The turbo on the 3.5L (OM603.970) is slightly different than the 3.0 SDL engine (OM603.961) as it doesn't have the ARV, so the .961 turbo will be slightly longer, you might want the rubber boot from the '87 car. Better still is to keep the '91 turbo, it is a slightly larger turbine and should improve performance / allow slightly more fuel & power to be tuned in.

The injection pump (and thus the engine) on your replacement engine will allow IIRC 400rpm more, so you'll be able to rev higher, except the automatic shift-points won't allow it. The stall-speed of the torque-converter is also slightly different to accomodate the different power curves and RPM limits of the two engines, might slightly affect launch performance. Best IMO to install the replacement engine and transmission together so that they are matched, not a big deal though.

The '91 glow-plug system has "afterglow" where the glow-plugs are on with the engine started in some instances, the '87 engine's glow-plugs will not last long in this configuration, best to replace them with "afterglow rated" plugs for the '87 190D turbo. If you are installing the head from your '91 on the '87 engine, disregard, but you'll need the complete head with injectors, glow-plugs, and injector lines to do so (good swap).

The '91 engine's oil pan is matched to the '91 engine, but has a windage-tray in the pan, I prefer the '91 pan on the '87 engine if you so choose.

The '91 engine also has a stronger front-cover / timing-chain case, if you are getting in that deep you will need to pull the harmonic-balancer which allows you the opportunity to renew the timing chain & guides (or at least check), and the oil-pump and chain.

The max RPM is limited mechanically inside the Injection Pump, no changes necessary there. Even with the original '91 transmission you will be able to use the extra HP from the 3.0L engine by shifting manually and using the entire RPM range. The power curve IIRC continues to increase as the RPMS rise, meaning the higher the RPM the more power output up to the limit (not including the throttling range of the governor = some power loss at the top). Idle RPMs are electronically controlled as you apparently are aware, and the same for both engines.

All I can remember at the moment.

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- Jeff