1971 Mercedes W108 280SE 3.5 V8 conversion to OM606 diesel.

johnty56 wrote: »

Very nice... would it be a shame to put a diesel in it? I have one of these and half the fun is in the set up.. I can't remember what the ratio is on the rear axle but how does it compare for a diesel conversion? Let me know if you are going ahead.. I might be interested in the engine!

Thanks for your interest. Definitely going ahead! I want to drive the car every day and I can't afford to do that on petrol at 16MPG!

I'm a bit reluctant to sell the original engine and 'box as the car would never be "matching numbers" again, but that reluctance may wear off as the budget stretches!

The rear ratio is indeed too short (3.46:1) for my conversion (the car will be slow), but I plan to put in a very tall ratio (.82:1) gearbox to compensate. I might increase the wheel size too if I need to, but the gearbox gets me back to the same kind of engine-to-wheel ratio as the original diesel donor car, so I'll see what its like once driving and decide then since the wheel change is relatively easy.

dgt wrote: »

This looks like an interesting venture! Where will you source the engine from?

I've got one in a '99 E300 Turbodiesel. Problem is that it's a bit difficult to get the auto gearbox which comes with it working properly. You can get a controller to run it but it can be clunky at best. I'll be bolting on a manual transmission, there are plenty which fit, and it will help me with my final drive ratio.

quietsailor wrote: »

Would you consider convertijg to gas? It'd keep original numbers and you'd get better value economy wise

I've thought about this alright, but I'd still be running a 40 year old engine and really want to put some miles on this thing. I like the idea that spares for the diesel are readily available, as well as the spectacular reliability reputation.

dgt wrote: »

If it's of any use, I have a M117 autobox sitting on a pallet if you're stuck for one. Having said that, the manual would appeal to me more!

Just to add, those engines take well to compounding and 4-500 bhp is a very achievable figure if that's of any interest...

Thanks, but I'm pretty fixed on the manual conversion for now. I've no plans to do anything in terms of power output. I'm aiming for economy and am more of a "Driving Miss Daisy" kind of guy. I'm very impressed by what these engines can do in standard form though, with just a heftier dose of fuel and a couple of turbos stuck on. It's incredible!

Capri wrote: »

16mpg /??miles per week / = €?? ,

My commute is 478 miles per week, at €1.50/L for both fuel types, that's nearly €100Euro per week saved. The diesel does about double the distance to the same volume of fuel. However, if you add vegetable oil into the equation, it gets even cheaper.

Capri wrote: »

Cost of engine/conversion/bits/time-labour = €??? , (not to mention the thought of returning it to 'matching numbers' in the future )

About €700 for the bits, doing the work myself, but it will certainly take a lot of time. I plan to be clever about any modifications, making them reversible where possible.

Capri wrote: »

Is it REALLY worth your time and money doing such a conversion

It's very well worth it, over a year, the conversion saves me about €5K in fuel alone, and that's without diluting it with chip fat.

Capri wrote: »

Maybe I'm just getting older and wiser but all this 'DAYSEL,DAYSEL,DAYSEL' talk that goes on here REALLY gets to me - diesels need more servicing and parts and oil changes. Slower and dirtier as well - yet the country seems fixated on DAYSEL as the saviour of motoring. Compare a 300SD and 350SE V8 over 3 years, initial price paid, fuel AND parts costs etc and see what wins

How does a diesel need more parts? In my case, I'm eliminating spark plugs, HT leads, distributor, points, coils, rotor arm and electronic fuel pump. I'm replacing them with glow plugs.
I'll have about 170HP and 330Nm of torque at just 1600RPM. That's plenty for my requirements!

Work is progressing slowly! I'm in the process of going through the wiring on the donor car to make sure that once the engine is removed, everything will still work. So far, things are going well, but its a very slow and painstaking process, particularly since I'm paranoid!

Here's the donor car, a 199 300E Turbodiesel:


And the power plant. It's had some boy racer treatment, but doesn't seem to have suffered too much abuse, it still runs very well, lovely and clean.

