Mercury Optimax 135

Having tested a run of Evinrude FICHT motors in recent months it was great to finally test an Optimax model specifically Mercury's 'little' V-six, the Optimax 135.

As most Trailer Boat readers know by now, the in-part Australian-developed Orbital Direct Fuel Injection system fitted to Optimax models differs markedly from OMC's FICHT system. For starters, there's an air compressor in addition to the pressurised fuel delivery system. According to Mercury Marine, this creates a better air/fuel swirl in the combustion chambers, finer atomisation of the mix and even cleaner running than the FICHTs.

But while Optimax models use a 400:1 petrol/oil mix at Dead Slow Troll (DST), this is increased to 44:1 at Wide Open Throttle (WOT) compared to 300:1 to 60:1 for the FICHTs.

Like the FICHTs, the Optimax models automatically double the petrol/oil mix for the first five hours running, provided the engine is kept above about 2000rpm. When the break-in period has finished the engine management system (ECU) automatically resets the petrol/oil ratio to normal levels.

Another example of this 'intelligent' engine management relates to idle speed. When cold, Optimax engines' idle speed is slightly higher than at normal operating temperature then after a minute or so the revs drop to a steady 550rpm in or out of gear, with absolutely no 'clunk' between shifts.

Interestingly, the standard water temperature gauge showed the Optimax to run hotter at low speeds than at WOT, unlike most other outboards. This substantially reduces exhaust pollution during extended low speed operation.

Although I could feel no difference in vibration levels between the Optimax 135 and the FICHT 150 models I've tested in the past (at least three of them to date) there was a distinct air compressor sound, somewhat like that of tyre air compressor pumps at service stations. This remained noticeable up to about 3000rpm when the normal Mercury V-six exhaust note took over. A favourite of some performance boaties, the trademark howl at WOT gave the Optimax 135 higher noise levels than the FICHT 150.

Mounted on a 5.2 metre Haines Signature 520R and spinning a 19-inch Laser II stainless steel prop, the Optimax 135 handled our estimated 950kg total (including two adults) with ease.

Because of the exhaust bleed holes let into the Laser's hub there was a slight delay in acceleration up to about 3500rpm until the prop started to bite solid water, but once planing, the acceleration out to WOT revs was unlike any carbie motor I've tested in this power range.

Over a sizeable opposing wind and tide chop on Newcastle Harbour at 4000rpm we cruised effortlessly at 56.4kmh and could hold a normal conversation at the helm. And through hard-over figure of eights there was absolutely no prop ventilation.

Although the harbour was like an obstacle course because of floating logs from recent rains, we managed a brief WOT run which returned 80.5kmh at 5400rpm consuming 48lt/h. According to Hamilton Marine's Hylton Hamilton (who supplied the demo rig) on calm water these figures improve to around 96kmh and 5800rpm.

Servicing the Optimax 135 may initially appear complex but in reality it is straightforward. The watercooled ECU is mounted to port at the front of the motor where it can be serviced (in an emergency) while the boat is afloat. There's a serpentine belt that drives the air compressor and car-type alternator and the spark plugs are quite easy to reach. To starboard are the main wiring harness and compact starter motor.

The only cause for concern in my opinion is the alternator. As it is now, it is exposed to salt air it should be enclosed with aircooling holes.

Servicing intervals are every 100 operating hours or once a year after the first 20, 50 and 100 hours or three, six and 12 months. The waterpump impeller should be replaced every 100 hours or annually.

With their air compressors and associated plumbing the Optimax models are more complex than their FICHT competition and the longshaft 135 weighs 9% more than a longshaft FICHT 150. This is still 13% lighter than Honda's BF130.

Note that the new Optimax appears better built and finished than either of these powerplants.

Apart from initial purchase cost the only hurdle Mercury Marine has to overcome is that (unlike the FICHTs) at the time of testing its Optimax models were not warranted for commercial applications.

Time will tell how reliable the Optimax models prove to be compared to their low emission competition!