Hi everyone! So far this forum has been more helpful to me than a stack of service manuals. But i have one question. My '84 300 D turbo revs at around 3200 RPMs at a steady 70 mph. Is this normal? My trans shifts flawlessly, so I know it's in high gear. (4th). I always thought diesels were not supposed to rev that high. I am planning a couple of 500-600 mile trips in the near future, and I am scared i'm gonna blow up my baby. Any help with this I will really appreciate. Pete Miller.

Working out the rpm/speed ratio for your car (assuming stock rear-end, wheels/tires) I come up with 70.4 mph. How come you're off so much? Have some nice safe trips. (Of course I WAS held back in math.)

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Sounds about right to me but I'm no expert.

What I'd like to know is why folks think their engines will explode if kept below redline. An engine is designed to run, not idle.

Sixto
91 300SE ... 3200rpm at 70mph, hasn't blown up yet
81 300SD

RPM's don't sound any different than my 300D.

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Jim

Pete:

MB diesels are over-square (read short stroke!) pre-chamber engines with variable injection timing. This results in maximum rpms much higher than the long-stroke, fixed injection timing, direct injection diesels typical of American truck makers. Those engines have a usable torque range from 2000 to 2200 rpm (or 1800 to 2000 for some), and have a rev limit of less than 3000 rpm. Why the don't make wide rpm/wide torque range engines with speed determined injection timing I will not speculate on -- I think this is changing!

To get the performance expected of diesels, especially in America, MB uses rather low rear end gears -- 3.72 or 4.00 ratios are common. This limits top speed (to 85 mph @ 5000 rpm on the 220D), but allows the use of a 4-speed tranny with overdrive rather than more gears.

My Volvo turns 2700 rpm at 70, the 87 300D turns about 2500 or so at 70 (stronger motor!), and I would guess the 220D is more like 3800 rpm. They are all known for running more that 300.000 miles with little or not engine work!

MBs run forever at these rpms -- very well designed and built, as even with a short stroke the rings are running above the expected "rapid wear limit" of 2500 ft/min, at which point engine wear increases rapidly.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Hi guys. Thanks for all the quick replys. I am a little confused by psfred"s reply though. You said your volvo turns about 2700 at 70, and the 87 300 D turns about 2500 at 70. as i said, mine turns about 3200 at 70. So far the concensus seems to be that this is normal for my car, so i'm gonna go ahead and drive it. thanks again, Pete

Pete:

Different final drive ratios and overdrive gears in the Volvo and 300D. Different redlines, too -- the Volly goes up to 5300 rpm, I've heard the MB stops at 4600 (electronically controlled).

I once figured out that theoretically the Volvo would do 120 mph before the engine ran at redline, but I suspect I'd be out of horsepower before then. It's only a 2.4 litre engine, 105 horsepower.

And I could be wrong, I've stopped looking at the tach most of the time.

3200 rpm is right at what everyone I've read on the list says. Probably got 3.72 rear end ratio, as that is what I have in the 300TE, and it runs about the same rpm at 70.

Drive all you like -- the TE has a little over 200,000 miles on it, has good oil pressure, only uses a quart of oil every thousand miles or so -- and it was not all that well taken care of before I got it!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!