words - Andrew Norton
In the next in his series of bi-monthly guides to choosing the right low pollution outboard for your boat, Andrew Norton compares 70–80hp four-strokes and DFI two-strokes

Up-rated from its DF60 counterpart, currently the DF70 is the only EFI four-stroke outboard in the 70–80hp range.

The sequential multipoint EFI enables the 70hp to run as well on Alpine lakes as it does at sea level because the engine management system automatically alters air/fuel ratios according to barometric pressure. The alternator has the highest output of all four-strokes in this power range, but about 5amp is used to power the EFI and management computer.

Since its Aussie release six years ago I've tested both the Suzuki and Johnson versions of the DF70 and each time they have performed flawlessly with strong midrange acceleration, very low vibration levels and no oil smoke appearing at any time. All engines started instantly hold or cold and warmed quickly from cold. A nice touch was the steady idling revs of 700 in or out or gear.

Initially I tested the DF70 and the discontinued carbie two-stroke Johnson 70 on identical 4.95m aluminium hulls and the DF70 easily outperformed the two-stroke for both WOT speed and fuel efficiency, despite the additional 50kg on the transom.

Most recently I tested the DF70 on a Kevlacat 2100 Hardtop where the twin engines provided excellent one ahead/one astern manoeuvrability, easy planing and a relatively high WOT average speed.

Spinning 15in-pitch alloy props and pushing a total of 2100kg including three adults, the DF70s trolled us at 5.0kmh using at total of 1.8lt/h and cruised quietly at 31.7kmh on 4000rpm using 18.0lt/h. There was no prop ventilation through tight figure-eight turns at 4000rpm.

The WOT average was 53.2kmh on 5750rpm using a total of 40.5lt/h with lower noise levels, than Honda's BF75 and the Mariner/Mercury F75.

Powerhead access is good and the dipstick and oil filter are easily reached. The toothed camshaft timing belt has a designed lifespan of 800 hours, and should it break, the engine is a "non-interference" type.

De-rated from its 90hp counterpart, the BF75 is based on the 1.6lt Civic car engine (which has EFI) and is very under-stressed for its output. It has electrically-operated manual chokes for simplicity with a manual override for hand-starting should the battery be flat, and unlike the DF70 a battery isn't needed to run the engine - just as well as the alternator charging capacity is a little low nowadays for a 75.

Mounted on a 5.8m fibreglass centre console Stinger 619-E longboat, spinning a 17in alloy prop and pushing a total of 800kg including two adults, the demo BF75 started easily hot or cold, warmed quickly from cold and didn't blow oil smoke. At 4000rpm it cruised effortlessly at 39.4kmh using 7.5lt/h and these revs through figure-eight turns no prop ventilation occurred. WOT average was 58.8kmh on 5800 revs using 23lt/h.

Although showing its age, the BF75 scores by being a simple engine to work on and has an impressive reliability record. Powerhead, dipstick and oil filter access are very good, while the toothed camshaft timing belt should last around 800 hours.

The result of a joint venture between Mercury Marine and Yamaha Marine, the F75 is de-rated from its F90 counterpart and uses the powerhead from the European Ford Sierra car but with four carbies instead of EFI. The DOHC powerhead is more throttle-responsive than Honda's BF75 and also quieter despite the additional valves and associated complication.

The F75 has an automatic choke system and an Electronic Control Module that advances the ignition timing for cold starting as well as normal operation.

The ECM reduces revs should the engine overheat or experience low oil pressure.

Mounted on a 4.8m Savage Envoy fibreglass runabout and pushing a total of 910kg including three adults, spinning a 16in Quicksilver Vengeance prop, the demo Merc F75 was significantly over-propped but never appeared stressed. It started easily hot or cold; warmed quickly from cold and didn't blow oil smoke. Noise and vibration levels were excellent.

Trolling at 4.9kmh on 850 revs, the F75 used 1.1lt/h and cruised quietly at 54.5kmh on 4000rpm using 10lt/h with no prop ventilation through tight figure-eight turns. Due to the prop, the engine couldn't make it past 4600rpm and used 24lt/h, where the Envoy averaged 62.3kmh.

