words - Andrew Norton
Tohatsu's new TLDI 50 represents a quantum leap forward in outboard engine technology, reports Andrew Norton

Tohatsu's new TLDI 50 is the most significant new outboard since OMC released its first FICHT model locally three years ago.

The beauty of the TLDI 50 is its simplicity. It's based on the popular Tohatsu M50D2 and apart from fuel and air delivery modifications and the addition of a sophisticated engine management system, it's still a two-stroke and has throttle response that only the DOHC Evinrude 50 comes close too, but still can't match.

Although there are some differences between the Orbital direct-injection system fitted to the TLDI 50 and Mercury Marine's Optimax models, in many ways the TLDI is a miniature Optimax. There's still the air compressor, relatively low fuel-injection pressure and the constant idle revs in or out of gear. At low speeds it sounds like an Optimax, although the compressor noise is not as prominent.

Basically there are six injectors. Three inject air into the fuel rail at 90psi, which is then mixed with the fuel pressurised to 70psi and directed to the three combustion chamber injectors. Says Tohatsu, the result is complete atomising without having to resort to high fuel-injection pressures (250psi with FICHTs, 750psi with Yamaha's HPDI models).

A single-throttle body air intake incorporates Throttle Position Sensors for load and barometric pressure (to ascertain the correct air/fuel ratio) and along with the air, lubricating oil is carried via the crankcase to the combustion chamber. Tohatsu Corporation claims that the fuel/oil ratio varies from 50:1 at WOT down to 450:1 at Dead Slow Troll (DST), though I haven't seen this in writing!

For ease of servicing the six injectors are contained in a neat cast block, held in place to the cylinder head by just two bolts. Really the only omission appears to be an oxygen sensor in the exhaust system.

In terms of diagnosing problems, the TLDI 50 leaves its FICHT, Optimax and HPDI competition for dead. There are a total of 28 troubleshooting codes and various combinations of the three warning lights in the analog tachometer that allow a servicing technician to work out where the problem lies without needing a laptop computer. This is a real advantage for commercial operators who may use the engine in remote locations.

Unlike FICHTs, where a laptop computer must be used to program a new or reconditioned engine to increase the fuel/oil ratio for the first five hours, all the Tohatsu dealer has to do is "force feed" the engine oil for the first half-hour. This is achieved by resetting the oil pump control lever and running the engine at idling speed in a test tank. When this is completed, the owner/operator simply follows the normal 10-hour break-in schedule.

Warning systems include overrev (the limiter is set at 6000rpm), low oil, engine overheat and low battery voltage. Should any of these problems occur, after a predetermined period the engine management system will initially reduce the revs to 2800-3200, then 700-900. The engine must be stopped and the problem rectified before the system resets.

The engine management system has at least 300 hours of memory. Should an operator not follow the break-in schedule (ie: limit WOT operation), overrev and overheat the engine or not keep the engine oil level above the minimum level this information will be recorded. If the owner then complains of poor performance or a breakdown, the service technician can access the memory and see if the engine was abused u which will, of course, void the two-year recreational warranty (one year for commercial operators).

Late in June I tried the first TLDI 50 in the country (supplied by Lakeside Marine, the national Tohatsu distributor) which was set up on a Stessl Striker 4.4 runabout. Okay, I know the hull is normally rated to 40hp, but Lakeside specified a factory upgrade to 50hp and it still provided a reasonable testbed for the engine. Because of the relatively short planing length, full trim-in was needed to plane us then the engine could be quickly trimmed slightly out past vertical before prop blow-out occurred.

Spinning a 13-inch pitch stainless steel prop, the engine provided as much performance as you would ever want on an aluminium hull this size. It started instantly cold and warmed quickly at a steady 700 revs with a slight oil smell (though no smoke) for the first few minutes. There was absolutely no clunk when reverse was engaged and with the anti-ventilation plate immersed, the engine edged us out from the weedy ramp at the Toukley (NSW) Aquatic Centre.

When forward gear was engaged there was a slight clunk, but nothing like that on a carburettored 50. But so bad was this weed that within 15 minutes the three tacho warning lights lit up, letting us know the engine was overheating due to a cooling intake blockage. A quick burst in reverse and this was cured.

