Transmission transplant

[tyrrellnick]

Hi everyone,

So now my 280SEL is purring like a kitten I'm looking at some potential improvements. I do a fair bit of highway driving, and the 280 sits at about 3,200RPM at 100kph in 3rd gear (it's a 3 speed auto). Ideally, in the interests of fuel economy and engine longevity I'd like to find a transmission option which saw the cruising RPM a bit lower.

Has anyone had any experience replacing the 3 speed tranny with a 4 speed of the same era? I'm guessing it would be a fairly simple transplant, but I'm keen to hear any feedback or advice. Also, if anyone knows what the code is for a 4 speed auto which would easily transplant into my car I'd be very grateful. As far as I'm aware the W116 280 was available with an optional 4 speed auto.

Cheers!

Nick

[CraigB]

Pretty sure 3 and 4 spd top gear will be 1:1 ie. not an overdrive gearbox and no diff to revs in top. Are you sure it is 3 speed? Try pulling the gear lever to low, then take off and move gear lever through the gears - I reckon you will count 4. Just starts in second unless you pull the lever back. What will change your revs is a taller diff. But i wouldnt' be confident it will lower your fuel consumption or effect longevity. I trust the Benz engineers that if this suited the engine they would have done it. These engines are designed to rev and not sloggers like the falcadore, but because people are used to that they think the Benz is wrong. Risks are that everything is under more strain with the taller diff and working harder under the strain may decrease economy, when you go to pass cars at 110 instead of revs being right in the pocket you have to wind it up etc. Years ago i had a rotary Mazda and to try and get economy numbers on paper for the standard tests they put a taller diff in it. It was common knowledge that put the earlier model diff in and greatly enhanced performance and under normal usage it used less fuel. Back then I don't reckon Benz played those sort of games and put in the diff it should have. But a v8 diff will make it rev lower.

[tyrrellnick]

Hi Craig, thank you.

It's definitely a 3 speed. From the manual there were three options for this model - 4 speed manual, 3 speed auto, 4 speed auto.

Maybe I should get a 450 :)

[AMG]

What is it that you are actually trying to achieve?

[tyrrellnick]

What is it that you are actually trying to achieve?

I do a lot of highway driving, I'd prefer the car cruised in a more relaxed way, and was more fuel efficient.

[AMG]

What is it that you are actually trying to achieve?

I do a lot of highway driving, I'd prefer the car cruised in a more relaxed way, and was more fuel efficient.

Sorry if this sounds harsh, but the m110 is not a fuel efficient engine. It's also not going to make any difference by swapping out diff ratios etc, because you'll actually be pushing the throttle harder to maintain the same engine RPM. It's also going to make the engine work harder.

Insofar as transmission is concerned - they all have a 1:1 final gear. so no point.

Suggestion would be to have it tuned, but again, the m110 was a revvy engine and it may sound busy but it's not under much stress at australian highway speeds - it was designed to run all day on the redline travelling along the autobahns.

And a 450 won't be any easier on the fuel - 17-20L/100km is a reasonable figure to achieve.

If you want efficiency, then convert it over to a Diesel OM605 / OM606, plus a 722.6 automatic, which has an overdrive gear. I doubt the cost of conversion will return any net gain on the difference between fuel consumption of the m110 and the more modern turbodiesel.

a more sensible 6 cyl swap would be an m104 3.0 or 3.2L with a 5 speed 722.6 automatic. That combination has an overdrive, and approx 170kW in factory specification.

However, undertaking a gearbox swap to obtain an overdrive will be just as expensive as swapping in a late model drivetrain from a donor vehicle. (and you'd buy the complete donor vehicle, because there are far too many components required beyond just the engine / transmission.


I know the intention is good, but undertaking these things and doing them properly is not a simple or cheap or even straightforward exercise.

If you were taking say, a w126 500SEL donor engine and transmission, You'd still want the differential, for the better economy ratio, you'll need a different propeller shaft front section, then stuff like the thermostat neck and water pump hoses, brakes will almost definitely need upgrading, the list goes on and on.... and because you have an m110 w116, the drivetrain and brakes will basically need wholesale changing to the heavier duty v8 parts.

In this particular application, achieving better fuel economy is not going to be easily solved, without the change bringing a negative.

Pretty much leaving you with two options...

1. sell the 280sel and buy a later more fuel efficient S class - maybe a 420SEL w126. or jump right into a modern S class.
2. drive the w116 less often, live with the fuel bill, keep the car original, understand it for what it is, and what it isn't.

These 6 cyl cars were never a 'lazy' sounding engine, and they are pushing a sizeable mass with a reasonably large frontal area compared to modern vehicles.

to find a 5 speed which matches the m110's bellhousing, I think you might be looking for some kind of manual gearbox conversion, or a really good fabrication shop to refab a 722.6 5 speed auto... and then you'll need a standalone TCU to control the shifts.

Again, comes down to cost. Fuel is still cheaper than the net gain from any swap cost and the resulting fuel economy isn't going to offset the costs by as much as you'd probably like.

Yes, the 3 speed auto seems like a dinosaur. and the 4 speed auto's aren't much different, and those were in cars right up to 1995. the first 5 speed autos were horribly fragile and the later ones got electronic shift control, which makes them an expensive proposition to swap (and fix.... conductor plate, solenoids, yada yada yada...) because you'll need a standalone TCU and an electonic speedometer drive, plus trigger setup because the trans doesn't have a VSS..

The bottom line is that it is only worth it, if you understand the cost, the complexity and you're prepared to live with whatever result you get, which inevitably will have one or two detractions from the original transmission.

Have a good think about whether or not you're prepared to live with the cost - without factoring it into the equation. You'll be lucky to get out of it under $3K just in bits n pieces, not including the cost of the transmission itself. That's a LOT of fuel.