Two-Stroke or Four-Stroke - Part One

To give Honda Marine its due, over the past nine years it's done a great job of educating boaters about the clean, low-pollution and fuel-efficient operation of four-strokes. But for some boaters four-strokes are an absolute waste of money and the traditional two-stroke still has a lot to offer.

When buying a new outboard, you really should analyse how much performance you need and how frequently the motor will be used. With the exception of Honda's VTEC-equipped BF225, four-strokes still don't provide the brilliant hole-shot acceleration and upper rev-range performance of comparable two-strokes, and really, they are only cost-effective when used frequently.

TWO-STROKES
Contrary to Honda's claims that two-strokes would die out, my testing of these in the past few years has proven anything but that. Two-strokes come in a wide range of choices to suit all boating needs.

For example, if you use your motor only 50 hours a year and don't do any waterskiing, then a carburetted premix two-stroke (such as Yamaha's CV85) will do a great job. The carbies rarely go out of tune and are easy to clean and repair should the motor ingest watered-down fuel. And with the advent of Isopropyl Alcohol, you don't have to worry about premix 'going off' during periods of disuse and having to ditch a tankful of expensive fuel before the summer season.

However, from being towed by an old Merc 80 while learning to ski, I know what mouthfuls of premix taste like, and I recommend skiers opt for an oil-injected motor. Instead of the normal 50:1, depending on the outboard brand, the fuel/oil ratio is anywhere from 100:1 down to 200:1 when idling, which makes life a lot more pleasant when you're close to an outboard's exhaust relief.

Oil-injection also eliminates the tiresome task of mixing fuel and oil and reduces oil costs.

But if you want true turn-key starting like a car, then consider an EFI two-stroke. If you ski high-altitude dams such as Eucumbene, it will operate at the same efficiency as at sea level and at Wide Open Throttle (WOT) will consume about 5% less fuel than its carburetted counterpart.

And if you pick up a tank of fuel that's been 'cut' with ethanol or cleaning fluid, which may reduce the fuel's octane, the EFI's anti-knock sensor will automatically retard the ignition timing to compensate. In a carbie engine, low octane fuel will result in pre-ignition or 'pinking', which will eventually erode the piston crowns.

If you ski a lot on freshwater dams and/or care about the environment, then a DFI two-stroke is the way to go. Because DFI motors such as FICHT and Optimax operate on a mix of stratified and homogenous combustion, the fuel savings are enormous even compared to EFI motors.

For example, EFI two-strokes such as those made by Mercury and Yamaha operate on air/fuel ratios of 14-15:1. But FICHT and Optimax motors have air/fuel ratios of about 40:1 below 2000rpm and 25:1 above. When they're idling, the fuel consumption is about one third that of an EFI motor, and even at WOT an Optimax 200 uses 20% less than an EFI 200.

Both FICHT and Optimax motors use a lot less oil than EFI motors and have fuel/oil ratios of 60:1 - 300:1 and 44:1 - 400:1 respectively. But EFI and DFI motors must be fitted with effective water-separating fuel filters to prevent any water or dirt reaching the fuel-injection system.

Correctly propped all two-strokes can be operated at WOT for extended periods without any powerhead damage.

FOUR-STROKES
Until four-strokes were fitted with EFI, most were 'slugs' compared to their two-stroke counterparts. But the testing I've conducted of DFI two-strokes and EFI four-strokes shows much less of a gap.

For example, I recently tested a Mercury EFI F115 against a FICHT 115 on identical 5.5m fibreglass hulls. Using similar prop pitch/gear ratio combinations, the Merc was 7% faster at WOT than the FICHT and returned 12% better fuel efficiency. The only downside was that the Merc lacked the hole-shot acceleration of the FICHT, but for social skiing it would be more than adequate.

Honda's EFI BF225 returns 10% better fuel efficiency than an Optimax 225, both when idling and at WOT, and provides comparable throttle-response, but only because of its variable-flow air intake system and VTEC, which ensures the correct valve lift and opening dwell at all revs. In comparison, Yamaha's EFI F225A has relatively lacklustre performance.

The main strength of four-strokes still appears to be around 2000-4000rpm - the wakeboarding to social skiing range. Tested on identical 5m hulls, a carburetted Merc F90 achieved a clean plane at 13% less speed and 100 fewer rpm than a DFI Tohatsu TLDI 90. At 4000rpm the Merc was 11% faster while returning 35% better fuel efficiency. But the Tohatsu had better hole-shot acceleration and when idling returned 80% better fuel efficiency!

All four-strokes 75hp and above are based on automotive engines, which are designed to operate under entirely different (overall much lighter) load conditions to marine engines. An example is Honda's BF130, which is limited to 4500rpm for maximum continuous operation.

DON'T FORGET SERVICING COSTS!
After the initial service, a two-stroke normally requires servicing only once a year, whereas in a four-stroke the oil and filter must be changed every six months regardless of how few hours are clocked up during this period.

This is essential with automotive-based four-stroke outboards, which run at least 20° cooler than they were originally designed to do. The lower operating temperatures increase piston ring blow-by because the rings have not expanded to their designed diameter, allowing water vapour from the combustion process to reach the sump. An emulsion called 'sludge' is created, which can block the oil pump strainer and reduce lubricating oil pressure.

An example of the difference in two and four-stroke servicing costs was provided recently by Hamilton's Marine of Wickham NSW. Excluding parts, servicing costs for a two-stroke Merc 90 are $396 for the first year then $132 for each year thereafter, compared to $528 and $352 for a Merc F90. It would sure take a lot of fuel to recoup the difference!