Engine Review: Tohatsu BFT150A
No doubt about it, Tohatsu's Honda-origin four-stroke 150 is a complex engine. Based on the Accord Euro car engine, it's equipped with components that not only improve mid-range fuel efficiency but also increase bottom-end grunt and top-end performance.
The BFT150A incorporates BLAST, ECO-mo and V-TEC to provide upper-end performance found wanting in some of the direct competition. But in true Honda fashion it has sensible features such as chain-driven camshafts, relatively simple valve clearance adjustment and a massive oil sump capacity to improve oil performance between oil and filter changes.
THE NITTY GRITTY
BLAST works by altering the air/fuel ratio from the normal 14.7:1 to 12:1 to pour in more fuel and increase bottom-end torque. Once engine load has fallen off when planing, ECO-mo kicks in by leaning the air/fuel ratio to 18:1 for unmatched mid-range fuel efficiency. As the rpm increases towards Wide Open Throttle the air/fuel ratio reverts to 14.7:1, the normal air/fuel ratio in engines not having BLAST and ECO-mo.
But there's way more to this coordination. Below 4000 rpm the intake air travels further to the valves to increase torque and above 4000 the air passage is shorter for more power. At 4500rpm the V-TEC system holds intake valves open for longer to get more air into the combustion chambers.
Unlike some of the direct competition the BFT150A has an oxygen sensor probe in the exhaust system. This measures the density of the air/fuel mix and advances the ignition timing when premium fuels such as 95 or 98 are used. The engine develops more torque and power than listed in the Tohatsu outboards brochure because of the denser fuel and in reality can provide better overall fuel efficiency than when using standard 91 unleaded, negating the higher fuel cost. Fuel is injected in the traditional EFI manner with injectors mounted in the intake manifold and relatively low injection pressure.
Yet though appearing to be highly stressed the engine is way underrated compared to the version in the Accord Euro, which developed 197 BHP at 7000rpm.
The self-adjusting chain drive for the double overhead camshafts eliminates the fear of camshaft belt breakage (this engine is an interference type where valves could contact piston crowns) while the twin balance shafts overcome vibration issues inherent in all petrol engines over 2.2lt piston displacement.
A 40A voltage regulated alternator provides adequate power for on board electronics with around 5A to 10A needed to power the multipoint EFI. Unlike Direct Fuel Injection two-stroke outboards only a normal car starter battery is needed and I recommend a 60Ah unit with 600 CCA (Cold Cranking Amps) that weighs around 15kg. Having additional CCA over the normal 450 CCA for an engine of this piston displacement ensures faster cold starting and a way longer battery life, as I've found going down this track in my 2.6lt Mitsubishi Triton ute.
The BFT150A has Tohatsu's On Board Communications system for connection to sounders and instrumentation such as range to empty for real time fuel usage monitoring.
Clearance adjustments for the 16 valves are done using a 10mm spanner and feeler gauge, a good compromise between Mercury Marine's roller cam followers and the Japanese competition's bucket and shim adjustment.
Four-stroke outboards are prone to oil sump dilution during extended trolling periods. The engine runs much colder than it would in a car, resulting in fuel blow by past the piston rings and into the sump under light loads. The oil is diluted by fuel preventing the oil from providing adequate lubrication. However the massive sump capacity of 6.7lt (some automotive engines of similar piston displacement have 4lt sumps) helps absorb this sludging between oil and filter changes, though a lengthy run at WOT after trolling periods will reduce this dilution.
The recommended oil is FCW (Four Cycle Water cooled) SAE 10W30 for all but tropical climates, with oil and filter changes every 100 running hours or annually after the first 20 hours or three months. The spin-on canister oil filter is easily removed and replaced. Access to the rest of the power head is good with wiring neatly routed as expected of a Japanese-made engine.
The gear ratio is a usefully low 2.14:1 enabling coarser pitch and more thrust efficient props to be swung compared to 2:1 and taller ratios, yet not too deep requiring a larger gear case that would create more hydrodynamic drag.
Earlier this year Lakeside Marine, the national Tohatsu distributor, extended the recreational warranty coverage for its four-stroke outboards to five years in line with other outboard manufacturers. Long warranty coverage is an important consideration when buying a new outboard, though with fresh water flushing after every usage and scheduled servicing there's no reason why the BFT150A won't return trouble-free boating long after the warranty has expired.
ON THE WATER
The demo engine was mounted on a Stejcraft 580BR Bahama bowrider and was swinging a 17-inch pitch four-bladed stainless steel Solas Titan prop, perfect for our total displacement of 1400kg including two adults and 100lt of Premium 95 fuel.
The engine started instantly hot or cold with no oil smoke appearing at any time, nor was there an oil smell when backing upwind. The beautifully balanced engine sent barely a tremor through the hull structure (fibreglass is excellent at transmitting engine vibration) and when idling the engine was so quiet it could barely be heard. Providing the anti-ventilation plate was kept at least three-quarters immersed power astern was good, great for backing out of skinny water. The shift quality into forward or reverse was excellent with barely a clunk, reinforcing the overall quality feel of this engine. No fear of losing my copious flab with this engine!
Once warmed up the BFT150A planed us in just 3.4 seconds, with another 16.7 seconds out to WOT. Through tight turns the Titan held on with no indication of prop ventilation and the leg could be trimmed well out before signs of prop blow out. Through the rpm ranges there was absolutely no transition through the stages of BLAST, ECO-mo and back to the normal air/fuel ratio, or a surge where the V-TEC cut in. Only smooth progressive power delivery as expected of an EFI engine. At or near WOT the exhaust note was low enough to hold a normal conversation at the helm.
Though I grew up in an era of noisy two-stroke outboards I must confess quiet, effortless power delivery is very nice. I'm sure my tinnitus wouldn't have become so vocal had engines like the BFT150A been around when I was younger.
THE WRAP
Until Tohatsu Corporation develops its own four-stroke outboards in this power range the BFT150A is a good addition to the line up. The engineering in it mirrors Tohatsu's thorough approach to designing and making outboards, a tad conservative but worth the effort in the long run. The build quality and finish are excellent and from my experience of long-term testing Honda outboards for the past 33 years the salt water corrosion resistance should be excellent.
In my opinion the BFT150A is best suited to lighter performance trailer boats that can utilise the free-revving nature of this engine. Mounting it on a heavier boat just wouldn't enable it to show its performance characteristics, although Tohatsu does offer an ultra-long shaft (30 inch) version for use on power cats.
For the location of your nearest dealer Google Tohatsu Outboards Australia, click on “find a dealer”, enter your postcode and hit “GO”.
QUICK SPECS
- TYPE Four cylinder EFI DOHC 4 stroke
- RATED BHP/MHP*147.9/150.0 at 5500rpm
- WOT RPM RANGE 5000 to 6000
- DISPLACEMENT 2354cc
- BORE X STROKE 87 x 99mm
- GEAR RATIO 2.14:1
- WEIGHT 220kg dry, long shaft
- RRP $21,398
- OEDA STARS 3
*Brake horsepower/metric horsepower
Check out the full feature in issue #505 of Trade-a-Boat magazine. Subscribe today for all the latest camper trailer news, reviews and travel inspiration.