RPM's - PeachParts Mercedes-Benz Forum

rdc5000 · #1 09-27-2002, 10:36 PM

I was wondering if my transmission is set right or what. I have 84' 300SD with a 4 speed auto, and at 67 I'm turning 3000 RPM's and I wanted to know if that to high?

Dave

psfred · #2 09-27-2002, 10:49 PM

Probably. You can get 5500 rpm out of these engines (for short periods), and they will run happily at 5000, so you won't run out of speed. Top speed is on the order of 117 mph, I think.

Peter

Piotr · #3 09-27-2002, 11:52 PM

I know it doesn't help, but in my 190 D I get 3000 rpm at about 65mph and 4000rpm at about 75mph. The fastest I ever went was 85 mph at 4700rpm.

P.E.Haiges · #4 09-29-2002, 08:52 PM

RDC500,

There is no setting of the transmission that controls RPM in high gear. It is the gear ratio of the differential that determines engine speed in high gear (direct drive on these transmissions).

67 MPH at 3000 RPM is about right for 300SDs.

P E H

leathermang · #5 09-29-2002, 10:08 PM

PEH, I think he was asking if he might be stuck in less than high gear..... thus turning too many rpms for the speed...

84300DT · #6 09-29-2002, 10:16 PM

i also get about 67mph @ 3000rpm

lrg · #7 09-30-2002, 01:17 AM

That sounds about right. My 300 TDT does an indicated 70+/- at 3,000 RPM and I know the speedo is a few mph fast. I think the tranny is designed to keep the cruise rpm in that range so if you need to pass the turbo spools up very quickly. I'm not sure but it wouldn't surprise me if the fuel economy is about optimum there too.

P.E.Haiges · #8 09-30-2002, 02:37 AM

Leather,

He said "Transmission is set right" so I interpreted that as some kind of an adjustment or setting of the transmission.

P E H

lorenztl · #9 01-30-2003, 01:53 PM

I recently purchased a '84 300D and thought the engine rev's a bit much too. Mine is about the same. It is noiser as RPM's rise as well. 3000+ at 70mph is nearly 900 revs higher than my '88 6.2 Chevy Diesel turbo Conversion Van.
It seems it could easily drop 500-800rpm's while increasing fuel mileage and still have plenty of power without downshifting. This would also increase longivity. Even my gas powered Buick Roadmaster rev's lower than this. Diesel torque is higher at lower rpm's than gas engines and should rev slower at highway speeds.
-
Tom

rwthomas1 · #10 01-30-2003, 04:59 PM

The best possible mileage/power with a diesel comes when the engine is operated at its torque peak. Operation at rpms significantly above this peak will result in low mpg. Judging by the way MB geared their US diesels they peak at 2800-3000rpm. Lowering the cruise rpm too far on an MB likely will lug the motor too much. The MBs I have driven only get up on the boost at about 2300-2400rpm at the earliest. A GM diesel is not a good comparison as they start making boost at 1600rpm with a torque peak at 1900rpm, redline at 3600rpm. I know, my truck turns 2100rpm at 72 mph and gets 21mpg on the highway. The best MB mod it would be to use a rear diff from an 85 or a wagon with the 2.88's (i think) as opposed to 3.07's. This should drop your cruise to 3000rpm at 75mph. Not low enough to lug the motor and still allow you to keep up with traffic. RT

artpb · #11 01-30-2003, 09:39 PM

I hope it's not to high because my 300sd is right abt there.

Rick Miley · #12 01-31-2003, 10:43 AM

There seems to be a lot of concern lately about highway cruising speed. Please remember that during the early to mid eighties when these cars were built, the U.S. had a 55mph national speed limit that didn't look like it would ever get raised. Try cruising at 55, as these cars were designed, and I think you might be surprised. This is not to say that they won't go faster, because they will. But 55 - 60 should be the sweet spot.

rwthomas1 · #13 01-31-2003, 12:49 PM

Hmmmm, 55mph..... Now where can I find a place to go that slow. Maybe a school zone? Seriously though, driving 55 around here will get you rear-ended in a hurry. RT