Review: MerCruiser 4.3L MPI

By: Andrew Norton

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  • Trade-A-Boat

The MerCruiser 4.3l MPI shows that there’s nothing wrong with old engine technology when it’s well packaged.

Review: MerCruiser 4.3L MPI
If it ain't broke, don't fix it: the MerCruiser 4.3 has been around since the 1960s.

According to my old grey cells the MerCruiser 4.3L has been around since the sixties, when it was installed in a front-wheel-drive Oldsmobile. The engine soldiers on in marine form and has been incredibly popular, both in single and twin installations.

Compared to low-emission petrol outboards of the same output, the 4.3L seems positively primitive. But it has simple engineering that’s hard to kill, even with minimum maintenance. And more to the point, the MPI version (that enables Mercury’s brilliant SmartCraft system to be used) pumps out 15 per cent more power than the old two-barrel carbie version.

 

MERCRUISER DESIGN

The 262 cubic inch (4.3lt) MerCruiser MPI is a 90-degree V6 version of the long-running 350 (5.7lt) V8 and develops 219.8 propshaft horsepower at 4600rpm with a Wide Open Throttle range of 4400 to 4800rpm. This rpm range is more akin to a current common-rail diesel than a petrol engine and combination of with the way oversquare bore and stroke dimensions of 101.6 x 88.4mm and hydraulic lifters driven by pushrods from the centre camshaft, make this one low-maintenance and long-lasting engine.

Freshwater cooling reduces cylinder wall condensation and adds 24kg to the dry engine/drive weight of 393kg, not bad considering the cylinder heads and block are cast iron – none of that newfangled alloy here!

The 65amp voltage regulated alternator on the MerCruiser 4.3L MPI is driven by a serpentine belt that also drives the power steering and watercooling pumps.

The 4.3L is available with legs from the ageing Alpha up to Bravo 3 but frankly, for twin installations the 3s are overkill as they prevent the classic one ahead/one astern technique for spinning a hull in its own length. For trailerboats, the cooling water intakes are in the drives but the 4.3Ls I tested in a Sea Ray 315 Sundancer had through-hull intakes with strainers and seacocks. Just remember to open them before starting the engines!

In single installations, access to the oil filter and spark plugs is good but in the 315 Sundancer, which had a beam of 2.87m, getting to the inboard plugs definitely needed a very flexible wrist and fingers. The price you pay for security in a smaller cruiser!

Servicing intervals are every 100 hours or annually after the first 20 hours, though, as most cruiser owners clock only around 50 hours a year from the second year onwards, I would change the oil and filter every six months and use Quicksilver SAE 25W40 oil designed for sterndrive engines. Standard 20W50 oils just won’t handle the high loads marine engines experience. Also premium unleaded (95) should be used as MerCruiser petrol engines need 87 AKI (Anti Knock Index in the US) which translates to 92 RON and standard unleaded in Oz is 91 octane.

 

PERFORMANCE

The review freshwater-cooled MerCruiser 4.3L MPIs had Alpha legs instead of Bravo 1 or 2, but these worked well in the 315 swinging 14.5 x 17in stainless steel Quicksilver Vengeance props driving through relatively tall 1.62:1 reduction ratios and being slightly underpropped for heavier loads. The engines started instantly hot or cold with no oil smoke appearing and the shifting was reasonably smooth despite the dog clutch drives. For any real power astern the drives had to be left down, but at least with around 1000rpm on the tachos the hull could be spun in its own length in calm conditions.

The engines quickly got the 4.6-tonne 315 out of the hole when the throttle levers were pushed down hard from trolling rpm, and across the entire rpm range the 4.3Ls had low vibration levels and only became raucous approaching WOT. As with all tests I perform on sterndrive cruisers the trim tabs were left on neutral and the drives trimmed as needed.

Through tight turns at 3000rpm no prop ventilation occurred and a nice touch was how the engines maintained their rpm due to the MPI pouring in more fuel to compensate for the increased load.

 

THE TRADE-A-BOAT VERDICT

I don’t have a problem with petrol engines in cruisers, providing they’re well maintained and the bilge blowers are run for several minutes before starting them. I used this technique (and opened the engine compartment hatches) with the twin V8 inboards in a Halvorsen 40 I skippered for private charter work on Sydney Harbour 30 years ago and never had an issue. And unlike mechanically-injected diesels, freshwater-cooled petrol engines don’t suffer from cylinder bore glazing when run under light load for extended periods. Perfect for pottering around waterways.

The MerCruiser 4.3L MPI makes good economic sense in infrequently used sportscruisers. The exhaust note is nowhere near as satisfying as a V8 but under load the V6 noise ain’t half bad!

Thanks to Lifestyle Marine, Toronto, NSW – phone (02) 4959 1444 – for supplying such well setup engines.

 

MERCRUISER 4.3L MPI SEA TRIALS

Sea Ray 315 Sundancer with twin 220hp MerCruiser 4.3L MPI petrol V6 engines. Average of two-way runs using SmartCraft instrumentation and Raymarine GPS.

RPM

SPEED

FUEL BURN

610 (trolling)

2.5kts

3.3lt/h

1000 (fast idle)

4.4kts

4.9lt/h

1500

5.8kts

7.3lt/h

2000 (offshore troll)

7.6kts

12.9lt/h

2800 (min. plane)

13.2kts

26.2lt/h

3000 (offshore cruise)

19.5kts

28.7lt/h

4000 (max cruise)

30.1kts

41.0lt/h

5000 (WOT)

35.3kts

61.6lt/h

* Sea-trial data supplied by the author. Fuel burn is per engine.

 

MERCRUISER 4.3L MPI SPECIFICATIONS

TYPE V6 petrol engine

RATED HP 220

DISPLACEMENT 4.3lt

WEIGHT 353kg

BORE x STROKE 101.6 x 88.4mm

 

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Originally published in Trade-a-Boat #446, November / December 2013

 


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