90 to 100hp four-strokes and direct-injection two-strokes

FOUR-STROKES

HONDA BF90
Up-rated from its 75hp counterpart, the BF90 is based on the 1.6lt Honda Civic car engine. It has electrically-operated manual chokes for simplicity with a manual override for hand-starting should the battery be flat, and unlike the Johnson/Suzuki DF90, a battery isn't needed to run the engine - which is just as well given the alternator charging capacity is low nowadays for a 90.

Mounted on a 5m Horizon aluminium centre console spinning a 19in alloy prop and pushing a total of 900kg including four adults, the demo BF90 performed well on this hull despite being over-propped. It started easily hot or cold, warmed quickly from cold and didn't blow any visible oil smoke.

The trolling average was 4.6kmh on 800 revs using 1.1lt/h. At 4000rpm the averages were 45.0kmh and 9.0lt/h. Through tight figure-eight turns at these revs no prop ventilation occurred.

The WOT average was 62.0kmh on 5300 revs using 26lt/h and for a carbie four-stroke noise and vibration levels were pleasantly low.

Although the BF90 is still a "first generation" four-stroke it scores by being a simple engine to work on and has an impressive reliability record. Powerhead access is very good and the dipstick and oil filter are easily reached, while the toothed camshaft timing belt should last around 800 hours before it needs replacing.

JOHNSON/SUZUKI DF90
De-rated from its 115hp counterpart and based on the Suzuki Vitara 4WD engine, the DF90 is the only four-stroke in this power range to have EFI, which enables it to operate as efficiently on high altitude alpine lakes as it does at sea level and equally well on hot humid summer days or cool dry winter ones.

Another advantage is the two-stage gear reduction, achieved by offsetting the driveshaft from the crankshaft with a reduction in between these and an additional reduction in the gearcase where drive changes from vertical to horizontal. This system enables coarse-pitch props to be swung without increasing gearcase diameter and subsequent hydrodynamic drag. Both sets of reductions are chaindrive, as is the camshaft drive, and Suzuki says the chains are designed to last the life of the engine.

The alternator produces 24 amps more than the BF90's alternator and twice that of the F90's system, but some of that current is used to power the computer-controlled EFI.

The drawback of the DF90 is its weight. The DF90 is best suited to mounting on aluminium hulls that have full-width transom pods and are over 5m in length.

On a 5.3m Clark Abalone Centre Console the demo DF90 proved almost as quick as a DF115 on a Clark 570 Abalone. The DF90 started instantly hot or cold, never blew any visible oil smoke and was so quiet when trolling it could barely be heard from the helm, while vibration levels across the entire rev range were very low. Slightly under-propped spinning a 19in-pitch alloy Suzuki prop and pushing a total of 950kg including three adults, the Suzy trolled at 4.8kmh on 650 revs using 1.1lt/h and averaged 38.2kmh on 4000 revs using 12.5lt/h.

Through tight figure-eight turns at 4000 revs there was no prop ventilation.

The WOT average was 57.9kmh on 5600 revs using 30.5lt/h. Powerhead access is good and the engine oil dipstick and oil filter are easily reached.

The large oil sump has sufficient capacity to effectively reduce oil sludging between oil/filter changes when used for extended trolling periods.

MERCURY/MARINER F90
A joint venture between Mercury Marine and Yamaha Marine, the F90 is up-rated from its F75 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 F90 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 also reduces engine revs should the engine overheat or suffer low oil pressure.

Mounted on a 5.05m fibreglass Signature 520R Runabout and spinning a 14in Quicksilver Vengeance stainless-steel prop the demo F90 provided a good blend of performance and fuel efficiency. It started easily hot or cold, warmed quickly from cold and didn't blow any visible oil smoke.

Pushing a total of 920kg including two adults the F90 trolled us a 3.8kmh on 750 revs using 1.1lt/h and averaged 45.5kmh on 4000rpm using 10.5lt/h. Through tight figure-eight turns at 4000 revs there was no prop ventilation. However for some reason the rev limiter was set at 5300 revs where we averaged 66.0kmh using 28lt/h.

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

YAMAHA F100A
Up-rated from its F80A counterpart, the F100A was Yamaha's next "big" four-stroke after the release of the F50 in the mid-90s and remains the lightest four-stroke outboard in this power range. The alternator output is acceptable for a 100hp outboard and 12 amps are produced at a low 1000 revs, useful for fast trolling.

On a 5.8m fibreglass Southern Star longboat, the demo F100A was little affected by widely varying loads and provided significantly better performance than its F80A counterpart. Spinning a 19in stainless-steel Yamaha prop and pushing a total of 850kg including two adults it cruised at 39.9kmh on 4000 revs using 13.0lt/h and through tight figure-eight turns at these revs no prop ventilation occurred. The WOT average was 65.7kmh on 6000rpm using 31lt/h and with pleasantly low noise levels.

Although trolling trials were not conducted, the fuel consumption should be very similar to the F90.

Powerhead access is good with the dipstick and oil filter easily reached although the carbie balancing linkages appeared more fiddly to adjust than the F90. Again, the toothed timing belt should last about 800 hours but as the F100A probably has an "interference" engine (should the belt break the valves could contact the piston crowns) the belt should be checked for any damage every 100 hours. The first service is at 10 hours.