old, but I have been experimenting with a Nissan diesel engine, and I spent a fair bit of time playing with the intake. I didn't use any fancy measuring equipment other than the seat of my pants.
Intake mods I have tried so far have been:
1 - Bypass the air filter.
2 - Bypass the air filter and remove intake resonator boxes.
3 - Remove all intake piping before the "throttle body" plenum intake.
4 - Dismantle the intake plenum to run open intake runners.
The results were pretty clear. Low to mid-range torque dropped noticeably as soon as the resonators were eliminated from the system. However there was noticeably more punch at the top end with #1 through #3. Intake noise is increased too. With #4 the engine was sluggish over the entire rev range. I think the open stacks were too short and turbulent. I would like to try #4 again with some longer intake runners when summer rolls around (out of interest and possible application for my race car).
These were tried out on an RD28 Nissan engine, which is a 2.8 litre straight 6, non-turbo, straight mechanical diesel. The intake is approx 3" diameter from 'throttle body' through to the air filter (except inside resonator boxes), and is approx 500mm long.
I'm not sure what is inside the resonator boxes (if anything), but they are the key to making this engine work properly. Most likely removing them caused it to run too lean.
The only mod that possibly improved economy was #1. Now that I am recording my mileage I will make an effort to try it out for a few tanks. I would imagine that a nicely ducted cold or even ram air setup would have more effect though.
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