Two Stroke or Four Stroke? What’s the Difference?
We are asked this question nearly everyday!
Generally the person asking does not want the technical answer, but instead the practical answer to the question - what does it mean to me?
Basically - a four stroke outboard is like your car motor.
Just like your car, there is a pan at the bottom of the motor that holds the oil when not running.
When you start the motor, a pump quickly recirculates the oil to lubricate the internal moving parts of the motor.
After 3,000 miles or so, you drop your car off at JiffyLube and they drain the oil, install a new filter and refill the motor with fresh oil.
You, or your mechanic will do the same after so many hours of run time with a four stroke outboard motor - generally every 100 hours.
Conversely, a two stroke outboard boat motor requires oil to be mixed with the gas in order to lubricate the internal moving parts.
Modern two stroke motors mix the oil and gas automatically, so you don’t have to worry about gas to oil ratios while at the fuel dock.
I can remember as a kid trying to figure out the amount of oil I needed to add when topping off the 6 gallon tank with 3.4 gallons of gas at a 50:1 ratio..
Anyway, instead of keeping oil in the belly of the motor like a four stroke and recirculating it over and over, with a two stroke outboard the oil is kept in a separate reservoir and injected as needed.
So, this oil is actually burned off with the gasoline, which is why two strokes can be a bit smokey, especially at start up. This smokey smell is a common complaint of two stroke owners.
More modern two strokes however, burn very little oil and are considerably less smokey than older two strokes.
Two stroke motors generally weigh less and have better power to weight ratios than heavier four strokes of the same horsepower.
When re-powering an older hull, it is important to remember that the manufacture’s power rating may have been prior to the days of often considerably heavier four stroke motors.
Four stroke motors in general are quieter, more fuel efficient, cleaner running and less smokey.
While these numbers can vary widely, the average life span of a two stroke outboard motor is reportedly 1300-1500 hours, while a well maintained four stroke motor may provide up to 3000 hours.
Now, putting this into perspective, if the average boat owner only adds 70 hours per year to their boat, either motor has the potential to provide many years of boating!
Fuel Injected models of either four or two stoke outboards tend to be the easiest to start, and generally have a built in computer or ECM, that amongst other things can calculate hours and run diagnostics.
Neither carbureted four or two stroke motors have this onboard diagnostics computer, so accurately assessing their engine hours is rarely possible.
Some, but not many boats will have an hour meter added to the console when new, and some tachometers will have a built in hour meter, but these are often inaccurate and most will keep ticking away when your 4 year old leaves the key in the “on” position!
Also, when such an hour meter is changed out, it all goes back to zero..
As you make your boat buying decisions, remember, two stroke outboards have been around for well over a 100 years and are well proven, four strokes are the new comers, both if maintained are capable of being very reliable for a very long time.
YCM a different kind of boat dealer
Sidebar -
In an effort to not upset our Evinrude friends out there, the (now discontinued, please see our May 8th post) Evinrude E-TEC two stroke motor is extremely fuel efficient, clean running and not stinky..