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19th November 1987
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Page 52, 19th November 1987 — TO BLOW OR NOT TO BLOW
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Mercedes-Benz doesn't offer turbocharging on its 307D van — but Omicron Engineering does.

We took one round our test route in standard and blown guises to assess the benefits. • Despite a swing by the leading van manufacturers towards turbocharged, direct-injection diesel engines, MercedesBenz has stuck steadfastly to the 53kW naturally-aspirated, ID!, 0M616LG for its 307D 3.5-tonne GVW van.

Of the vehicles in this weight range sold in Britain, the 3071) falls a long way behind the born-again Transit, but it is still a widely used van.

It has been around since 1977 in dieselengined form and, during that time, has gradually earned a reputation as a durable and fairly economical performer, without the rate of acceleration to put it among the sprinters.

With such a moderate output — maximum net torque 137Nm at 2,400rpm DIN — it is not surprising that it has been targetted by at least one turbocharger conversion specialist.

Omicron Engineering is such a company and offered Commercial Motor one of its converted vehicles as an unusual beforeand-after subject for roadtest.

The MuIbarton, Norwich-based firm is the UK importer for the Swedish STT diesel-turbo system which has been designed, developed and manufactured specifically for Mercedes CI engines.

When applied to the 3071), the 0M616LG's power is increased by one third to 70kW, while the nett torque is increased by nearly 40% to 190Nm.

In graphical form the improvement of output is startlingly clear. There is more power and torque, beginning lower down the rev range and continuing for a much longer period.

On the road, the improved performance of the converted 307D is equally striking but, like most such benefits, there is a price to pay: increased fuel consumption.

Two-and-a-half years ago, Commercial Motor tested a factory-prepared 307D high-roof van with the revised 4.4:1 final drive instead of the previously standard 4.9:1 unit (CM 1 June 1985).

It returned a moderate fuel consump tion of 14.3lit/100km (19.7mpg) at an above-average speed of 66.9km/h (41.6rriph) and pulled down to the 48km/h (30mph) town limit in top (fifth) gear without difficulty.

rirrinvissiffi This time our three-year-old test vehicle, with a history of local-delivery runs, has 32,000km on the clock and is owned by Garrett Turbo Services of Cheadle Hulme. It is one of the later 3071) highroof models.

Around the Kent light van test route, with its mixture of motorways and Aroads, fully-laden and in its unmodified state (naturally-aspirated), it produced figures of 12.81it/100km (22.2mpg) at a rather slow overall average speed of 63.31cm/h (39.4mph).

Apart from its heavy steering at low speeds, the overriding impression was of the engine's total lack of punch. This seemed worse at such critical moments as when pulling away from traffic lights or when accelerating in fast urban traffic.

The engine felt very flat on long gradients such as the M20 hill climb at Kingsdown, where it flagged quite early on, forcing a change down into fourth gear to maintain reasonable progress on a clear motorway. Having completed the route the 307D went to Omicron for a turbocharge conversion and was retested.


The effect of the turbocharge conversion is startling. The acceleration is now much quicker than anticipated. It makes the difference between simply poodling along, holding 'faster vehicles back, and being able to accelerate vigorously and hold station with fast moving traffic.

It has a much stronger, positive response at lower road speeds.

Instead of having to pull away from the kerb in first gear, it is now possible in second and without too many engine revs.

There is no longer any problem with falling revs over KUigsdown Hill. Thanks to the extra torque, the engine feels far more vigorous, holding its speed at 88km/h in top gear and clearing the brow half a minute quicker than before.

Had a rev counter been fitted originally, it would have been possible to be more specific about exactly where the benefits in engine performance lie.


As far as the overall fuel consumption is concerned the result is something of a false dawn.

It registers a 12% drop in fuel useage with figures of 14.51it/100km (19.5mpg) at a faster, 66.41cm/h (41.3mph), average speed. This could be quicker but, during our test, there were a few traffic hold-ups due to fog.


The conversion itself has been developed by STT (Svensk Turbo Telmik) of Njurunda, Sweden, mainly for Mercedes cars — but for vans also. Since 1981 it has transformed over 3,800 vehicles throughout Europe.

Five years ago Omicron became the UK representative, converting M-B cars and the G-wagen, and this April, following requests from customers, it began working on the 207/307D vans.

The work entails the removal of the radiator, fuel injection pump, exhaust system and oil filter.

While the fuel pump is away being recalibrated to STT's own test plan, which is known only to their fitting agency (Omicron is looking to appoint its own fitting agents) the turbocharger is mounted on the nearside of the engine. It locates beneath the Fl pump-mounting flange in a sturdy cradle bracket.

STT specifies the British-made Garrett T03 turbo-blower unit which is supplied by the van-owner, Garrett Turbo Services. Once it is in position, the exhaust gases are re-routed from the manifold on the off-side of the engine behind the sump pan to the turbocharger.

To match the 63mm bore of the gasturbine outlet, Omicron fits a larger diameter exhaust system. It costs £.321 of the total price.

The filtered air is rerouted via the turbo's cold side into the inlet manifold.

Like most turbocharged engines, the STT conversion requires oil cooling, so the existing oil filter is re-worked to accommodate the extra plumbing. The new cooler matrix is mounted in the cold air stream between the front grille and the water radiator lowering the temperature by as much as 10°C, holding it at just below 80°C under all operating conditions. Apart from catching sight of it through the front grille, and the larger diameter exhaust, there are no obvious signs of any change, other than an additional STT logo.

From under the bonnet or from inside the cab there are no obvious hindrances to routine service checks. Indeed, there are no clues to it being turbocharged.

It does not appear to interfere with oil changes or other maintenance tasks. Infact, the additional bulk has been added without any loss of access and is altogether a neat, compact conversion.


Anyone considering having this work carlied out on a 307D van still under its Mercedes-Benz warranty must guard against their position in the event of a defect claim.

The guarantee will continue to operate on those parts not affected by the installation, but where a fault can be traced back to the conversion, the original warranty is invalidated. Omicron says, however, that STT's experience in this matter makes such problems As a precaution, STT will take over the manufacturer's warranty under its own insurance and offer a second year driveline cover at an all-in cost of 2120.

The turbo kit itself has a 12-month guarantee and there is a comprehensive UK/European service support from the turbocharger supplier.


Standard Mercedes-Benz vans are durable and a little on the heavy side. Priced currently at £11,650, the 307D is not among the cheapest and it does offer a fairly ordinary performance.

Turbocharging is one way to sharpen it up, but at £1,420 for the sTT conversion, which takes the van out of commission for two days, it begs the question why? — particularly when there are no apparent fuel savings to be made.

The increase in power and torque gives obvious benefits in journey times for the express parcels carrier, with less stress for the driver.

For those operating with trailers and running at, or near, the vehicle's 5.5tonnes-gross-train limit the 0M616's 70kW output equates to a 32% increase in power-to-weight ratio and here, especially with such a jump in torque, it makes long, laden journeys easier work.

by Bryan Jarvis


People: Bryan Jarvis
Locations: Njurunda, Norwich

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