Hi Tom
I have been trying to get a handle on turbo technology. I saw your video of you hogging out the housing, impressive machine work

Why would you just not get a bigger turbo unit? I hope it isn't an ignorant question. Maybe you said but it went over the top.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

Hello Silber,
The eternal question, eh? (Thanks for the kudos...)
Keep in mind that the following information is theoretical and has yet to bear results that I can demonstrate. But the results will be coming soon...
My take is that the larger the compressor/turbine/shaft, the more energy it takes to spin it up to the point where you start making boost that you can feel. The original Garrett T3 in my '84 300D was sized well for that engine and above 2500 rpm, it can make way more boost than the injection pump can feed with fuel, consequently it has a very conservative wastegate setting (My stock wastegate was set to 9 psi !?!). The problem is that with the technology at that time, the turbine design was slow to build boost.
The newer Stage II and III turbine (exhaust) wheels are better designed and can extract more energy from the exhaust stream down low - so if you can adapt your exhaust housing for them, you can fix the slow spooling nature of that T3 turbo design.
Doktor Bert has suggested that adding a larger 50 or 60 trim compressor wheel might also help. But I have heard stories of 60 trim wheels causing turbo lag.
Keep in mind that my goal is to improve low end torque and turbo response - not create a coal rolling, tire shredding drift machine. I feel that the factory set the IP governor on these engines a little too conservatively for my taste, so I will be adjusting the governor to try and increase the low to mid range fueling to match the hoped for improvement in boost response.
To be honest, not everyone has their own machine shop or access to one - so many try to adapt later design turbos to their 617a engines, which for me is way more work than I wanted to get into. A variable vane turbo like the one outfitted in the new Mercedes Sprinter vans is a good example.

Here are some more photos:

Old turbine wheel in the opened up turbine housing:
https://picasaweb.google.com/lh/photo/fs5q6kMLiwJK0TpzhfcARtMTjNZETYmyPJy0liipFm0?feat=directlink

New turbine wheel in same:
https://picasaweb.google.com/lh/photo/VpcBtT1EKC2W95DYlIBWQtMTjNZETYmyPJy0liipFm0?feat=directlink

air/water intercooler shots:
https://picasaweb.google.com/lh/photo/PoFYA5dXBc3Ce5xbq11CGtMTjNZETYmyPJy0liipFm0?feat=directlink
https://picasaweb.google.com/lh/photo/kimAw1eC5z6eQa9E6hhng9MTjNZETYmyPJy0liipFm0?feat=directlink
https://picasaweb.google.com/lh/photo/iL4IkUwtBluqDIxz7t0wYtMTjNZETYmyPJy0liipFm0?feat=directlink

This is a stainless a/w intercooler that I found at the metal recycle yard for about 10 bucks. It has the same inlet / outlet diameters as the stock turbo-to-intake manifold, and has the capacity for a lot of coolant flow. I plan to use a second oil cooler mounted under the front bumper as a radiator for the coolant, and a second heater recirculator pump to move it around and around.

Enjoy!