I have seen numerous postings on the internet regarding the boost pressures at which people have been setting their engines. While it is true that most engines are running well below the factory specified pressure, there are quite a multitude of statements as to what the actual specification is. Various people have stated in various postings that the factory specification is 10, 11, 12, or 14 psi, or .9 or 1.0 bar. Obviously there is only one correct answer as to the *actual* factory specification. I have included the following chart sourced from the MBZ service CD.

As you can see, the maximum pressure (at the high side of spec) tops out at .75 bars, which equals about 10.9 psi.

Many people feel that the factory specifications are conservative in consideration of reducing stress & thereby prolonging the life of the engine. My question is whether anyone has done any *actual* testing with a pyrometer to see what is truly a safe range of pressures at which to set these engines. Of course the fuel enrichment setting for any particular engine will have a great deal of influence on the exhaust temperatures, but does anyone either have data, or know where to find data on higher boost levels? I’m sure that I would not be the only one to want some definitive information on this subject.

Thanks in advance!

RTH

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If I remember correctly the C111 that ran the mileage at speed record with a 617 turbo motor was running at around 50psi boost.

The 603 motors had a higher overboost pressure switch and they easily trip when the waste gate hose is off. That means that pressures over 20psi are easily attainable with the stock turbo. They really fly untill the overboost kicks in.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Hi Steve:

If I remember correctly, there were several modifications to the engine in that iteration of the C-111 in addition to a larger turbo. If memory serves me correctly, the compression ratio was reduced, the piston crowns were plasma/ceramic coated, and who knows what else. I’m sure that there had to be a sizable intercooler – can you imagine the temperature of the charge air coming out of the turbo at 50 psi (!).

I just replaced the head on the old ’84 300D at 207,000 miles. When I reassembled it, the boost pegged my gauge at probably 17+ (the gauge only reads to 15). I’m not sure if the overboost circuit kicked in or not. This condition was traceable to the wastegate hose, which had a huge tear, so the wastegate valve probably never budged off its seat. The car definitely moved out nicely for a diesel – but I don’t really want to melt down this engine, hence the question of a *reasonable* boost setting.

Thanks for your reply; I’m really happy to see that you’re sticking around here – your knowledge, insight, & willingness to help is invaluable to everyone here!

RTH