Turbo conversions on Mercedes Hymer motorhomes

I was recently asked about turbo conversions on pre 95 Mercedes Classic Hymer motorhomes, so decided it was about time to pull together all the info I have collected over the years.

These vans left the factory normally aspirated, but many owners back in the 1990s had aftermarket turbo conversions fitted. But are they any good?

I've owned vans with the German STT turbo conversion, with no intercooler, and also the TB Turbo conversion from the now defunct TB Turbos of Lancaster. I have also driven non converted vans, and hand on heart, there is a noticeable difference, but it's not huge, like you would expect from, for example, a turbo vs non turbo saloon car.

I finally had a very illuminating conversation with the owner of a specialist company in Lancashire called Diesel Bob. He told me that the problem with adding a turbo to these old Merc engines (the OM602 etc), is that they don't have ECU controlled pumps - just old fashioned mechanical pumps. So when you put your foot down and generate boost, there is no signal to the pump to push more fuel into the injectors to give the power. So while you do get a welcome extra kick from the raised inlet pressure, you don't get it's full potential.

The OM602 and its close relatives were the last generation of light commercial diesels before ECUs came in and direct rail injection - which is why from the 90s onward you could get a diesel car or van with a 2 litre turbo diesel that was almost twice as powerful and more economical than the old technology. In Mercs case this was around 95 when the Sprinters were introduced. The first generation Sprinter had 5 cyl engines based on the 602, but with factory fitted turbo and better pumps - they gave around 150bhp, compared with the 95bhp of the Hymer 602 engine.

The turbo conversions came about because by the time you had bunged a 6 or 7 meter high quality Hymer body on a 410D chassis rated at 4.6t, what you ended up with was a light commercial vehicle that basically spent its life fully loaded, at around 3.7 to 4.3 tons, depending on model and payload. Even by the standards of the day they were underpowered, and slow. But they were also the top of the range vans of the day in Germany - around 100,000 Dmarks, and buyers were well heeled, so thought nothing of splashing out on a turbo conversion offered by the dealer.

Unfortunately - and it is a question that is asked regularly - there is no easy way to do a similar turbo conversion today - both STT and TB are out of business, and more importantly, the custom exhaust manifolds onto which the turbos mounted are no longer available. The only alternative is to have a custom exhaust manifold made by a specialist company. A few people have done it though, so I have heard over the years. Of course there are motorsport outfits that would do it, but it's also a question of money. The guys that have done it are usually in the motor trade and DIY. It's a fair amount of labour, plus parts, and at commercial rates of £50+ per hour I reckon a custom turbo conversion would cost around £5k or more.

But for the power hungry there are alternatives. The first is to re-engine with a donor engine that is factory turbo'd. Donor engines are either from first generation Sprinters or the Ssangyong Musso from the 1990s - a 4x4 car that used the Merc 5 cyl engine under licence. I know of at least 2 guys that have done this, but it's quite a job because the Hymer engine compartment is quite tight.
The second, and quite recent, option is to fit an uprated diesel pump. The OM602 (and its brothers) engines were fitted in early Merc G Wagens, and there is a healthy industry in customising and uprating these. Also, for some weird reason, Merc saloon cars of this era are favourites for fans of "drifting". There are 2 firms that specialise in this - Dieselmeken in Sweden and DieselPump UK here in the UK. Both have informative websites, and both claim to offer an uprated pump that will up-rate a 602 to 150, 200 or even more BHP. The cost is reasonable - around £1500. I know of 2 UK Hymer owners that have done this. The only slight reservation I have about this though, is the effect on the rest of the van - the drivetrain and cooling systems, of suddenly upping the power to 150bhp, and purists will also point out that brakes should also be upgraded to cope.

Edir 2020 - here is a very interesting Youtube from DieselpumpUK - the engine is a 617 - similar to the the 602. It's a good fun watch!

https://www.youtube.com/watch?v=tfvzwWP0WXE

Another issue is that the gearboxes on Merc Hymers are quite low geared, so even if you had power to spare, it would only be useful on hills - you wouldn't get much extra top speed cruising because the engine would be at its rev limit. Some owners have changed their gearbox to a higher ratio version - myself included. It's a straight swap, if you can find one, and the result is that you lose the low crawler gear and get a higher fifth gear, and thus lower revs at high speed. It's still useless on hills though.

There is also another school of thought that says - why bother! Trundling along at 55 is all part of the fun! This is also the point at which owners of Fiat family engines start to crow - Fiat were earlier to the market with stock turbo engines, and there is no doubt that the turbo Fiat engines of the same era, in a similar body weight, are more powerful.

Finally it also must be said that anybody who is disappointed with the power of their Merc 602 based Hymer - usually S660 and S700 owners - the heaviest - should first ensure that their fuel delivery system is in tip top condition. There are at least 3 filters between the tank and the injector pump, and as most vans are now in the 20 to 30 year old age bracket, these items can get clogged with age. I have seen many a story online where a sluggish van eventually started to give up the ghost - usually on long hills, and all it turned out to be was either a clogged fuel lifter in the tank, the inline filter, or more rarely the main fuel filter. Once these have all been attended to, a van can spring to life in a very satisfying way. There is no point in contemplating power modifications until these items have been checked and serviced.
Sorry this reply has turned into a bit of an epic, but I have been meaning to write this up for quite a while as it is a regular question. I shall turn it into an article for my blog and for this group also.

If you google "Hymer Mercedes turbo conversion" or similar, you will get hits from many forums going back years - it's been quite a hot topic over the years, and unlike Facebook, all the old archive stuff on the forums is still available and searchable with google.

EDIT NOV 2020 - There is now a dedicated Mercedes Hymer engine upgrade Facebook group.

https://www.facebook.com/groups/846310116138685