At DST and at an almost vibration-free 700rpm (similar to the old two-stroke four-cylinder Mercury 50) the demo engine pushed our 600kg rig at 4.8kmh and consumed an incredibly-low 0.5lt/hr. This was about what a four-stroke 20 would consume. It is also 37% and 44% less than the Evinrude and Honda 50 respectively. And compared to the Yamaha F50, the TLDI 50 used half the amount of fuel!

Don't forget that the Evinrude, Honda and Yamaha all DST at least 50 revs faster u which for freshwater anglers can be the difference between holding onto a lure or not.

The four-strokes also don't have the TLDI's innovative trolling feature where depressing the starter key for one second increases the trolling speed by increments of 100 revs out to 900. So you can set you favourite trolling speed without having to touch the throttle!

At 1000 revs the TLDI 50 averaged a wake-free 7.1kmh and at 2000 revs 10.4kmh. The minimum planing speed was 24.5kmh on 3000 revs, while at an effortless cruise of 4000 revs and 38.9kmh the engine consumed 8.8lt/hr. This was 26% and 29% more than the Evinrude and Honda, but 19% less than Yamaha's F50 which doesn't develop any real power until about 4500rpm.

Above 4000 revs the power came in and the engine wanted to reach WOT very quickly. At 5000 revs the Tohatsu maintained a fast cruise of 49.0kmh and at 5800 revs and WOT 56.8kmh, consuming 19.2lt/hr. This was 20, 10 and 7% higher than the Evinrude, Honda and Yamaha respectively.

Of course, comparing two and four-strokes on fuel consumption alone doesn't give a true picture of an engine's fuel efficiency or how much fuel is used for distance travelled. For example, at 4000 revs the Evinrude and Honda averaged 26 and 31.6kmh, while the Yamaha averaged 39kmh. But at 4000 the Tohatsu averaged 38.9kmh and based on the fuel figures recorded it was 19 and 22% more fuel efficient than the Evinrude and Yamaha. Only the Honda was more fuel efficient ? by 5%.

At WOT the Tohatsu was 3% more fuel efficient than the Evinrude, 7% over the Honda and an impressive 11% over the Yamaha. Noise levels were also significantly lower than the carburettored Honda and Yamaha and slightly lower than the EFI Evinrude.

Through hard-over turns there was absolutely no prop ventilation, unlike the four-strokes which were all fitted with alloy props. In weedy waterways such as Lake Budgewoi the sharp blade edges of the stainless steel prop really were effective weedcutters!

The TLDI's undercowl access is excellent, with all components such as the two fuel pumps, vapour separator, injector block and throttle body assembly well laid out.

Recommended servicing intervals are every 50 hours or three months after the first 10 hours. The waterpump impeller should be changed every 150 hours or once a year.

According to Lakeside Marine, servicing the TLDI 50 should be no more expensive than the carburettored model and based on personal experience about 60-70% that of a comparable four-stroke - an important consideration in these days of GST-inflated servicing charges.

Tohatsu's TLDI 50 is the best news in several years for family boaters and anglers alike. It combines the performance of a two-stroke with the fuel efficiency of a four-stroke, but is an inherently simple engine. Sure it doesn't run quite as cleanly as its direct Evinrude (four-stroke) competition, but then there are no valves to adjust or sump oil to change.

As Trailer Boat readers know, I'm a lover of four-strokes in mid-range outboards. But that's only because up until now there's been nothing to compare them to. Tohatsu's TLDI 50 changes that.

longshaft tldi 50-epto
Engine type: Direct-injection loop-charged, two-stroke
Cylinders: Three inline
Prop hp/rpm: 49.3/5680
WOT rev range: 5500-5680
Piston displacement (cc): 697
Bore x stroke (mm): 68 x 64
Ignition system: CD with electronic engine management
Charging circuit (amps): 19 amps regulated
Fuel type: Straight ULP
Oil type: TC-W3
Oil tank capacity (litres): Undercowl 2.0lt
Gear ratio: 1.85:1
Transom height/weight: 508mm/94kg
Approx. GST-inclusive retail: $9500
Test engine from Lakeside Marine, Charmhaven (NSW), tel (02) 4392 6110.

Published : Sunday, 1 October 2000

Prices and specifications supplied are for the market in Australia only and were correct at time of first publication. boatsales.com.au makes no warranty as to the accuracy of specifications or prices. Please check with manufacturer or local dealer for current pricing and specifications.

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