Yanmar's mechanically injected 6LPA-STP2 still has a following.
There are now more boats than ever before on the water with electronic engine management for their diesel engines, offering up info such as real-time fuel flow and range-to-empty.
Heck, even I now have a car with EFI, central locking and an engine immobiliser, not to mention my first automatic transmission in 35 years.
But my work vehicle, a 1991 Mitsubishi Triton, is carburetted and all manual. The reasoning behind this is that should this vehicle break down, I can quickly fix it with minimal expense.
I shudder to think how expensive the 1999 Corolla I was recently given would be to fix, if it had an engine management computer failure or issues with the electronically-controlled transmission.
Avoiding potentially expensive repairs down the track is the main reasons why Yanmar's straight six 6LPA-STP2 is still popular.
Sure, it is an interference engine with 24 valves driven by a toothed rubber belt, but providing this is checked annually and replaced every 800 to 1000 running hours there's no reason why it shouldn't be as reliable as an engine having a gear-driven camshaft and push-rod valve actuation.
And from my experience of replacing cylinder head gaskets on engines with push rods, it’s my opinion that a cylinder head with overhead camshaft is way simpler to remove and refit.
Best of all, the Yanmar 6LPA-STP2 is a Toyota engine that was used in the Landcruiser 100 Series, so spares are easily obtained. Those 100 Series engines had an impressive record for reliability.
The Nitty Gritty
The electronically-managed turbo intercooled 1HD-FTE engine in the 100 Series Cruisers developed 202 brake horsepower at 3400rpm and 430NM of torque at 1800rpm, so in comparison, the 6LPA-STP2 is highly stressed and requires careful propping to reach its designated Wide Open Throttle rev range.
It develops 311 BHP at 3800rpm, with a maximum continuous rating of 283 BHP at 3600rpm.
The maximum torque is 700 NM at 2200 to 2400rpm with 580 NM still available at WOT.
The nearest comparable petrol inboard is the discontinued 8.1 litre 8.1S HO Horizon MerCruiser V8, which developed 420 BHP at 4600rpm, but only around 700NM at approximately 3200rpm.
When powering any cruising boat, I believe the maximum torque output is way more important than maximum output.
After all it's torque that planes a hull, not maximum BHP.
And unlike the 8.1 which shouldn't be run continuously at more than 3500rpm, the 6LPA-STP2 has a continuous rating only 200rpm below WOT.
Of course, with its EFI and electronic engine management, the 8.1 had real-time fuel flow and range-to-empty instrument displays but at 513kg with gear box, the freshwater-cooled 8.1 actually weighed 62kg or 14 percent more than the 6LPA-STP2 with box.
At 1194mm long it was only 63mm shorter but 130mm wider.
The real difference was its height of 610mm compared to 812mm for the Yanmar; a big difference when the engine is mounted under the saloon floor.
Apart from the heavier weight was the issue of fuel flow.
The 8.1s I tested in a Sea Ray 40 used 133 Lt/hr each at WOT compared to 60 Lt/hr each for twin Yanmars I tested in a mate's Cresta 32.
Obviously the MerCruisers would have provided a higher WOT speed, but top end performance is irrelevant in an offshore fishing boat, where the ability to hold a plane at the lowest possible speed is far more important.
The Yanmar has an 80amp/hr voltage regulated alternator – adequate considering none of that output is powering the engine, as would be the case with electronic management.
Apart from the belt-driven camshaft which reduces noise compared to gear driven camshafts, the two-stage fuel injectors further cut noise, while the crankshaft with 12 balance weights lessens engine vibration across the entire rev range.
Oil and filter change intervals are every 100 to 200 running hours per year depending on the amount of low speed operation.
All four-stroke water-cooled engines, petrol or diesel, suffer from blow-by of combustion chamber gases past the piston rings during extended low speed running.
Mechanically injected diesels such as the 6LPA-STP2 also can suffer glazing of cylinder bores due to the fuel being injected too early at low revs so a run-up to or near WOT is essential to raise engine temperature to reduce sump oil dilution and possible bore glazing, which would then require re-honing of the cylinder walls.
Yanmar recommends using SAE10W30 or 15W40 oil depending on the anticipated ambient temperature range between oil and filter changes, but my mate who supplied the test engines uses mono-grade SAE30 oil because it doesn't have viscosity index improvers to break down under heavy loads.
On The Water
The twin Yanmars I tested were in a 1980 Cresta 32 flybridge cruiser and replaced twin 7.2 litre Chrysler 330hp V8s.
Unusually for this type of boat, the engines were located under the cockpit with V-drives running a 12-degree shaft angle to the props.
Engine access with this layout is far superior to being under the saloon floor, though accessing the stuffing boxes under the engines is a bit tight.
I have a lot of time for the old 7.2 litre Chryslers, having run twin 300hp “Golden Lion” versions of this engine in a 1967 Halvorsen 40 I skippered in 1983.
I've never had a problem with petrol inboards in timber hulls, but fibreglass is a different matter.
I’ve seen firsthand how a petrol-engined fibreglass boat burned to the waterline in 10 minutes. So I fully understand my friend’s desire to go diesel.
The Yanmars were a perfect match to the Cresta and able to plane the hull below the peak torque band – good for planing fuel efficiency.
After glow plug warm up, the engines started instantly with just a puff of grey smoke from condensation in the exhaust system.
Alas, the classic V8 exhaust note wasn't there, but the engines had a low rumble that wasn't annoying.
The standard single lever controls that were originally fitted in 1980 worked smoothly and one engine ahead-one astern quickly spun the hull in its own length at idle speed.
Once warmed up, the fully-run in engines were opened out with a clean plane occurring at 2200rpm with no trim tabs fitted, but some black smoke did appear around 2000rpm as the engines came under heavier load.
Once up however, the engines ran as cleanly as the Chryslers with no diesel smell in a following sea.
At the minimum cruising revs of 2500 the engines maintained through tight turns and even out to WOT, they allowed us to hold a normal conversation in the cockpit. My mate's coastal game boat had been transformed to a canyon runner!
It's good that there's still sufficient market demand for a mechanically injected powerboat diesel.
And with its Toyota heritage it should return a long service life providing they’re maintained.
The 6LPA-STP2 is an excellent re-power engine for older boats, particularly those originally fitted with carburetted petrol inboards.
It's light for output so it shouldn't adversely affect hull trim.
The test engines transformed my mate's Cresta and I can just imagine how it could transform other hulls.
Twin 6LPA-STP2 engines in a Cresta 32, with four-bladed 19 x 21in props, driving through 2.08:1 reductions and pushing a total of 7.8 tonnes including 3 adults and 1100L fuel. Average of two runs on Lake Macquarie NSW, calm water.
Range is in nautical miles with a 10% reserve. Fuel usage for both engines combined.
Twin 7.2 litre Chrysler 330 HP V8s in a Cresta 32, with three-bladed 18 x 21in props through 2:1 reductions and pushing 7.8 tonnes. Average of two way runs on Lake Macquarie NSW, calm water. Range in nautical miles. Full fuel tank with a 10% reserve. Fuel usage for both engines combined.
ENGINE TYPE Straight six turbo intercooled diesel
RATED BHP/MHP* 311/315 at 3800rpm
BORE X STROKE 94 x 100mm
DRY WEIGHT 451kg (shaftdrive); 452kg (V-drive)
*Brake hp/metric hp