With DOHC powerhead, access isn't as good as the BF75 but the dipstick and oil filter are easily reached. Unlike the Honda there's no provision for operating chokes manually, so a "healthy" battery is a must.

De-rated from its F100A counterpart, the Yamaha F80A was one of the company's early "big" four-strokes after the release of the F50 in the mid-nineties.

It still has carbies, although the F80B released last Christmas will have EFI, in my opinion long-overdue for freshwater anglers who have had to re-jet the carbies above 1200m.

The alternator output is reasonable for an 80 and 12amps are produced at a low 1000rpm, useful for fast trolling.

As with its BF75 and F75 competition, the F80A is a very under-stressed engine and is capable of easily handling heavily-laden workboats. On a Southern Star UB520 the demo F80A provided plenty of power and had slightly lower noise and vibration levels across its rev range than the BF75 on the Stinger.

Although trolling trials weren't conducted, the fuel consumption should be very similar to the F75. The demo F80A had similar performance to the F75 and spinning a 17in prop and pushing a total of 800kg including two adults, it cruised quietly at 37.5kmh on 4000rpm using 10lt/h. Through tight figure-eight turns at these revs there was no prop ventilation.

The WOT average was 59.5kmh on 5800rpm using 26lt/h, and as with the F75 a normal conversation could be held, and heard, at the helm.

Powerhead access is good with the dipstick and oil filter easily reached, although the carbie balancing linkages appeared more fiddly to adjust than the Merc. Again, the toothed timing belt should last about 800 hours, but as the F80A probably has an interference engine, the belt should be checked for any damage every 100 hours. The first service is at 10 hours.


The second generation of Tohatsu Corporation's direct fuel-injected two-strokes, the TLDI (Low Pressure Direct Injection) 70 is de-rated from its 90hp counterpart and utilises the Orbital Engine Corporation's OCP or Orbital Combustion process which Tohatsu modified in-house for the TLDI series of engines.

The OCP functions by injecting air into the fuel delivery rail at 80psi, which then mixes with fuel pressurised to 90psi, and is directed to the three combustion chamber injectors. This results in complete air/fuel atomisation (down to a fuel droplet size of only five microns) without having to utilise high fuel-injection pressures.

A single throttle body air intake incorporates Throttle Position Sensors for load and barometric pressure (to ascertain the correct air/fuel ratio). According to Tohatsu, the multipoint oil injection system varies the fuel/oil ratio from 50:1 at WOT right down to 450:1 at DST and an electric oil pump accurately delivers oil where it's needed. The stratified combustion system below about 3000 revs gives an air/fuel ratio of about 40:1, richening to 25:1 when the engine operates on homogenous combustion.

Standard are warning systems for engine over-rev (6000 revs), low oil, engine overheat and low battery voltage. Should any of these problems occur, the electronic control unit (engine management system) will progressively reduce revs to 900rpm. The ECU has at least 300 hours of memory and records any operating problems encountered during this period, and 28 troubleshooting codes and various combinations of the three warning lights in the analog tachometer allow problems to be analysed without a laptop computer.

The 70 should provide slightly better performance than the Johnson/Suzuki DF70 and use less fuel when trolling.

Powerhead access is good and the air compressor drive belt, fuel pump, vapour separator, injector block and throttle body assembly are easily reached. The first service is at 10 hours. But as the management computer and electric oil pump draw a fair amount of current and the voltage must be maintained above 10.8, a battery of at around 110amp/h capacity should be used.

De-rated from its 90hp counterpart, the 75 is essentially an Evinrude/Johnson V6 cut in half and has a big bore, short-stroke design for low piston speeds at WOT.

