603 motor, what's this thing inline with the turbo and air intake?
 

This box here, what does it do? Air recirculation? What would happen if I bypassed it and ran just an intake+pipe to the turbo?

Hard to tell from your photo but I believe that's the air flow meter which tells your computer how much air is going into the engine. The later diesels use more electronic controls in place of some of the vacuum systems found on the W123 and earlier cars. I doubt it would still run, or at least run properly, without a flow meter so it is NOT something you want to remove. It's similar to the unit that a lot of gassers with fuel injection use and shouldn't hurt performance any (even if you could remove it).

Is the airflow sensor used for anything more than idle speed, EGR and ARV control in an 87 300SDL? The only influence the EDS has over the IP is through the ELR which is an idle control system, right?

It seems to me that if the EGR and ARV systems are disabled and the car idles properly with the ELR disconnected then you can remove the airflow sensor.

This is conjecture, NOT A RECOMMENDATION.

Sixto
91 300SE
87 300SDL
83 300SD

I'm pretty weak on the electronics of these things so I'm not sure. Past experience with the electronic stuff is that it is seldom as simple as it first appears. If you're inclined to try it I guess you could unplug the connector at the unit and see what happens. I had one of these apart before on an old TR8 and I can tell you they don't provide much of any restriction so removing it, even if it is deemed to be unnecessary, is likely to provide little, if any, benefit. If it were me I'd just leave it alone and move on to some other, better performance enhancements. Just my 2cents.

I wouldn't recommend removing it either since these cars aren't starved for air by any measure. But I don't think it's doing much once the EGR and ARV are disabled.

Sixto
91 300SE
87 300SDL
83 300SD

Sixto is right, it's the mass airflow sensor and exists to control the EGR and ARV. If those two are disconnected (and they SHOULD be!), the airflow sensor can be removed. I wouldn't bother, you won't get a performance gain (at least I seriously doubt it). I heard of someone else doing this, the engine ran fine, but it wasn't any faster. Just leave it alone (my $0.02.)

:D

I give up, what's ARV?

P E H

Air Recirculating Valve!