I'm very excited, I've finished the electrical strip down, wohoo!
I can make the engine run now with only 4 12V connections and 2 fuel pipes. It's such a relief as the electronics are the part I was most worried about for the whole job. I just have 2 permanent 12V feeds, 2 which are switched by the ignition key and 1 for the starter solenoid. Its very satisfying.
The wiring all still looks a bit of a mess but I can tidy it up once I see where to put everything in the project car.

This spaghetti is mainly the old instrument cluster connections (I hope to find a tacho and glow plug indicator light in there) and the computer diagnostic port connection.


On the other side of the donor engine bay is another box of tricks, power for the ECU comes from here and some other stuff.


The interior isn't looking great. It turns out I didn't need anything from in here and could have left the dash intact, but I needed to get in there to trace some wires in order to learn this! I'll know for the next one ;-)


So, this is it with all the required electronics sitting on top of the engine. Attention now moves to the mechanical strip down!

Another big step along the way. The donor engine liberated from its former home. The auto 'box is going to a new home elsewhere and I've ordered what I hope will be all that is needed to add a flywheel and cluch. Time will tell!

mannurse wrote: »

Good work! Very interested to see how this turns out.
Do you know if something similar could be done to the common rail engines? I have a c220 cdi with manual trans and it returns 60mpg and would love to use it for my old w123.
You are right about the mech IPs with an alda are hard to find, this is a good solution.
Are you going with a dualmass flywheel? I think the manual transmissions from the early 90s all used DM so the gearboxs have short input shafts as the spigot bearing is in the flywheel.

I've no idea about the commonrail stuff, I'm afraid. The guy did say that my ECU was pretty basic compared to some of the modern stuff coming out.
I'm going to use a single mass flywheel. I've ordered what is a conversion kit for a dual-mass sprinter van. So, hopefully, it takes into account the shorter gearbox shaft!! I just got a text saying its sitting in Parcel Motel in Finglas because it's "Oversize". Might try to spin up there tonight to pick it up! If it all works, I'll post up photos and the part numbers, etc so others can copy.
Thanks for your interest in my project, it's great to get a bit of encouragement, especially when I get to wondering if I'm nuts to be bothering with it at all!

I bought a bunch of brownie points with an iPad last week. I've been cashing them in hand over fist over the long weekend, and made some more good progress!

The old 280SE was very reluctant to give up its 3.5L V8. It looks like the designers intended for the engine & 'box to come out through the bottom of the car. Well, I don't have the head room nor the equipent to get the car that high up, so I had to pull the gearbox out from under the car and then lift the engine straight up. It was a real pain getting it all out as every electrical connector was corroded solid, the bolts holding the 'box to the engine were all but inaccessible, the exhaust joints had all rusted solid and I couldn't upen the connections to the oil cooler and didn't want to cut them.

Anyway, it's out now!


I haven't made a detailed study yet, but some stuff I've found out over the weekend:
The 280SE engine bay is about 3cm narrower than the E300 donor car. This shouldn't be a big problem.
The manual gearbox I intend to use is about 11cm shorter than the auto I'm removing. no problem in terms of mounting, but I'll need to extend the prop shaft to meet it.
The rad from the donor is way too big. I'll wait to see what space is like with the new engine fitted before doing naything drastic, but I might have to go buy an aftermarket one.
It looks like the steering box in the 280SE is enormous and might get in the way of the exhaust, but again, I'll see what its like with the engine fitted. I might even explore the possibility of insalling the rack out of the E300 if it works out anyway handy as the old steering box is incontinent and making a mess of my garage floor.

This is going to be a long post, I hope I've explained what's going on well enough for others to replicate it if they want to!

I'm not sure what you call this plate, but this is the original plate which had the ring gear attached to it and sat behind the torque convertor. You might or might not be able to see there are 6 tabs protruding from it which whizz past a sensor which I guess is for an engine speed feed to the ECU rather than a crank position.


The ring with the tabs on it looks pretty crude, but it was obviously machined after being welded on to make it perfectly circular, at least on the outside!


Anyhow, that bit is getting dumped as I'm changing to a manual gearbox. So, I bought one of this on a wing and a prayer. It was listed as a dual mass flywheel conversion kit for a Merc Sprinter van, 2.2CDI and others from 2002 to 2006:



It contained all of these bits:


The first problem I encountered was that there is a protruding flange on the end of the crank shaft, and a corresponding recess in the flywheel. The flange was 11.2mm and the recess only 10mm. This meant that the flywheel could not be tightened right up against the end of the crank, so the crank shaft flange was duly ground back to be shorter than the recess in the back of the flywheel.


