TALK­ING TORQUE

VW’S 3.0 litre Amarok diesel suits boats too...

trade a boat - - CONTENTS - AN­DREW NOR­TON with

It didn’t take long for VW to start pro­mot­ing its 3.0 litre diesel in the Amarok 4WD. Orig­i­nally the twin turbo 2.0 litre diesel was pro­moted but why stop at that when the 3.0 litre en­gine is so good? This high revving (for a diesel) en­gine suits per­for­mance hull ap­pli­ca­tions, es­pe­cially when mated with the Mer­cruiser

Bravo 3 leg.

In fact, this en­gine was marinised for marine usage way be­fore VW of­fered it in the Amarok, un­usual as nor­mally au­to­mo­tive ver­sions of engines ap­pear be­fore their marine coun­ter­parts.

Sea Ray is large enough as a boat builder to of­fer a wide range of en­gine al­ter­na­tives in its hulls. Though petrol V8s are tra­di­tion­ally the way to go in its bow rid­ers, Sea Ray took the ini­tia­tive of fit­ting a TDI 3.0L to its 250 SLX bow rider, whereas nor­mally this hull would have had a Mer­cruiser 6.2 MPI V8 which de­vel­ops sim­i­lar torque but at around 1000 higher rpm.

THE NITTY GRITTY

When check­ing the specs the first thing I no­ticed about the TDI 3.0L is that its pis­ton stroke is big­ger than the cylin­der bore, un­usual in V6 engines which tra­di­tion­ally are way over square, hav­ing larger bore di­men­sions than stroke. How­ever VW was smart in do­ing this as it al­lowed for more torque at lower rpm whereas bot­tom end torque is nor­mally lack­ing in V6 engines.

Another as­pect was VW’S con­ser­va­tive ap­proach to en­gi­neer­ing. The TDI 3.0L is rated to 500 hours of run­ning per year com­pared to around 250 for the au­to­mo­tive-ori­gin diesel com­pe­ti­tion. And the 3.0L can be run at Wide Open Throt­tle for one hour in ev­ery eight or a to­tal of 12.5 per­cent. Not bad for a high revving en­gine which was ob­vi­ously de­signed for Au­to­bahn driv­ing!

Once the plas­tic shroud cover­ing the en­gine has been re­moved, ac­cess to nor­mal ser­vic­ing points is very good. The en­gine sump oil dip­stick is for­ward with the power steer­ing pump reser­voir and coolant over­flow tank (fresh wa­ter cool­ing is stan­dard) to port and the wa­ter sep­a­rat­ing fuel fil­ter to star­board. Neatly grouped for twin en­gine in­stal­la­tions...

Al­though the elec­tron­i­cally man­aged com­mon rail fuel in­jec­tion sys­tem is nor­mally self-bleeding a hand pump is fit­ted for ini­tial prim­ing. Fuel is in­jected at 26,000 psi or 1770 bar, right at the top end of in­jec­tion pres­sures, so main­tain­ing fuel lines in good con­di­tion is es­sen­tial with this en­gine.

Ser­vic­ing in­ter­vals are ev­ery 100 run­ning hours or an­nu­ally af­ter the first 20 hours which should be ad­hered to as turbo in­ter­cooled diesels don’t like dirty oil. Per­son­ally, if us­ing the en­gine in­fre­quently I’d change the en­gine oil and fil­ter ev­ery six months. Fre­quent oil changes are the se­cret to gain­ing longevity from any en­gine.

If in doubt about the right oil to use, con­sult Mercury Marine’s rec­om­men­da­tions as mod­ern turbo in­ter­cooled diesels can use sur­pris­ingly low vis­cos­ity oils.

The en­gine-only di­men­sions are 927 mm long, 813 wide and 853 mm high, slightly longer and wider than the Mer­cruiser 6.2 MPI but sub­stan­tially taller. The 6.2 mea­sures 813 x 762 x 559mm though at 423 KG with Bravo 3 drive the 3.0L is 63 KG or 13 per­cent lighter than the Mer­cruiser 6.2 MPI with Bravo 3 drive.

ON THE WA­TER

Though slightly over-propped the demo 3.0L in the 250 SLX ap­peared a very good match. There was no smoke or diesel smell on cold start up or at any other time, even when climb­ing out of the hole. The sin­gle Bravo 3 drive gave ex­cel­lent con­trol com­pared to twin Bravo 3 drives, where us­ing one ahead/one astern sim­ply crabs a hull side­ways. Per­son­ally I think sin­gle-prop Bravo 2 drives are bet­ter for twin in­stal­la­tions be­cause they al­low for true one ahead/one astern han­dling. The shift ac­tion of Mercury’s DTS with “driveby-wire” tech­nol­ogy was smoother than ex­pected with no “clunk” into ahead or astern. How­ever the usual diesel rumble meant noise lev­els when trolling were slightly higher than a petrol V8.

