Sorry for the long post!
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You already have OD in your E4OD so a GV would be useful only as a gear-splitter, not for OD. Superfluous IMO. Yes, both "systems" (much more to either one than just a "part") are expensive. The best thing to do is make sure what you have now is working as well as possible, then evaluate any needs/wants for improvements. Most of my opinions/experience of the IDIs are towing, not in ambulances, but I think of ambulances as always "towing" with the wind-resistance of the big bodies and extra weights they always carry. An ambulance will never drive like an empty pickup; it's always loaded/towing.
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IDI tuning is a more technical process than the newer, computer-controlled Diesels. Find a mechanic in your area that can check/adjust your pump timing. A small change there can alter your power curve and output significantly. It requires special and spendy tools like a Ferret pickup and digital timing light or a Luminosity probe and pickup. When a new pump was installed on my truck (stock pump was worn out) the timing was bumped a bit and it really made a difference in the trucks driveability. I'd owned it from ~5k (bought it a year old) to ~150k (sold at just over 200k) and believe me when I say it had never driven as powerfully or towed as comfortably as it did with that small change in timing. It never smoked unless you were really on the pedal hard or towing very heavy and uphill and even then it was a haze, not a cloud. The mileage was as good if not slightly better (too many mixed loads to calculate exactly, but the average didn't change). I always drove with the tach needle just under 2000
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If your Ambo has some mileage/hours on it, a fresh pump and injectors may be in order and you can replace them ALL for under a grand. There are hi-po versions of each available and the IDI's can make some strong, respectable power pretty cheaply through these mods.
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I've ridden in/worked out of some modded IDIs but never owned one, with the exception of the timing bump mentioned above. With an ATS turbo, "Moose pump" and bigger injectors, my buddy's single-cab, short-bed F250 play truck can really embarrass the "coal rollers" around here. It's the drivetrain from a totalled tow-rig that got rebuilt and dropped in a stripped work-truck (rubber floor mat, no stereo, etc) that lost a 351 and had its frame shortened to fit a parking-lot sweeper unit. Lightweight + Strong = FUN. Mine was on stock 200k+ mile injectors when sold and running very very well, but when the pump got tired it was obvious. The pump was replaced by a busy/popular local diesel shop that replaced it and retimed the motor but I was unhappy with the "tune". It just never seemed to settle down at speed and didn't run as smoothly as it had originally. I spent some time looking for a different shop and found a real "IDI guy". When I got the truck back and hauled that first load of hay I was grinning from ear to ear.
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Long story short, an IDI is never gonna be a modded Powerstroke for 70 mph hillclimbs but they can run very well, make plenty of power to move serious weights reliably, and get good mileage. The original turbo IDI's were of the non-wastegated, non-intercooled type and liked more RPM to build boost (at least ~2400 is where the ones I've driven like to pull) but they sure do tow nice, especially with automatics. It's possible to build a more modern wastegated system that builds boost and tows well at lower RPM (right below 2000) but I think that system would favor a manual transmission. Stock non-turbo engines are happy well below 2000 cruising with a dense load like rock or lumber, and need more than that for a high-drag load like a heavy 5th wheel camper but obviously want more RPM in either case when ya get to a hill.
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If I were in your shoes I would: 1. Get the timing checked and set by an IDI guy. Drive it hard and heavy and see what you think. Take the mechanic for a ride if you can and talk to him. Get the timing set where you like it. A pyrometer is essential for these changes. Melting pistons will never happen or even be a concern if you have a gauge and tune/drive accordingly. 2. Shop around for a turbo. There are used systems for the pickups available often but I don't know about the vans. They may use the same parts? I wish I'd turbo'd my truck when it was new. It would have had more power and gotten better mileage all that time I owned and used it. Those heavy 5th wheel campers and bulldozers and loads of hay would have been more manageable and I probably would have kept the truck a little longer. With an automatic and/or on a budget I'd probably find a non-wastegated system but with a manual I'd build for maximum low rpm grunt and get a small turbo to spool fast and a big wastegate. With the packaging hassles and low power/heat levels we're talking here I wouldn't bother with an intercooler. Just not enough room to fit one easily and not enough benefit to justify going to a lot of trouble fitting one at the power levels you're seeking. IF (big if) heat management became a concern with a turbo IDI (ESPECIALLY in a van) I'd do what works so well with 6.5 turbos: Water/methanol injection works great for charge cooling and is way easier to install than an intercooler. It's beautifully simple. Sure you have to fill the tank but with an ambulance body you can fit as big of a tank as you want (20 gallons would be way overkill and not take up that much room really) and put it some place easy to fill and water/meth is cheap. Filling water/meth is NO big deal in my experience. Use just enough alcohol for antifreeze properties and you'll see optimum cost/benefit. Snow Performance is the brand I know best. Lots more info on these options if you're interested. Just ask and I'll bore ya! LOL
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Oilburners.net is the best resource I've found for these engines.