What separates the 75 from its TLDI 70 and Mariner/Mercury OptiMax 75 competition is that the DFI system is completely self-contained and doesn't need a battery to function. So should the battery be flat the engine can be manually started, if your arms are strong enough! Other features include a massive-output variable voltage alternator and Speed Adjusting Failsafe Electronics (SAFE) which reduce engine revs should the oil level in the undercowl tank drop below a preset level or the engine overheats. On Bombardier XD50 the fuel/oil ratios vary from 360:1 trolling to about 60:1 at WOT.

On a Stacer 475 Bay Master Sports runabout and spinning a 14x17in pitch alloy prop, the demo E-Tec 75 was completely unaffected by the total load varying between 860 and 910kg. It started instantly hot or cold and didn't blow any visible oil smoke. Vibration and noise levels were extremely low, particularly from trolling speeds to 4000 revs, but another E-Tec 75 I tested recently had quite a flat spot on transition from stratified to homogenous combustion.

Although since testing the first E-Tec 75 last December I have found the fuel-flow gear fitted to be inaccurate below 2000rpm, the consumption would have been around 0.7lt/h trolling at a steady 500rpm, where we averaged 6.1kmh. Through tight figure-eight turns at 3500rpm there was no prop ventilation. Cruising effortlessly at 4000rpm, averages were 53.5kmh and 15.6lt/h. WOT average was 68.5kmh on 5150rpm using 25.2lt/h.

To ensure gearcase oil longevity the lower unit is from Evinrude's/Johnson's 115–130hp V4 range. While Bombardier says changing the gearoil is not needed for three years, I recommend a once a year check for water contamination, as fishing line caught around the propshaft could chew out the gearcase seal.

E-Tec dealers recommend lubricating the throttle and gearshift linkages and cables at least once a year.

The 75 is Mercury Marine's smallest DFI model and is the least powerful of a trio of OptiMax engines from 75–115hp, all utilising the 3lt powerhead from the V6 OptiMax 200–250hp but essentially cut in half to provide the 1.5lt powerhead.

The 75 uses Orbital Engine Corporation's OCP, which mixes fuel pressurised to 80psi with air at 90psi which is then injected directly into the combustion chambers, providing an atomised spray of around five microns per droplet. A mix of stratified combustion where the air/fuel ratio is about 40:1 is used below around 1500 revs and homogenous (25:1) above for hole-shot power. As with Tohatsu's TLDI system, engine oil is injected electrically where needed but the fuel/oil ratios vary between 400:1 when trolling out to 44:1 at WOT. A laptop computer is needed to diagnose any running problems saved in the ECU's memory.

The 75 has automatic additional engine oiling for the first five hours and is SmartCraft compatible, giving operators fuel-flow and range to empty info and providing full engine protection including automatic rev reduction should the engine overheat or suffer low oil pressure/level. The 75 can be fitted with TrollControl for 400rpm ultra low-speed trolling.

So far I've not been able to test, or even sight, the 75, but with massive piston displacement for output and under-stressed powerhead, it might outperform the E-Tec 75 while returning excellent midrange fuel efficiency.

Like the TLDI 70, the OptiMax needs a battery to power the ECU so a capacity of at least 110amp/h is required to prevent low-speed engine misfiring.

All engines profiled comply with US EPA 2006 exhaust emission regulations. In addition the Honda BF75, Johnson/Suzuki DF70, Evinrude E-Tec 75 and Mariner/Mercury OptiMax 75 meet California Air Resources Board (CARB) 2008 requirements. The DF70 also meets EU2005 requirements and the E-Tec 75 EU2006, which regulate noise emissions.

Standard are two-year warranties for recreational usage, while the E-Tec 75 has three years and in addition the Mariner/Mercury F75 and OptiMax 75 have three years against corrosion perforation and four for selected ignition components.

Recommended servicing requirements for the four-strokes are every 50 hours (100 for the BF75) with water pump impeller replacement every 100 hours.

Apart from the Yamaha F80A, the first service is at 20 hours when the valve clearances should be checked. All the four-strokes have canister-type oil filters.

The Tohatsu TLDI 70 and OptiMax 75 require servicing every 100 hours while the E-TEC 75 doesn't require any servicing for the first 300 operating hours, or three years, whichever comes first.

