I'm putting my wagon back on the road. I have egr -less exhaust and intake manifolds from the early w116 which the exhaust is jet hot coated, a small IC, MYNA pump and increased boost from the stock turbo. I'd like more performance as a daily driver and want to upgrade the turbo and install a larger exhaust to match. What are your suggestions?

BorgWarner 179355 (EFR 7064B) with a 3 inch exhaust is what I have in mind for 'Desert Rose"...eventually.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Mitsubishi TD05 is practically the same size as the stock turbo (besides a larger turbine housing)
I'd get an 8 or 10cm housing for that. They make enough air for about 250-260 hp on a 603. The only downside is the wastegated versions have very small exhaust openings but you can bore it out.

i might look at a holset HE221W it supports up to around ~250hp and with the 6cm housing spools past 10psi by 1500. it should also be a good deal cheaper than an efr and doesn't require water cooling.

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1985 300D om617: 8mm M pump 175cc 5200rpm, holset he221w @ 30psi, large A2W ic, compounds on the way.
KD9AFT
A&P

Which of these is easiest to replace the stock turbo? Where’s the best place to buy?

the TD05 is likely the easiest and cheapest as it is a t3 flange look on ebay for it but beware there are many Chinese clones and i wouldnt trust running it hard for long. the HE221w is a bit smaller and tougher however you would need a t25 t t3 adapter for it they are available on: Home | Powered by Holset. and finally the EFR can be had in a t3 flange but would cost between 1200-2000 depending on options they can be had on: https://www.full-race.com/product-category/borg-warner-efr/turbos-efr-series/

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1985 300D om617: 8mm M pump 175cc 5200rpm, holset he221w @ 30psi, large A2W ic, compounds on the way.
KD9AFT
A&P