Trolling at 800rpm the 3.0L used only 40 per­cent of what the 6.2 MPI 320 HP model uses when idling. The per­for­mance ta­ble shows how fuel efficient the 3.0L was up to 1000rpm.

Only when the hull started to plane with the en­gine un­der a fair amount of load was the char­ac­ter­is­tic V6 ex­haust note ap­par­ent and once plan­ing, the only real noise was from the drive unit with its two sets of right an­gle gears. Mercury Marine now needs to work on re­duc­ing NVH or Noise Vi­bra­tion and Harsh­ness lev­els in its stern drives to make them as quiet as the

engines driv­ing through them.

The 250 SLX planed cleanly below where max­i­mum torque is de­vel­oped – al­ways good as once plan­ing the en­gine is op­er­at­ing in its peak fuel ef­fi­ciency zone.

I’ve tested in­board diesels in some hulls where plan­ing oc­curred above where max­i­mum torque was de­vel­oped and this se­ri­ously af­fected plan­ing fuel ef­fi­ciency. The 3.0L planed us around 1000rpm lower than a com­pa­ra­ble-torque V8 and used 16.7 L/hr at 2000rpm whereas at a 6.2 MPI I tested planed a hull at 3000rpm us­ing 41 L/hr. Again the 3.0L used only 40 per­cent of the fuel. Cruis­ing at 2500rpm com­pared to 3500 for the 6.2 MPI the 3.0L used 44 per­cent of the fuel.

Ob­vi­ously de­vel­op­ing 320 HP (and now 350 HP) the 6.2 MPI would give higher top end speeds but the per­for­mance of the 3.0L sure wasn’t dis­ap­point­ing. Through tight turns at 3000rpm the 3.0 main­tained its revs with­out us touch­ing the throt­tle. At WOT the 3.0L used 59 per­cent of the fuel of the 6.2 MPI at WOT, in­di­cat­ing the gap nar­rows the higher the revs are. This is a typ­i­cal sit­u­a­tion as petrol engines have max­i­mum fuel ef­fi­ciency higher up in their rpm ranges than com­pa­ra­ble-out­put diesels.

Even at WOT the 3.0L had way lower ex­haust noise lev­els than a four stroke petrol out­board of sim­i­lar out­put and showed just how far diesels have come in re­duc­tion of noise. Vi­bra­tion lev­els were also low and for fat­ties like me out there, the 3.0L sure won’t re­duce any flab!

THE WRAP

The mat­ing of the 3.0L and Bravo 3 for sin­gle in­stal­la­tions is ex­cel­lent. Based on price alone the 250 SLX would be pow­ered by more 6.2 MPI engines than the TDI 3.0, but for boaters se­ri­ous about con­serv­ing fuel the diesel is very appealing. Ob­vi­ously main­te­nance and ser­vic­ing costs will be higher and de­spite be­ing elec­tron­i­cally man­aged the 3.0L won’t like pot­ter­ing around bays and in­lets for ex­tended pe­ri­ods, whereas the fresh wa­ter cooled 6.2 MPI would be right at home.

Fresh­wa­ter-cooled petrol engines don’t suf­fer cylin­der bore glaz­ing as can diesels, un­like raw or sea­wa­ter-cooled four strokes which run rel­a­tively cold, these suf­fer blow-by past the rings from the com­bus­tion cham­ber to the sump dur­ing ex­tended low-load pe­ri­ods, di­lut­ing en­gine oil and re­duc­ing its lu­bri­cat­ing ef­fec­tive­ness. En­gine wear is also ac­cel­er­ated and one owner of a Caribbean 26 pow­ered by twin raw wa­ter-cooled Mer­cruiser 4.3 litre stern­drives told me the engines had

“worn out” by 1400 hours.

How­ever the point is that a diesel can pro­vide com­pa­ra­ble low to midrange per­for­mance as a petrol en­gine of sim­i­lar torque. Top marks to Sea Ray for pro­vid­ing an al­ter­na­tive power source to the ubiq­ui­tous petrol V8!

For your near­est Mer­cruiser dealer Google Mercury Marine Diesels, click on Find a Dealer then en­ter your sub­urb and post­code. The clos­est deal­ers will be dis­played with dis­tance as the crow flies.

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