I am confused as to how a Turbo operates in my 1985 300TD. The car is quite slow up to about 2500 RPM when it suddenly starts pulling. Is this turbo unit pedal actuated or does it engage only after certain RPM? I drove an Audi recently where turbo engaged as you pressed the gas pedal but my car does that only after it passes 2500 RPM.

Igor Zeljic, Minneapolis.

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1984 MB 300D Turbo Diesel W123 Sedan 132K
1987 MB 300D Turbo W124 Sedan 295K
1983 Porsche 944 104K
1989 Audi 80 Quattro 180K
2002 Audi A6 3.0 Quattro Avant 49K (Wife's car with warranty )

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Thats the way it is with MB Diesel turbos. They don't start to boost until the engine revs to about 2500 RPM. They are not actuated by any mechanical connection, there just isn't enough exhaust to spin the turbo fast enough at lower engine speed. Once the turbo starts to boost, there are more exhaust gasses generated and the intake manifild pressure builds up quite fast, thus the noticible increase in power.

The turbo lag is much more noticeable at high altitudes but once the turbo gets to maximum boost, the engine then runs like it was at sea level.

P E H

I'd like more information on turbos at altitude. I live in the Mile High city. My 85 turbo is about twice as fast off the line as my 77 non-turbo. I have attributed this to the fact that the 85 has a turbo. However, I can really feel the turbo kick in at about 2300 rpms. So,is the turbo having an undetectable effect (except by comparison with a non-turbo engine) at lower rpm's or is there some other factor making the non-turbo so much slower. Can someone comment ,who drives or has driven both at sea level? Is there a detectable difference in power between the non-turbo and turbo at rpm's below 2000?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Oh hear we go again, turbo talk!! Turbos are a compromise. The housings on the compressor and turbine sections are both closely matched to the engine for each application. The turbo must be small enough to spool up quickly yet large enough to provide enough flow and boost at high rpm. This results in a compromise as a smaller turbine section would provide more boost at lower rpms but not flow at high rpm and a large section would flow like gangbusters and make plenty of boost but the lag would be huge waiting for the thing to spool up. Various schemes have been tried. Mazda tried a twin-sequential turbo system on their RX7. A small turbo worked alongside a large turbo with a whole pile of electronic wizardry to make plenty of boost all over the rpm range. There are also VNT turbos from Garret that have a "vane" that directs the gasses. It directs more exhaust gas against the turbine at lower rpms to spool up quickly and then opens up as rpm/boost rises to flow better. Finally there are modern computer controlled engines that seamlessly blend the power curve so that you really don't feel the turbo lag. VW TDI's are a good example of this. Our "classic" MB's are based on older technology and ideas so yes there is turbo lag but its really quite mild compared to some other turbo systems. At lower rpms your turbo simply isn't spinning fast enough yet to provide significant boost. RT

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84' Mercedes Benz 300D Anthracite/black, 171K
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