A few thoughts on engine conversions

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G-Restorations's picture
Joined: 07.04.2011
Location: Blackpool
GWOA Groups: Committee, Members

The G Wagen engines.

The engines found in early Gs, though reliable and capable of serious offroading, really don't do the job for 21st century on road driving conditions. A 2500Kg un-aerodynamic car needs much more power to keep up with modern day traffic. The information in this article is written with fitting an OM606 Turbo from the Mercedes W210 E-Class in mind.

This is a super reliable and potentially powerful replacement.


How much power?

At G Restorations we want to produce a conversion which works on a daily basis, starts up when you go to it whatever the weather, smooth when you drive it, and reliable to own. We want your G to be able to tow a heavy load up a big hill without overheating or cruise on the Autobahn without killing the engine.

The power from a standard OM606 turbo is 177 hp;  we advise 210 hp. You will see silly power outputs quoted for these engines, but what are they like to live with on a daily basis - overheating when pushed, black smoke at low revs when there's no boost, a big step in power between no boost and boost and a loud crackly idle.

In short, we want you to forget you've had an engine upgrade and just enjoy the benefits.


What engine and gearbox do you have?  Which generation of G do you have?

The engine and box you have in the G now affects the difficulty of the conversion to a new more powerful engine.

Long or short wheelbase has no effect.  The easiest conversion is a W460 Diesel with a 5 speed manual gearbox. This is because the wiring, fuel system and propshaft are completely compatible.

The most difficult would be a W463 Petrol with automatic transmission.


What transmission should I choose? Benefits and pitfalls

To get the best from an engine conversion in your G the transmission choice is critical. The original transmission on your G will be geared to cope with off-road conditions and a fairly low power to weight ratio, typically 35-60 horsepower per tonne. This means they are usually are quite low geared. So when an engine with twice the horsepower and torque is dropped in, they are very low geared. To get some idea of what gearing your car has at the moment,  look what speed your G is doing in top gear when the engine is at 2000rpm. Ideally, this should be about 58MPH.

You may be advised "just put some bigger tyres on". However, this has little effect and just makes your G drive badly.



G Restorations don't recommend automatic gearboxes. Although we have nothing against autos in principal, this is because every auto gearbox option available is compromised in some way.

722.4xx gearbox This is found in w124 saloon cars, too weak and no overdrive top gear....not an option.

722.3xx gearbox This is the W463 autobox found in petrol and diesel versions. strong enough if its in good order but no overdrive top gear, so only just suitable if you have 4.11 axles in your G. It will still feel under-geared.

722.6xx gearbox This gearbox is found in several Mercedes models including the G300TD 96-98. It is strong enough and has the overdrive 5th gear.  The bad news is that it is electronically controlled from an ECU in the donor car. To make it work in a G, it needs an aftermarket stand alone controller. These are available from Offgear in Denmark. G Restorations won't use this controller because of reliability and licensing issues (Offgear state "for off-road use only")


Manual Gearboxes

G Restorations recommend a manual gearbox. We use six speed gearboxes from Mercedes saloon cars, these being very strong with the overdrive 6th gear and very easy to swap out in the event of a failure, and cheap to replace.



What Engines are available for upgrades?

OM602 2.9 Litre 10 Valve 5 Cylinder turbo or normally aspirated. This engine is a longer stroke with good low down power, original fitment in G Wagens W461 models, civilian and military and lots of  Mercedes Sprinters.


OM662 2.9 Litre 10 Valve 5 Cylinder turbo or normally aspirated.  This engine is the same as the OM602 above, but is made under licence by Ssangyong Motor Co and is fitted to Mussos and Rextons. It is configured differently to the Mercedes version so a bit more work is needed to fit it.


OM603 3.0 Litre 12 Valve 6 Cylinder Turbo or normally aspirated. This engine is original fitment in the W463 G300, G350TD. and the W124 saloon car. A good reliable engine but no real point when there are so many OM606s about.


OM605 2.5 Litre 20 Valve 5 Cylinder. turbo or normally aspirated. This  is a 5 Cylinder version of the OM606, ideal if you have a SWB G.  It also leaves a bit more room in the engine bay.


OM606, 3 Litre 24 Valve 6 Cylinder,  turbo or normally aspirated.  The best option by far, smooth, powerful,  immensely strong and reliable,  readily available (in the UK). Found in Mercedes E-Class W210 (96-2002)


jdring's picture
Joined: 12.04.2004
Location: South of Oxford, UK
GWOA Groups: Members
Re: A few thoughts on engine conversions
Nice summary. Could you add some clarity on diesel/petrol options and a word about economy/performance maybe. See pm (x2)
Nick123's picture
Joined: 12.01.2004
Location: Argyll, Scotland
GWOA Groups: Members
Re: A few thoughts on engine conversions
Nice piece and some food for thought. As I understand it "OM" = "Oil Motor" i.e. Diesel. Mine is a very basic OM606A ("A" = "Turbo") coupled to a SLK 6 speed gearbox (I don't know the designation reference number) and we get mid 20s m.p.g., keeping up with modern traffic, without being too light footed. 1st gear is pretty much a crawler. Until you get to a hill 6th gear means that you can almost drive it as you might an automatic, without sliding up and down the 'box. We cruise at 65 - 70 m.p.h. comfortably.
jdring's picture
Joined: 12.04.2004
Location: South of Oxford, UK
GWOA Groups: Members
Re: A few thoughts on engine conversions
I remember my 230 SWB petrol cruising easily at 70-80. 5-speed manual made it a dream. My recent 230 auto manages only 55-60 because of the 4 speed auto. Both returned about 18mpg with 'keeping ahead' driving. My mistake not knowing that OM meant diesel.