Because the output ratings vary across 10hp, the specifications tables list engines from 70–80hp, not alphabetically as with outboards 60hp and below.

Having not yet tested the TLDI 70 or OptiMax 75, I can't comment on how well these engines compare with the E-Tec 75 or the four-stroke competition. But of the other engines it's a toss-up between the DF70 and E-Tec 75, which are the cleanest-running engines in the 70–80hp range. The DF70 has a proven reputation for reliability and an extra longshaft option for cats, while the E-Tec 75 scores for power-to-weight and throttle-responsiveness.

From my testing of three of each, the DF70 has been the more consistent performer and at this stage of E-Tec development my preference must go to the DF70.

That said, the Honda 70 has a proven track record for reliability. Stay tuned for tests on the 75hp OptiMax and Tohatsu outboards - they're bound to give the E-Tec and DF70 a run for their money.

70–80hp four-strokes
Engine Johnson/Suzuki 70 Honda 75 M/M 75 Yamaha 80
Engine type All crossflow four-cylinder
Valves 8 12 16 16
Valve actuations SOHC SOHC DOHC DOHC
Drive All-belt driven
Prop HP @ rpm Carburetted 73.9/5500 75.1/5000 78.8/5500 --
EFI 69.0/5500 -- -- --
WOT rev range Carburetted -- 5000–6000 4500–5500 5000–6000
EFI 5200–5500 -- -- --
Piston displ. (cc) 1298 1590 1596 1596
Bore x stroke (mm) 74x75.5 75x90 79x81.4 79x81.4
Ignition system Carburetted All CD with electronic timing advance
EFI All electronic engine management
Charging circuit (amps) Carburetted -- 16 reg 20 reg 20 reg
EFI 25 reg -- -- --
Break-in period (h) 10 10 10 10
Fuel delivery Carburetted All four single-barrel
EFI Seq. multipoint -- -- --
Fuel type All 91 RON ULP
Oil type (SAE) 10W30/40 10W30/40 10W30/40 10W40
Oil capacity (lt) 4.5 4.5 4.5 4.5
Gear ratios 2.42:1 2.3:1 2.33:1 2.31:1
Transom heights (in) 20/25 20/25 20/25 20/25
Weights (kg) Longshaft 162* 169* 175* 174*
Weights (kg) Extra-long 167* 174* 180* 178*
Rec. retail Longshaft $10,895* $12,390* $12,997* $12,243*
Rec. retail Extralong $10,995* $12,895* $13,450* $12,536*
* Weights and prices are electric start remote control versions with power trim and tilt, current at time of publication

70–80 two-strokes
Engine TLDI 70 E-TEC 75 OptiMax 75
Engine type All three-cylinder DFI two-stroke
Prop hp @ rpm 69.0/5500 75.1/5000 75.1/5375
WOT rev range 5150–5850 4500–5500 5000–5750
Piston displacement 1267 1295 1524
Bore x stroke (mm) 86x72.7 91x66 92x76
Ignition system All electronic engine management
Charging circuit (amps) 23 reg. 75 reg. 60 reg.
Break-in period (h) 10 5 5
Fuel/air scavenging All loop-charged
Fuel delivery Low pressure dual stage High pressure single stage Low pressure dual stage
Fuel type All 91 RON ULP
Oil type Valvoline Premium TC-W3 Bombardier XD50 or XD100 Quicksilver Premium TC-W3
Oil capacity (lt) 4.0 undercowl 2.8 undercowl N/A
Gear ratio 2.3:1 2.33:1 2:1
Transom heights (in) 20/25 20 only 20/25
Weights (kg) Longshaft 143* 138* 164*
Weights (kg) Extra long 147* -- 169*
Rec. retail Longshaft $12,504* $12,375* $13,102*
Rec. retail Extralong $12,687* -- $13,706*
* Weights and prices are electric start remote control versions with power trim and tilt, current at time of publication

Published : Tuesday, 1 March 2005

Prices and specifications supplied are for the market in Australia only and were correct at time of first publication. 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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