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Author Topic:   Yamaha 225 Four Stroke
grandmufti posted 05-27-2002 10:04 PM ET (US)   Profile for grandmufti   Send Email to grandmufti  
I got pulled over by the Coast Guard today for a safety inspection while out on Lake Erie.I passed with no problem.What really impressed me was the motor they were running.They had a 225 Yamaha Four Stroke hanging on the back of their rig.Prior to seeing and hearing this motor I thought my Yamaha 115 Four Stroke was quiet.They actually ruined my day.That motor is so quiet at idle that you do not hear it at all.How can a bigger motor be quieter than mine?
jimh posted 05-27-2002 10:22 PM ET (US)     Profile for jimh  Send Email to jimh     
It does seem that virtually all 4-stroke outboards are described as quiet. This seems a bit odd, since there does not seem anything obvious about the 4-stroke (in lieu of 2-stroke) operation that would make it produce less noise.

Now better fuel economy and lower emissions, these seem easily understood; a 4-stroke does not waste any unburned fuel like a 2-stroke.

But why they seem to be inherently quieter I don't see a simplistic explanation.

Perhaps it has to do with the fewer number of power cycles (half as many). Yet if the engine has the same horsepower, then each of the power cycles must be twice as powerful since they only happen half as often.

Or perhaps it has to do with the ability of the 4-stroke to tolerate much high back pressure in the exhaust path. It may just be there is better mufflering on a 4-stroke because it can tolerate higher back pressures.

Anyone know the answer to this?

Dick posted 05-27-2002 10:52 PM ET (US)     Profile for Dick  Send Email to Dick     
I can't answer the question as to why the are quieter, but my 4 stroke is quieter than any outboard I have ever run.

The King County Sheriffs Dept runs a big aluminun hog here on Lake Washington. After blowing several 225 Yamaha 2 strokes they put on a pair of Yamaha 225 4 strokes and they love them. At this point they have more trouble free hours on the 4 strokes than they ever did on the 2 strokes. I don't know how this relats other than it looks like the 4 stroke is a better motor for heavvy duty use.

LarrySherman posted 05-29-2002 10:24 AM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     
If I remember correctly, on 2 cycle motorcycle engines, the exhaust is tuned to provide the correct amout of back pressure on the cyclinder to allow the cyclinder to draw in the fuel air mixture and expel the exhaust. If tuned incorrectly, you loose massive amounts of power. That is why 2 stroke exhaust manifolds and canisters are always so strangly shaped comparred to 4 strokes. the same must be true for 2 stroke outboards...thus the noise.


crashq posted 05-29-2002 01:16 PM ET (US)     Profile for crashq  Send Email to crashq     
One of the reasons why two strokes are so noisy is that they expel their exhaust on the second half of the power stroke. The outgoing gas is under higher pressure than a four stroke when it exits from cyclinder. The pressure in the cylinder quickly reaches equilibrium with the manifold pressure, but then decreases as the piston continues its downward stoke (and increases the cylinder's volume). The motors are designed so that as the pressure drops, the piston travels past the reed valves (i.e. intake port) and the lower pressure draws fuel and air into the cylinder sweeiping the remainder of the exhaust out of the cylinder.

Some unburned fuel and fresh air from the intake are also swept out of the exhaust port in the same motion. That is why standard two strokes have poor emissions performance. With the injected two strokes, only air comes in through the intake port (and therefore no fuel from the intake port is swept out), thereby improving emissions performance

lhg posted 05-29-2002 06:38 PM ET (US)     Profile for lhg    
The Yamaha 225 4-strokes, at 3500 RPM, are not as quiet as my 200 Merc 2-stroke EFI's.
At idle yes, at speed, no. Same for the Opti's. Definitely more decibels at cruising speed.

Any internal combustion outboard is going to make mechanical noise at 4000 rpms. The 4 strokes have 10 times as many moving parts. The exhaust noise is mostly buried in the wake.

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