W126 300SE HWY CRUISING RPM - PeachParts Mercedes-Benz Forum

92300SE · #1 08-20-2009, 03:33 PM

I know that those w126 300se/sel are quite underpowered,however,I try to figure out if this is a common rpm for cruising on the hwy at 65 mph...I have about 3000 rpm,car shifts excellent,very smooth and crisp,I belive it starts in the second gear?(1990,300se) then it goes in third at about 30 mph if not pushed hard,then in the highest gear at 40 mph and thats it,runs at about 2000 rpm at 45 mph and ,as mentioned above, at 3000 rpm at around 65 mph...feels really that is missing overdrive the way the M103 engine sounds.Any input?

pawoSD · #2 08-20-2009, 04:45 PM

Quote:

Originally Posted by 92300SE

I know that those w126 300se/sel are quite underpowered,however,I try to figure out if this is a common rpm for cruising on the hwy at 65 mph...I have about 3000 rpm,car shifts excellent,very smooth and crisp,I belive it starts in the second gear?(1990,300se) then it goes in third at about 30 mph if not pushed hard,then in the highest gear at 40 mph and thats it,runs at about 2000 rpm at 45 mph and ,as mentioned above, at 3000 rpm at around 65 mph...feels really that is missing overdrive the way the M103 engine sounds.Any input?

Nope that sounds right. I think they have a 3.65 rear diff ration on that vehicle. The W124 300E is better on the highway as it has a 3.07 diff....so around 3100rpm at 75mph.

Perfectly normal. The M103 is a very high revving engine...it can run all day long at 3-5k rpm with no issue.

lee polowczuk · #3 08-20-2009, 05:44 PM

surprised me too... when i got my first 124 some years ago....

it seems like they should have put another gear in there ...a super overdrive...

that said... you rarely hear of a transmission failure...

go figure

pawoSD · #4 08-20-2009, 05:46 PM

High rpm's does not affect the trans....it spins the same either way.

A higher overdrive would take the M103 out of its ideal power band/torque range for pushing a car that size at the given speed. The MB engineers designed it as such.

Brian Carlton · #5 08-20-2009, 05:50 PM

Quote:

Originally Posted by pawoSD

A higher overdrive would take the M103 out of its ideal power band/torque range for pushing a car that size at the given speed. The MB engineers designed it as such.

A taller overdrive would benefit the 300SE. It would reduce it's highway engine speed to a much quieter 2500 rpm or so. It's got more than enough torque at that speed to propel the vehicle at a steady state. If necessary, it can downshift for hills.

However, since a five speed wasn't available at that time, and accelerating the rather heavy vehicle at an acceptable rate is of paramount importance..........it gets the short axle for that purpose.

drosen · #6 08-20-2009, 10:39 PM

My 300 SEL runs at 3000 rpm on the highway at 65mph also, always has, so you can feel confident that everything is ok.

wbain5280 · #7 08-21-2009, 12:43 AM

Don't worry about the engine speed. The M103 is an excellent engine and can run all day at high RPM. If there is excessive noise and/or vibration, consider replacing the engine and transmission mounts. From there, the flexdiscs, check the rear differential mount and then the center bearing.

johnathan1 · #8 08-21-2009, 12:55 AM

300SE/SEL's are very high revving on the highway...mine would buzz down the highway, but the engine was always smooth and quiet.

It did take a while to get used to...

eatlead · #9 08-21-2009, 02:08 AM

Sounds normal to me....the M103 power kicks in around 3-4K.

Sometimes...I like to keep her in 2nd gear around town to get more zoom...

Lead
1991 300SE wht/gry

Hit Man X · #10 08-21-2009, 01:01 PM

I drive mine mostly in 3rd in the city up to around 3000rpm. This keeps the car from shifting in and out of 4th gear and from falling on its face.

A member here installed the SDL's 2.88:1 into his 300SE and loves it. It is far more balanced on acceleration and highway driving. Gets you to around 2900rpm @ 70mph... I've been wanting to do this since I purchased it, along with a permanent first gear start.

Remember these cars were built in the fuel crunch with freeway speeds around 55-60mph. So 3k @ 60mph pulls it along fine, it has plenty of power to pull it at 70mph @ 3k also.

chinny4290 · #11 08-21-2009, 04:06 PM

You guys think you have it bad? I have a W116 280S with the 722. 4-speed. 70 MPH = 3600 RPM

Hit Man X · #12 08-21-2009, 09:41 PM

Quote:

Originally Posted by chinny4290

You guys think you have it bad? I have a W116 280S with the 722. 4-speed. 70 MPH = 3600 RPM

3.69:1 or so versus 3.46:1 I do believe.

pawoSD · #13 08-21-2009, 11:53 PM

My grandpa had a 280S.....my dad can recall him being highly dis-pleased with its unquenchable thirst for fuel.....

He went from a 220 diesel to a 280S....I imagine that was quite a shock. Though, he only kept his cars for 1-2 years, as a lead paint dept supervisor he could get a new MB practically every year with heavy discounts back then.

My dad got to drive it a few times though, and said it could FLY on the autobahn! (and you could literally see the tank needle move while at high speed

)