Re-gearing a 300SE for the Highway

[S-Class Guru]

My '91 300SE turns 3500 rpm at 70. This is finally starting to bug me, and it eats up water pumps, alternators, tensioners, etc. I had the idea of regearing it 10 years ago, but never got around to it.

I thought about trading the 3.73 diff gear for a 2.80.
Then modifying the tranny so it starts off in low gear.
Modifying the speedo would be the last part.
I really think the engine could pull the car down the road at 2700 rather
than 3500, with acceleration just a downshift away if required.
I live in Texas, so I don't need the "Granny" low gearing.

So, questions:

1. Anyone ever heard of doing this?
2. Can I buy ring/pinion gears for my diff anywhere, or do I need
to find a complete diff that would bolt into my late 126 chassis?
3. Would I need to find the exact speedo, with the proper internal gearing?
Or can the electronic speedo somehow be adjusted to match different gearing?

thanks,
DG

[Duke2.6]

I think the axle ratio is 3.46:1.

The problem you have is a nearly 4000 pound car with a 3L engine that produces about 175 lb-ft of torque at 4000 RPM.

If you regear, it will probably be a slug, even with first gear starts, and the cost would be very high relative to the value of the car.

You're better off just driving it into the ground or selling it and getting a V-8 S-class.

Most of the M103 engine cars with automatics are turning 3000 or more revs at 70 except for the later 300Es with 2.87:1 axles.

The manual transmission versions are much more relaxed with their overdrive fifth gear.

Duke

[Mike Murrell]

I own one of these cars and Duke hit the nail on the head. You'd spend a fortune reengineering a car to do something that's not feasible given it's weight.

You'd be better off selling it and looking for a 420 or 560 if the revs are bothering you.

In spite of 3450 rpms @ 70mph, I think this is one of the best cars MB ever built. You get the simplicity of the M103 system and the ride/size/comfort of the W126 body/chassis and in my case, 20-22mpg on the highway. Not bad for a big beast.

[Duke2.6]

I agree. These inline sixes are quiet and smooth as silk. Don't worry about the revs. They are designed for sustained high revs. If you have to, cover up the tach.

Duke

[S-Class Guru]

Thanks guys. Yeah, you're probably right. If I did it the poor engine would probably downshift just to fight a 5 mph headwind.
But you gotta admit, even 15 years ago, a tranny with no overdrive, no lockup, and a 2nd gear start was kinda silly. Of course, it's still working perfectly after 160k, and probably will for twice that.

DG

[fahrgewehr2]

I use my SE for highway driving and find a it a superb high speed car.

I cruise at 90 effortlessly and the low gearing allows for great throttle response at 70mph and up, unlike other cars where you are either coasting or downshifting.

267k of high revving and it still spins right up to 6500rpm in the first three gears.

Great sounds from this motor too- a nice mechanical purr.

[schwarzwagen]

i am still on my OE trans at 270k at it shows no signs of slip or any other ailments.

the rear axle is in fact a 3.46, you probably could get a way with a 3.27, but anything else would make the car too slow.

[fahrgewehr2]

Originally Posted by schwarzwagen

i am still on my OE trans at 270k at it shows no signs of slip or any other ailments.

the rear axle is in fact a 3.46, you probably could get a way with a 3.27, but anything else would make the car too slow.

Schwartz- do you know the weight difference between the SE and the SEL?

Always wish I had bought the SEL, as the rear seating in the SE is really pretty tight for such a big car. What repairs have you done to your SEL?

[schwarzwagen]

Originally Posted by fahrgewehr2

Schwartz- do you know the weight difference between the SE and the SEL?

Always wish I had bought the SEL, as the rear seating in the SE is really pretty tight for such a big car. What repairs have you done to your SEL?

fahrgewehr,

the weight difference is actually quite small. i believe its about 50-60lbs or less. overall weight is about 3750-3800 in US trim, i removed the US bumpers and installed late model euros that shaved at least 100lbs from the curb weight. i think my car is about 3650 right now.

i have not had to do anything crazy to it, i have owned it since 1994 and put on 220k miles of the 270k it currently has. i am really finicky about the suspension so i replace shocks every 50k miles, brakes when 20% of the pad remains, control arms and bushings every 2 years regardless... the a/c condensor and a/c high pressure hose went bad a couple years ago, replaced that, ACC mono valve went out once, flex discs and center support bearing once per year, very soon i am going to buy a new stainless exhaust from timevalve.com, and soon the head will need to be rebuilt, the fuel distributor is also getting weak...

all in all i am very pleased with this car, it has paid for itself more than once.

[S-Class Guru]

Okay, I'm convinced. If I'm only at 160k, I should have a lot of 4000 rpm mile left. Maybe I'll just disconnect the tach.

DG

[fahrgewehr2]

Originally Posted by schwarzwagen

fahrgewehr,


i have not had to do anything crazy to it, i have owned it since 1994 and put on 220k miles of the 270k it currently has. i am really finicky about the suspension so i replace shocks every 50k miles, brakes when 20% of the pad remains, control arms and bushings every 2 years regardless... the a/c condensor and a/c high pressure hose went bad a couple years ago, replaced that, ACC mono valve went out once, flex discs and center support bearing once per year, very soon i am going to buy a new stainless exhaust from timevalve.com, and soon the head will need to be rebuilt, the fuel distributor is also getting weak...

all in all i am very pleased with this car, it has paid for itself more than once.

Seems about like what I've had done. I also had a valve job- Makes a difference. Rear suspension needs somework- all I've done is schocks and sway bar links. This summer I will redo it.

Aiming for 300K, then I will retire this car.

[lyonstexas]

My 1992 300SE has the 5th gear overdrive and it runs 2700rpm at 70mph. When the 5th gear was not working it worried me to run it at the higher rpms. Perhaps the 5-speed trans will fit. Anyone know for sure?

Tim

[fahrgewehr2]

I'm trying to figure that out in another post (titled m103 question).

The only short coming of this car is the transmission that needs more gears at low speeds.

Someone must know if the 5spd tranny will bolt up??

[wbain5280]

The addition of a first gear start would really help. When I move the selector from 4 to 2 and back, the first gear start really helps from a stoplight.

How can the trans be changed for a first gear start?

[fahrgewehr2]

You can make it start in first gear everytime automatically if you change the valve body. Bergwerks does it.

Does first gear feel ackward and jerky in everyone else's cars? The 1-2 shift feels very different than the other shifts, for me.

I still want to know if the later 5spd auto (the hydraulic unit with elec overdrive, not the 722.6) will bolt up to the 103 motor. Or even the 4 spd out of a 400E or 124 e320- They shift smoother and start in first.