Problem number 2 is that the new flywheel is completely surrounded with ribs which, I can only suppose are used to generate a signal for the Merc Sprinter it was intended to be used in.

These ribs also slightly contact the sensor. Plan "A" was to machine away the ribs I don't need in order to leave 6 remaining, simulating what was in place before, I could then put a spacer behind the sensor so that it didnt stick out so far and wasn't hit by the remaining ribs. The main issues with plan "A" were that the ribs weren't in the same place on the flywheel as the tabs on the original item (I'm not absolutely sure that there isn't some kind of timing function along with RPM!), and that I would then need to have the flywheel balanced.

I have a great engineering friend who came up with a far superior plan "B": We would drill the 6 'valleys' which correspond to the tabs we had removed, tap them and screw in bolts which would protrude above the existing ribs. We'd have to move the sensor even further out, but that's no big deal, and we wouldn't be disturbing the balance of the flywheel. So, to the implementation of plan "B".

We marked the valleys which were to be drilled and tapped, then mounted the flywheel on the engine. We turned down a big bolt so that it matched the shaft size of the sensor and would fit snugly in the sensors mounting hole. We then drilled a hole down the centre of the bolt and poked the bolt into the sensor hole. Now, we had a guide which we could use to make sure the holes we drilled were directly beneath where the sensor would be.

Here's the sensor fitment:


and the bolt we guntered into a tool:


and some drilling underway:


Here's a bolt inserted in the hole. I plan to get longer bolts, loctite them home and then remove the heads so that the part left sticking out proud of the ribs is closer to the 5.5mm of the tabs which were originally present.


Aside from these alterations, Everything else seems to work out in terms of the position of the bearing which accepts the end of the spigot shaft, the position of the clutch friction plate in relation to the splined part of the spigot shaft, and the position of the pressure plate in relation to the release bearing/concentric slave culinder which is mounted in the gearbox. Happy days!

Well, here it is, the "new" engine in its new home. It's only sat there, but it fits in well. The engine bay looks a bit empty but will soon fill up with a battery, air box, computer box and screen washer bottle. the blue shopping bag contains the wiring harness for now!!





This intercooler pipe had to be sacrificed as it was sitting on the anti-roll bar, I'll just re-route it, or might even get away with squashing it a bit:



Lots of clearance between subframe and sump:


And the exhaust clears the steering box:



It's difficult to get a decent photo of it, but here's the attachment for the shifting mechanism on top of the gearbox. It's pretty tight to the top of the transmission tunnel. That long self-tapper is someone else's legacy!


This photo shows the original V8 engine mount and the new, improved wooden version.

Bigus wrote: »

Very nice so far, I wonder is there another alloy engine bracket from another model that would allow you to use the original engine mounts ?
German auto parts in walkinstown behind Brennan's bakery have a load of merc engines with brackets on the shelf,pre export .

Some sort of aluminium heat shield / heat dissipator might be in order to take heat away from the steering box.

That'll be one quick economical car when it's finished. They handle like a sports cars too , compared to later models and some more modern yokes.

Well done

Thanks for your interest Bigus!
Thought about swapping brackets, but the existing ones line up so well with the subframe that I'll either move the existing mounts in to meet them or find some off something else and stick them in.

I kept a big lump of heat shielding off the donor car which I'll use to protect the steering box and engine mount.

Hopefully it'll go well, I'm particularly looking forward to the 6 speed manual box.

commited wrote: »

Great project and great progress! I know a chap that does a fair bit of work on these engines and he's not a fan of the OM605/606 as the heads can give trouble.

Thanks commited, that's the first time I heard that. Have you seen there are guys running 2 and 3 turbos with getting 400 & 500HP from these without changing the internals!?
Do you know what kind of head problems there are and anything I can do in terms of prevention?

Ah, I have heard about guys snapping glow plugs off in the head alright, I thought that you were talking about head gasket/valve gear troubles.

I'm not looking for big power, I'll be using the engine in standard form.