MerCruiser D1.7L DTI turbo diesel sterndrives

A couple of diesel manufacturers offer repowering packages for older planing-hull cruisers such as the Caribbean 28. But what about smaller sterndrive-powered boats such as the Caribbean 26?

Normally the 26 retails for around $105,000 with twin 210hp V-six EFI petrol MerCruiser sterndrives - around $35,000 cheaper than the same boat with twin 130hp turbo-aftercooled diesel Volvo sterndrives. However, if you have an older 25 or 26 with the twin petrols and want to go diesel, there's a big cost differential to fit new engines.

This is where the 120hp (crankshaft rated) MerCruiser D1.7L DTI becomes very good value for money. Admittedly even in twin installations they won't provide the power of twin 130hp Volvos, but then at around $42,000 for a pair they're a lot cheaper!

Depending on fitout, a Caribbean 26 with petrols displaces about 2-2.5 tonnes and the 130hp diesels add 80kg. Again that's not much in a 26-footer, but opting for twin D1.7L models actually reduces the weight by more than 200kg over the petrols and almost 290kg over the 130hp Volvos. The reduced weight aft should enable the hull to plane faster or hold a clean plane at much lower speeds.

Considering that the MerCruiser D1.7L has a piston displacement of only 1.7lt compared to 2.4lt for the 130hp Volvo, do twins provide adequate power for the 26?

The answer is yes. Based on my testing of a single D1.7L in a 5.5m Savage Mako 55 SP, which displaced a total of 1320kg during trials, the motor was more than adequate for this hull and would have no trouble powering the 26 in a twin installation.

The Alpha leg had a 2:1 gear ratio and a four-bladed, 18-inch alloy prop was fitted. The demo D1.7L started instantly each time, although as expected with direct-injection diesels having fixed-injection timing there was some smoke on start-up. There was also the traditional diesel 'rattle' at the trolling speed of 900 revs in gear but this all but disappeared at 1000 revs.

The Mako trolled at 2.9kt and at 1000 revs the average was 4.0kt. The turbo started to come on boost at 1800 revs and at 2000 revs we averaged 7.3kt with the fuel consumption a low 4.0lt/hr.

The D1.7L planed us cleanly at 2750 revs and 15.4kt, which was maintained into the blustery sou'easterly-driven 1.5m swells off Newcastle Harbour, while consuming only 7.5lt/hr. The planing revs were 350 above maximum torque of 264Nm, ensuring that at peak engine load while coming onto the plane the engine was operating at maximum efficiency. This factor makes the automotive-based D1.7L better suited for pushing heavy hulls than purely marine engines.

Back in calmer water at 3000 revs the D1.7L cruised quietly averaging 19.2kt using 9.0lt/hr and through tight figures of eight at 3300 revs there was no prop ventilation.

The standard power steering was direct yet reasonably light.

'Flooring' the throttle at 3000 revs created no black smoke and at 4000 revs the D1.7L averaged 27.5kt using 17lt/hr, while the Wide Open Throttle (WOT) average was 31.8kt on 4600 revs consuming 27lt/hr and at these revs we could still hold a normal conversation at the helm.

Based on an Isuzu diesel, the D1.7L develops its maximum power at 4400 revs compared to 3800 revs for the Volvo. It has a cast iron block, a crossflow alloy cylinder head and DOHC actuation for the 16 valves.

It displaces 1686cc and has a bore of 79mm x 86mm with a compression ratio of 17.2:1.

Unusually for an automotive diesel of this displacement, it has direct fuel-injection (with glow plugs for cold-starting ease).

Combined with the standard wastegate turbo and intercooling, the D1.7L complies with current IMO diesel exhaust emission requirements. Closed circuit (heat exchanger) cooling and a 50-amp alternator are fitted.

The engine itself measures only 726mm (long) x 746mm (wide) x 689mm (high).

In comparison the 190/210hp MerCruiser measures 838mm x 813mm x 533mm, while the 130hp Volvo measures 1008mm x 749mm x 758mm.

In the case of the Mako, access to the engine oil dipstick and filler cap, oil filter, leg oil level tank, sedimenter, water-separating fuel filter and engine coolant overflow bottle was very good. A serpentine belt drives the alternator, freshwater pump and engine-mounted seawater pump.

In a twin rig application such as would be the case in our imagined 25/26, this ease of access should be retained.

Recommended servicing intervals are every 100 operating hours or once a year after the first 20 hours or three months, but as turbocharged engines tend to dirty the oil quickly, the engine oil and filter should be changed every 50 hours or six months, whichever comes first.

AND THE NEW MERCRUISER PETROL MX 6.2...
Developed from the Mercury Racing Scorpion 377, the MX 6.2 is a 'stroked' version of the long-running 350ci or 5.7lt, 90? petrol V-eight.

The MX 6.2 is available in two versions for cruisers, the MPI Sterndrive and MPI Inboard. Both have multipoint fuel-injection and develop 320 prop hp. To counter the leg losses the sterndrive version achieves this output at 4800-5200 revs, while the inboard peaks at 200 fewer revs.

In comparison the 350 MAG MPI in both sterndrive and inboard versions developed 300 prop hp at 4600-5000 revs and 4400-4800 revs respectively.

Both MX 6.2 models have 70-amp alternators as standard (up five amps from the 350) and the fuel-injection systems are watercooled to reduce vapour locks.

The cylinder blocks are cast iron, the crankshafts forged steel and the roller follower camshafts were designed specifically for this engine. Other features include ceramic coating and electro deposition paint (EDP), process exhaust elbows and manifolds, and plastisol-coated oil sump pans. Starter motors are positioned higher on the block, away from any bilge water.

Closed circuit cooling is standard on Horizon models and optional on other models. The system adds just 36kg to the overall weight.

The MX 6.2 Sterndrive is available with Bravo I, II or III legs and complete with the Bravo III weighs 474kg, while the engine itself measures 940mm x 737mm x 559mm. These figures are identical to the 350 MAG MPI Sterndrive.

Complete with a Hurth 630A gearbox, the MX 6.2 Inboard weighs 390kg and measures 1143mm x 762mm x 559mm, again identical to the 350 MAG MPI Inboard.